GRAVITY AND MAGNETIC STUDY OF NORMALLY AND REVERSELY POLARIZED INTRUSIONS, MENOMINEE COUNTY, MICHIGAN Thesis for the Degree of M. S. MICHIGAN STATE UNIVERSITY * PEDRO LEON TORRES 1976 W Wee; J04? . .AUpmz 0 I330 I W r j ! DD53 ABSTRACT GRAVITY AND MAGNETIC STUDY OF NORMALLY AND REVERSELY POLARIZED INTRUSIONS, MENOMINEE COUNTY, MICHIGAN BY Pedro Leon Torres Aeromagnetic anomalies of positive and negative magnetic amplitude were investigated by surface gravity and magnetic methods. This leads to an interpretation of such parameters as total mass of the anomalous bodies, density contrast, volume, depth, magnetic susceptibility, remanent and induced magnetization components, Koenigsberger ratio, and possible magnetite content. The main purpose was to investigate the remanent magnetization and its polarity by indirect means, using gravity to define the size and depth of the two anomalous bodies, and magnetics to interpret polarity and magnitude of remanence of the two intrusions. The anomalies are adjacent, have roughly concentric pat- terns, and have high and low magnetic intensities. They are suggestive of stock-like intrusive bodies in the basement rock. Such a study of localized intrusions use- fully complements other work that has been done on inter- pretation of dike-like, or other "two-dimensional" features. Pedro Leon Torres Ground gravity and magnetic surveys of the area in the southern Upper Peninsula of Michigan (Menominee County) were carried out from November, 1975 to January 1976. One hundred and fifty-two gravity stations were occupied with a station spacing from a half mile in swamp areas to a quarter mile on the central and western portion of a 16 square mile grid pattern. One hundred and thirty seven magnetic stations were occupied with a station spacing similar to the gravity survey. Vertical ground magnetic and Bouguer gravity maps of the area surveyed were prepared after applying the normal corrections. The vertical- intensity ground magnetic map indicated the presence of a magnetic high and a magnetic low which coincided with two well marked gravity highs. Only fragmentary direct geo- logic information was available on the basement complex in this part of northern Michigan, because of overburden of glacial drift, Paleozoic sedimentary rock, and poorly distributed basement drill holes. However, depth to the basement surface was estimated using the geophysical methods. The few well logs from drilling confirmed that the basement surface under the area surveyed was not deeper than 200 feet for both anomalies. The bottom of the pos- sible causative bodies can be modelled as being on the order of 10,000 feet. These anomalies were believed to be originating from mafic intrusives possibly coming from PreKeweenawan Pedro Leon Torres or Keweenawan igneous activity. The interpretation of the anomalies was begun by curve fitting various size vertical cylinders with an assumed minimum density contrast of 0.3 gm/cm3 and a maximum of 0.45 gm/cm3. This resulted in a fit of the sizes for the southern anomalous body (3.57 milligals) of 17.7 cubic kilofeet, and for the north-central anomalous body (4.47 milligals) of 25.2 cubic kilofeet. 14 The mass excess was 2.15 x 10 gm for the first one, and 14 3.06 x 10 gm for the second one. On the basis of well logs from drilling, a "Fence Diagram" was constructed and a maximum depth to the base- ment structure was calculated to be 200 feet. Induced and remanent magnetization were calculated by the formula J = Ji t Jr, where Ji is the intensity of magnetization due to induction by the earth's present magnetic field, and Jr is the natural remanent magnetization. The calculated Jr was stronger than Ji in both cases. The susceptibility 3 contrast assumed for both bodies was 1.9 x 10- e.m.u/ cm3. The induced magnetization so calculated for both bodies gave a value of 1.05 x 10-.3 e.m.u/cm3. The remanent magnetization for the positive anomalous body 3 gave a value of 2.58 x 10- e.m.u/cm3, and a value of —4.24 x 10-3 e.m.u/cm3 for the negative one. The Koenigsberger ratio, for the positive magnetic anomaly, was 2.46 and for the negative one, it was 4.04. Pedro Leon Torres For this value the volume percent of magnetite was esti- mated as 2.9% and 3.5% for each respectively. GRAVITY AND MAGNETIC STUDY OF NORMALLY AND REVERSELY POLARIZED INTRUSIONS, MENOMINEE COUNTY, MICHIGAN BY Pedro Leon Torres A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geology 1976 ACKNOWLEDGMENTS It is with sincere appreciation that thanks are extended to Dr. Robert S. Carmichael, my thesis Chairman, for his stimulation, enthusiasm, assistance, and criticism during the course of this study. Thanks are also extended to Drs. F. W. Cambray and J. W. Trow for helpful suggestions and critical review of the manuscript. The author wishes to express his sincere gratitude to Dr. R. Reed, who suggested this work, provided aero- magnetic maps of the area surveyed, and gave freely of his time for consultation. I would also like to thank Dr. C. E. Prouty who provided literature and gave advice. Special thanks are due to the invaluable assistance of Mr. N. E. Thurber, Mr. C. G. Ryan, and Mr. W. M. Phillips, Michigan State University students, for help with the measurement and some calculation of results of field work. Special thanks are also due to the Venezuelan government (Ministerio de Obras Publicas), for scholarship support and financial assistance for this work. ii Finally, I would like to extend my most sincere and deep appreciation to my wife, Migdalia. iii TABLE OF CONTENTS LIST OF FIGURES O O O O O O O O O O O O O O O 0 CHAPTER I. INTRODUCTION . . . . . . . . . . . . . . II. ACCESSIBILITY AND EXTENT OF THE AREA S U RVEY ED 0 O O O O C C O O O C O O O 0 III. GEOMORPHOLOGY OF THE AREA . . . . . . . Terrain O O O O O O O O O O O O O O O 0 IV. GENERAL GEOLOGY OF THE AREA . . . . . . V. GRAVITY FIELD SURVEY . . . . . . . . . . Equipment . . . . . . . . . . . . Description of and Determination of Altitude by the Wallace and Tiernan Altimeter O O O O O O O O O O O O O 0 Survey Sampling Area . . . . . . . . . . Measurement Method . . . . . . . . . . . VI. CORRECTIONS TO GRAVITY OBSERVATIONS TO YIELD THE BOUGUER GRAVITY RESIDUAL Free Air and Bouguer Corrections . . . . Latitude Correction . . . . . . . . . The Bouguer Gravity Map and Remova of Regional Effects . . . . . . . . . . . Regional Effects . . . . . . . . . . Estimation of Maximum Gravity Error . . VII. MAGNETIC FIELD SURVEY . . . . . . . . . Equipment . . . . . . . . . . . . . . . Survey Sampling Area . . . . . . . . . . The Zero Level . . . . . . . . . . . . . iv Page vii 13 13 15 19 19 22 27 29 32 35 36 37 38 40 41 41 43 44 CHAPTER VIII. IX. XI. XII. REDUCTION OF MAGNETIC DATA TO YIELD THE VERTICAL INTENSITY GROUND MAGNETIC MAP 0 O C O O O O O O I O O O O O O C 0 Correction of Magnetic Values for Instrument and Diurnal Variations . . . Normal Corrections . . . . . . . . . . . . FENCE DIAGRAM OF MID-MENOMINEE WELLS AND ITS INTERPRETATION . . . . . . . . . GRAVITY INTERPRETATION . . . . . . . . . . Approximation by a Cylinder . . . . . . . Vertical Cylinder Approximation by Axial Line Element and Solid Angles . . Vertical Line Element Formula . . . . MAGNETIC INTERPRETATION . . . . . . . . . Reversals of the Earth's Magnetic Field . Self-Reversals . . . . . . . . . . . . Field Reversals . . . . . . . . . . . Assumptions . . . . . . . . . . . . . Simple Method of Determining the Magnetic Susceptibility and the Induced Magnetization . . . . . . . . . . . . . Positive Residual Relief . . . . . . . Negative Residual Relief . . . . . . . Method to Determine the Remanent Magneti- zation of Approximately Cylindrical Bodies . . . . . . . . . . . . . . Koenigsberger Ratio Calculation . . . . . Magnetite Content . . . . . . . . . . . . CONCLUS ION O O O I O O O O O O O O O O O O Approximations Made . . . . . . . . The Reverse Nature of Anomaly Related to GeOIOgy O O O O O O O O I O O O O O O Page 46 46 49 51 56 56 57 58 66 67 67 68 71 76 77 78 79 82 84 86 86 88 APPENDIX I. II. DATA TABLE--GRAVITY DATA TABLE-~MAGNETIC BIBLIOGRAPHY . . . . . . . . vi Figure 1. 1-1. 10. 11. LIST OF FIGURES Index Maps . . . . . . . . . . . . . . Topographic Map . . . . . . . . . . . Gravity Stations . . . . . . . . . . . Bouguer Gravity Map . . . . . . . . . Magnetic Stations . . . . . . . . . . Vertical Intensity Ground Magnetic Map Total Intensity-~Aeromagnetic Map . . TheLaCoste-Romberg meter, with the carrying case back, the concave base plate, and the eliminator charger connected to it . . . . . . A simulated reading, taken with the lZ-volt battery attached to it . . . From left to right, connection cable, battery charger, 12 volt battery, compass, and voltimeter . . . . . . A Wallace and Tiernan Altimeter. Zero on the scale equals -1000 feet, each small division is equivalent to 5 feet . . . . . . . . . . . . . . . An altimeter showing the table of cor- rections inside of the instrument . A second altimeter together with the last one was used in this survey also 0 O O O O O O O O O O O O O O 0 Graph of Deviation vs. Time for Altitude Correction . . . . . . . . . . . . . vii Page 10 11 12 20 20 21 23 23 24 26 Figure 12. 12-1. 12-2. 12-3. 12-4. 13. 14. 14-1. 15. 16. 16-10 17. 17-1. 18. Graph of Deviation vs. Time for Cor- rection of Gravity Instrument and Diurnal Variations . . . . . . . . . Gravity Profile C to C' . . . . . . . Flux-Gate Magnetometer showing the main switch off. Battery cable attached on instrument's base . . . . . . . . Flux-Gate Magnetometer showing the "+" position (positive, for vertical field directed downward) . . . . . . Flux-Gate magnetometer, battery pack (16 C-cell, 1.5 volt) and tripod are attached to the instrument . . . Graph of Deviation vs. Time for Cor- rection of Magnetic Instrument and Diurnal Variations . . . . . . . . . Schematic Block Diagram of Mid- Menominee County . . . . . . . . . . Fence Diagram of Mid-Menominee Wells . Theoretical Vertical Cylinder Approxi- mated by Thin Horizontal Disc . . . Vertical Gravity Profiles Over Cylindrical Models . . . . . . . . . Vertical Gravity Profiles Over Cylindrical Models . . . . . . . . . Magnetic stratigraphy during the last 5 million years . . . . . . . . . . Field reversals from Tertiary to Permian . . . . . . . . . . . . . . Vertical Intensity Ground Magnetic Profile Laid in Northeast and Southwest Direction . . . . . . . . Residual Magnetic Anomalies Laid in Northeast and Southwest Direction . viii Page 34 39 42 42 44 48 52 53 60 61 62 70 70 74 75 CHAPTER I INTRODUCTION The objective of this work is to study the nature of vertical-component positive and negative magnetic anomalies, in particular their remanent magnetizations, which are caused possibly by magnetiferrous or ferromagnesian intru- sions in Menominee County of the Upper Peninsula of Michigan. The main reason to do this work was to investi- gate specifically the negative character (1ow magnetic anomaly) shown by the aeromagnetic map provided by Michigan Geological Survey (Department of the Interior, USGS, 1970). The total and vertical components of the field will give anomalies in opposition to those to be expected from the present field. Another subject of the present work will be to find the possible extent of magnetic rock deposits by making depth determinations to the top and bottom of theoretical cylindrical gravity models. A sixteen-square-mile area of Menominee State Forest of the southern Upper Peninsula of Michigan was surveyed gravimetrically and magnetically (Figure l) by up , '9' " A. e- ' e“ :)\‘:e um I'M" \\ "T 535* I h“ ’ I s; .0 ' - ‘ - 4 K“ I.” I‘ ' MI, ./ I . I b“ \ , I g ) j g .fmh \ 1 s \ 0 K I 4 \ I. - 41‘ I IL V v-h—f .qg m. I \ \ \ I \ : LID II L\accmu ma AIIEA m sum as Ill-B O FIGUREJ INDEX MAPS the writer in cooperation with Michigan State University students, in a project of geophysical interpretation. The gravity and magnetic measurements were done concurrently. The gravity anomalies exhibited large positive values with respect to the regional trends drawn. These positive anomalies could be interpreted as intrusive masses of higher density than the surrounding basement rocks. The position of the magnetic anomalies with respect to the position of gravity anomalies was observed to be very close to each other. However differences in size, center of maximum and minimum, and elongation were observed to be different, surely indicating the presence of a component of magnetization other than that induced by the earth's field. As the appreciable susceptibility and density contrast give rise to quite high anomalies, the gravity and magnetic method were adopted for the present work using a LaCoste Romberg gravimeter and a Fluxgate magnetometer. This proposed research will involve the application of gravity and magnetic geophysical exploration methods which will require assumptions about density and susceptibility contrast of rock material, and the relation of induced magnetization to the remanent magnetization with regard to polarity and intensity. CHAPTER II ACCESSIBILITY AND EXTENT OF THE AREA SURVEYED That portion of the Menominee County covered by the accompanying index map (Figure l) is located approxi- mately between longitude 87° 39' W and 87° 44' W and lati- tude 45° 33' N and 45° 36' 30" N in the southern part of Menominee County of the Upper Peninsula of Michigan. The south part of the area is well delineated by the south line of T37N; Michigan. On the east side of the area, on section 29, is the "Wire Grass Lake" which is connected to the "Shakey River” to the south. At the north-northwest corner, there is another small river which together with the Shakey poorly drains the area (Figure 1-1, topographic map). The area surveyed from its starting point (base station N°l) is spatially located approximately five miles east from Carney, Michigan. It is, geomorphologically speaking, located in an area of sandy till moraines to the west and till sheets and till plains to the east, surrounded in its majority by lowland swamps. Close to Carney, Michigan (outside of the surveyed area) small drumlins were observed covering till sheets and till plains. Most of the area surveyed 4 T37” T36" R27W R 2 8 W SCALE m was VII/'0 0 MTUI l8 mu IA LEVEL WIMIZOFEET QUADW LOCATIW TOPOGRAPHIC FIGURE l'l MAP is accessible with the exception of low swamp areas that are uninhabited and accessible with difficulty only in the winter time. A medium duty road (base line) ran directly east-west cutting our coordinate system exactly in half, thereby serving as our first traverse for markings and gravimeter-magnetometer readings. This medium duty road crossed the area at the middle of the grid pattern from east to west, linking the town of Carney with other little towns (i.e., Nathan and Hammond). In addition light duty roads connect with this medium duty road, thereby providing a larger degree of accessibility to the surveyed area. Three railways serve this county, the main line of the Chicago and North-Western Railways Co. parallels U.S. Highway 2 (eastward of the grid pattern, passing by Carney, Fig. 1-1) and the Wisconsin & Michigan Railway (not shown on this figure) linking the first two at the northwestern side of the area. The total area covered was approximately sixteen square miles (four square mile coordinate system). The series of points, at which the geophysical measurements were taken, were marked off at regular intervals of 1/4 mile. However some points (therefore measurements) were inaccessible due to extremely dense wooded areas and swamps. Therefore, these areas necessitated the usage of 1/8 or 1/2 mile intervals from which to obtain data (Figures 2 and 3). After doing all corrections, a Bouguer gravity and vertical magnetic maps (Figures 2-1 and 3-1) were drawn in order to make the interpretation and comparison with the total aeromagnetic map provided by the U.S. Geological - Survey, which was flown at 500' above the earth's surface (Figure 4). comma m- can. OONTROI. mu av EVERY V4 MILE men REPRESENTS A GRAVITY STATION LOCATION m as: mAvITY muons - FIGURE 2 AK mm-m 1"" mew “W ’--~ mm Imam mm 6233 mms‘dnsEOLow mvrrv mm (:2 mm mmmvm uTEIIsTY BOUGUER GRAVITY MAP GRAVITY sumo" MENOMINEE COUNTY - NATHAN ANOMALIES FIGURE 24 (FIGURE 2. GRAVITY STATIONS sCALE anus: m” I W W O l W COIIT IITERII . WMBYEVERYWOVZM NR L m, WHICHWAWTIC STAT“ m W. fiWW‘FfiS A» \ CONTOUR INTERVALS . O MAXIMUM ORMI M MMIuETIc MTEMsm moss men on 61% Low /--- INDICATES INCOMPLETE on m MOIcATEs CLOGDLOMMAOETICMTEMSITT wmm Cwscnmmmm MAGNETIC sTATION VERTICAL INTENSITY GROW MAGNETIC MAP MENOMINEE COUNTY - MTHAN ANOMALES FMS-I (new: 3. MAGNETIC STATIONS 'uIIJ 12 AEROMAGNETIC MAP OF THE AREA SURVEYED GOAL: III MILEG (TOTAL F 'E LD) I W "A 0 1' (U.S.G.S MAP GP- IEEEEEEHiiiIEEEEI-IIIIIIIIIIIIF 7II) LEGIMO ’50‘ COIITOIIII IMTEIIVALG X "7 IAXIIUI OII MIMIMIIM MAGNETIC IMTEIIGITV IMOICATEG Low MAGMETIC IMTEMGITY Fig. 4.--Total Intensity--Aeromagnetic Map. CHAPTER III GEOMORPHOLOGY OF THE AREA Terrain The uppermost sediments over the majority of the county were deposited by glaciers which pushed across the county many thousands of years ago. The surface features formed as a result of glacial activity are therefore com- posed of sand, gravel, clay, glacial till, boulders, etc.; which in general, have not been compacted appreciably since their deposition in Pleistocene or later time. The more common sediments were till sediments consisting usually of sandy and stony clay, numerous boulders throughout the area, and drumlins to the east side (close to Carney) similar to those produced by glacial erosion. Till is a mixture of sediments which have been deposited directly by the glacier (Vanlier, 1963). Generally, the moraines left by the glacier constitute approximately 50% of the area surveyed. The elevation of the moraines was variable between 802 feet and 892 feet. The maximum relief was 90 feet. Approximately 80% of the sixteen square mile quad- rangle was covered by forest. 13 TIErII 4:: .‘III’I!(III. ’I‘lll'l I [it I . {I’ll ( I III 'III lllllllll'. {Isl-Ill]. III 14 Low lands, between glacial deposits, developed marshy or boggy conditions due to the lack of adequate drainage. These boggy conditions stimulated growth of dense black spruce, tamarack and berry bushes. As one moved up and outwards from these areas of high water saturation, basswood, maple, and birch trees were common; intermingled with hemlock, Norway pine, and balsam firs that became more abundant as one went near the more cool, wind swept mounts of the glacial deposits. An area of particular interest was the boggy portion of the two northeast sections of the quadrangle. These sections were covered by extremely dense vegetation consisting of thorny berry bushes, small black spruce and tamarack. The density of these constituents was such that any attempt at traversing the area was pre- vented, rendering the gathering of data impossible. Since the primary determinant of vegetation is climatic condition with soil type being of secondary consideration any corre- lation of vegetation to bedrock would be a difficult task (especially when dealing with basement irregularities and glaciation). CHAPTER IV GENERAL GEOLOGY OF THE AREA Though studied for nearly a century, the regional geology of the Upper Peninsula has not been completely defined primarily because of the covering of Paleozoic sediments and glacial drift. In view of this, several assumptions will be taken into account with the available geological data of the surrounding area. The Menominee district (northwest of the area) is chiefly underlain by lower and middle Precambrian rocks, formerly designated Archean and Algonkian rock, which are covered extensively by Pleistocene glacial deposits, and locally by lower Paleozoic sandstone and dolomite. The lower Precambrian rocks occur in two separate areas, one in the northeast quarter composed of granitic gneiss (the Carney Lake Gneiss) and one in the southwest quarter composed chiefly of basaltic and felsic metavolcanic rocks (the Quinnesec greenstone) formation (Bayley, Dutton, and Lamey, 1966). Lyons, Prinz, and Cain, quoted in William (1963) have investigated areas immediately north of the twelve-foot Falls Quartz Diorite. These writers agree that the 15 16 Newingham Granodiorite was emplaced by intrusion of a silicate melt, but their interpretation concerning other plutonic units differ markedly. Lyons concludes that this intrusion produced extensive metasomatism within the Quinnesec formation greenstones forming the Marinette Quartz-Diorite and Hosking Lake Granite in situ causing a strong local metasomatic effect. Prinz and Cain postu- lated a magmatic origin for all plutonic bodies of the area. In the Menominee district (Bayley et al., 1966; Leith and Allen, 1915) found that the granite south of the Menominee River intrudes a series of basic volcanics called the Quinnesec schist which he correlated with the Keewatin. Although it was realized that the correlation of the Quinnesec schist as Keewatin introduced a conception of structure quite out of accord with natural inferences, it remained for Corey and Bowen (Leith and Allen, 1915) working under the direction of Van Hise and Leith in 1905 and Hotchkiss in 1910, to show conclusively that the Quinnesec schist is partly intrusive into, but in greater part interbedded with, the upper part of the Huronian (i.e., Animikie). Evidence of the present subsurface geology and structure has not been discovered or studied in Menominee County (Nathan area). Because of the veneer of glacial till in this area, subsurface geology will be correlated with well drillers records and outcrops that 17 occur away from the surveyed area. Some of these drilling records will be described in Chapter IX, where they were used to draw a fence diagram in order to find an approximate depth to the top of basement rocks. A general geology will also be inferred and assumed by reports that have been done by Dutton, Lamey, and Bayley (1966) and Russell (1906). Precambrian rocks form the bedrock in most of the Menominee district. They are overlain unconformably by Cambrian and Ordovician sandstone and dolomite in some areas, and all rocks are concealed in part by unconsolidated glacial deposits of Pleistocene age in most of the area. The rocks forming the Paleozoic system in the Menominee region include the Potsdam sand- stone, the Calciferous cherty limestone and the Trenton limestone. Chamberlin (1883) quoted in Russell (1906) described two great lobes of Pleistocene glaciation, the Green Bay lobe and the Chippewa lobe. The Green Bay lobe occupied the basin of Green Bay and extended southwest to the vicinity of Madison, Wisconsin and expanded westward. The Chippewa lobe expanded to the south now occupied by the western portion of the northern Peninsula of Michigan and adjacent portion of Wisconsin. By an inspection of all measurements of drumlin trends and striations on bedrock, they served to show the general trend of ice motion which appeared to advance from 18 the northeast and as it moved over the land tended to change its direction (Nathan, Menominee County, secs. 26 and 35, 2-3 miles south of T38W; R28E). Dominant direction was N60E. Although this particular area has not been examined in detail, enough is known concerning it to indicate that its covering of till is a part of the effects of the Green Bay lobe, and is possibly of the nature of an interlobate moraine. Naturally, these two great lobes mentioned and described by Chamberlin, must be correlated with the upward and subsidence movement described before by Russell. CHAPTER V GRAVITY FIELD SURVEY Equipment The instrument used in this study was a LaCoste- Romberg Geodetic Gravity meter (Figures 5 and 6). With a range of about 7000 milligals and a sensitivity of about 0.01 milligals. The meter has been designed to operate at a thermostated temperature of 48°C kept constant by a 12-volt battery or the eliminator (Figures 5 and 6). Compact rechargeable motorcycle batteries were used during this survey. Batteries were changed every night before the Gravity meter was to be used. The temperature of the instrument was controlled by its attachment to the battery eliminator/charger throughout the night. A concave base plate (Figures 5 and 6) was used. Extreme care was exer- cised to always reclamp the spring mechanism after three readings were taken for each station. The moving hairline was centered on the calibration-scale value of 2.60 and was always brought in from the right side of the scale to prevent hysteresis. It was necessary to turn the dial scale considerably to compensate for the change in latitude from East Lansing to Menominee County. Instrument drift l9 1Ef[[[[tl[fl[(([tlll‘[(.lld[l[[Il} 'lnlll'lll [II II I I 'I 20 Fig. 5.--The LaCoste-Romberg meter, with the carrying case back, the concave base plate, and the eliminator charger connected to it. Fig. 6.--A simulated reading, taken with the 12-volt battery attached to it. 21 Fig. 7.--From left to right, connection cable, battery charger, 12 volt battery, compass, and voltmeter. (assumed linear with time) and earthtide effects were removed using correction curves (Figure 12) that were made by reoccupation of base station every 1-2 hours. Measure- ments between stations were either obtained by (1) car, (2) pacing, or (3) a sound producing object (in this case a car horn) which was in a known location. Two Aneroid altimeters (the Wallace and Tiernan altimeter, the more sensitive and reliable extension of the aneroid barometer) were utilized during the survey to measure the elevation contrast of the gravity stations. 22 Description of and Determination of Altitude by the Wallace and Tiernan Altimeter Within this instrument, changes in atmospheric pressure are exerted upon an evacuated beryllium copper capsule. Deflections of the capsule (caused by pressure variances) are transferred by a linkage to a geared sector which meshes with a pinion on the pointer shaft. The pointer is read on a dial that is calibrated in feet or in meters. Any lag in the reading is prevented by a device which eliminates backlash between the sector and the pointer pinion. A desiccant is enclosed in the instrument to absorb moisture that might otherwise collect and alter the readings. The instrument is mounted in a shockproof metal case and provided with an outer case and strap for carrying it in the field. An adjusting screw is provided in the face of the dial so that the instrument may be made to display a given bench mark elevation. Due to the low temperature during the survey period, time consuming operation, and high fixed costs, both repetition of station altitude readings and continuous monitoring of base station deviation were not possible. This stipulation necessitated the method of determining elevation with one altimeter, a second one being utilized in order to check the reliability of the data. The altimeter is taken to a starting point of known elevation, with the instrument adjusted to display the 23 Fig. 8.--A Wallace and Tiernan Altimeter. Zero on the scale equals -1000 feet, each small division is equivalent to 5 feet. Fig. 9.--An altimeter showing the table of corrections inside of the instrument. 24 Fig. 10.--A second altimeter together with the last one was used in this survey also. elevation of the base station (in this case, station number one, on the main road, corresponded to a bench mark, there- by providing an absolute measurement of altitude). The instrument is then taken to the various stations where an elevation is required, with the readings of time, altitude, and temperature being recorded. After a period of time, usually no more than two hours, the observer returns to the base station and records the aforementioned data again. Due to change in atmospheric conditions, the first and last readings at the base station will usually not agree. ' A correction must therefore be applied to the intermediate station readings. It is usually assumed that the change in the atmosphere has occurred uniformly with time. The II[J[(.[I[{I[I[[[[IIL.({ lllu‘ll\[( [ff [1! II I I .III 1 25 corrections are therefore computed in proportion to the elapsed time from the first observation. To obtain the correction factor, the base station deviance from absolute altitude (multiplied by a correction factor for temperature determined from a graph inside the altimeter) is compared to the time of day. This graph produces a curve within which intermediate stations may be plotted. The amount of deviation that corresponds to the station is either added or subtracted to the recorded altitude of that station. (The addition or subtraction of the deviant value is dependent upon the changing atmospheric pressure.) If a low pressure system is approaching, one measures an increase of base station readings. Correction factors would then be subtracted to adjust for the higher recorded elevations. The opposite would be performed, and addition, for the occurrence of a high pressure system. The accuracy of altitude determination by this method was determined by analyzing the most deviant curve (as determined from a linear comparison of base stations). The graphs connecting the base stations were drawn from little data available for the exact determination of such curve (Figure 11). Data points are represented by crosses and these are not meant to indicate error ranges. The comparison of this linear representation of base station deviation (Figure 11) to the curved .ucaguazu» so? u: :0... .83 «08.2 20:85.00 Snag .2 S 82 20.2.30 #30»: us» 023...»; :2: 26 32.4.50 2; .2. 3 20.258 —_ U m D Q _m Angvmirr “Us: 00.0 00.0 00;. 09.» 03 00.. 00a. 00.: 3.9 r I- b p p n p _ . h p b . _ . p p q . . q u 4 _ A — A d A I q d . no.» .03 0..- on.» .000 3 .08 n... 8... .30 :20 0. .8.» an- 8% .05 0 20 J .08 a- 8.. .000 «.2» .03 .u- 8.0. .20 .3... .08 0.- 8.: .08 h a.» - . . IT0~ m .08 __ 0.... .03 a 20 A .to o. 9.0. .30 n .2» m .0. 0 0 0.0. .20 ._ .00 m 33:3 3:858 us: 3.3, 5.5m 0n n 05-8.7: 30E 9.0.5.538 5.33 m 0.2» t!» .36 4100 zoFommmoo wear—.334 mo“. m2: .m> 20.._.<_>mo m0 Ia 2......» .02» I 00. P. T F 2.1.3. 30.: 0:053:33 miss. .30 mzo.._.<_¢<> 132135 024 Egg. >._._><¢@ mo ZOE-cummoo mo... NZ:- .m> zo....<_>wo mo $28 ("31%") NOIIVIABO 35 corresponds to a maximum deviation of .016 milligals from a corrected reading. This correction was made by a calculator machine as were the corrections for Free-Air and Bouguer, elevation, and latitude. Also, here, data points are represented by crosses. They are not meant to indicate error ranges. Free Air and Bouguer Corrections An elevation correction was made to take account of the fact that gravity changes with elevation, because a higher station is farther away from the center of the earth than a lower one and gravity is affected by the attraction of material under the station. The correction has two components: (a) The Free-Air, which is due to the vertical gradient of gravity. The magnitude of this effect is 0.09406 mgal/ft and it was added to the observed gravity values. This magnitude was obtained subtracting the gravity difference between sea level and a point of ele- vation "h" (Dobrin, 1960). 2 R2 (R+ h)2 Go - —-lL———-— Go = Go (1 - ) (R+h)2 where: Go Gravity value at sea level R = Mean radius of Earth Since h is much smaller than R, 2 Goh Change in gravity = (approximately) R 0.09406 milligal/ft 36 (b) The Bouguer Correction, which accounts for the attrac- tion of the rock material between sea level and the sta— tions. The attraction in milligals of an infinite slab of thickness h (ft) is Znych (Dobrin, 1960), where c is the assumed average density for crustal rocks, and Y is the 8 dyne-cmz/ universal gravitational constant (6.670 x 10- gm ). It was necessary to assume an average density value for these rocks. This average density is about 2.6 gm/cm3 (Anderson, 1974), so the Bouguer correction was .033 milligals per foot and it was subtracted from the observed gravity. The combined Free-Air and Bougher correction was .061 milligals per foot and its error was calculated at plus or minus .10 milligals. Latitude Correction The international gravity formula for the variation of normal gravity along the geoid with the latitude O is: G = 978.049 (1 + 0.0052884 sin2 ¢ - 0.0000059 sin2 20) gal where: 978.049 = value of gravity at the Equator. It is seen that the gravity at the equator (O = 0°) is about 5,000 milligals less than at the poles (O = 90°). The rate of change of gravity along a north-south line is obtained by differentiation of the equation above mentioned, and it is: 37 W = 1.307 sin 2¢ milligals per mile In the Nathan anomaly area the latitude value is approximately 45.58°, thus W = .33 mgals, for the change for every 1/4 mile (approximately 1,320 feet) and was added for stations to the south of the main medium duty road (base line), and subtracted for stations to the north of the same road. Latitude distances were mapped and measured with the help of a grid pattern (4 x 4 miles, each 1/4 mile) and a topographic map of the area with a maximum error of 100 feet causing a gravity error less than plus or minus 0.03 milligals. The Bouguer GravityMap and Removal of Regional Effects After the application of all corrections already mentioned, a Bouguer gravity map was constructed. This is the ordinary "observed" gravity map on which the primary results of a gravity survey are presented (Figure 2-1). The gravity stations (Figure 2) plotted on a grid pattern, with (4 x 4 miles), was used to contour the gravity readings. It was so regular that this simple method can give accurate enough results for exploration work. The contour interval was chosen to be .5 milligals and the scale was in miles. 38 Regional Effects To bring out the local anomalies in relief, the regional effects were removed by graphical smoothing as illustrated in Figure 12-1. This gravity cross section was drawn through both centers of the main anomalies. After several other calculations, it was shown to be the best gravity cross section to represent the closer sym- metrical pattern. This regional effect was sketched by the regular method of smoothing. On subtracting it from the observed curve, the local anomalies were obtained. Regional gravity lines were connected across the interval of the anomalies on each profile with the provision that the difference of regional and anomalous gravity at each point on the profile, corresponded to a perpendicular pro- file (Anderson, 1974). One of these profiles, together with others, were necessarily drawn along a North-East, South-West direction (Figure 2-1) to obtain the best gravity cross-section under the provisions already mentioned. This procedure could also be used to remove the effects of two nearby low gravity anomalies on both sides of the Bouguer gravity anomaly (3.77 mgals, Figure 2-1). However, because of the practically unknown geology and the impossibility of establishing a constant regional pattern, the decision was to draw the best gravity cross-section where the closer symmetrical pattern could be estimated. Discussion about nearby anomalies will be done in a later chapter (Gravity Interpretation). 39 \/ TN. 500...; ./ I... i/ e... a...” .00. .... /.. E... .3... -.I . . \\ 4. .\-M.\ . . __ _._ .; /. .\e K. @ .. .. ! e .. .. .. ... . ... A. .0 C ... ... .. .. w. .. . c. 7... .I - e .0 E 0 3.0.9... >550 I. S‘IVOI'HIN 4O Estimation of Maximum Gravity Error The reading of the gravity meter is sensitive to about .01 milligal. The error in elevation was a maximum of 4 feet combined with instrument error; corresponding to a maximum deviation of approximately .37 milligals. Mapped station locations were accurate to within 100 feet, causing a gravity error of plus or minus 0.03 milligals. The total maximum error was 0.40 milligals, not inclusive of terrain errors that arise mainly from the error in elevation determination. CHAPTER VII MAGNETIC FIELD SURVEY Equipment Magnetic stations were read using a Flux-Gate Magnetometer (Scintrex, Model MF-2-100). With its internal sensor, it measures the vertical component of magnetic field intensity. The full-scale meter ranges are 100 gammas (resolution i .5 gammas), and 300, 1000, 3000, 10000 (10K), 30000 (30K), and 100,000 (100K). Resolution is i .5% of full scale on each range. In our case, for the northern hemisphere, the range is set for +80,000 to -20,000 gammas. Temperature stability is l gamma per °C. A battery pack was required (16 C-cell, 1.5 volt, alkaline batteries) for this survey, and it was not necessary to use more than one set in order to complete the area sur- veyed. It was necessary to turn the meter scale several times in order to set the scale at some range which would be slightly larger than the maximum gammas exhibited by the airborne survey. The aeromagnetic map published in 1970 by the U.S. Geological Survey (Map GP-7ll) provided the aeromagnetic contour lines of high and low values which were used in the first instance to test and see if 41 42 Fig. 12-2.--F1ux-Gate Magnetometer showing the main switch off. Battery cable attached on instrument's base. ,.,. A .. Fig. 12-3.--Flux-Gate Magnetometer showing the "+” position (positive, for vertical field directed downward). { [It'll Ill-I'll .IIIlllllll-lI l it'll. :[llll lull.- III II‘ l‘l: 43 the readings were of an appropriate value, such that they would not necessitate the continual changing of scale. As in gravity, a base station was reoccupied every 1-2 hours to correct for diurnal variation, any instrumental drift, and temperature coefficient. Survey Sampling Area Certain precautions were taken in carrying out all magnetic field work. All metallic objects were removed from our wearing apparel (i.e., wristwatch, keys, penknife, etc.). Other objects often avoided were steel wires in Spectacle frames, power lines, pipe lines, nails in field shoes, etc. The grid pattern (rectangular coordinate system) used for gravity readings, was used for magnetic readings also. Stakes at these stations were colored with spray paint and reoccupied at various times. It was possible to reach only 133 staking points of the gravity grid pattern to obtain magnetic readings. A tripod for fixing the magnetometer naturally was used several times, and an error of i 5 gammas was observed. The Flux-Gate magnetometer was used most of the time by attaching the carrying strap to the instrument and using the upper buttons to maintain the same height at the same location for more accuracy in the control of daily (diurnal) vari- ation of the Earth's magnetic field. 44 Fig. 12-4.--F1ux-Gate magnetometer, battery pack (16 C-cell, 1.5 volt) and tripod are attached to the instrument. The Zero Level One of the most important points in considering the magnetic anomalies in the area was the zero level. (The reading of the magnetometer at points where there are not appreciable disturbances from subsurface masses so that only the ”normal" geomagnetic field is present.) It is often difficult to decide about a suitable zero level, however with the aeromagnetic survey flown and compiled by the U.S. and Michigan Geological Surveys (flown at 500 feet above ground, 1966), it was possible to choose a suitable beginning "working zero." All readings were corrected at the end of the survey by adding to or 45 subtracting from them, a constant amount arrived at from the study of the resulting anomalies. The suitable "working zero" was chosen at base station number one, along the main medium duty road where disturbance from subsurface masses was negligible. CHAPTER VIII REDUCTION OF MAGNETIC DATA TO YIELD THE VERTICAL INTENSITY GROUND MAGNETIC MAP Before magnetic readings can be mapped, several corrections must be applied. These are the diurnal cor- rection, the temperature correction, the normal correction, and the terrain correction (under special conditions). Correction of Magnetic Values for Instrument and Diurnal VariatiOns In this survey, the values obtained from the magnetometer were corrected for variations caused by instrument and diurnal effects. Diurnal variations are those initiated by the sun's emission of high energy particles and effect on the ionosphere, and the moon's gravitational effect upon the ionosphere. They constitute the predominant effect upon reading deviation. Instrument variation originates from any inability of the magnetometer to reproduce a reading. Because of this, mitigating factors such as leveling and temperature have an effect on instrument.variation. 46 47 The procedure utilized in this survey that allowed for the correction of these variations involved the repe- tition of readings at a base station (or any previously occupied station whose relation to the base station was known) at intervals of 1-2 hours. The base station devi- ations (and deviation of other stations if used) were plotted on a graph against time (see curve sample, Figure 13). In cases where more than one base station was monitored on a given day, it was related to the first base station by incorporating its deviation into the base station deviation at the time the secondary station was first read. Once this had been accomplished, a smooth curve was drawn through (or as near as possible to) the points. If the repeated readings showed an increase, the amount of increase was subtracted from that intermediate station's value at the time the station was read. If the repeated readings exhibited a decrease, the amount of decrease at the time an intermediate station was read, and was added to that station's value. During the survey, negative and positive values over the anomalous bodies were recorded. To more easily facilitate the correction of these readings, a suitable zero level station was arbitrarily chosen; all values greater, being positive, and lower, being negative. After the corrections were completed, the values were once more assigned their respective positions in exhibiting 48 005 00? 09» oofi 09.. oo..«. .— 1 4F 1 . n + w ITVOHT m. mmawri .o\\\\ \‘\ u: .m.m®\\\\ \\ .. o oo... 09» 3.. .3 .. R. 2.. one u- «:3 .. ab. 8. 8.» so «add .. an. 6. 9% an. 2.1a .. 2.. 8. 9.. t. «and 3d \ .. o 8. o... 8.. 5d a no N, 8N. H4 «:3 .. on. 09 8.“. a. 8.66 8.5.8 o o 9.: 0 find 3.23 SE. zoEuEB ma: 33> 255 23%.. :2... 9.0.2.533 3.53 mzo_.—. 3213.0 024 g. 0.5.9.642 to zoFommmoo m0..— wZF M) Sta->8 B 1155 l l l L l L r I I F I T 8 8 9 S 8 9 l r O F 4100. 11.00. (mum uouvmaa 49 the polarity of the anomalous mass. The maximum deviation due to instrument and diurnal variations on a given day was found to be 520 gammas. These values were discarded as being due to a magnetic storm; value displayed for those stations coming from readings of a later expedition. The deviation of utilized days was approximately 60 gammas. Upon fitting of various smooth curves through the data points, an error of approximately 12% could be apparent. This corresponds to a maximum deviation of approximately 15 gammas from a corrected reading. In this survey, a compensated magnetometer was used and a temperature coefficient was included together with the drift in the diurnal variation correction thus the temperature correction was not necessary. Normal Corrections Throughout historical time, continual changes in the magnitude and direction of the earth's main or "permanent" field has occurred as one moves about the surface of the earth. These changes correspond in a sense, to the variations of the earth's gravity with latitude. However, normal magnetic changes over the earth are not regular functions of latitude, as is the case with gravi- tational field. This variation, which cannot generally be correlated with known geologic features, is in many cases similar to the large scale regional variations often observed in gravity work. Where the survey was confined 50 to a small area of sixteen square miles, and where the geologic structure are unknown and possibly of large horizontal scale, this reduction was neglected. Terrain corrections were also neglected because there was not an appreciative difference in elevation capable of necessitating such a correction. In general this area can be considered flat, which means that a correction of such magnitude will not affect our readings appreciably. CHAPTER IX FENCE DIAGRAM OF MID-MENOMINEE WELLS AND ITS INTERPRETATION Since there is an absence of "upper limestone" and "middle limestone and sandstone" to the north of the quadrangle surveyed (Fig. 14-1, Fence Diagram), one could expect a somewhat similar lithology within the quadrangle. As the "Fence Diagram" exhibits, there is a transition from glacial drift to the lower sandstone which rests upon the Precambrian rocks. From figures (N° 14, 14-1), one notices that the stratigraphy dips towards the southeast in the same direction as the occurrence of thickening beds. The fence diagram was constructed with six logs of wells (the closest to the area surveyed) and their description is as follows: Well N° 1 (38N 26W 16-1), Well N° 2 (35N 27W 23-3), Well N° 3 (37N 28W 11-1), Well N° 4 (35W 28W 19-1), Well N° 5 (35N 25W 35-1), and Well N° 6 (38N 28W 9-1) (Vanlier, 1963). At well N° 6, approximately 9 miles to the northwest, the Precambrian rests directly beneath the glacial drift at a depth of 19 feet. At well N° 4 approximately 10 miles to the southwest, the Pre- cambrian resides at 162 feet below the surface. This 51 52 SCHEMATIC BLOCK DIAGRAM OF MID-MENOMINEE COUNTY QUADRANGLE SURVEYED LEGEND 571""W‘ UPPER LIMESVOIES “IDOL! LIMESTONE! AID SANDSTOIIS LOIER SAIDSVOII HGUREJ4 ‘PnecAuanuu nocus FENCE-DIAGRAM OF MID-MENOMINEE WELLS LEGEND OLACIAL DRIP T I QUADRANOLE ' UPPER mutton: s u a v e v E o .- 3' 7... Eamon mutton : ., ‘ no “Inflow! ' ‘.'. :l . 1 . .00. ”of .. (5.37.13: Lona “Huston: ' A .‘u ' s cl . %.f 3.. i:- I. ...... . ... ...‘t. _ ". 1 \’\,\{\/ . . ..,- . ,- ..; . {3"} vacuum“ ‘ -‘O'_'.......-.o .- '1' .. \lz \ . ’ J '03:: \ ‘- ‘ 10".. ( ~‘ 3'. \H " ’ VERTICAL IINCHUZOO.FEET MW!“ HORIZONTAL I INCH 8 4 MILES 54 indicates a gradual inclination of the basement rocks, that when extrapolated to the quadrangle surveyed, gives on an approximate depth no greater than one thousand feet to the top of the Precambrian (assuming no irregularities). Since well N° 3 obtained a lower sandstone to a depth of 62 feet and was abandoned, one cannot predict exactly how far the drilling was from Precambrian rocks. However in view of the fence diagram, one could expect Precambrian rocks to lie at a depth of not more than a maximum of two hundred feet below the quadrangle (Figure 14-1). To the south of the quadrangle, the Precambrian rocks consist of granite. To the north of the quadrangle, one finds the Precambrian to consist of cherty limestone. This irregularity leads one to believe that perhaps in or near the area of the quadrangle, an event has occurred which has resulted from a change in the Precambrian struc- ture. Perhaps a fault running east-west near the quadrangle and parallel to the Menominee range to the north, with the rock to the north being the graben, might explain this feature. The graben and horst assemblages could have then been covered by a Precambrian sea, resulting in both being covered by limestone. As time progressed, erosion down to the previous basement granite of the horst block did not wear through to the granite of the graben. Since the graben was filled with limestone, limestone was at the same erosional level as the granite to the south of the ‘l.ll[([ Iii I'll 55 quadrangle. At this time, the postcambrian sea and current resulting stratigraphy were developed. The anomalies in the quadrangle could be due to the intrusion of ferro- magnesian dikes, resulting from the deformational process surrounding the fault. CHAPTER X GRAVITY INTERPRETATION Approximation by a Cylinder In this chapter an attempt has been made to inter- pret positive gravity anomalies due to causes such as rock masses emplaced in rocks of higher density, sediments with density lower than that of the surrounding rocks, or any other process which can account for a density contrast between rocks of different sources. In order to determine the parameters of the anomalous masses it was necessary to fit a regional trend, which when subtracted from the Bouguer anomaly, would yield the residual anomaly of the mass. The best regional value (on a profile North-East, South-West, Figure 2-1) was fitted graphically taking advantage of the good anomaly boundaries on this profile. The interpretation procedure was to treat the anomalous masses in the basement as vertical cylinders, because of the concentric symmetries of the geophysical anomalies. Two approximation techniques were used; the vertical line element or axial line element, and solid angles. 56 57 Vertical Cylinder Approximation by Axial Line Element and Solid Angles An important class of geologic structure that is more or less vertical is intrusive igneous plugs. These are often represented for the purpose of analysis by vertical right cylinders. Unfortunately, calculating the gravity effect of a vertical cylinder is easy only on the axis. At all other points the calculation is difficult, involving elliptic integrals or series expansion of Legendre polynomials. Therefore, approximation to the gravity effect of vertical cylinders are important in practice (Hammer, 1974). Approximation was done by a vertical cylinder to best fit the observed radial profile values of gravity at various depths and radius. A minimum density contrast of .30 to a maximum of .45 grams per cubic centimeter was necessary because of the higher density of iron-rich for- mation possibly present there (Bacon, unpublished report). The formula used for calculating gravity over a finite vertical right cylinder is: ‘1) A92 = 6.39 RZAO 1 - 2 2 1]: 21 (1 + x /zl ) 1 2 22 (1 + x /z2 2)1/2 where: Agz vertical gravity anomaly (milligals) R radius (Kilofeet) 58 A0 = density contrast (gm/cm3) N II depth to the top of the cylinder (Kilofeet) N II depth to the bottom of the cylinder (Kilofeet) horizontal distance from the center (Kilofeet) Vertical Line Element Formula The above expression was used for cylinders of finite length (top at depth Z bottom at depth 22) by 1: simply subtracting the effect calculated for depth 22 from that calculated for depth Zl' All distances are expressed in Kilofeet (convention for numerical constant) and the vertical gravity (Agz) in milligals (Nettleton, 1942; Hammer, 1974). For solid angles: (2) gz = 2.03 WAot where: 92 vertical gravity anomaly (milligals) A0 density contrast (gm/cm3) t thickness (i.e., 22 - 21) The thickness or length (i.e., Z2 - 21) of each cylinder must be less than or equal about half its mean depth, so its gravity effect can be approximated closely enough for most geophysical purposes by considering the mass to be condensed upon its median plain and calculating the effect in terms of the solid angle subtended by the boundary on the plane. Values of "W" can be read from the 59 solid angle chart (Nettleton, 1942; 1971), from the ratio Z/R (which is constant for any given case) and the ratio X/Z. Calculation of solid angles for even simple boun- daries is mathematically difficult and tedious but they can be calculated for circles. This is useful for calculating gravity effects for any body which can be approximated by horizontal circular slabs. The calculated profiles of gravity over the cylinders (by the vertical line and the solid angles method) were compared to the residual gravity anomalies (Figures 16 and 16-1). The cylinder that best fitted the positive gravity anomaly (3.57 milligals) southwest of the area surveyed had a radius of .8 Kilofeet (Figure 16). This close fit of calculated and actual gravity was possible applying the solid angle method, with a density contrast of .45 gm/cm3 (the maximum utilized) and calculating the sum of the solid angles (Hammer, 1974) of 8 thin circular slices to achieve an error on the axis no greater than -6%. Axial errors (expressed as a per- centage of the central anomaly magnitude for the cylinder) for a family of vertical cylinders are shown in Figure 17 (Hammer, 1974). This close fit of calculated and actual gravity permits the calculation of an approximated value for the volume of the cylindrical model. This volume was determined by the formula: 60 THEORETICAL VERTICAL CYLINDER APPROXIMATED BY THIN HORIZONTAL DISCS XSKIIquI) IBIMHIleO) 3% 4.35 . 3.90 . O .57 t I. 5 I . 40 T: 2. O O. 97 3 2. 5 O. 66 E 5 £9 .& Kilofeet Fig. 15.--Theoretical Vertical Cylinder Approximated by Thin Horizontal Disc. 61 VERTICAL GRAVITY PROFILES OVER CYLINDRICAL MODELS U $2.2: _o 3 .2369: mo . “.3652: .....Baco .o o x :35: arcane c :0 lxocaao Soc: : # u m if... 552 h '03:, —‘-Q— o Bus 6 @luolo > KHoteeI 63:3. 233:6 5:3 Cu .6 60209.6 fame S .I...I.. . 4.3 3:59;. 02...: Co coZoE TITI... \ A 1:333 Lonnie .ou_Co> IIII .. \A\-z .aEhmfiKZZVLosagmom «Ii .. J c o. FIGURE I6 .J , round Surface L f 1% 62 RTICAL GRAVITY OVER CYLINDRICAL VE 51 an d! 4 o 1 a // / 2:25.: 4 4 round Surioc 2:39 339.... watt. :2. «cheat—.6303“... «29.0 ...-cm . E o ....fi.....~....u.fi..fixx.......cmuw @IIQIIG co_.oE.onno 3.530 _.ou. III” .oEb¢.¢v:_>eL0 .oaummom «vi I I .... .. Av Q Aw O 6 . _. R L L u . = .w G .. «T “T lwl. F. 63 V = nRz h where: V = volume in cubic Kilofeet R = radius (.8 Kilofeet) h = thickness or height of the cylindrical model (8.8 Kilofeet) 17.7 cubic Kilofeet 4.8 x 1011 cm3 v = 3.14 x (.8)2 x (8.8) The excess mass was easily determined once the volume was known and the density contrast assumed. The excess mass of the cylinder was: X mass V x A0 4.8 x 1011 cm3 x .45 grams/cm3 2.2 x 1014 grams These values of volume and excess mass of the cylindrical bodies may be in error because of the uncertain in the real depth to the bottom, and they only must be considered as approximations to the true values. The cylinder that best fit the positive gravity anomaly (4.47 milligals) northeast of the area surveyed had a radius of .9 kilofeet, a depth to the top of .1 kilofeet, a depth to the bottom of 10 kilofeet (Figure 16-1). This second approximation was also possible by applying the solid angle method with a density contrast of .45 grams/cm3 and calculating the sum of the solid angles of 11 64 thin circular slices to achieve also an error on the axis no greater than -6% (Figure 15). On this figure the numbers from 1-11 represent the 11 thin circular slices with dif- ferent depths to the top and bottom of the theoretical cylindrical body. The radius and the density contrast had to be changed in order to best fit the theoretical gravity curve to the actual gravity. It was also apparent when calculation for finite cylindrical body was done (by the vertical line element) that the error on the axis exceeds in most cases 100% or greater, so the calculation for this method is considered very poor (Hammer, 1974). More generally, the range of maximum gravity error as a function of length and diameter of a cylinder is shown in a chart developed by Hammer (Figure 3, p. 209, 1974). The curve is plotted in dimensionless-unit H/d with dimensionless unit R/d as variable parameter, where H represents the thickness or height of the cylindrical body (kilofeet), d (kilofeet) is the depth to the top, and R is the radius also in kilofeet. Another important disadvantage in this approxi- mation is complete lack of flexibility with respect to irregular shape and variable density. Naturally a similar method was applied to the "solid angles" approximation, using another chart developed by Hammer (1974, p. 223). The excess error on the axis was calculated for each thin slice of the theoretical cylindrical body and it was not 65 greater than -6%. The error on the flank can be estimated subtracting the solid angle curve approximation from the actual residual gravity. Perhaps if one more assumption (such as that the radius will be not constant throughout the intrusive body) is taken into account, a final best fitting close to the flanks of the anomalous body could be possible (Nettleton, 1942, pp. 309-310). However, the above sample will serve to show the ambiguity in the possible source of a given residual gravity curve where geological and structural controls were not well known. It will also illustrate why approximation methods such as have been outlined here are sufficiently accurate to answer most questions regarding possible sources of anomalies when other control is not available. A third gravity anomaly approximately 1 1/2 miles northeast from the center of the 4.47 milligals gravity anomaly (Figure 12-1) resulted from the gravity field measurements. This anomaly is also positive with a high and low negative to the right and left side of the 4.47 milligals gravity anomaly respec- tively and will not be analyzed in detail like the first two one because of its small influence over the main anomalous bodies. It will be discussed later and its possible correlation with the main bodies will be estab- lished. CHAPTER XI MAGNETIC INTERPRETATION Frequent occurrence of rocks whose NRM (natural remanent magnetization) is in the direction nearly Opposite to the present geomagnetic field was a puzzling fact in the early days of rock magnetism studies. Some authors have pointed out that the phenomenon could be explained by possible reversals of the earth's main magnetic field in the past. The investigation of David (1904) and Brunhes (1906) into the magnetization of lava flows and their underlying baked clay led to the first observation of directions of magnetization in rocks roughly opposed to that of the present field. This led to speculation that the earth's magnetic field had reversed itself in the past. Since then, the study of many rocks formations around the world and throughout the geological column has revealed directions of magnetization roughly opposed to one another. The increasing amount of paleomagnetic work sup- ports the validity of the viewpoint that the most recent world-wide reversal of the earth's magnetic field took place about 700,000 years ago during the early Pleistocene or very late Pliocene age. Hence the potential value of 66 67 this reversal as a key to settling the Plio-Pleistocene boundary may well be emphasized. Intrusions of igneous rocks in nature are common, and the rocks which are intruded are reheated to some extent. This process will cause a remagnetization of the intruded rock as a result of cool- ing from above the Curie temperature. It had been well established that the earth's field reversed itself many times. When rocks of equal or different sources cooled or have deposited, it can produce reverse polarity at times when the earth's field has been reversed. Naturally, it will explain the present case if one body was intruded when the earth's field had different polarity. Reversals of the Earth's Magnetic Field Self-Reversals Neel (1955) quoted in Strangway (1970) has con- sidered theoretically several mechanisms by which rocks would undergo self-reversal of their remanence. He pointed out that to have self-reversals requires the coexistence and interaction of two ferromagnetic materials. These two constituents need not be different ferromagnetic materials, they may represent two interwoven sub-lattices of a ferri- magnetic. Work by Nagata (1961) on the extract from the self- reversing Haruna dacite was pursued by Uyeda (1958) using synthetic materials. He found that self-reversal is an 68 intrinsic property of the ilmenite-haematite solid solution series in the region of .45 to .60 ilmenite. Other workers have showed that the reversal is connected with the ordering and disordering of Fe and Ti ions in the lattice. The magnetic characteristics of rocks having reverse NRM are the same as from those of normally magnetized rocks. Then the occurrence of such rocks having reverse NRM may most plausibly be interpreted on the hypothesis that the geomagnetic field has occasionally reversed and that the rocks were magnetized during the periods of the reverse field. Field Reversals Wilson (1962a) has produced a particularly con- vincing piece of evidence in favor of field reversal from an investigation of a doubly heated rock. A band of laterite had been heated by an overlying lava, the direction of magnetization of both agreeing and having reversed polarity. Subsequently both the laterite and the lava were intruded by a basic dike. The direction of this dike was also reversed. The second heating by the dike did not however exceed the Curie temperature, so that the heating effect could be removed by partial thermal demagnetization. As a result each laterite sample contained two independent magnetization, one for each of two different temperature ranges. In the same sample, both of these superimposed magnetization were of reversed polarity. It seems almost 69 impossible to explain this fact by any known or theoretical self-reversed mechanism. Strangway (1970) says that in contrast to the search for self-reversals in nature much evidence has been produced to show that the field did in fact reverse. The most convincing evidence of all has been produced by the combined study of paleomagnetism and careful isotopic age- dating, since basaltic rocks have been found to have suf- ficient potassium to be suitable for potassium-argon dating. In very young rocks, less than about 5 million years old, many investigators have found consistent evidence on a worldwide basis to show that reversals take place simul- taneously around the world (Figure 17, data after Opdyke, 1972). The rocks which are younger than 700,000 years show mostly "normal” polarity, if the present field is called ”normal." This last period has been called the ”Brunhes normal" epoch. From .7 to roughly 2.5 million years ago, the field was essentially reversed, although some inter- ludes of normal periods have occurred. This epoch (-0.7 to -2.5 x 106 years) has been called the Matuyama reversed epoch. Before that (to -3.3 x 106 years) follow the Gauss normal epoch and (to -4.5 x 106 years) the Gilbert reversed epoch. However, in all these epochs frequent short inter- vals of opposite polarity (”events") to that corresponding to the character of the "epoch" occurred. The pattern of 70 mun jaiao‘nmu 70 g 80 2 § 3 90 U 2 _§ -mo 3 g 3 110 > U a o 2 “‘ 2 u 3 no O-r- «on g in 130 g a r “O 14- § — unmuo tvmr - 150 2 3 g 5 3 'mo 2 s . I OLDUVAI cum 3 .‘ 170 ,L ‘ g 'mo -‘ 190 KAENA tvmt c 3..- ... MAMMOMH EVEN! t 200 z 5 g g u 210 3 — cocum sum 5 n :- .. 220 41,. .. - uumvm EVEN! :5 . c. - tvcm g 23° Ca- WIN? i " I “ "° sh _E_ Figure 17 Figure 17.1 Fig. l7.--Magnetic stratigraphy during the last 5 million years. White: normal magnetization; black: reversed magnetization (data after Opdyke, 1972). Fig. 17—1.--Field reversals from Tertiary to Permian. White: normal; black: reversed; cross-hatched: uncertain (due to frequent reversals) magneti- zation (data from Irving et al., 1973). 71 frequent back and forth reversals of the geomagnetic field with a frequency of one change in 400,000 years seems to have been the rule for about the last 70 x 106 years. The polarities have been followed back further to the upper Permian. The reversal frequency has apparently been much less (once in 107 years or more) before the Tertiary time than in the Tertiary and Quaternary (Figure 17-1, data after Irving et a1). This evidence is strong proof that the field reversals do occur, since many different rock types (basalts, sediments), and areas show consistent results. This resulted includes data from many places and several laboratories, and although additional features may yet appear, the general sequence of events for the last 5 million years (at least) is now clear. Assumptions On examining the magnetic anomaly contour maps of both vertical and total components, it is found that a single vertical cylinder does not suggest itself to be the best representation of the anomalous bodies. A lengthy and arduous computation such as was employed for gravity interpretation can satisfactorily explain the observed trends of the components. Figures 3-1 and 4 show the contour maps of vertical and total anomaly components respectively. A magnetic profile was drawn in the same Northeast-Southwest direction as the Bouguer gravity 72 profile (Figure 3-1). In order to determine the magnetic susceptibility, and induced and remanent magnetization some assumptions were made. This survey will illustrate that it is very important to know the above parameters mentioned, and the ratio of remanent to induced magnetization and the direction of remanent magnetization for the correct inter- pretation of magnetic anomalies. In general, it is known that Jr (the remanent magnetization) is not in the direc- tion of the present earth's field. In recent years, laboratory measurements have shown that the direction of natural remanent magnetization is parallel to the earth's present field only in rocks of Quaternary and late Tertiary ages (Girdler, 1960). This must be taken into account for the following assumption. One of the main assumptions was done, taking into account the effect of Induced and Remanent magnetization. It is inferred that the natural remanent magnetization is much greater than the induced magnetization since one magnetic anomaly is negative. This suggests the presence of igneous rocks with a strong reverse magnetization. In one case it was assumed that the anomaly is due to an intrusive body (which can be represented by a vertical finite cylinder), and both Jr (remanent magneti- zation) and Ji (induced magnetization) are parallel to the present earth's field (55,000 gammas) and therefore the cylinder will be vertically polarized. In fact the earth's 73 magnetic field in this area has an inclination about 75-76 degrees, which means that a vertical polarization may be expected for a first approximation. In this case the formula J = Ji + Jr was applied. In a second case, it was assumed that the anomaly was also due to an intrusive body (like the first one) and both Ji and Jr were parallel to the present earth's field, but not in the same sense. This assumption was made since the aeromagnetic and ground vertical maps showed the north-seeking pole pointing up (instead of down for the northern hemisphere), so the vertical component of the field gives anomalies in oppo— sition to those to be expected from the present field. Then the formula above mentioned will be of opposite sign, J = Ji - Jr. It will be seen later that the value of the angle of the lower surface will be sufficiently small not to affect any considerable change in the final calculation. If these assumptions are allowed, then the formula: V J (W1 - W2) gammas (Finite Cylindrical Body) residual vertical magnetic anomaly (Figures 18, 18-1) where: V J = intensity of magnetization (e.m.u./cm3) W1 = solid angle subtended by the upper surface W2 = solid angle subtended by the lower surface can be used to calculate the magnetic remanence, which will be of opposite sign for the two anomalies treated. 74 VERTICAL INTENSITY GROUND MAG- NETIC PROFILE LAID IN NE AND SW GAMMAS 2000‘ I 730' 0500. I 000. 500‘ DIRECTION I” —__/ NILES afov 4000‘ «3500* 4864‘ 40004 I/2 xy-Reqlonal Anomaly SCALE (MILES) I -|/z PW Fig. 18.--Vertical Intensity Ground Magnetic Profile Laid in Northeast and Southwest Direction. 75 RESIDUAL MAGNETIC ANOMALIES LAID IN NE AND sw DIRECTION CANNAS 2000 i / I700 —— —- I500' l000q 500‘ ° ‘In I I acc- -I I 00" fl 00¢u___. _____. ._____. ______ _____. ... 4 0004 SCALE (MILES) I u: Ila o I W Fig. l8-l.--Residua1 Magnetic Anomalies Laid in Northeast and Southwest Direction. .1! l. ‘4'! llllllll .ll. III III! II .I'III All!!! ll IIIIIIJI. 5"! [III 76 Moreover, the application of the formula before mentioned (which is an application of the "solid angles” methodology) was so a continuation of the same technique employed in gravity interpretation. Then the margin of error will be less, and the value of the parameters so calculated will be closer to the reality. In any practical job like this (where measurements on orientated rock samples were impos- sible to do), several assumptions must be in mind to make the best approximation to resolve J into Jr and Ji. Simple Method of Determining the Magnetic Susceptibility and the Induced Magnetization The method described below is rough but very useful for obtaining quick estimates of the susceptibility and induced magnetization of magnetic rocks. For a vertically magnetized, vertical cylindrical body, the volume magneti- zation can be considered as replaced by a surface magneti- zation of its upper and lower faces. For each such face the magnetic effect at the point being considered is pro- portional to the solid angle subtended; hence, the magnetic effect of the cylindrical body is proportional to the dif- ference between the solid angles subtended by its upper and lower faces. If the bodies had no components of remanent magnetization, the susceptibility contrast for the positive and negative anomalies was determined by the formula before mentioned: 77 V = J (W1 - W2) Values for W1 and W2 were obtained from a chart (Nettleton, 1942; 1971) which was also employed for gravity interpre- tation. With this chart a quick estimation of the angles W1 - W2 can be done, knowing the depth to the top and to the bottom of the theoretical cylindrical bodies, so as its radius. From the gravity calculation (for the positive magnetic anomaly) these values were: 21 equal to .l kilofeet, 22 equal 10 kilofeet, and R equal to .9 kilofeet. It should be understood that the value mentioned above have been applied from gravity approximation to magnetic inter- pretation. In other words, the size and shape of the causative gravity anomalies have been assumed to be approxi- mately the same for the magnetic anomalies. The position and shape of the Bouguer gravity anomalies and vertical ground magnetic anomalies, surely indicated a close approx- imation, assuming equal dimensions for the magnetic inter- pretation. From the values above mentioned, the ratio Z/R was obtained and the reading was taken on a horizontal coor- dinate (chart for solid angles, Nettleton, 1942; 1971) for points on axis (X = 0). Positive Residual Relief (1700 gammas) The formula before mentioned was rearranged to calculate the magnetic susceptibility: 78 V (W1 - W21 J = AK HE = V AK HE (W1 - W2) where: J intensity of magnetization (e.m.u./cm3) AK = susceptibility contrast (e.m.u./cm3) V = vertical magnetic anomaly (residual, 1700 gammas) HE = earth's magnetic field (55,000 gammas) W1 = solid angle subtended by the upper surface (4.7) (obtained from the ratio Zl/R, equal to .25) W2 = solid angle subtended by the lower surface (.025) (obtained from the ratio ZZ/R, equal to 11.25) The calculated value for the susceptibility con- trast so calculated will be a maximum value, because the remanent magnetization was not taken into account. This 3 e.m.u/cm3 units. This value of value was 6.612 x 10' magnetic susceptibility, corresponding to the maximum measurements on 200 oriented samples from the Keweenawan gabbro, diabase, and associated rocks due by Charles (1965) near Duluth, Minnesota, may be a clue for later interpretation. The calculated value for Ji, the induced magnetization, in a magnetic field of 55,000 gammas is 3 3.64 x 10- e.m.u/cm3 units. Negative Residual Relief (-1804 gammaS) A similar calculation was done with W1 equal to 5.5, W2 equal to .028. The calculated value for AK (the 79 magnetic susceptibility contrast) was 5.994 x 10"3 e.m.u/ cm3 units. The calculated value for Ji, the induced mag- netization, in a magnetic field of $5,000 gammas was 3.29 x -3 10 e.m.u/cm3 units. Method to Determine the Remanent Magnetization of Approximately Cylindrical Bodies Once an approximated maximum susceptibility con- trast was determined by the method outlined before, the effect of induced and remanent magnetization was applied. The formula J = Ji i Jr will depend upon whether the induced intensity, Ji, has the same or the opposite direc- tion as the remanent intensity, Jr. Suppose that to start with J =IJi + Jr for the vertical residual positive anomaly (1700 gammas), and then for the residual negative anomaly, J = Ji - Jr. To continue with the application of induced and remanent magnetization, and looking the values of susceptibility contrast and induced magnetization calculated before, it was concluded that: (l) The calculated values for the susceptibility con- trast were almost the same. (2) Therefore, the induced magnetization were also almost the same. (3) The above mentioned conclusions were expected, since the high and low total and vertical magnetic intensity maps (Figures 3-1 and 4) showed almost the same range of magnitude. It tells us that 80 the induced magnetization for both anomalous bodies will be the same. So, for the second calculation, to interpret these anomalies the best possible, several susceptibility values were assumed. Naturally, these values of susceptibility con- trast were chosen according to magnetic susceptibility measurements, which have been already made around the area surveyed (Meshref and Hinze, 1970; Charles, 1965). First of all, this range of susceptibility contrasts was assumed taking into consideration that the anomalous bodies are Keweenawan basic intrusives rocks. Meshref (1970) gave an average of susceptibility (K), for intru- 3 3 sives (basic) of 5.683 x lo' , while Charles 3 e.m.u/cm (1965) gave an average range from 0.3 x 10' to 6.0 x -3 e.m.u/cm3. It is apparent that there exists a strong 10 correlation between these two values which were calculated from different sites, but for equal rock formations. Susceptibility contrast values (AK), were assumed taken in consideration those maximum values of susceptibility (K) given by Meshref (1970) and Charles (1965). The Koenigsberger ratio was also taken into account, and each calculation for different susceptibilities, were compared with those observed by the above mentioned workers. So, to interpret these anomalies the best possible, 3 a susceptibility contrast of 1.9 x 10- e.m.u/cm3 was 81 assumed to be the best representative average value for these two anomalies. This arbitrary value was used by the writer after several calculations, with different sus- ceptibility contrasts, were made. Naturally, it was a lengthy and arduous computation employed to find the best representative susceptibility contrast value. Values, below 1 x lo“3 e.m.u/cm3 were discarded because of the high Q (Koenigsberger ratio) values that they produce and which do not agree with those already calculated by Hinze et a1. (1966). Also, we must have in mind that (Q) of igneous rocks is usually between 2-10, and it will exceed 100 in some basaltic effusives: also, it rarely is below unity (Nagata, 1961). In our case, any value between 2-10 will be acceptable according to the data given by Hinze (1966). He made several calculations in an external field of .6 Oersted for Keweenawan and pre-Keweenawan rocks, finding a maximum value of 10 for pre-Keweenawan iron formation. Also the gravity interpretation and the available geological information of the surrounding area were useful to choose approximately the intrude rock type. With the new sus- 3 ceptibility contrast (1.9 x 10- e.m.u/cm3), a final approach was done. The induced magnetization so calculated 3 e.m.u/cm3. With this new gave a value of 1.05 x 10- approach, the formula: V = J (W1 - W2), was again applied taking into consideration that J will be different for both anomalous bodies. Rearranged the formula above 82 mentioned to calculate the remanent magnetization for the positive magnetic anomaly: V = (Ji + Jr) x (W1 - W2) where V is the vertical component of the magnetic anomaly (1700 gammas), Ji is the induced magnetization (1.05 x 10- e.m.u/cm3), Jr is the remanent magnetization, and W1 - W2 are the angles subtended by the upper and lower surfaces (4.68). The remanent magnetization gave a value of 2.58 -3 3 x 10 e.m.u/cm . A similar calculation was done for the negative magnetic anomaly where V has a value of -1804 3 e.m.u/cm3, and W1 - W2 is 5.47. 3 gammas, Ji is 1.05 x 10- The remanent magnetization gave a value of -4.24 x 10- e.m.u/cm3. Koenigsberger Ratio Calculation (Q = Jr/Ji) Vacquier (1961) suggested that, by carefully repeating a survey across the span of a decade in regions where the secular variation is pronounced, it might be possible to determine in situ the ratio of remanent to the induced polarization, given rise to large magnetic anomalies. After the induced and remanent magnetization were calculated, a quick approximation of their ratio was done to indicate the range within which its value falls. This approximation was possible applying the formula: Q = Jr/Ji, where Ji is the magnetization induced in the 3 83 sample by the earth's magnetic field (HE) at the surveyed area, and Jr the remanent magnetization. Quantities Jr, Ji and K were expressed on a volume basis in c.g.s units throughout this work. From the formula before mentioned (Q = Jr/Ji), it was found, for the positive residual magnetic anomaly (Figure 17-1) a value of 2.46, and for the negative residual magnetic anomaly, a value of 4.04. A summary of magnetic properties (after Hinze, O'Hara, Secor and Trow, Report of Investigation No. 12, Michigan Geo- logical Survey, 1966) was done, and the Koenigsberger ratio was calculated for rocks of Keweenawan and pre- Keweenawan ages in an external field of .6 Oersted (60,000 gammas). If a comparison is established between them and the present calculation, it will be seen that the rock type of the area surveyed may be considered of Keweenawan age (basic intrusives), naturally taking into consideration that the susceptibility contrasts were assumed from the range established for this kind of rock type, given by Hinze and O'Hara (1966). Also, the Koenigsberger ratio for both anomalous bodies were compared with those given by the above mentioned workers. Those values so calculated resulted in greater than the maximum calculated for basic intrusives (2.0). This difference may be due to the external field of .55 Oersted (55,000 gammas) which will be different than that applied by Hinze et a1. (1966). 84 Magnetite Content (Fe3g4) Magnetite is by far the most common and most mag- netic of the magnetic minerals. It is probable that the magnetic properties of most rocks are directly dependent on the amount of magnetite that they contain. Slichter's (quoted in Dobrin, 1960) careful study of the properties of magnetite indicates that its effective susceptibility in a field of the strength of that of the earth's when in powdered and highly disseminated form, as it would be expected to occur as a constituent of rock is around .3 (i.e., 300,000 x 10'5) c.g.s. units. probably it is the most useful figure for our consideration until it is only a small fraction of the total rock volume. On this basis it might be possible to estimate the per- centage (by volume) of disseminated magnetite (Nettleton, 1940; 1942). From the general formula: J = Ji i Jr, the amount of magnetite was calculated. Rearranging this formula, an approximation was done as follows: J K HE + Q K HE K (1 + Q) HE .3 P (l + Q) HE (for the positive magnetic body) = .3 P (1 - Q) HE (for the negative magnetic body) For the positive magnetic body: .017 .3 P (1 + 2.46) .55 P .029 = 2.9% (magnetite by volume) 85 For the negative magnetic body: -.01804 .3 P (1 - 4.04) .55 P = .035 = 3.5% (magnetite by volume) Parasnis (1975) calculated percentages of rocks and ores with a maximum value of 3.4% of magnetite for basic rocks. Taking into account these values and comparing with the above calculated, it will be seen that the present percent of magnetite in the actual rock will be in this range observed by Parasnis. However it must be remembered that in any particular case these values may differ enormously, depending on the amount of magnetite and grain size. I‘IIIII .. II [III I till-..., . |I[ I CHAPTER XIII CONCLUSION Approximations Made One of the most important approximations made in the gravity model interpretation was that the observed gravity highs were approximated by cylindrical vertical bodies using two different methods (the vertical line method and the solid angles method). The approximation was made assuming structures of approximately circular cross section, varying the density, size, and shape of the stocked cylindrical bodies. The volume and excess mass calculated from the best model (Figure 16 and 16-1) was slightly higher for one body than the other. In both cases, these values should not be considered exact, because of the causative error due to the application of the solid angles methods, and the possible error due to the calcula- tion of the other parameters, like depth, size, density contrast, etc. Secondly, approximations were made with different density contrasts using a maximum value of .45 grams/cm3. This is a high value of density contrast, corresponding to a body considerably denser than the surrounding rock. 86 87 Values like this have been found by Bacon (Michigan Geo- logical Survey, personal report) at the eastern half of the Northern Peninsula of Michigan. However this high value which is suggestive of a more basic and more dense rock type is meant to represent a basic rock such as gabbro or diabase, intruding rock of lower density. It is impor- tant to remember that a body need not have a uniform den- sity contrast throughout it as was assumed for the best final fitting models. One more approximation made for the present work was the calculation of remanent magnetization, assuming a susceptibility contrast of 1.9 x 10.3 e.m.u/cm3. Using this value and having in mind that the direction due to the earth's vertical magnetic field will be unchanged but that of the remanence will be reversed (for the negative magnetic anomaly), an approximation of Jr was carried out for both anomalous bodies. This calculation of remanent magnetization showed that it was stronger than the induced magnetization in both anomalous bodies. Also, the remanent magnetization of the (-) body is stronger than the first one (positive anomaly). This value was expected by the writer because of the reverse magnetization that occurs there. This value may not be sufficiently high to repre— sent the best possible the negative magnetic anomaly. However, we must understand that in any practical job, where several assumptions have been made (because of the 88 unknown geology and the real susceptibility contrasts), a margin of error must be expected. The Reverse Nature of Anomaly Related to Geology (1) the negative magnetic anomaly shown by the aeromagnetic map (U.S. Geological Survey, 1970, Figure 4) and by the vertical intensity ground magnetic map (Figure 3-1) might be explained by greater remanent magnetization having opposite direction to that of the induced magneti- zation. (2) The negative anomaly is everywhere magnetized in a direction opposite to the present field whereas older and younger bodies nearby showed apparently directions of magnetization conforming to the present day field. As the south-seeking pole points down (-1804 gammas) instead of up for the northern hemisphere, the vertical component of the field gives anomalies in opposition to those to be expected from the present field. (3) The depth and shape of the best fit cylindrical gravity models, their density contrasts, and their high magnetite content, suggested to the writer that the anomalous bodies may be intrusive basaltic rocks within the granitic basement. (4) Quantitative interpretation is further handi- capped because the proportion of induced and remanent magnetization is rarely well known (Heiland, 1940). The 89 above statement still holds true in general for an area with complex Precambrian geology such as the Upper Peninsula of Michigan. (5) This study has shown in conclusion that aero- magnetic interpretation together with gravity and magnetic ground survey, are extremely useful tools in estimating approximately the local structure of Precambrian terrain consisting of widely diverse magnetic formations. However the complexity of the magnetic rock properties (like susceptibility contrast, induced and remanent magnetization) reduce the interpretation to a semiquantitative approach based upon the integration of several factors. These factors, like surface and subsurface geology interpretation, Bouguer gravity anomalies and analytical studies of gravity and magnetic data will be useful for the final interpreta- tion. Analytical studies useful in the interpretation include gravity and magnetic depth determinations and trend analysis, total and theoretical intensity anomalies, and correlation of theoretical magnetic and gravity anomalies with the observed gravity and magnetic data. In reality, how deep those intrusive bodies can prevail will be a geological problem. It might suffice to pay more attention to the possible density contrast of the intruded bodies and the surrounding rocks and also to the susceptibility contrast of them. 90 It will be interesting to continue with this research, since the purpose of gravity and magnetics is simply to localize interesting anomalous areas and give approximate values of properties to the causative anomalous bodies. Further exploration with seismograph and borehole drilling would be interesting to obtain a better evaluation of the shape and size of these anomalous bodies. APPENDIX I DATA TABLE--GRAVITY APPENDIX I DATA TABLE--GRAVITY Free-Air, Bouguer, Drift Bouguer iii? .222 $3533; 6:33? 3553333“ mgals W-l 816 4401.53 0 Wl-Wl7, westward W-2 848 4401.95 +0.42 along the main W-3 817 4399.94 -1.59 medium duty road W-4 817 4400.00 -1.53 (base line, which W—S 822 4399.80 -l.73 cut the grid pat- W-6 832 4400.02 -l.51 tern in exactly W-7 840 4400.76 -0.77' half). W-8 872 4401.88 +0.35 Interval, 1/4 mile W—9 863 4401.55 +0.02 W-lO 843 4400.64 -0.89 W-ll 828 4400.34 -1.19 W-12 829 4399.85 -1.68 W-13 853 4400.15 -1.38 W-14 888 4401.46 -0.07 W-lS 868 4401.25 -0.28 91 92 Free-Air, at. gig: ”233“§§é13§3§t 33233:; Aggggggggte corr (GraVlty mgals mgals W-16 870 4401.58 +0.05 W-l7 852 4403.17 +1.64 N-18 849 4400.17 -1.36 Nl8-N22, northward N—l9 879 4399.52 -2.01 perpendicular to the N-20 875 4399.02 -2.51 base line, right from N-20 875 4399.34 station number seven, N-21 885 4401.53 +2.19 interval, 1/4 mile. N-22 882 4400.82 +1.16 N-23 .880 4402.21 +2.55 N-23-N24, northward N-24 861 4400.57 +0.91 perpendicular to the base line, right from stations six and five respectively, 1 1/2 mile from base line N-25 872 4400.87 +1.21 N25, northward N-26 844 4398.48 -1.18 perpendicular from N-27 892 4403.43 +3.77 base line, right from station four, 1.75 miles from it. N25: exactly two miles from station one, [{[I‘l IIIl‘lIII III III til III 1 II l I 93 Free-Air, Stn gig: 3233“§§é13$3§t 2323??? Agggggiggte corr (GraVlty) mgals mgals northward N27, northward perpendicular to the station number nine, this station has the maximum + gravity value, 1/4 mile from station nine. N-19 879 4399.86 Base station N-28 864 4399.14 -1.06 N28, half mile from N-29 820 4397.60 -2.60 station eight N-30 853 4402.30 +2.10 N29, 1/4 mile from station eight N30, half mile from station nine N-31 820 4395.87 ,-4.33 N31, 1/4 mile from station six N-32 819 4398.33 -1.87 N32-N34, perpendicular N-33 817 4398.42 -1.78 to the station number N-34 819 4398.07 -2.13 five every 1/4 mile. N-35 826 4397.47 -2.73 N35, .75 mile from station six 94 Free-Air, Bouguer, Drift Bouguer iii?! 3221;222:236 6:22:12 Aié’ii’éi'éite mgals N-36 818 4397.15 -3.05 N36, half mile from six N-37 841 4397.24 -2.96 N37, one mile from six N-38 826 4397.66 -2.54 N38, 1 1/4 mile from six N-39 892 4398.81 -1.39 N39, 1 1/4 mile from five N-40 853 4399.43 -0.77 N40, 1 mile from five N-4l 824 4398.09 -2.11 N41, .75 mile from eight N-42 830 4399.94 -0.26 N42-N43, perpendicular to 9, .75 and 1 mile N-43 836 4399.49 -0.71 from it. N-44 834 4398.26 -l.94 N44, 1 mile from eight (northward, perpen- dicularly) N—45 827 4400.32 +0.12 N45-N48, perpendicular N-46 832 4401.07 +0.87 to 10 (northward) N-47 832 4399.68 -0.52 N-48 844 4400.80 +0.60 N-49 842 4398.31 -l.89 N49-N52, northward 95 Free-Air, Bouguer, Drift Bouguer 231' .22: Isms, 6:221? mm mgals N-SO 855 4399.23 -0.48 perpendicular to 13, N-51 830 4399.67 -0.18 interval, 1/4 mile N-52 878 4399.23 -0.97 S-53 826 4400.99 +0.79 SS3, southward from 11, interval, 1 mile S-53 826 4401.51 Base station E-54 826 4400.91 -l.12 E54-E58, eastward E-SS 812 4400.30 -1.73 from 53, interval E-56 811 4399.96 -2.07 1/4 mile E-57 815 4399.29 -2.74 E-58 816 4399.63 -2.40 S-59 817 4399.68 -2.35 SS9, southward from E58, interval, 1/4 mile S-60 814 4400.62 -1.41 S60, southward from 11, interval, .75 mile W-61 813 4400.75 -1.28 W61-W66, westward W—62 814 4401.48 -0.55 from $60, interval, W-63 822 4402.56 +0.53 1/4 mile W—64 827 4402.71 +0.60 W-65 825 4401.85 -0.18 W-66 818 4402.00 -0.03 96 Free-Air, Stn gig: Bifiguiiéiifiéit 332333; Agggggiggtion corr (GraVlty) mgals mgals S-67 817 4402.29 +0.26 867-869, southward S-68 818 4403.04 +1.01 from W65, interval, S-69 836 4401.32 -0.71 1/4 mile W-70 833 4401.69 -0.34 W70, westward from 869, interval, 1/4 mile E-71 827 4401.27 -0.76 E71, eastward from $69, interval, 1/4 mile S-72 833 4400.63 -1.40 $72-$73, southward S-73 814 4401.43 -0.60 from $53, interval 1/4 mile W-74 826 4399.78 -2.25 W74—W75, westward W-75 839 4400.66 -l.37 from S73, interval, 1/4 mile S-76 848 4400.92 -l.1l S76, southward from $74, interval, 1/4 mile S-77 815 4400.43 -1.60 S77, southward from S75, approximately .37 mile W-78 832 4401.32 -0.71 W78, westward from S75, interval, 1/4 97 Free-Air, 5a. gig: Bifiguiiéi'ifiéit 232325;; Agggfiggte corr (GraVlty) mgals mgals mile E-79 821 4401.88 -0.15 E79, eastward from S68, interval, 1/4 mile S-80 825 4399.88 -2.15 $80-$81, southward S-81 828 4400.21 -l.82 11, interval, 1/4 mile W-82 829 4399.65 -2.38 W82, westward from N51, interval, 1/4 mile W—83 818 4399.57 -2.46 W83, westward from N52, interval, .75 mile N-84 810 4399.33 -2.70 N84-N87, northward N-85 808 4399.17 -2.86 from W83, interval N-86 831 4399.20 -2.83 1/4 mile N-87 826 4399.71 -2.32 88 - -- - Missing point W-ll 828 4400.87 Base station N-89 836 4401.66 +0.25 N89-N90, northward N-90 846 4402.04 +0.64 from W11, interval, 1/4 mile N-91 822 4399.90 -1.50 N91, .75 mile from W12 98 Free-Air, Bouguer, Drift Bouguer iii: $221635???) 6:33;" 3553333“ mgals N-92 836 4401.13 -0.27 N92, 1/4 mile from W12 W-12 829 4400.60 Base station N-93 837 4399.97 -l.38 N93, 1/2 mile from W12 W-94 870 4401.41 +0.06 W94, westward from N91, interval, .12 mile N-95 864 4402.45 +1.10 N95, 1 mile from W12 N-96 860 4400.29 -1.06 N96, northward from W14, interval, 1/4 mile N-97 843 4401.81 +0.46 N97, northward from W14, interval, 1/2 mile W-98 852 4403.21 +1.86 W98, westward from N97, interval, 1/4 mile S-99 881 4400.75 -0.60 S99, southward from W98, interval, 1/4 mile W-7 840 4401.59 Base station S-100 859 4402.63 +0.21 8100-8101, southward S-101 856 4402.58 +0.16 from W7, 1/4 and .37 I I I. ' 'II I'll. I!!! [III- 1 III I. ll J'llll. II In 99 Free-Air, Stn gig: 3233“§§é1353§t 33232:; Aggggggggte corr (Gravity) mgals mgals mile interval respectively S-102 828 4400.31 -2.11 3102-8104, southward S-103 849 4402.25 -0.17 from W8, interval, S-104 848 4402.79 +0.37 1/4 mile W-105 852 4402.27 -0.15 W105, westward from $103, interval, 1/4 mile S-106 824 4400.14 -2.28 5106-8107, southward S-107 812 4399.34 -3.08 from W6, 1/4 and .37 mile respectively S-108 825 4400.05 -2.37 8108-8109, southward S-109 842 4402.03 -0.39 from W7, 1/2 and .75 mile respectively S-110 832 4400.77 -0.65 8110, southward from W9, 1/4 mile interval S-lll 843 4401.99 -12.3 $111, southward from SS3, 2 mile interval W-112 857 4401.25 -l.l7 W112, westward from $110, 1/4 mile interval W-ll 828 4400.96 Base station. W-113 810 4401.45 -0.13 W113-1l4, westward 100 Free-Air, sen gig; Bifiguiiéiifiéit 33232:; Aggggggggte corr (Grav1ty) mgals mgals W-114 808 4402.08 +0.50 from 8110, 1 1/2, W-llS 831 4402.17 +0.59 1.75 and 2 miles respectively S-116 826 4401.87 +0.29 Sll6, southward I from W115, 1/4 mile E-ll7 855 4402.25 +0.67 E117-119, eastward E-118 836 4401.99 +0.41 from W113, 1/4 mile E-119 846 4400.75 -0.83 interval S-120 822 4401.54 -0.04 8120, southward from W12, 1/2 mile interval S-121 836 4402,08 +0.50 $121, southward from W13, 1/2 mile interval W-l 816 4403.11 0 Base station S-122 812 4399.93 -3.18 5122, southward from E54, 1 1/4 mile inter— val right to the bottom outside of the grid pattern E—123 825 4399.65 -3.46 E123, eastward from 8122, approximately .12 mile outside of the grid pattern I I I... I I I I‘ ‘l .J I l‘ [J I I'll 101 Free-Air, 5.. gig: Bifiguiiéiifiiit 33232:; Agggggiggte corr (GraVlty) mgals mgals E-124 842 4399.97 -3.14 E124-125, eastward E-125 832 4399.46 -3.65 from E123, right to the bottom along the 2 mile line from the medium duty road N-126 843 4400.09 -3.02 N126-128, northward N-127 857 4400.51 -2.60 from E125, 1/4 mile N-128 837 4400.97 -2.14 interval W-129 845 4401.10 -2.01 W129, westward from N128, 1/4 mile inter- val N-130 831 4401.70 -1.41 N130, .12 mile interval S-13l 849 4401.01 -2.10 8131, 1/4 mile from W129 S-132 892 4401.75 -1.36 8132, .75 mile from E55. S-l33 869 4399.66 -3.45 8133, southward from E54, approximately .60 mile interval W-l 816 4403.12 Base station N-134 825 4403.67 +0.54 N134—137, northward N-135 810 -0.90 from W17, 1/4 mile 4402.23 102 Free-Air, Bouguer, Drift Bouguer 3131 3231;225:133, 6:32:13 3553333“ S mgals N-136 802 4401.20 -l.93 interval N-l37 822 4401.34 -l.79 N-138 893 4402.09 -l.03 N138-139, northward N-139 822 4401.81 -l.32 from W16, .75 and 1/2 mile interval respectively N-l40 842 4400.57 —2.55 N140, 1/4 mile from N95 N-14l 818 4401.18 -1.94 N141, approximately 1 mile from N90 N-142 844 4400.24 -2.88 N142, 1/4 mile from N141 N-143 837 4400.35 -2.57 N143, .75 mile from N52 N-l44 807 4399.87 -3.25 N144, .85 mile from W82 N-l45 810 4400.11 -3.01 N145, .55 mile from W82 S-l46 822 4403.81 +0.69 8146-149, southward S-147 842 4403.57 +0.45 from W1, 1/2 mile S-148 824 4403.88 +0.76 interval (146-147 S-149 846 4403.80 +0.68 outside of the grid lifIlll‘i-‘Ili Ettllilli' Ill Iii-Ill III. 103 Free-Air, Bouguer, Drift Bouguer ’33? .22: 1:23:23; 6:33:13 A5553???“ mgals pattern E-150 846 4403.66 +0.54 E150, 1 1/4 mile from N126 N-151 832 4403.59 +0.47 N151, 1/4 mile from 3150 W-152 816 4406.32 +3.20 W152, 1/2 mile from 853 APPENDIX II DATA TABLE--MAGNETIC {{ll.?l‘ln|‘ll[[[l\ll[r[ lt‘lllII.I I III I III I APPENDIX II DATA TABLE--MAGNETIC Meter Corrected Stn Readings Approximate Location (Gammas) W—l 0 Wl-Wl7, westward along the main W-2 -22 duty road (base line for Gravity W-3 -53 Readings) W-4 -14 W-S +4 W-6 +33 W-7 -149 W-8 -1601 W-9 -204 W-10 +214 w-ll +233 W-12 +182 W-13 +31 W-14 +30 W-15 +18 W-16 -34 104 105 Meter Corrected Stn Readings Approximate Location (Gammas) W—l7 +153 N-18 +263 N18-N24, the same location as N-l9 +72 Gravity stations N18-N24 N-20 -102 N-21 +3 N-22 +107 N-23 +134 N—24 +116 N-25 +167 N25-N28, the same location as N-26 +150 Gravity stations (N49-N52) N-27 +130 perpendicular to W13 N-28 +182 N-29 +183 N29-N33, the same location as N-30 +45 Gravity stations (w83, N84-N87) N—3l +147 N-32 +98 N-33 +190 S-34 +360 834, the same location as E-35 +641 Gravity station (SS3) E-36 +392 E35-E37, the same location as E-37 +343 Gravity stations (ES4-E56) N-38 -16 N38 = N35 (Gravity station) N-39 -1604 N39-N40 = N27-N30 (Gravity 106 Meter Corrected Stn Readings Approximate Location (Gammas) N-40 -1383 station) S-41 +1730 S41-W4S = W62-W66 (Gravity W-42 +998 station) W—43 +299 W-44 -l W-45 +51 S-46 +42 846-S47 = 867-868 (Gravity S-47 +453 station) E-48 +53 E48 = W70 (Gravity station) E-49 +250 E49 = S69 (Gravity station) E-SO -43 E50 = E71 (Gravity station) E-Sl +148 E51 = W78 (Gravity station) E-52 +88 E52 = W75 (Gravity station) E-S3 +9 E53 = W74 (Gravity station) E-54 +109 E54 = S73 (Gravity station) N-SS +90 N55 = S72 (Gravity station) N-56 +12 N56 = $60 (Gravity station) W-57 +263 W57 = W61 (Gravity station) N-58 +13 N58 = S80 (Gravity station) N-S9 -635 N59 = 881 (Gravity station) N-60 +341 N60-N66 = N89-N93, W94, N95 N-6l +351 (Gravity station respectively N-62 +216 107 Meter Corrected Stn Readings Approximate Location (Gammas) N-63‘ +248 N-64 +212 N—65 +204 N—66 +140 N-67 +246 N67-S70 = N96, N97, W98, S99 N-68 +140 (Gravity station respectively) W-69 +162 S-70 +117 s-71 +61 871-580 = 3100-5109 (Gravity S-72 +41 station respectively) W-73 +201 S-74 +51 S-75 +51 S-76 +151 S-77 +11 S—78 +21 S-79 +51 S-80 +81 W-8l +151 W81-W86 = SllO-WllS (Gravity S-82 +151 station) W-83 +151 W-84 -57 W-85 +83 108 Meter Corrected Stn Readings Approximate Location (Gammas) W-86 +83 S-87 -343 $87-W92 = 8116-3121 (Gravity E-88 -442 station) E-89 -512 E-90 —217 S-91 +13 W-92 +256 N-93 -1564 N93-N96 = N45-N48 (Gravity N-94 +446 station) N-9S +276 N-96 +166 N-97 +156 N97 = 1/4 mile from Gravity station N43 S-98 +96 898-899 = N43-N44 (Gravity E-99 -54 station) S-100 -44 3100 = N42 (Gravity station) E-lOl -384 E101 = N41 (Gravity station) S-102 -284 8102 = N28 (Gravity station) s-103 -454 ' 3103 = N29 (Gravity station) S-104 —514 8104-8106, station points between S-105 —1254 $103—N39 (Magnetic stations), S-106 -1854 southward, close to one of the magnetic anomalous bodies. 109 Meter Corrected Stn Readings Approximate Location (Gammas) S-107 ~50 3107—8118 = 8122-8133 (Gravity E-108 -50 stations). Stations 107 and 108 E-109 +30 outside of the grid pattern E-llO :0 (see Figure 3) N-lll +125 N-112 +73 N-113 —62 W-114 +22 N-llS +44 S-116 +84 S-117 +88 S-118 +92 N-119 +405 N119-N124 = N134-Nl39 (Gravity N-120 -26 station) N-121 -5 N-122 +15 N-123 +30 N—124 -10 N-125 -134 N125-N130 = N140-N145 (Gravity N-126 -70 station), left corner of the N-127 -150 grid pattern, northward N-128 —183 N-129 —152 110 Meter Corrected Stn Readings Approximate Location (Gammas) N-130 _+45 S-131 +452 Sl3l-W137 = Sl46-W152 (Gravity S-132 ~85 stations), 8131-132, outside of S-133 —210 the grid pattern. S-134 —26 E-135 —142 N-l36 -34 W-137 +1730 W-137 (Positive higher magnetic value) BIBLIOGRAPHY BIBLIOGRAPHY Acharya, N. ”Interpretation of Local Magnetic Anomalies of an Iron Ore Deposit in Podagada Range of Hills near Umerkote, Koraput District Orissa." Bulletin of the National Geophysical Research Institute of Hyderabad (NGRI), Vol. 7, pp. 1-73, 1969. Ade-Hall, J. M. & Wilson, R. L. 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Geophysical Prospecting, V01. 8' pp. 98-110, 1960. 113 Hammer, S. "Approximate Gravity Calculations." Geophysics, vol. 39, pp. 205-222, 1974. Heiland, C. A. Geophysical Exploration. New York: Prentice Hall, Hener Publishing Co., 1968. Hinze, W. J. "Geophysical Studies of Basement Geology of Southern Peninsula of Michigan." Am. Assoc. Pet. Geol. Bu11., vol. 59, no. 9, pp. 1562-1584, 1975. Jakosky, J. J. Exploration Geophysics. Trija Publishing CO. ’ pp. 160-165, 19610 John, S. K. & Cannon, F. "Geological Interpretation of Gravity Profiles in the Western Marquette District, Northern Michigan." Geological Society of America Bulletin, vol. 85, pp. 213-218, 1974. Larson, E. E. & Strangway, D. W. "Magnetic Polarity and Igneous Petrology." Nature, pp. 756-7, 1966. Leith, C. K. & Allen, R. C. "Discussion of Correlation of the Huronian Group of Michigan and the Lake Superior." Journal of Geology, vol. 23, pp. 716, 1915. McElhinny, M. W. Paleomagnetism and Plate Tectonics. 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"The Surface Geology of Portions of Menominee, Dickinson, and Iron Counties." Geo- logical Survey, Michigan, Annual Report, pp. 7-82, 1906. Scheidegger, A. E. Foundations of Geophysics. Elsevier Scientific Publishing Co., New York, 1976. Shurbet, D. H. et a1. "Remanent Magnetization from Com- parison of Gravity and Magnetic Anomalies." Geophysics, vol. 41, no. 1, pp. 56-60, 1976. Singh, C. L. & Ram, A. "A Study of the Vertical Magnetic Anomaly Caused by Magnetite Deposits in Sua Area of Palamau District Bihar (India)." Pure and Applied Geophysics, vol. 85, pp. 283-289, 1971. Smellie, D. W. "Elementary Approximations in Aeromagnetic Interpretation.” Geophysics, vol. 21, pp. 1021- 1040, 1956. Strangway, D. W. History of the Earth's Magnetic Field, Earth and Planetary Science Series. New York: Vacquier, V. et a1. "Interpretation of Aeromagnetic Maps. Memoir, 47 of The Geological Society of America, pp. 6-7’ 1951. Vanlier, K. E. "Ground-Water in Menominee County." Michigan Geological Survey Section, Department of Conservation, 1963. Verma, R. K. et a1. "Results of Vertical Magnetometer Surveys over Raniganj Coalfield, India." Geo— physics Research Bulletin, vol. 11, no. 3, pp. 167- 168, 1973. Watkings, N. D. & Haggerty, S. E. "Oxidation and Magnetic Polarity in Single Icelandic Lavas and Dykes.” GeOphysics. J. Roy. Astron. Soc., 15, pp. 305-15, 1968. 115 William, B. W. "Textural Variation within a Quartz Diorite Pluton (Twelve-foot Falls Pluton), North- eastern Wisconsin." Geological Society of America Bulletin, vol. 74, pp. 243-250, 1963. I’IICHIGRN STATE UNIV. LIBRARIES IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIllllllIIlII 31293102774746