A Marine magnetic survey off the east coast of the United States : Project N-20

TECHNICAL REPORT

A MARINE MAGNETIC SURVEY
OFF THE EAST COAST OF THE UNITED STATES

PROJECT N-20

Geomagnetics Branch
Marine Surveys Division

SEPTEMBER 1962

U. S. NAVAL OCEANOGRAPHIC OFFICE
WASHINGTON, D. C.
Price 40 cents

Ca

FOREWORD

The results of the survey described in this report are

considered to be of significance to both the Navy and the

scientific community. The region investigated is a transition

zone between a continental mass and a true oceanic basin. Geo-

physical investigations of such regions may lead to a better
understanding of the earth's major crustal features and their
origin. The use of geophysical exploration techniques such as

described here provide a means of deducing information about

the earth's deeper structures lying beyond the limits of

direct measurement.

. C, STEPHAN
Rear Admiral, S. Navy
Commander

ACTA

O 0301 0044491 oO

iii

HOON

CONTENTS

Ibo INTRODUCTION
A. Purpose
B. Background
C. Survey Area
II, SURVEY OPERATIONS
A. Conduct of Survey
B. Control
C. Instrumentation
D. Personnel
III, DATA PROCESSING
A. Preliminary Data Processing
B. Magnetic Temporal Variations
C. Total Magnetic Intensity Contour Chart
D. Residual Magnetic Intensity Contour Chart
E. Profiles
IV. SURVEY RESULTS
A. General
B. Discussion of Data
Wo SUMMARY OF FINDINGS
REFERENCES

APPENDIX - Fredericksburg Magnetograms, July 18-24, 1961

Page

24

25

FIGURES

Track Chart and Profile Index

Total Magnetic Intensity Contour Chart

Residual
Magnetic
Magnetic
Magnetic
Magnetic
Magnetic
Magnetic
Magnetic
Magnetic

Magnetic

Magnetic Intensity Contour Chart
and Bathymetric Profiles A-A'
and Bathymetric Profiles B-B'
and Bathymetric Profiles C-C'
and Bathymetric Profiles D-D'
and Bathymetric Profiles E-E'
and Bathymetric Profiles F-F'
and Bathymetric Profiles G-G'
and Bathymetric Profiles H-H'

and Bathymetric Profiles I-L'

vi

Page
12

13
14
15
16
17
18
19
20
21
22

23

I. INTRODUCTION

A, Purpose

In July 1961, the USS PREVAIL (AGS-20) conducted total magnetic
intensity, bathymetric, and bathythermal survey operations for the
U. S. Naval Oceanographic Office.* The purpose of the magnetics
phase of this survey was to define more precisely the characteristics
of the earth's magnetic field over the continental shelf, slope, and
adjacent deep-water area off the east coast of the United States.

Coincident with the geomagnetic and bathymetric measurements,
bathythermograph observations and continuous recordings of sea
surface and injection temperatures were taken. These observations
were part of a project to develop a method for predicting the
ocean's thermal structure. Thermal structure data are reported
in U. S. Naval Oceanographic Office Technical Report 113 (in prepa-
ration) and are not included in this report.

B. Background

Several airborne magnetometer survey suaeus had been flown by
Oceanographic Office Project MAGNET survey aircraft over the east
coast of the United States and the adjacent ocean area. Analysis
of the data recorded along these tracks indicated that a magnetic
anomaly is characteristically present at or Me: the location of
the continental slope. Distinct magnetic anomalies of about twenty

‘

miles in horizontal extent and with various shapes and amplitudes

*In accordance with Public Law 87-533 effective 10 July 1962, the
U. S. Navy Hydrographic Office was redesignated as the U. S. Naval

Oceanographic Office.

always appeared on magnetic profiles flown transverse to the slope.
In this area, seismic investigations by others indicated the presence
of a ridge in the crystalline basement rocks. However, it was not
known whether this seismic ridge was the source of the magnetic
anomalies. Likewise, neither the detailed configuration of the
magnetic anomalies nor the exact positional relationship of the
anomalies and the continental slope were known.
C. Survey Area

The survey was conducted in an irregularly shaped area lying
between latitudes 35°N and 40°N and longitudes 70°W and 76°W. The
survey track lines were run approximately perpendicular to the con-

tinental slope. Specific survey tracks are shown in Figure 1.

II. SURVEY OPERATIONS

A. Conduct of Survey

The PREVAIL departed New York on 17 July and arrived in
Washington, D. C., on 25 July after completing almost 2500
miles of survey track. As shown in Figure 1, average track
spacing was approximately 30 miles with the tracks trending north-
west and southeast. This particular survey track configuration was
established to best meet both magnetic and bathythermal survey
requirements. The average speed of advance over the survey track
was 12.5 knots.
B. Control

Survey control was by Loran-A with additional position deter-
minations by radar where possible. The position of the ship was
determined every fifteen minutes and then plotted on Mercator
Plotting Sheets (H.0O. 3000 series). On the shoreward side of the
survey area, both Loran-A and radar were used. Here, the probable
position accuracy is estimated as being within + 1 mile. On the
seaward side of the survey area, radar fixes were not available,
and only Loran-A fixes were taken. Here, the probable position
accuracy is estimated as being within + 2 miles.
C. Instrumentation

A Varian nuclear resonance magnetometer, Model XN~4901, was
used to measure the earth's total magnetic field intensity. With
this instrument, the total field intensity can be measured to an

accuracy of about + 1 gamma (0.00001 oersted). Magnetometer equip-

3

ment consisted of a power supply, preamplifier, counting circuits,
analog recorder, and towed sensing unit. The sensing unit, a
Varian Model X-49-813 using 700 feet of Simplex #310 two-conductor
cable, was towed 400 feet astern. This sensing unit was streamed
and recovered manually. Console electronic equipment was installed
in the drafting room on the after part of the ship. Data were
recorded in analog form on a Varian G-11 recorder in units of "mag-
netometer counts". These units, an inherent property of the magne=
tometer design, are an inverse function of the total magnetic field
intensity. In the survey area, one magnetometer count is equal to
approximately 1.3 gammas.

Bathymetric instrumentation aboard the PREVAIL consisted of an
Edo AN-UQN-1B sonar receiver-transmitter, the output of which was
recorded directly in fathoms on a Mark V Precision Depth Recorder
(PDR). This type of recorder can be read to the nearest one fathom.
The bathymetric recording instrumentation was located in the ship's
drafting room.
D. Personnel

Two geophysicists from the Geomagnetics Branch, U. S. Naval
Oceanographic Office installed and operated the magnetometer system.

PREVAIL personnel operated the bathymetric instrumentation.

III, DATA PROCESSING

A, Preliminary Data Processing

The magnetometer recorder traces were scaled at time intervals
of fifteen minutes and also wherever maximum and minimum magnetic
intensity values were recorded. These values were converted from
Magnetometer counts to gammas and plotted on the smooth plot of the
survey track. The Precision Depth Recorder traces were scaled in
a similar manner.

B, Magnetic Temporal Variations

No attempt was made to remove temporal variations of the
earth's magnetic field from the data. Records of the Fredericksburg,
Virginia, Magnetic Observatory indicate that no severe disturb-
ances occurred during the time of the survey. Magnetograms and
calibration data are reproduced in the Appendix.

The Fredericksburg observatory is approximately 150 miles
from the shoreward side of the survey area and about 500 miles
from the seaward side. Because of these distances, it is not pos-
sible to determine accurately the magnitude of the errors intro-
duced by the temporal variations. Nevertheless, the magnetograms
should indicate times when the data cannot be considered completely
reliable. Variations that occurred will introduce small errors
in the location of contour lines, particularly in the areas of
shallow magnetic relief. However, it is considered that they had
little effect on the magnitude and position of the most signifi-

cant anomalies.

C. Total Magnetic Intensity Contour Chart (Figure 2)

The total intensity values plotted on the Smooth Track Chart were
contoured at 50-gamma intervals. The contours are shown in Figure 2.

Dashed contours represent extrapolated data.

D. Residual Magnetic Intensity Contour Chart (Figure 3)

In order to more clearly define the anomalies, the regional
gradient of the total magnetic intensity was removed from the original
values. To accomplish this, the total intensity contours from H. 0.
Chart No. 1703, The Total Intensity of the Earth's Magnetic Force
(for the year 1955) were corrected to the year 1961 and then inter-
polated at 50-gamma intervals. These interpolated contours were
then reproduced on the total intensity contour sheet of the survey
area. At each point on the sheet where survey plot contour lines
intersected charted contour lines taken from H. 0. 1703, the differ-
ence was computed. If the survey contour value was greater than the
charted contour value, a plus (+) value was assigned to the differ-
ence; if smaller, a minus (-) value was assigned. An overlay was
placed over these two contour representations, and the differences
at contour intersections were plotted and contoured at 50-gamma
intervals. The Residual Magnetic Intensity Contour Chart for the
survey area is shown in Figure 3.

E. Profiles

Profiles of the total magnetic intensity and the measured
bathymetric depth along each track are Se sccar ee in Figures 4
through 12. These profiles were constructed using the smooth-plotted
survey tracks as base lines. An index to the geographical location

of each profile is shown in Figure 1.
6

IV. SURVEY RESULTS

A. General

A significant advantage of a shipborne magnetic survey is
that bathymetric measurements can be taken simultaneously with
the magnetic measurements. Direct comparison of magnetic and
bathymetric data relative to each other is thus possible,
irrespective of the certainty of the ship's true position.

The data contained in this report provide useful infor-
mation relating to the geologic structure pattern jn this area.
These data can be correlated with similar information from
adjacent regions. Such correlation may yield clues leading
to a better understanding of the relationship between conti-
nents and ocean basins.

B. Discussion of Data

The magnetic field contour pattern in the survey area
(see Figure 2) contains a large, elongate, magnetic anomaly
on the western side. This anomaly has lineations corresponding
closely to those of the continental slope. On the eastern
side, the increasing complexity of the contour pattern suggests
the existence of a magnetic feature lying just outside the
survey area. Between these two features is a broad area
void of magnetic anomalies.

In the survey area, the bathymetric data indicate that

the sea bottom has no topographic features capable of accounting

for the observed magnetic anomalies. Figure 1 shows that a
portion of the survey track connecting points C and D passed
directly over Baltimore Canyon. Similarly, the track connect-
ing points E and F passed directly over Norfolk Canyon. In
neither case was there any magnetic field change to correspond
with these prominent topographic features.

Using the data shown on Profiles G-G', H-H', and I-I',
depths to the source of the large magnetic anomalies that were
found near the continental slope were estimated. These depth
estimates were made in accordance with empirical slope methods
of Vacquier et al (1951). The average depth estimates to

magnetic sources for these profiles are as follows:

Profile G-G' 19000 feet
Profile H-H' 18900 feet
Profile I-L' 19980 feet

Depth estimates made from magnetic data from a single
survey track are at best only approximate. It was found,

"magnetic depths'’ estimated above are in

however, that the
reasonable agreement with the depth to the crystalline basement
complex, as determined from seismic and drilling data by Ewing
et al (1950). It appears that the top of the magnetic source

is probably closely coincident with the basement surface.

North of this survey area, seismic data (Ewing et al,
1950) indicate the existence of a ridge on the surface of the
erystalline basement. The possibility has been considered
that this ridge may extend into the survey area and may be the
source of the magnetic anomalies found in the vicinity of the
continental slope. However, King et al (1961) computed values
of the magnetic polarization intensity that this ridge would
be required to have in order to produce the magnetic anomalies
observed over it. These computed values were too large to be
plausible.

In the southern part of the survey area, the magnetic
anomalies peak more sharply. This phenomenon may indicate a
shallowing of the basement in that region.

It appears that the most probable general explanation for
the continental slope magnetic anomaly is that advanced by
King et al (1961). These investigators suggest that although
basement topography probably contributes to the magnetic pro-
file, the continental slope magnetic anomaly may be partly the
expression of a large mass or series of masses of more highly
magnetic rocks within the basement.

Another significant feature is the relative position of
the peak of the magnetic anomaly. Profiles H-H' and I-I'
(Figures 11 and 12) are representative of the southern part of

the survey area. These profiles show the peak of the anomaly

to lie seaward from the break between the continental shelf and
the continental slope. In the northern part, profiles A-A' and
B-B' do not show any peak lying seaward but indicate that the peak
lies shoreward from this break. This difference in trend sug-
gests that the lineation of the magnetic anomaly is not directly
related to that of the continental slope. Instead, the lineations
of both are probably related to a subsurface structural trend.

The small, broad anomalies occurring about 60-80 miles sea-
ward from the continental shelf have been reported previously
by Keller et al (1954). They were noted as occurring in approxi-
mately the same location as an increase in isostatic gravity
anomalies. Bathymetric data reveaied no topographic features to
account for the anomalies. Consequently, they may be reflections
of some type of deep-seated lithologic contrast. In profile view,
these anomalies are best seen on Profiles A-A', D-D', and F-F'

on Figures 4, 7, and 9 respectively.

10

V. SUMMARY OF FINDINGS

Magnetic measurements across the continental slope and
adjacent deepwater area off the east coast of the United States
revealed the presence of an elongate anomaly of a few hundred
gammas amplitude. This anomaly hasa lineation corresponding
closely, but not exactly, with that of the continental slope.
Depth estimates made on this anomaly are in reasonable agree-
ment with seismic depths to crystalline rocks. This agreement
suggests that the anomaly is caused by contrasts in intensity
of magnetic polarization within the basement.

The center of the survey area is void of magnetic features.
However, small, broad anomalies occur about 60-80 miles east
of the continental slope. Bathymetric data revealed no
topographic features capable of accounting for these anomalies.
Consequently, these anomalies may be reflections of some type

of deep=seated lithologic contrast.

11

U.S. NAVY HYDROGRAPHIC OFFICE

PROJECT N-20

MARINE MAGNETIC SURVEY

TRACK CHART AND PROFILE INDEX

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PROJECT N-20
MARINE MAGNETIC SURVEY
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2400 —4

MAGNETIC AND BATHYMETRIC PROFILES G-G’

VERTICAL EXAGGERATION 30:1

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21

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23

REFERENCES

Ewing, M., Worzel, J. L., Steenland, N. C., and Press,
F., "Geophysical Investigations in the Emerged and
Submerged Atlantic Coastal Plain, Part V, Woods Hole,
New York, and Cape May Sections." Bulletin of the
Geological Society of America, Vol. 61, No. 9 (Sept.
1950), pp. 877-892.

Keller, F., Meuschke, J. L., and Alldredge, L. R.,
"Aeromagnetic Surveys in the Aleutian, Marshall, and

Bermuda Islands.'' Transactions of the American

Geophysical Union, Vol. 35, No. 4 (Aug. 1954), pp.
558-572.

King, E. R., Zietz, I., and Dempsey, W. J., The
Significance of a Group of Aeromagnetic Profiles Off
the Eastern Coast of North America. U. S. Geological
Survey, Professional Paper 424=-D. 1961.

Vacquier, V., Steenland, N. C., Henderson, R. G.,

and Zietz, I., "Interpretation of Aeromagnetic Maps."

The Geological Society of America Memoir 4/7. 1951.

24

APPENDIX

FREDERICKSBURG MAGNETOGRAMS, JULY 18-24, 1961

25

Table of Base-line and Scale Values
for Full-size Magnetograms

Fredericksburg Magnetic Observatory
Standard Magnetograph

PRELIMINARY VALUES

Declination Horizontal Intensity Vertical
(D) (H) Intensity (Z)
Interval Base-line Scale Base-line Scale Base-line Scale
value value value value value value
° ! ' /mm x & /mm x 7 /mm
Jul 18-24 6 22 0.49 19165 73.65) 53055 3.0

1961

Base-line separation distance on original magnetograms Z-H 111 mm.

D=B+5S d H=B+S_h Z = BL+S 2
ip) 1D) wee H 4H mm Z 2 mm

D (gamma) scale value = 2.7 %/mm

Directions of increase on magnetograms: D (W) up: H up: Z up

26

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