MAGSAT Investigator-B tapes were preprocessed by (1) removing all data points with obvious erroneous values and location errors; (2) removing smaller spikes (typically 15 nT or more), and deleting data tracks with fewer than 20 points; and (3) removing a linear trend from each track. The remaining data were recorded on tape for use by the equivalent source mapping (ESMAP) program which uses a least squares algorithm to fit the magnetization parameter of the grid of equivalent source dipoles in the crust to satellite data acquired at different times and locations. ESMAP was implemented on the TASC computing system and modified to read preprocessed MAGSAT tapes and interface with TASC plotting software. Some verification of the software was accomplished. Gridded 1-degree mean values of gravity anomaly and sea surface undulation computed from SEASAT radar altimeter were obtained and brought on line

Lazarewicz, A. R.

Sailor, R. V.

English

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19820016702 2020-03-21T09:09:19+00:00Z
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THE ANALYTIC SCIENCES CORPORATION
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Quarterly Progress Report
for the period
1 September 1981 to 31 December 1981
22 January 1982
(E82-10304) (INVESTIGATING 1 ECTGNIC AND
	 NB^ X4578
BATHYMETRIC FEATURES Of ThE INDIAN OCEAN
USING MAGSAT MAGNETIC ANGMA1Y LATA]-, 
fi
	
O	 0'lQuarterly Pro g ress lielio.rt. 1 S (-2 ri
 . - 31 Dec,
a Unclas
1981 (Anaiytic scic-nc"es C -01P.) 17 P	 G3/43 00304
Prepared Under:
Contract No. NAS5-26424
(Magsat Scientific Investigation)
for
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
Goddard Space Flight Center
Greenbelt, Maryland
RECEIVED	
Prepared by:
FEB 4 I^Frz-	 Andrew R. LazarewiczI	 Richard V. Sailor
MS/ 90M	 Approved by:
Robert F. Brammer
THE ANALYTIC SCIENCES CORPORATION
One Jacob Way
Reading', Massachusetts 01867-3992
nTHE ANALYTIC SCIENCES CORPORATION
1.	 INTRODUCTION
The TASC Magsat investigation w)vers an area in the
eastern Indian Ocean which contains several. major bathymetric
and tectonic features (Fig. 1-1). The overall objectives of
this investigation are:
•	 Determination of the optimum resolution
of Magsat anomaly maps of the study
region.
•	 Production of magnetic anomaly maps from
Magsat data covering the study region(0 - 50 degrees S, 75 - 125 degrees E).
•	 Comparison of Magsat and satellite alti-
meter (geoid undulation) data in this
area, and quantification of their
relationships.
•	 Interpretation of the Magsat data using
satellite altimeter and other geophysical
data in order to determine the origin
and sources of the observed magnetic
anomalies.
The investigation area consists of a variety of geo-
logic features including two ridges ) a trench, fracture zone
and plateau.. Figure 1-2 is the contoured bathymetry with a
1-degree resolution (Rapp, 1981), showing the locations of the
major bathyx ietric features.
The investigation during this quarter focused in
three areas:
1-1
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• PREPROCESSING MAGSAT DATA
The Magsat Investigator-B tapes were
preprocessed by l) removing all data
	
t
points with obvious erroneous values and
	
}
location errors, 2) removing smaller
spikes ( typically 15 nT or wore), and
delet,^.:' g data tracks with fewer than 20
poin ts and 3) removing a linear trend
from each track. The remaining data was
recorded on tape for use by the ESMAP
program.
r
I
• IMPLEMENTING BSMAP
The NASA-supplied program, NSMAP (g^quiva-
	 x
lent Source MAPping), uses a least-squares
	 ;.
,Algorithm to- t the magnetization param-
eter of a grid of equivalent source di-
poles in the crust to satellite data
acquired at different locations and tim es .
RSMAP was implemented on the TASC comput-
ing system and modified to 1) read pre-
	 k'
processed Magsat data tapes and 2) in-
terface with TASC plotting software.
Some verification of the software was
accomplished by testing against model
anomaly fields,
a► 	 ACCESS TO GRIDD ED GRAVITY DATA
Gridded 1-degree mean values of gravity
anomaly and sea-surface undulation com-
puted from the Seasat radar altimeter
were obtained and brought on line for
use in the upcoming Quarter,
{
Interpreting magnetic and gravity anomaly maps usually
begins with spatial correlation of anomaly features with geo-
logic features, ;Inferring more detailed geophysical, inform ►.-
Lion from magnetic data, requires estimation of the magnetic
susceptibility for regions of the crust from the computed
► Sag aetixation values for the equivalent source dipoles. Grav-
ity anomaly and. sea- surface undulation (approximation to the
:t	 geoid) data are used. to estimate or limit density struc ture.
^t3 (	 1-2
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300
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Gridded gravity anomaly data at sealevel can be upward con-
tinued and compared with gridded ) reduced to the pole, tuag-
netic anomaly data at the same altitude. Using the combined
magnetic and gravity data, estimates of the contrasts in den-
sity, susceptibility, and perhaps rock type may be made, The
progress for this quarter focused on the acquisition or gener-
ation of gridded anomaly maps.
JAVA TRENCH sac
WALLABY
PLATFAU
BRVKEN
RIDE
DIAMANTINA
FRACTURE ZONE
I
SOUTHEAaT
INDIAN RiDGE
5001	 1	 1	 -_ -	 I	 -i
750	 no	 950	 10510	 1150	 1250
EAST LONGITUDE
Figure 1-1	 Investigation area showing
major tectonic features
1-3
1-4
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ORIGINAL PAGE IS
OF POOR QUALITY
N tit It
0
10
ul
p 20
g
0 300
40
60 ^ rti	 nn o	 ^_ .
75	 85	 95	 105	 115	 125
EAST LONGITUDE
Figure 1-2	 Investigation area showing bathymetry at a
one degree resolution. The area without
data in the middle of the right-hand side
of the plot is over Australia.
THE ANALYTIC SCIENCES CORPORATION
2. TWO DIMENSIONAL MAPPING OF THE MAGNETIC ANOMALY FIELD
2,1
	 METHOD
The NASA-supplied Investigator-B tapes ("quiet time"
Magsat data for the TASC investigation area) were previously
found to contain bad data points which were categorized as
follows (Ref, 1): 1) known missing data values (flagged with
a 99999.0) and 2) isolated data spikes with values greater
than about 15 nT. There are also points with incorrect values
for latitude, longitude, or altitude. These points and all
tracks consisting of less than 20 points were deleted from the
investigation data set. A linear trend was removed from each
track of data to reduce the effects of external fields. Since
we believe that the data sampling rate can be lowered (Ref.
1), the remaining edited and de-trended data was averaged over
five consecutive points and stored on a new and condensed ver-
sion of the Investigator-B tapes. This averaging increased
the sampling interval from 36 to 180 laii.
The goals of this research include producing a magne-
tic anomaly map and forming a joint interpretation of magnetic
and gravity anomaly data. To accomplish these tasks, it is
necessary to reduce the Magsat data to a regular grid at a
constant altitude and to infer the associated crustal magneti-
zation. The computer program ESMAP, obtained from NASA, is
particularly well suited for these purposes. ESMAP uses a
least-squares fitting algorithm to adjust a magnetization
parameter (dipole moment per unit volume) of individual tracks'
arranged in a grid of dipoles such that the resulting field
fits the input satellite scalar data. The dipoles are located
2-1
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2-2
R
THE ANALYTIC SCIENCES CORPORATION
in the crust and oriented along the Earth's main magnetic field
to simulate induced magnetization, Once the magnetization
parameters have been determined for the equivalent source di-
poles, u gridded magnetic anomaly map can be computed from
those dipoles. The resulting anomaly map can be reduced to
the pole by reorienting the dipoles to a vertical. position,
During this Quarter, ESMAP was modified to work on the TASC
computing system, tested, and interfaced with TASC's contour
plotting package whose results are in the figures in this
report.
Since ESMAP is a complex software package, several
tests were run to check for internal consistency, and an
attempt at independent verification of the computed field
points was made, The purpose of the testing was to 1) check
that ESMAP could reproduce a model anomaly field (internal
consistency), and 2) convert the magnetization parameter
computed for the model dipoles into an estimate for magnetic
susceptibility. Given the magnetization parameter of a single
dipole, a computation independent of ESMAP for the resulting
field can be made and compared: with ESMA',P computations.
Three tests were run, lea-d-ng to a preliminary equiva-
lent dipole source model and estimated anomaly -map for the
TASC investigation area. Test 1 fit a matrix of 25 dipoles in
the investigation area to a small number of data tracks and	 t
computed an anomaly field, Test 2 fit a single, centered di-
pole to the resulting field from Test 1 and computed an anomaly
field and a reduced-to-pole anomaly field, Finally, Test 3
fit a matrix of 25 dipoles to the anomaly field from Test 2.	 {
These tests confirmed the internal consistency of the ESMAP
algorithm, Each test: (Sections 2.2 - 2.4) and the preliminary
model of the field (Section 2.5) covered the entire 5r by 50
degree area and produced anomaly maps on a 1 by 1 degree scale
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at 350 km altitude. All of these magnetic anomaly maps have
inaccuracies near the edges (a 12.5 degree border) as described
by Horner (Ref. 2).
	
2,2	 TEST 1: INITIAL SOFTWARE TEST
.
Thirty tracks (3621 points) of data were processed to
fit 25 dipoles (10 degree spacing). The results are shown in
Fig. 2-1a. The first; astimate for the anomaly field compares
satisfactorily with the published two degree Magsat map (Ref, 3)
regarding the location and amplitude of the major feature lo-
cated near the Broken Ridge, However, the broad spacing of
the dipole locations leads to a "ripple" effect visible in the
anomaly map and clearly indicates the need for a denser dipole
grid in the firther stages of this investiga tion.
	2.3
	 TEST 2: SINGLE DIPOLE FIT AND REDUCTION TO POLE
The gridded anomaly map resulting from Test 1 was
used as i.npvt data for this test. A single, centered dipole
was fit to the 2601 input data points. The purpose of this
test was to generate a single dipole field. It was simpler to
let ESMAP fit a single dipole field to an arbitrary field,
than to generate a single dipole field analytically. Since
only one dipole was fit, the magnetization has a very high
value which is not geophysically significant. However this
particular dipole will be used to test the internal consis-
tency of ESMAP. After solving for the dipole parameters, the
dipole was rotated to a vertical position, and its field com-
puted (Fig. 2-1b), The resulting fi^gld shows that the reduc-
tion to the pole worked correctly. The computed magnetization
parameter and the field are in a form which can be, but has
2-3
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not yet been, verified in a straightforward manner using stan-
dard far field formulas for a. simple dipole. This verifica-
tion requires conversion of the magnetization parameter to a
dipole moment in the appropriate units, and will have geophysi-
cal significance when related to the magnetic susceptibility
of the local rock.
Figure 2-1c shows the resulting field when the dipole
is not reduced to the pole. The resulting fields are consistent
with those expected from a single dipole field. The shape
distortion from the input field is due to the use of a single
dipole, and the large field magnitude is due to the attempt to
fit the magnetization parameter of a single dipole to relatively
large values (10 nT) of the field far from the dipole. This
is an extreme case of using an insufficient number of dipoles
to fit the input field data, as can be seen in the unrealistic
value for the magnetization parameter, and an obviously poor
fit to the input magnetic field model.
2.4	 TEST 3: LEAKAGE AMONG MODELED DIPOLES
A final, consistency test was performed by using the
computed field shown in Fig, 2 -1c as the input data, and solv-
ing for a matrix of 24 dipoles, the center one being located
at the same location as the single dipole of Test 2. The di-
poles in this case have a 10 degree spacing, The resulting
field is shown in Fig. 2-1d, and is visually indistinguishable
from Fig. 2-1c. The value of the magnetization parameter of
the centered dipole is 25 times the value of the single dipole
(as the volume is 25 times smaller), and the surrounding 24
dipoles have magnetization parameter values more than 5 orders
of magnitude smaller than the value for the central dipole.
2-4
f „i^	
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'r!HE ANALYTIC SCIENCES CORPORATION
Thus ESMAP is a self-consistent program to generate gridded
anomaly fields and does not significantly "leak" magnetization
from dipole to dipole,
2.5	 PRELIMINARY ANOMALY MAP OF THE INVESTIGATION AREA
A first computation of the anomaly field for the entire
investigation area was made using the maximum number of dipoles
(software limited) which could be fit in the area with a single
computer run. Final mapping will subdivide the investigation
area to decrease the dipole spacing, thins requiring multiple
computer runs. For the preliminary map, 144 dipoles with a
spacing of 4.67 degrees were used. A significant portion of
the TASC Magsat ir►vestigator -B quiet time data (50 104 points
in 379 data tracks) was used as the input data. The field
estimate was computed (Fig. 2-2), and due to the large spacing
among the modeled dipoles, the "ripple" effect is still evident.
The overall fit of field magnitude and feature location corre-
sponds well, by visual inspection, with the published Magsat
two-degree anomaly map (Ref. 3). The latter was mapped using
averages at satellite altitude of data within, 2 by 2 degree
squares, and did not use equivalent dipole source modeling.
Tho strongest features evident in the computed
anomaly map are located near the Broken Ridge, the west cost
of Australia, and the Diamantina, Fracture Zone (Figs. 2-1,
2-2). Surprisingly, there is no major anomaly feature visible
at the Ninetyeast Ridge, which is adjacent to the Broken Ridge.
v
As previously mentioned, the computed anomaly values in the
12.5 degree border are degraded due to edge effects of the
ESMAP modeling algorithm. Subsequent maps will be designed to
be overlapped so that the inaccurate anomaly values in the
'	 borders are not used.
2-5
It `
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ORIGINAL PAGE 19
OF pO6R QUAL"
00
1	 1	 -,3
50
	
90•
OO
	
76e	
(a)	
1250	 759	 (b)	 1250
M
0
	
Cc 00"	 00
50	
i	
500 t-	 1, 14
750	 10	 1250	 750	 (d)	 1250
EAST LONGITUDE
Figure 2-1
	 a) A 25 dipole fit to 3621 points; b) a 1 dipoleFit to the result computed in (a), the output
field is reduced to the pole; c) same as (b) but
not reduced to the pole; d) a 25 dipole fit to
the result computed in (c). Contours are in nT
with 0.25 nT spacing
2-6
10
20
g
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40
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THE ANALYTIC SCIKNCES CORPORATION
0
60
75	 05	 86	 105	 115	 126
EAST LONGITUDE
Figure 2-2	 Preliminary Magsat anomaly map with 144 dipoles
fit to 50104 data points. Contours are in nT
with a 0,25 nT spacing
ORIGINAL PAGE I3
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THE ANALYTIC SCIENCES CORPORATION
3.	 TWO DIMENSIONAL MAPPING OF GRAVITY DATA
Gridded data of sea-surface undulation and graqity
anomaly values were obtained from Professor Richard Rapp of
the Ohio State University (Ref. 4). These data represent mean
values in one by one degree squares and were computed from
Seasa •
 radar altimeter data. Figure 3-1 shows the sea-surface
undulation map and Fig. 3-2 shows the ,,ravity anomaly map,
presented at the same scale as Figs. 1-1 and 1-2. The major
features in the gravity anomaly map are associated with the
Ninetyeast Ridge and Java Trench. The Broken Ridge, corre-
lated with a strong magnetic anomaly feature, does not have a
pronounced correlated feature in the gravity anomaly map. For
proper comparison with magnetic data, the gravity anomaly map
must be upward continued to the altitude (350 km) of the mag-
netic data. This operation will be performed daring the next
quarter.
0
3-1
o
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10
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THE ANALYTIC SCIENCES CORPORATION
60	 '
76	 86	 86	 106	 116	 126
EAST LONGITUDE
Figure 3-1	 Seasat derived sea-surface undulation map.
Contours are in meters with 10 meter intervals
"AWS0
a, 
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THE ANALYTIC SCIENCES CORPORATION
ORIGINAL P,4gF u
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50i I I-J,1 1 1.1 I Aj I T, s_1 I V ► :=Vj `"t 	 11-,L tI — U 'i' i	 1 i V a o 1 %d" 1 tj
75	 85	 95	 105
	 1111	 175
EAST LONGITUDE
Figure 3 - 2	 Seasata derived gravity anomaly map. Contours
are in mgals, with 10 mgal intervals
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THE ANALYTIC SCIENCES CORPORATION
UNRESOLVED PROBLEMS AND FUTURE DIRECTIONS
tr
	
	 ESMAP was designed for gridding satellite da,ta by
computing parameter values of the equivalent source dipoles.
The major issue not solved during this quarter was a deter-
mination of the relationship of the computed model magneti-
zation parameter for the dipoles (as oatput by ESliAP) with the
magnetic susceptibility of the rock.
The resolution limit of the magnetic anomaly data was
previously estimated at 250 laii (Ref, 1). Work will continue
towar^ the determination of the optimum dipole spacing for
equivalent source modeling, which is not equivalent to the
resolution limit of 'the anomaly maps. This information will
be used to produce a reduced-to-pole magnetic anomaly map of
the investigation area, The gravity anomaly data will be up-
ward continued to the altitude of the magnetic anomaly data,
allowing comparisons o ,r the two data sets, The first major
geophysical implications of these analyses will be from clas p i-
fication of the major tectonic features in the investigation
area.
4­ 1
THE ANALYTIC SCIENCES CORPORATION
REFERENCES
1,	 "Quarterly Progress Report for the Period I July to 30
September 1981, 11
 The Analytic Sciences Corporation,
Report No, PR-1325-3, 15 August 1981.
2. florner, R.J., Jr., "The Equivalent Source Magnetic
Anotualy Program (ESM.AP) User's Guide," Prepared by
Computer. Sciences Corporation for Goddard Space Flight
Center, 1980,
3. Lzingel, R.A., "Magsat Anomaly Map Relative to MG680982
Model, Selected Quiet Data Below 400 Km," NASA GSFC,
1981,.
4,	 Rapp, R.11,, 11 1-Degree Mean Gravity Anomaly and Sea-
Surface Undulation Data Base," Personal communication,
1981.
I.
R-1


Investigating tectonic and bathymetric features of the Indian Ocean using MAGSAT magnetic anomaly data

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