A. VERTICAL MAGNETIC INTENSITY STUDY
05 THE GRAND RAPIDS ANOMALY

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ABSTRACT

A VERTICAL MAGNETIC INTENSITY STUDY
OF THE GRAND RAPIDS ANOMALY

by Jack G. Stevenson

A large magnetic anomaly was mapped in western
Kent County, Michigan in the area of the City of Grand
Rapids. The residual vertical magnetic intensity anomaly
is approximately 1650 gammas and the areal extent is
roughly 144 square miles. The feature chosen to represent
the Grand Rapids body is an infinite vertical cylinder of
20,000 foot radius, 7300 foot depth and susceptibility
contrast of .00553 (cgs), plus three additional smaller
cylinders flanking the main body. Regional gravity data
is at variance with the magnetic interpretation, suggesting
a false basic assumption in the magnetic study. The exis-
tance of remanent magnetization is suSpected. Further
gravity work is recommended to provide a unique solution

to this problem.

A VERTICAL MAGNETIC INTENSITY STUDY

OF THE GRAND RAPIDS ANOMALY

By

Jack C. Stevenson

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Geology

19611r

ACKNOWLEDGMENTS

I wish to express my sincere thanks to Dr. William
J. Hinze for his guidance, suggestions, and effort that
helped to initiate and shape this study. I also wish to
thank him for his invaluable assistance during the organ-
ization and construction of this paper.

I also wish to thank Dr. James W. Trow and Dr. Harold
B. Stonehouse for their suggestions and criticisms. I also
wish to thank them and Mr. George Secor for giving of their

time so that this thesis could be completed.

TABLE OF CONTENTS

CHAPTER PAGE
I. INTRODUCTION . . . . . . . . . . . 1
II. GEOLOGY AND GEOGRAPHY . . . . . . . . 3
III. FIELD PROCEDURE . . . . . . . . . . 5
IV. REDUCTION OF DATA . . . . . . . . . 9
V. RESULTS OF THE SURVEY . . . . . . . . 10
VI. THEORY OF MAGNETIC SURVEYING . . . . . . 13
VII. INTERPRETATION OF THE GRAND RAPIDS ANOMALY . 16
VIII. CONCLUSIONS . . . . . . . . . . . 25

BIBLIOGRAPHY . . . . . . . . . . . . . 27

FIGURE

II.

III.

LIST OF FIGURES

Isogonic Map of the Southern
Peninsula of Michigan

Location of Area of Investigation .

Comparison of Profiles

Plan View on Assumed Body Causing the

Grand Rapids Magnetic Anomaly

PAGE

22

2A

LIST OF PLATES

PLATE

I. Magnetic Vertical Intensity Map All Plates in
Back Pocket
II. Second Derivative Map
III. Station Location Map
IV. Plan View Comparison of Anomalies
V. Bouguer Gravity Anomaly Map of Kent

County and Adjacent Areas

CHAPTER I
INTRODUCTION

The isogonic map of the Great Lakes area (see Fig. 1)
shows a rare, but classical isogonic anomaly in the
Grand Rapids, Michigan area. The declination pattern
strongly suggests an intense distortion of the earth's
magnetic field by a local magnetic field.

w. P. Jenny (1934) describes a magnetic vector study
of the Southern Peninsula of Michigan in which he defines
and studies an anomaly in the Grand Rapids area. From
seven magnetic stations established by the U. 8. Coast
and Geodetic Survey Jenny interprets that the Grand Rapids
anomaly is due to a large domal uplift along a regional
anticlinal trend. However, he also recognizes that some
of the Michigan anomalies may be caused by a variation
in the magnetic permeability of the basement rocks. He
finds nearly the correct anomaly (1500‘) for the Grand
Rapids feature so he apparently assumed a large domal up-
lift was capable of causing this magnitude of anomaly.
More recent work points out that‘the sedimentary rocks of
the Southern Peninsula of Michigan do not contain sufficient
magnetic minerals to create an anomaly of this magnitude

(Hinze, 1963). A topographic feature of the basement rocks

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ISOGONIC MAP or THE SOUTHERN PENINSULA
or MICHIGAN“

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also could not produce an anomaly of this size. Therefore,
a.horizonta1variation in susceptibility of the basement
rocks is the only possible source.

The purpose of this study is to interpret geologically
the Grand Rapids anomaly. To accomplish this purpose it
was necessary to measure and map the magnetic field of
Grand Rapids and surrounding areas. This required the
develOpment of a procedure of observing the magnetic field
in industrial-urban areas. Then it was necessary, from the
magnetic map obtained from the survey, to determine the
characteristics of the source of the anomaly and to make

a reasonable geological interpretation of the anomaly.

CHAPTER II
GEOLOGY AND GEOGRAPHY

The Sourthern Peninsula of Michigan is structurally a
sub-circular basin. The area under investigation (see Fig.
2) is to the southwest of the center of the Michigan Basin.

The bedrock is Mississippian Marshall Formation in
the western part of this area and Pennsylvanian Saginaw
formation in the north and east, the contact occurring a
few miles east of the Grand Rapids area. The subcrop in
the local Grand Rapids area is a commercial gypsum of the
Grand Rapids Group and is extensively mined underground.

The basement rock where encountered in the known
drill holes is a Precambrian granite. Wells which pene—
trate to the Precambrian basement rocks in the Southern
Peninsula of Michigan are rare, only seven are known at
the present, all in the southeast portion of the State.

The morphology of the land is gently to steeply
rolling glacial drift with numerous glacial moraines usually
covered with good to poor tOp soil. The area is drained
by the Grand River and numberous minor tributaries which
provide light to medium dense drainage patterns. The major

topographic features are the entrenched river valley and

 

 

 

 

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floodplains of the Grand River. The only major city is
Grand Rapids, located in the west-central portion of Kent
County. Grand Rapids and the outlying suburbs are a
sprawling urban area with light to medium industry. The

only other major industry in this area is farming.

CHAPTER III
FIELD PROCEDURE

During the winter of 1963-64 a vertical magnetic
intensity survey was carried out in Kent County and adja-
cent areas, from TEN to TlON and from R9W to RlAW. The
stations composing this survey were located at two mile
intervals on north-south lines and at three mile inter-
vals on east—west lines whenever possible. In all, 250
stations were established covering approximately 31 town-
ships. The ideal station spacing was changed only because
of difficulties in establishing stations due to uncontrol-
lable factors. Also the station spacing was changed in
order to obtain additional detail of the observed anomaly.
This was hindered by the obvious difficulties in locating
magnetic stations within a highly industralized, urban area.

Readings at each station were taken at different
locations until three observation sites in a dog-leg pattern
were within a 10 to 12 gamma variation. Occasionally sev-
eral observation sites at a station were necessary. A
few stations were abandoned. The method of obtaining mag-
netic field observations outlined above nearly eliminates

the danger of including extraneous surface effects in the

readings, such as those from "tramp" iron and steel, power
lines, pipe lines, steel structures and other surface mag-
netic bodies.

The station locations were located on the 1961 Michigan
Department of Conservation County Road Maps. Stations al—
ways were observed at road intersections for ease and accur—
acy of location.

At each individual station the distance from the
intersection of the roads to the points of observation was
paced off on north—south and east—west lines. This infor-
mation together with a diagram of station location and the
time of reading of at least three separate observation
sites was recorded in a field book under the station desig-
nation. Base stations were established wherever convenient
for the purpose of tying together all of the field stations.

The time variations in the earth's magnetic field were
determined by checking the magnetic readings of base stations
at different times during the day. These variations, or
drift, can then be eliminated from all the station readings.
In the preliminary stages of the survey magnetic drift was
checked at base stations at two hour intervals. The daily
drift or diurnal variation was found to proceed slowly and
the total drift during the day was small enough, in view of
the comparative accuracy of the survey, to allow for longer

time intervals between station checks. Therefore the diurnal

drift was checked before and after the noon meal and
before and after each individual day's work.

At the conclusion of the survey all base stations
were tied together by the method of leapfrog 100ping.

The base stations then were tied to the Kent County mag-
netic base station established on the 1960-61 Michigan
State University regional magnetic survey of the Southern
Peninsula of Michigan. On this survey this primary
station has an arbitrary value of 1481 gammas.

The regional normal magnetism of the earth, or the
"latitude and longitude" vertical magnetic intensity
variation was obtained from the 1962 map in the GeOphysics
office of the Geology Department of Michigan State
University. The secular magnetic variations over the
survey area since 1962 are inconsequential so a new map
was not constructed. This correction was made on all
stations relative to the primary base for this survey.

The instrument used on this survey was an Askania
Vertical Torsion Magnetometer sz which measures relative
vertical magnetic intensity. In this instrument a magnet
is suspended by two torsion fibers. When the arresting
system is released any magnetic field acting on the magnet
will cause it to move. The magnet is brought back to an
equilibrium position by applying torque to the fibers through
a calibrated screw system. The amount of torque applied is

prOportional to the vertical magnetic field acting on the

lO

magnet. The levels of the Askania Vertical Torsion Magneto-
meter are adjusted in such a manner that when the instrument
is leveled the magnetic axis and the axis of rotation of the
magnet system arehorizontal. In this position the observed
reading is only a function of the vertical magnetic intensity.
The Askania Vertical Torsion Magnetometer is tempera-
ture compensated. However a test was made to determine the
effects of extreme temperature shocks on the instrument.
This was done by bringing the instrument to equilibrium under
an 80°F condition and then quickly removing it to an 18°F
condition. Readings were taken approximately a minute apart
to check on the compensation for this large temperature
change. In two minutes the readings were within the accur-
acy level of the survey and within seven minutes they were
back to normal. In view of the fact that the magnetometer
kept at air temperature during the survey, no abrupt temper—
ature variation was experienced by the instrument. There-

fore, no temperature correction was applied to the survey.

CHAPTER IV

REDUCTION OF DATA

Magnetic diurnal drift curves were plotted for each
day's observations and all stations were tied into the
base stations and then corrected to the primary base.

The maximum daily magnetic drift observed was about
25 gammas and the maximum drift between stations from the
beginning to the end of the survey also was about 25

gammas.

CHAPTER V
RESULTS OF THE SURVEY

A vertical magnetic intensity map, (Plate 1) was
prepared for the area under investigation from the cor-
rected data. The regional magnetic intensity for this
area was calculated by the cross—profile method using
fourteen smoothed observed profiles to obtain a regular,
smooth regional curve. A normal vertical magnetic intensity
of 1400 gammas was established for the area. After this
regional value was removed from the vertical intensity map
a large magnitude residual anomaly was found centered between
T6N and TTN and Rllw and R12W. This anomaly has a magnitude
of over 1600 gammas and covers an area of roughly 144 square
miles in a sub-square pattern.

Several magnetic lows surround this major anomaly.

A major low lies directly west of the anomaly. This low
has a magnitude of about —350 gammas. Another major mag-
netic low is centered in T5N and R10 and 11w and can be
explained by an extension of a regional low extending into
this area from the southeast. A large positive anomaly
exists in the southwest corner of the intensity map with

attendant lows to the northwest and southeast. Another

13

large positive anomaly exists on the extreme northwest
portion of this map. Several minor positive and negative
anomalies show trends of directly east-west direction.
This parallel belt of anomalies lies generally north of
the center line of the intensity map, from T7N to TSN.

A second derivative map of the area was constructed
from the vertical magnetic intensity map as a guide to in-
terpretation and particularly for comparison to Vacquier's
(1951) theoretical models. Henderson and Zietz's (1949)
second derivative formula was used in obtaining Vacquier's
theoretical maps. Their formula was adapted to fit the
grid radii used in this study. The grid radius is so
adjusted that anomalies with sources above a certain
depth are screened out, leaving only those anomalies which
are created by deeper bodies.

Several grids were tried in preparing the second
derivative map, including two, three, and four mile
radius grids. It was found that a three mile radius
floating grid using known station values as centers came
the closest to fitting the ideal case of Vacquier.

The basic formula for the second derivative method

of Henderson and Zietz (1949) is:

14

2
r

K K
ZT = 5; A - _—_£ A

K
A
E. k
K 2
:E Ak Uk = the partial second derivative or slope at
the center of grid

. I2 23
' Atr "'A¢o F ‘H" gggé'
Using a second derivative grid radius of three miles, the
average values of the field at a distance of three and 3(5’

ZSTr - average value of the field at distance r

the central value

miles respectively become:

_I 2
AT3 =Ato — .13.. mg 2?:
combinin these
by equation%

18 22 At xg
IE: 3I§'= Alto - If' £932

2
a At = 2 _

This is the formula used in the second derivative calcu-

lations.

CHAPTER VI
THEORY OF MAGNETIC SURVEYING

Magnetic anomalies are usually caused by variations
in the magnetite content of rocks. Rocks which have
sufficient magnetite to cause measurable anomalies are
the igneous rocks, magnetic iron ores, and rocks with
sedimentary concentrations of magnetite (Jakosky, 1950).
Therefore, magnetic anomalies in the Michigan Basin are
principally caused by variations in magnetic susceptibility
in the igneous rocks and metasediments of the Precambrian
basement complex.

When using a torsion magnetometer the measured mag-
netic intensity is the sum of the magnetic fields associ-
ated with magnetic bodies, both local and regional, super-
imposed on the earth's vertical magnetic field.

The interpretation of the field intensity map is
based on the fact that the earth's normal vertical magnetic
field will be greatly distorted in areas of horizontal
variations of magnetic susceptibility of the underlying
rocks. This fact is used to interpret the source of the
anomaly as to configuration, geometry, depth, rock type,
and other parameters. From this interpretation an attempt

is made to determine the relative magnetic susceptibilities

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16

of the rocks and the magnetite content, both of which
indicate the type of rock present.

What is necessary then is data that can be used in
a quantitative study.

Various components of the earth's magnetic field can
be measured but in the Grand Rapids area vertical magnetic
intensity was measured. The vertical intensity is measured
in the unit of gamma, which is 10-5 oersted.

The intensity of magnetization of a rock unit is a
function of the susceptibility of the rock and of the
strength of the inducing magnetic field, whenever remanent
magnetization is absent. When a rock is formed it takes
on properties inherited from the inducing magnetic field
and from its own internal composition at the time of
formation. When these properties are at variance with the
magnetic properties expected of the same body in the
earth's magnetic field at present, remanent magnetic
characteristics exist. However, no quantitative information
of these characteristics may be obtained directly from the
rock in this study. Therefore, the earth's present mag—
netic field of roughly .6 oersteds is used as the inducing
field and the present direction of polarization is used on
the induced body. If no remanent magnetization is assumed

the formula for the intensity of magnetization is I=kH.

The basic premise behind magnetic exploration is
that the detectibility of the magnetization of a feature
depends on the contrast of susceptibilities between the

feature and the surrounding rocks.

17

CHAPTER VII
INTERPRETATION OF THE GRAND RAPIDS ANOMALY

The magnetic vertical intensity map of the area
under study (Plate 1) shows a portion of the magnetically
positive trend roughly extending northeast-southwest as
first described by Jenny (1934). This study will be
concerned with the major anomaly along this trend, the
anomaly centered near the intersection of T6 and 7N and
R11 and 12W. Hereafter in this study this anomaly will
be called the Grand Rapids magnetic anomaly.

The interpretation of the Grand Rapids magnetic
anomaly is based on the dimensions and configuration
of themagnetic anomaly, the limiting and the suggestive
geologic conditions, and the assumption that remanent
magnetization is not influencing the observed magnetic
field.

Remanent magnetization is due to retention of mag-
netic characteristics in a magnetic body after the inducing
field has changed. This inducing field may be an earlier
earth's magnetic field and may bear no resemblance to the
present earth's field. Retention of magnetization may be
due to rapid cooling of the body from a molten to a solid
condition in a magnetic field, or simply the result of the

retentivity characteristics of the body. No quantitative

19

information of remanent magnetization in the causative
feature under study was available and no direct measure-
ments could be made. Therefore no remanent magnetization
was considered in the interpretation.

It is possible to have a variety of solutions
derived from the Grand Rapids anomaly. This is true
because single field potential data has no unique
solution. Several interpretative processes however are
used to delinate the parameters of the causative body and
to define the possibilities concerning the anomaly.

The simplest procedure to determine depth of the
causative feature is the half-width method. This tech-
nique gives maximum possible depths from well defined
anomalies.

L. J. Peters (1949) also develOped a method of
estimating depths to magnetic features based on half
lepes tangent to the observed anomaly curve. Both the
half-width and Peters' methods gave maximum depths for
this feature in excess of 14,000 feet.

Vacquier (1951) has determined theoretical magnetic
total intensity and second derivative anomalies for a
series of models as a guide to magnetic interpretation.
Observation of the models selected a 6 x 6 parallelOpiped
model intensity map at 75°N as most closely approximating
the observed anomaly. The size of the models is based on

the depth to the distorting feature. Vacquier uses

20

depth indices determined from theoretical magnetic maps

to estimate depths. This method is based on the relation-
ship of the observed indices on the map to the length of

the theoretical map indices, the quotient observed/theoreti—
cal being the depth to the feature. These depth indices

can be taken from both the intensity maps and the second
derivative maps.

Vacquier (1951) states that this method is useful
only where constant polarization with depth exists and the
sides of the body are vertical or near vertical. The
other simplifying assumption that Vacquier and this study
make is that the magnetic polarization of the disturbing
body is in the same direction as the present direction
of the earth's magnetic field.

The second derivative map constructed from the observed
magnetic intensity map bears a resemblance to Vacquier's
model second derivative but containes irregularities which
suggest that the causative body is more complex than the
model.

Calculations of Vacquier's depth indices also indi—
cates depths between 14,000—16,000 feet.

All of the depth determinations of the feature were
considerably deeper than the acceptable value from geologic
information, therefore the depth to the feature was assumed
from geological information. Cohee (1945) published a

basement contour map of the Michigan Basin. This map shows

21

an average depth of 7300 feet to the basement rocks in the
anomaly area. This value was accepted rather than the
excessive depths determined by the interpretational methods.

One major fact gained from the second derivative
map is that the zero contour line tends to outline the
feature when the inducing magnetic field is at a high
angle. The radius of the feature as indicated from the
zero contour line is from 18,000 to 23,000 feet.

The second derivative map suggests that the most
obvious ideal representative of the body is a vertical
cylinder. Several different depths and radii of vertical
cylinders were tried, at 7000 feet and 7300 foot depths
and 22,500, 20,000, and 18,000 foot radii. The results
were matched against profiles constructed across the
observed anomaly.

The vertical magnetic intensity from vertical
cylinders was computed from.Nettleton's (1942) solid
angle formulas which assume vertical polarization. This
assumption is valid because the indicing field is at a high
angle, 75°.

The best fit of observed and computed curves occured
when the vertical cylinder was assumed to be at a 7300 foot
depth, have a 20,000 foot radius, and have a 5530 x 10-6
(cgs) susceptibility contrast, as shown in Fig. 3.

In general the curves match closely, but the observed

curve has an abnormal peak, Fig. 3. Curves drawn across

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23

the anomaly north of the high crest match very well both
as to width and magnitude of the anomaly curve. A close
inspection of the intensity and second derivative maps
show this peak to be anomalous. A smooth curve drawn over
the anomaly from control on the sides shows this peak to
be highly anomalous to the rest of the curve. Therefore,
this particular vertical cylinder was chosen as a first
partial representative of the causative body.

The original assumption of depth is supported by
using both Peters' method and the half-width method on the
7300 foot depth theoretical curve. Both methods give
depths in the same range as the previous attempts on the
observed curve. This illustrates that Peters' method and
the half-width method for depth determinations bothigive
excess values on this type of feature. This close com-
parison of values also suggests that the chosen theoretical
depth is a good approximation of the actual depth.

To extend further the ideal theoretical representation
of the causative body the vertical field intensity map of
the theoretical body was placed over the observed intensity
map. It was found that additional magnetic sources were
needed to adjust the theoretical map to the observed map.
These sources were placed at the corners of the observed
map and the effects were calculated and added to the theor—
etical map until a close representation of the variation

in the intensity of the anomaly could be found. The effects

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PLAN VIEW OF ASSUMED BODY CAUSING

THE GRAND RAPIDS MAGNETIC ANOMALY
BASED ON INFINITE VERTICAL CYLINDERS

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24

of several vertical cylinders were tried at each corner of
the anomaly. The corner on the southwest was best fitted
to a 7000 foot radius cylinder tangent to the body cylinder
of the anomaly. Similarly a 6000 foot and a 4000 foot
vertical cylinder were established at the southeast and
northeast corners respectively. The northwest corner did
not need any additional adjustment. The shape of the
resulting body is shown in Fig. 4. These infinite vertical
cylinders were all applied to form a resultant anomaly.

The overlapping effects were algebraically summed with the
cylinders in the assumed positions. The resulting anomaly
when contoured and laid over the intensity map provides

a very close fit, as shown in Plate 4.

To determine the actual susceptibility of the causa-
tive body it is necessary to add the calculated suscepti-
bility contrast to the susceptibility value for the sur-
rounding country rock. In this case it is assumed that
the normal country rock is granite because nearly all wells
in the Southern Peninsula of Michigan entering the Precam-
brian basement encounter granite. The susceptibility of
granite ranges fkwm1.00002'to .0029 (cgs). However,
according to Jakosky (1950), 60% of granite samples have a
measured susceptibility of less than .0001 (cgs). So
taking a mean value for granite of .0001, the body causing
the Grand Rapids anomaly has a susceptibility of .00563
(cgs). This susceptibility suggests a diabasic or basaltic

25

composition for this rock. Measurements of susceptibility
by Mooney and Bleifuss (1953) and Jakosky (1950) suggest
this range of composition. Basalt samples have more
susceptibility values near the calculated susceptibility.
Calculating the per cent of magnetite in the rock by the
formula, % magnetite (vol.) = susceptibility of the rock/
susceptibility of magnetite (.3), gives a value of 1.9%.
Therefore, the rock is likely a basic rock intruded into
the basement complex, perhaps during the Keweenawan intru-
sive period in late Precambrian as suggested by Hinze (1963).
Quite possibly extrusives were issued from this source.

A comparison may be drawn between the Grand Rapids
magnetic anomaly and the Sauble anomaly of Lake County,
Michigan as interpreted by Meyer (1963). Strong parallels
do exist. The Sauble anomaly consists of a large magnetic
anomaly which extends over several dozen square miles and
the theoretical source of the feature is represented by a
vertical cylinder. Strong differences also exist. The
areal extent, magnitude, and the total representation of
the Grand Rapids anomaly is greater than that of the
Sauble anomaly. The depth to causative feature and the
susceptibility contrast of the Sauble anomaly is greater
than the Grand Rapids anomaly. The chief characteristic
of the Sauble anomaly that is missing from the Grand Rapids
anomaly is the large positive gravity anomaly associated

with the magnetic anomaly.

26

In the Grand Rapids area the regional gravity was
established by the Michigan State University Department
of Geology (Hinze, 1963), see Plate 5. No detailed gravity
data was available to this present study so interpretation
is based solely on the observed magnetic data.

The only gravity anomaly apparent on the regional
gravity map is a small nose on the regional gradient.

The anomaly causing this nose has a magnitude of about

1 milligal. The Sauble anomaly shows a gravity residual
anomaly of over 20 milligals (Meyer, 1963). This points
up a wide variance in the parameters of the causative
bodies. A calculation of the effect on gravity of the
assumed Grand Rapids magnetic body shows little comparison
to that gravity anomaly determined from the regional map.
This suggests an interpretation different from that
presented.

More detailed gravity work must be carried out in
the Grand Rapids areas to permit more quantitative inter-
pretation of this anomaly and to obtain data that when
combined with the magnetic data from this study will lead
to a unique solution of this problem. The known gravity
data in this area (see Plate 5) is too greatly at variance
with the magnetic interpretation. Either unusual conditions
exist, in terms of susceptibility and density, or one or

more of the basic assumptions must be false.

27

A high degree of remanent magnetization may exist
contrary to the assumption made at the beginning of this
study. If this is the case then the total field intensity
is much higher than was assumed and as a result the body
could have greatly different parameters. The case for a
high remanent magnetization is supported by the existance
of a strong magnetic low (see Plate 1) west of the Grand
Rapids anomaly, this low possibly indicating a different
declination for the strongest magnetic field. Also some
support for this assumption comes from the fact that the
depth indicators did not give reasonable values. A pos-
sible alternate interpretation of the causative body is
a flow with a high remanent magnetization instead of a
vertical intrusive. Therefore the anomalous peak of the
observed anomaly could be a vertical feeder system or a
volcanic pipe.

None of these ideas can be quantitatively inter-
pretated without further data from other sources.

In summary, the presented representation of the
Grand Rapids magnetic body has a very close fit to all
known magnetic parameters. This representation only can
be changed by further gravity work or drilling to the base-
ment rocks. Therefore, it is felt necessary to have further
detailed gravity work done in this area. From this gravity
information combined with the magnetic information provided

by this study, a unique solution to the problem may be found.

CHAPTER VIII
CONCLUSIONS

The Grand Rapids anomaly which lies on a northeast-
southwest trend of positive magnetic features in the
Kent County area has a vertical magnetic intensity mag-
nitude of 1650 gammas and an areal extent of roughly
144 square miles. The magnitude of this anomaly is

the greatest in the Southern Peninsula of Michigan.

The magnetic interpretation suggests that the source

of the Grand Rapids anomaly is an infinite vertical
cylinder having a depth of 7300 feet to the top
surface, a radius of 20,000 feet, and a susceptibility
contrast of .00553 (cgs), plus three additional

smaller vertical cylinders of the same susceptibility
contrast located on the flanks of the main body. The
calculated susceptibility suggests that the causative
bodies have a basic composition. The 7300 foot depth
was based on geologic information because normal methods
of magnetic depth determinations do not give geologically
reasonable results for the Grand Rapids anomaly.

The presently available regional gravity data does not
support the magnetic interpretation under normal condi-
tions. It is expected from the gravity data that one

or more of the basic assumptions made in the magnetic

29

interpretation are false. The existence of remanent
magnetization in the body is suSpected. Further
gravity work is recommended to detail the gravity
anomaly associated with the Grand Rapids feature.
Surface affects caused by urban—industrial areas are
not excessive if care is taken.in location of stations

and the accuracy of the survey is not too high.

BIBLIOGRAPHY

Cohee, G. V. (1945) Oil and gas investigations preliminary:
U. S. Dept. of Interior, Geological Survey, Chart 9.

Dobrin, M. B. (1960) Introduction to geophysical prospec-
ting, McGraw—Hill Book Co. 2nd Ed.

Heiland. (1940) Geophysical exploration, Hafner Publishing
Company

Henderson, R. G. and Zietz, I. (1949) The computation of
second vertical derivatives of geomagnetic fields:
Geophysics, Vol. 14, pp. 508-516.

Hinze, W. J. (1963) Regional gravity and magnetic anomaly
maps of the Southern Peninsula of Michigan: Rept. of
Invest. 1, Dept. of Conservation, Geological Survey
Division.

Jakosky, J. J. (1950) Exploration geophysics, Trija
Publishing Company, 2nd Ed.

Jenny, W. P. (1934) Magnetic vector study of Kentucky and
Southern Michigan: AAPG Bu11., Vol. 18, No. 1,

PP- 97-105-

Meyer, H. M. (1963) A combined magnetic and gravity
analysis of the Sauble Anomaly, Lake County, Michigan.
Masters Thesis, Michigan State University.

Mooney, H. J. and Bleifuss, R. (1953) Magnetic suscepti-
bility measurements in Minnesota: Geophysics, Vol. 18,

pp. 383-393.

Nettleton. (1942) Gravity and magnetic calculations:
Geophysics, Vol. 7, pp. 293-310.

Peters, L. J. (1949) The direct approach to magnetic
interpretation and its practical applications:
Geophysics, Vol. 14, pp. 290—320.

Vacquier, V. et a1. (1951) Interpretation of aeromagnetic
maps: Memoir 47, G. S. A.

(1914) Mineral resources of Michigan: Michigan Geological
and Biological Survey, Pub. 19, Geological Series 16.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I I I
THE GRAND RAPIDS MAGNETIC ANOMALY

PLAN VIEW COMPARISON OF ANOMALIES

Q j

VERTICAL INTENSITY .
I"=2Miles

ACTUAL
CALCULATED —-—- —__.. ___..

 

CONTOUR INT-'ZOOGAMMAS J.C. STEVENSON

Plate 4

 

 

 

  
   

 

mu umv v
IBRPRY
SING. MICHI

 

 

 

MICHIGAN STATE UNIV. LIBRARIES

III

4662

 

II I I III

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