Progress in software development and in preliminary analysis of MAGSAT tracks crossing the Indian Ocean is reported. Tracks crossing the Java Trench, Broken Ridge, the Southeast Indian Ridge, and the Ninetyeast Ridge show that magnetic anomalies correlate with some of these features. Preliminary study of anomaly profiles indicates that tracks of anomaly data (the observations minus a core field model) have a power spectrum decreasing as the inverse square of the spatial frequency. An apparent noise floor of about one to two gammas rms is reached at wavelengths of about 360 km, corresponding to approximately 10 samples of the decimated Investigator tape data at a sampling rate of approximately 4.9 sec/sample

Lazarewicz, A. R.

Sailor, R. V.

English

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THE ANALYTIC SCIENCES CORPORATION
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QUARTERLY PROGRESS REPORT
FOR THE PERIOD
2 DECEMBER 1980 TO 31 MARCH 1981
7 May 1981
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Prepared Under:
Contract No. NAS5-26424
(Magsat Scientific Investigation)
for
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
Goddard Space Flight Center
Greenbelt, Maryland
Prepared by:
RECEIVEU	 Richard V. Sailor
Andrew R. Lazarewicz
JUN 10
Approved by:
SIS/902.6
Robert F. Brammer
T(19E
THE ANALYTIC SCIENCES CORPORATION
One Jacob Way
Reading, Massachusetts 01867
T	 I
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SUMMARY
The main efforts during the first quarter of the TASC
Magsat invt,,Ligation have been in software development and pre-
liminary analysis of Magsat tracks crossing the study region
in the Indian Ocean. Tracks crossing the Java Trench, Broken
Ridge, Southeast Indian Ridge, and Ninetyeast Ridge show that
magnetic anomalies correlate with some of these features,
Preliminary study of anomaly profiles indicates that tracks of
anomaly data (observations minus a core field model) have a
power spectrum decreasing as the inverse square of the spatial
frequency. An apparent noise floor of about one to two gammas
rnis is reached at wavelengths of about 360 kin, corresponding
to approximately 10 samples of the decimated Investigator Tape
data (sampling rate —4.9 sec/sample).
ii
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INTRODUCTION
The TASC M-%sat investigation covers an area in the
eastern Indian Ocean containing several major bathymetric and
tectonic features (Fig, I.). The overall objectives of this
investigation are:
•	 Production of magnetic anomaly maps from
Magsat data covering the study region
(0 deg - 50 deg S, 75 E - 115 E)
•	 Comparison of Magsat and satellite altime-
ter data in this area, and quantification
of their relationships
•	 Determination of the optimum resolution
of Magsat anomaly maps of the study region
•	 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 first part of the work will be a study of individual pro-
files of Magsat data in order to characterize the quality and
resolution capability of the data and to locate major, anomalies.
Later, the analysis will proceed in two dimensions with gridded
Magsat and gravity field data.
1
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Figure 1	 Study Area, With Selected Magsat Passes
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THE ANALYTIC SCIENCES CORPORATION
2.	 PROGRESS
The main efforts during the first quarter of the TASC
Magsat investigation have been in software development and pre-
liminary analysis of Magsat tracks crossing the study region.
The software developed includes routines for reading and select-
ing data from both the CHRONICLE and Investigator tapes, for
plotting anomaly profiles and ground tracks, and for interfac-
ing the Magsat data with existing TASC filtering and analysis
software. When reading the CHRONICLE tapes, subroutines pro-
vided by NASA were used to compute spacecraft position and
values of the magnetic field from spherical harmonic models.
Several tracks of data were extracted fro4i the CHRONINT
and Investigator tapes for preliminary analysis. Anomaly pro-
files were examined to determine whether major anomalies corre-
late with known tectonic features. Spectrum analysis using
autoregressive model fitting was performed to determine the
spectral characteristics of the lithospheric anomaly signal
and of the noise.
3
RESULTS
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The region of the eastern Indian Ocean covered by
this investigation is shown in Fig. 1, which also shows sever-
al selected Magsat tracks. The major physiographic features
of this area include the Southeast Indian Ridge, Java Trench,
Diamantina Fracture Zone, Broken Ridge, and Ninetyeast Ridge.
One objective of the first part of this investigation is to
determine the characteristics of magnetic anomalies that might
be associated with these features.
The results shown in this section are from the Magsat
data provided in the Investigator-B tape format, having a samp-
ling rate of approximately 4.9 sec/sample. Each data point
represents the average value of 80 corresponding URONICLE
data points. The anomaly profiles shown in Figs. 2 through 9
are derived by subtracting values predicted from the 1.980 Ma-sat
field model from corresponding averaged observations of the
vector magnernmeter.
Figures 2 through 5 are anomaly profiles for Magsat
pass #45. Figure 2 is the residual total field magnitude,
determined by subtracting the value predicted by the model
from the observed field magnitude computed from the three vec-
tor components. The locations of the major physiog raphic fea-
tures are indicated on the anomaly profiles (see also the map
of Fig. 1). Some of the features in the anomaly profiles coin-
cide with the locations of physiographic features. One of the
most obvious features in the anomaly profiles is associated with
the B.:oken Ridge and Diamantina fracture zone. This anomaly
has an amplitude of about 10 gammas and. its form depends on4D
4
THE ANALYTIC SCIENCES CORPORATION
SCALAR R-12555
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0.0 20.0 40.0 60.0 90.0 100.0 120.0 140.0 160.0 180.0
DISTANCE (SAMPLES)
	
t	 !	 ^	 1	 a
	
0	 2000	 4000	 6000
KILOMETERS
Figure 2	 Anomaly Profile for JB) from Vector Data of
Pass #45, Average Sample Spacing is 35.3
Ian/sample. Physiographic features indicated
are: JT-Java trench, BR-Broken Ridge, DF-
Dimantina Fracture Zone, SE-Soutlleast Indian
Ridge	 X COMPONENT R•72556
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DISTANCE (SAMPLES)
	
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Figure 3
	 Anomaly Profile for X-Component, Pass 45,
See Fig. 2 for Key.
5
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THE ANALYTIC SCIENCES CORPORATION
V COMPONENT
	
15.0 ,1
	
1
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DISTANCE (SAMPLES)
	
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KILOMETERS
	
Figure 4	 Anomaly Profile for Y-Component, Pass 45.
See Fig. 2 for Key.
Z COMPONENT
Q	 J	 N	 BR
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 X15,0
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OZ -25.0
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0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0
DISTANCE (SAMPLES)
	
0	 2000	 4000	 5000
KILOMETERS
	
Figure 5	 Anomaly Profile for Z Component, Pass 45.
See Fig. 2 for Key.
6
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THE ANALYTIC SCIENCES CORPORATION
the component. Equivalent source modeling will be necessary
t	 for further study of the origin and nature of this anomaly.
Figures 6 through 9 show the same type of anomaly
profiles for pass #685 (see map of Fig. 1). Again, the loca-
tions of the major physiographic features are shown, and some
of these are associated with features in the anomaly profiles.
These profiles have more spikes then those for pass #45, but
the Broken Ridge-Diamantina Fracture Zone anomaly is still
apparent, particularly in JBI (Fig. 6).
Figures 10 and 11 are typical. power spectrum estimates
from the anomaly profiles shown. These spectrum estimates were
obtained by fitting autoregressive (AR) models to the anomaly
time series. One-sigma confidence bounds for these estimates
were computed from the AR model parameters and the residuals.
The optimum order for the AR model was chosen by an informa-
tion criterion. For the profiles analyzed thus far, the op-
timum order ranges between two and seven.
Figure 10 is the AR power spectrum estimate (of order
7) for the pass #45 total field magnitude profile in Fig. 2.
The total power in this spectrum is about nine gammas rms.
As found for all of the along-track MAGSAT spectra,
the spectrum may be divided into three portions. At low fre-
quencies (wavelengths greater than 2900 kin) the spectrum is
flat, as the core-field model has removed most long-wavelength
features. At intermediate frequencies, the spectra behave as
s:.	 a power law (f -b in this case (Fig. 10) with a slope (b) of
about 3.2. At high frequencies there is an apparent noise
floor, beginning at about 10 -i. sample -1 (corresponding to a
wavelenbth of about 360 km). If it is assumed that this repre-
sents a component of white noise, then this component contains
7
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THE ANALYTIC SCIENCES CO(gPORATION
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DISTANCE (SAMPLES)
Figure 6
10.0
0	 2000	 4000	 6000
KILOMETERS
Anomaly Profile for JBI from Vector Data of
Pass #685. ,average Sample Spacing is 36, 3  ltrnl
Sample. Physiographic FeatureF Indicated are;
SE-Southeast Indian Ridge, DF-Diama,ntina Fracture
Zone, BR-Broken Ridge, NF-Ninetyeast Ridge.
X COMPONENT	 8,72554
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.00.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 18 0
DISTANCE (SAMPLES)
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KILOMETERS
Figure 7
	
Anomaly Profile for X-Component, Pass 685.
See Fig. 6 for Key.
8
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THE ANALYTIC; SCIENCES CORPORATION
Y COMPONENT
0.0
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0.0 20.0 40.0 60.0 80.0 100.0 120,0 140.0 160.0
DISTANCE (SAMPLES)
0	 2000	 4000	 6000
KILOMETERS
Figure 8
	 Anomaly Profile for Y-Component 3 , Pass 685.
See Fig. 6 for Key,
Z COMPONENT
.0
DISTANCE (SAMPLES)
L	 I	 I	 I
0	 2000	 4000	 6000
KILOMETERS
Figure 9	 Anomaly Profile for Z-Component, Pass 685.
See Fig. 6 for Key.
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THE ANALYTIC SC;wNCES CORPORATION
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FREQUENCY (CYISAMPI
3000	 1200	 900	 72
WAVELENGTH Ikm1
Figure 10	 AR Spectrum Estimate With 95% Confidence Bounds
for Pass #45, JB(. The TotAl rms Power is Ap-
proximately 9.2y (including low frequencies not
shown), with rms Power in Different Sections as
Indicated. Dimensionless Frequency is Shown,
Sample Spacing is —35.3 km.
0.7 gamma rms. The preliminary interpretation is that this ap-
parent noise floor represents instrument noise plus uncorrelated
external field noise.
Figure 11 is the corresponding AR spectrum estimate
for pass #685 (from the total field magnitude profile of Fig.
6). This differs slightly from the spectrum of Fig. 10, having
a
a higher noise floor of 1.7 gamma (vs 0.7 gamma) and a less
steep slope. (Most of the anomaly spectra studied thus far,
with the exception of the one shown in Fig. 10, have power law
slopes of approximately 2.)
10
THE ANALYTIC SCIENCES CORPORATION
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FREQUENCY (CY/SAMP)
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3000	 1200	 300
	
72
WAVELENGTH (kin)
Figure 11
	 AR Spectrum Estimate with 95% Confidence Bounds
for pass #685, JBI. The Total rms Tower is Ap-
proximately 10.2y (including low frequencies not
shown), with rms Power in Different Sections as
Indicated, Sample Spacing is —36.3 km.
The difference in the noise floor levels of Figs. 16
and 11 may be due to differing number of spikes, which can be
seen by comparing the anomaly profiles; orbit 685 (Fig. 6)
has more and larger spikes and also has a higher noise floor
than orbit 45. Spike removal routines have already been ap-
plied to the anomaly profiles shown, but further aejustment of
their parameters is necessary. The future work will investi-
gate proper smoothing of the data,
Table 1 lists the values for total rms power and noise
level for spectra of all of the anomaly profiles of Figs. 2
through 9. The total, rms power ranges between 9 and 29 gammas
,
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1.1
aTHE ANALYTIC SCIENCES !CORPORATION
TABLE 1
RESULTS FROM SPECTRAL ANALYSES
FIELD COMPONENT
PASS # (gammas)
JB) X !	 Y Z
Total rms Field" 9.2 9.5 19.9 19.6
45 ^.
rms Noise's 0.7
10.2
1.2
56.3
0.9
29.4
2.5
11.1Total,rms Field
685
rms Noise 1.7 2.0 1.1 4.1
"The values given for JBI are the difference in magnitude be-
tween the observed field and the model ffield. Thesee values
are not equal to the magnitude of an anomaly vector formed
from th.e x, y, z anomaly components.
with one spectrum having 56 gammas. The rms noise level is
between 0.7 and 2.5 gammas, with one case having 4 gammas.
For all of the spectra, the noise floor is reached at a wave-
lengtb of about 360 km. These preliminary results can be com-
pared with the pre-Magsat mission specifications, i.e., a re-
solution limit of about 300 km for lithospheric anomalies and
a noise level of about 3 gamma for the vector magnetometer
measurements.
it should be emphasized that these results are pre-
liminary and that the noise "Level and resolution capability
for lithospheric anomalies must be determined from analysis of
more tracks. Furthermore, the spectra shown represent a one-
dimensional analysis of the along-track power for individual
tracks, and are appropriate for short wavelengths. For wavel-
engths longer than about 1000 km, spherical harmonic analysis
should be used. Future work will consider anomalies over a
.
12
TI-1177 ANALYTIC SCIENCES CORPORATION
spatial grid, two-dimensional spectra, and degree variance
spect.ra. An issue not, yet addressed is the degree and order
of tile core Held which is subtracted from the Magsat observa-
tions to comput.e anomalies. For these preliminary calculations,
a degree and order 14 field has been used, but use of .1 lower
order field should be considered if it can be demonstrated that
features of this wavelength might originate in the crust.
if.	 PROBLEM AREAS AND RECOMMENDATIONS
No significant problems have been encountered,
5.	 PLANS FOR TH`E NEXT REPORTING PERIOD
Analysis will continue on individual Magsat tracks,
extracted from both CHRONICLE and Investigator tapes. The
tracks will be studied for repeatability of anomaly features
and the relationships of these features to tectonic and gra-
vity field features. Further studies in spectrum analysis
will help to characterize the anomaly signal, the noise level,
and the resolution that can be obtained from Magsat data in
the study region. The relationship of these results to tile
D st index', and to the degree and order of the removed core
field, will also be considered.
13


Analyses of MAGSAT tracks crossing the study region in the Indian Ocean

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