Ground tracks from SEASAT were used in an effort to develop qualititative relationships between the gravity field and MAGSAT magnetic anomalies in the eastern Indian Ocean. Investigation of data quality led to analyses of the average value (over 80 vector data points, or approximately 36 km intervals) and of the standard deviation of this average, as a mean of identifying noisy portions of the data. It was discovered that the plots of the average value minus the individual (measured) point value are most useful for identifying noisy areas and data spikes. Spectrum analysis using edited (spikes removed) data show that the noise floor is less than 1 nT and the slope of the spectrum in the region of wavelengths between 1200 km and 250 km is approximately -3. Consequently the estimated resolution limit improved from approximately 360 km to approximately 250 km

Lazarewicz, A. R.

Sailor, R. V.

English

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QUARTERLY PROGRESS REPORT
FOR THE PERIOD
1 APRIL. TO 30 JUNE 1981
15 August 1981
(L82-10066) [MAGSA`I ANOMALY PROFILES OF THE
	 N82-20589
EASTERN INDIAN OCEAN) Quarterly Progress
.Liepo.rt, 1 Apr.
	 30 Jun. 1981 (Analytic
Sciences Corp.) 21 p HC A02/8E A01 CSCL 05B
	 Uncla
G3/43 00066
Prepared Under:
Contract No. NAS5-26424
(Magsat Scientific Investigation)
for
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
Goddard Space Flight Center
Greenbelt, Maryland
Prepared by:
V. Sailor
RE CEIVED
	
	 AndrewdR. Lazarewicz
Approved by:
SIS / 902.6	 Robert F. Brammer
I3'3 — 01-7
V /9 C-
 
ANALYTIC SCIENCES CORPORATION
One Jacob Way
Reading, Massachusetts 01867
THE ANALYTIC SCIENCES CORPORATION
TABLE OF CONTENTS
F age
No.
1,	 INTRODUCTION	 1
2. PROGRESS	 3
3. RESULTS	 5
4. PLANS FOR THE NEXT REPORTING PERIOD
	 17
REFERENCES
	 /6
ii
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THE ANALY'T'IC SCIENCES CORRORATION
1.	 INTRODUCTION
The TASC Magsat investigation covers an area in the
eastern Indian Ocean containing several major bathymetric and
tectonic features (Fig. 1). The overall objectives of this
inves1%.igation are:
•	 Production of magnetic anomaly maps from
Magsat data covering the study region
(0 0 - 50 0
 S, 75 0 E - 115 0 E)
Comparison of Magsat and satellite al-
timeter data in this area, and quanti-
fication 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 this investigation is a study of individual
profiles of Magsat data to characterize the quality and reso-
lution capability of the data and to locate major anomalies.
Preliminary results of this work are presented in Refs. 1, 2,
and 3 and in this report. Important results obtained thus far
include a preliminary estimate of the resolution capability
G250 km), identification of significant anomaly features, and
results bearing on the problem of data spike removal.
1
1 41
THE ANALYTIC SCIENCES CORPORATION
SELECTED SATELLITE TRACKS
 
R•70830
10°N 
•	
A 8 C
JAVA TRENCH
n
II
WALLABY
PLATEAU
NINETYEAST
RIDGE
II` 
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SOUTHEAST"ONE
INDIAN
RIDGE
SEASAT, 
TRACKS
NCX
MAGSAT TRACKS
0°
10°S
LU
20°S
X
30°S
40°S
50°S
MAGSAT TRACKS:
A	 214,645,861
B	 45, 1201
C	 722, 1170
D	 460,830,969
SEASAT TRACKS:
E	 1267
F	 722
	
70°E	 80°E	 90°E	 100°E	 110°	 120°E
LONGITUDE
	
Figure 1	 Study Area, With Selected Magsat
and SEASAT Tracks
UV PCOR
THE ANALYTIC SCIENCES CORPORATION
2.	 PROGRESS
The main efforts during this quarter of the TASC Magsat
investigation have been to verify the results obtained in the
first quarter and to continue the study of individual Magsat
anomaly profiles, using data from a newly received Investigator
tape. Previous results (Ref. 1) were based on a study of iso-
lated tracks using a preliminary Investigator tape. The recent
work used groups of tracks (Fig. 1) which are very close in
geographic location (though not necessarily close in altitude).
This work has demonstrated the repeatability of certain anomaly
features and has also resulted in improved estimates of the
anomaly spectrum.
Investigation of the data quality led to analyses of
the average value (over 80 vector data points, or Z 36 km in-
tervals) and of the standard deviation of this average, as a
means of identifying noisy portions of the data. It was dis-
covered that plots of the average value minus the individual
(measured) point value are most useful for identification of
noisy areas and data spikes.
The results of spectrum analysis were improved over
those reported previously. The earlier results (Refs. 1 and
2) showed a noise floor level of 1 to 2 y (nT), and spectra
with a slope of approximately -2. Results using edited (spikes
removed) data from the newer Investigator tape show that the
noise floor is less than 1 nT and the slope of the spectrum in
the region of wavelengths between 1200 km and 250 km is approxi-
mately -3. Consequently, the estimated resolution limit has
improved from 2360 km to 2250 km.
3
M1.
THE ANALYTIC SCIENCES CORPORATION
The results presented in the next section will also
be presented at the IAGA a,sembly in Edinburgh (Ref. 3.).
4
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4
THE ANALYTIC SCIENCES CORPORATION
3.	 RESULTS
One of the objectives of this investigation is to
develop quantitative relationships between the gravity field
and Magsat magnetic anomalies in the eastern Indian Ocean.
This analysis must be done in three dimensions, using data
gridded at a reference (constant) altitude. However, for a
preliminary and qualitative look at this question, a few in-
dividual profiles of satellite data have been considered.
Figure 1 shows the locations of several Magsat and
SEASAT ground tracks. The SEASAT radar altimeter observed the
sea surface height relative to a reference ellipsoid, which is
approximately equivalent to the geoid (perturbed by small con-
tributions from oceanographic and other noise sources). Geoid
features correlate strongly with bathymetric and tectonic fea-
tures and SEASAT data has the capability of resolving such fea-
tures with wavelengths as short as 30 km (Ref. 4). A spatial
correlation is expected between geoid and magnetic anomaly
features, and this will be shown qualitatively using the fig-
ures below. A direct comparison of individual SEASAT and Magsat
profiles is not possible because of the difference in inclina-
tion of the orbits of the two satellites (Fig. 1).
Figure 2 shows a portion of SEASAT Rev 722 with a
second order trend removed to emphasize features of interest.
The Java Trench and the Southeast Indian Ridge are the major
tectonic features, having geoid signatures characteristic of
their type. Significant bathymetric features such as Wallaby
Plateau, Broken Ridge, and Diamantina Fracture Zone are asso-
ciated with geoid signatures up to several meters in amplitude.
5
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zu
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SOUTHEAST
JAVA TRENCH	 INDIANRIDGE
BROKEN
,0	 RIDGE
DIAMANTINA
WALLABY
	
FRACTURE
PLATEAU	 ZONE
,0
0
0
0
H IWI
0.0) 100,0 200,0 300,0 400.0 500.0 600.0 700.0 800,0 900.0
TIME (SEC)
T	 _
	 T.
0	 1000	 2000	 3000	 4000	 5000	 6000
DISTANCE IKMI
Figure 2	 SEASAT profile Minus Second-Order Trend, Rev. 722
This is shown more clearly in Fig. 3, which is an expanded
view of a portion of Fig. 2. From this profile, it is clear
that geoid features as short as 50 km can be resolved by SEASAT.
The resolution limit of SEASAT data for observing geoid fea-
tures has been shown to be about 30 km (Ref. 4) and was deter-
mined from study of SEASAT repeat tracks (tracks separated by
less than 3 km in geographic Location). Spectral coherence
was computed for pairs of repeat tracks and the resolution
limit was defined as being the wavelength at which there is no
significant coherence between the two repeat tracks. This
technique will be applied to Magsat data, although differences
in altitude between Magsat "repeat" tracks will have to be
taken into account. However, under certain approximations, a
difference in altitude will not affect the spectral coherence.
(Upward continuation is a linear operation on the spectrum.)
6
	
	
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FTHE ANALYTIC SCIENCES CORPORATION
This approach to repeat-track analysis will be investigated
during the next quarter, as a final item to check before chang-
ing the emphasis of the investigation from along-track geome-
tries to two-and three-dimensions.
Figures 4 through 7 illustrate the repeatability of
certain anomaly features in Magsat data. Shown is the anomaly
in IB( relative to the MGS481#2 field model. Although the
scales on the plots are different, there is good qualitative
agreement in the form of the major anomaly features. (Linear
trends have not been removed from any of the anomaly profiles
shown.) These magnitude profiles are derived from the vector
data, but the anomaly profiles of the individual vector compo-
nents appear to be more noisy and show less visual correlation
H•75892
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PLATEAU
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RIDGE
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FRACTURE
ZONE
	
0,01	 5010	 10010	 150.0	 200.0	 250,0	 300.0
TIME (SEC)
0	 500	 1000	 1500	 2000
DISTANCE (KM)
	Figure 3	 SEASAT Profile Minus Linear Trend, Rev. 722
THE ANALYTIC SCIENCES CORPORATION
ANOMALY, 8214
300	 F4,70061
200
-30.0 L-
0.0 20.0	 40.0	 60.0	 80.0	 100.0	 120.0	 140.0	 160.0	 180,0
DISTANCE ( SAMPLES)
10.0
Q
C7
0.0
J
Q
Z	 . 10.0
Q
20.0
Figure 4	 Anomaly in (BI for Orbit 214 (Paired
with Orbits 645 and 861, See Figs. 1
and 5)
with the sas',ie component of the other "repeat" tracks. This is
because the individual components are more sensitive to satel-
lite position than the magnitude.
The most prominent anomaly feature in these tracks,
associated with Broken Ridge and Diamantina Fracture Zone, has
an amplitude of up to 10 y. The anomaly features associated
with the Java Trench and the Southeast Indian Ridge are not as
consistent. It may be that anomalies associated with the latter
features have longer wavelength components and thus do not show
up as well against the background of the time-varying effects,
which contribute long-wavelength trends to the profiles.
Comparison of Fig. 6 with Fig. 7 shows that the data
on the older Investigator tape are noisier. This is confirmed
8	 Ga'a",("+ ant r, l	 !^ M ^cw.
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	 20.0	 40,0 60.0 80.0 100.0 120.0 140.0 160.0 180.0
DISTANCE (SAMPLESI
L_	 I	 1	 I,	 I	 I	 I
0	 1000	 2000	 3000	 4000	 5000	 6000
KILOMETERS
20.0
MINU 861	 0OI S MODEL
	 Lr_ 
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2
a
0,0
a
O2
a
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-20.0
0,0
	
20.0	 40.0	 60.0	 80,0 100.0 120.0 140,0 160,0 180.0
DISTANCE (SAMPLES)
l	 i	 I	 I	 I	 t	 I
0	 1000	 2000	 3000	 4000	 5000	 6000
KILOMETERS
KEY: JT - JAVA TRENCH
BR -- BROKEN RIDGE
DF - DIAMANTINA FRACTURE ZONE
SE - SOUTHEAST INDIAN RIDGE
Figure 5	 Comparison of Field Magnitude Anomaly For
Orbits 645 and 861
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THE ANALYTIC SCIENCES CORPORATION
R425Db +
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.0
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DISTANCE (SAMPLES)
, ► 	 I	 I	 _j
0	 2000	 4000	 6000
	 v
KILOMETERS
Figure 6	 Anomaly ProZile for (BI from (Old 'Pape)
Vector Data of Pass #45. Average Sample
Spacing is 35.3 km/sample. Physiographic
features indicated are: JT-Java trench,
BR-Broken Ridge, DF-Dimanbina Fracture
Zone, SE-Southeast Indian Ridge
from a comparison of the PSD estimates (Figs. 11 to 13), where
the newer data has a lower noise floor, primarily due to better
spike removal.
For the purpose of identifying noisy portions of data,
it seemed appropriate to first look at the standard deviation
of the 80-point running average, which is provided on the Inves-
tigator tapes. However, as Fig. 8 shows, this quantity mainly
reflects the field gradient since the field changes signifi-
cantly in the "36 km interval over which the 80 observations
	
10	 -	 :' n e
	
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THE ANALYTIC SCIENCES CORPORA'T'ION
ANOMALY PROFILE COMPARISON
FIELD MAGNITUDE, ORSITS 46 AND 1201
300
	 N now
Z
a
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DISTANCE (SAMPLES)
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ORBIT 1201	
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	 OR	 se
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0
0
0
.0
010	 20.0	 40.0	 60.0	 60.0	 100.0	 120.0	 140,0	 180.0	 180.0
DISTANCE (SAMPLES)
0	 2000
	
4000	 6000
KILOMETERS
KEY:JT — JAVA TRENCH
WP — WALLABY PLATEAU
BR -- BROKEN RIDGE
OF — DIAMANTINA FRACTURE ZONE
SE SOUTHEAST INDIAN RIDGE
Figure 7
	
Comparison of LBI Anomaly for Orbits 45 and 1201.
Although the long wavelength components are sig-
nificantly different between these two orbits,
qualitatively, the anomaly features correlate well.
11
Op,1GIVAL PA"= 7' I
OF POOR QUALITY
k
THE ANALYTIC SCIENCES CORPORATION
400
	 (BIORBIT 45	
R•TG088
QS 30.0
QO
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H
Q 20.0
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0
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HN
0.0 1	 1
0.0	 20.0	 40.0	 6010	 80.0	 100.0	 120.0	 140.0	 160.0	 180.0
DISTANCE (SAMPLES)
Figure 8
	
	
Standard Deviation of the Running Average
Over 80 Observations
AVERAGE VALUE-POINT' VALUE-REV 1201
R•75827
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10,0
5,0
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—15.0
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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)
Figure 9
	 Field Magnitude Minus 80-Point (36 KM)
Running Average
12
(Jr.POOR QUALITY
S^
THE ANALYTIC SCIENCES CORPORATION
are averaged. Discussions with Dr. Langel led to the sugges-
tion by TASC that this standard deviation quantity be replaced
on future Investigator tapes, as it is not useful for showing
short-wavelength noise. For example, for orbit 45, the field
gradient in the Z component is large, and the standard devia-
tion of the averages has the form of a ramp, with the standard
deviation ranging from about 120 to 5 nT.
A more useful quantity for study of the quality of
the Magsat data is formed by subtracting the 80-point running
average from the observed values. This is plotted in Figs. 9
and 10, which show this quantity to be much less sensitive to
the field gradient and shows more clearly the data spikes and
the areas which are more noisy.
AVERAGE VALUE-POINT VALUE, REV 30
15.0
Z 10.
a
C7
W
U 5.
Z
W
1EW
LL
LL
0.
-5
	
0.0	 20.0	 40.0 60,0 80.0 100.0 120.0 140.0 160.0 180,0
DISTANCE (SAMPLES)
	Figure 10	 Z-Component Minus 80-Point Running Average
13	 0WQ1NAL PAGE I'S
THE ANALYTIC SCIENCES CORPORATION
For example, for the z-component of Orbit 30 (Fig.
10), this quantity shows only a slight long-wavelength trend
due to the field gradient. It is also apparent that the later
part of this track has a lower noise level.
The overall noise level for any track is shown by its
PSD, and since this noise level is sensitive to data spikes,
most are removed by a running average spike-removal algorithm
before fitting an autoregressive model for spectrum estimation.
(Work in progress includes implementation of an autoregressive
prediction-error data editor.) The newer data however, have a
lower noise floor than the older data, as can be seen by compar-
ing Figs. 11 and 12 -- the noise floor for orbit 45 was 0.7 y
SPECTRUM OF IBI ANOMALY, ORBIT 45
104
DEGREE 14
FIELD MODEL
103
a.
ay
L) 102
N
Q
C7
Q3 101d
100
95% CONFIDENCE BOUNDS
ESTIMATED PSD
6.9 y	 (SLOPE = 3,2)
.4 NOISE FLOOR — 01
10-1
10 -3
	10-2
	
10 -1
	
100
FREQUENCY (CYISAMP)
I
3600	 1200	 360	 72
WAVELENGTH (KM)
Figure 11
	
	
Spectrum Analysis of Magnetic Anomaly
(Orbit 45, Old Tape)
14
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THE ANALYTIC SCIENCES CORPORATION
SPECTRUM OF 181 ANOMALY, ORBIT 45
104
	 11.75825
DEGREE 14
FIELD MODEL
103
d
Q
v 102
N
Q
C7
cc
	 95% CONFIDENCE BOUNDS
3
d
ESTIMATED PSD
100
	
2.4-r	 (SLOPE	 3.7)
to
10-1
	
10 -2 	10-1	 100
FREQUENCY ICY/SAMP)
I	 I	 1	 1
	
3600	 1200	 360	 72
WAVELENGTH (KM)
Figure 12 Spectrum Analysis for JBI, Orbit 45,
New Investigator Tape. Noise Floor:
0.3 y rms
for the old data (Fig. 11), and is 0.3 y for the new data (Fig.
12). The slope of this spectrum estimate has also changed,
due to the change in the field model (MGST 12/80 vs MGST 4/81)
and the change in the noise floor. An effort has begun to
explain the observed along-track PSD slope in terms of simple
analytic models of the (spherical) anomaly spectrum. Prelimi-
nary results show that the observed. Magsat spectra are roughly
consistent with upward continuation of a spectrum model given
by Lowes (1974, Ref. 6).
Figure 13 is a power spectrum estimate for Orbit 1201
(profile shown in Fig. 7). The noise floor for orbit 1201,
15	 ^^ i`ORIGINAL. i r%P-
OF POOR Q , L.I' I' Y
z
THE ANALYTIC SCIENCES CORPORATION
SPECTRUM OF IBI ANOMALY, ORBIT 1201
104
	
N•7 tie 33
103
d
aN
v 102
cc
N
QC7
W 
101
CL
100
10
DEGREE 14
FIELD MODEL
95% CONFIDENCE BOUNDS
213y
ESTIMATED PSD
\^	 3 87	 ^`^ (SLOPE = 3.51
4	 NOISE FLOOR
5'y
10 -3 	10 -2
	10-1	 100
FREQUENCY ICYJSAMP)
	
I	 I	 1	 1
3600
	 1200	 360
	 72
WAVELENGTH (KM)
	Figure 13
	 Spectrum Analysis of Magnetic Anomaly
(Orbit 1201)
slightly higher than that for orbit 45, is reached in both of
these cases at about 250 km. This is observed for other exam-
ples not included here, so the conclusion is that the resolution
limit for lithospheric anomalies in the newer Magsat Investi-
gator data is about 250 km. This result will be used in the
process of filtering the data for producing anomaly maps.
16	 ORIGINAL PAGE Rr^
Or' POOR 0- 1. I'LI 7
THE ANALYTIC SCIENCES CORPORATION
4.	 PLANS FOR THE NEXT REPORTING PERIOD
The analysis of individual tracks will be concluded
with verification of a reliable automatic data editor and better
understanding of the along-track spectra. Spectral coherence
between pairs of close tracks will be computed. Work will be-
gin on editing and filtering the data for production of anomaly
maps. Gridded geoid data will be obtained for study of the
relationships to the gravity field.
,. A
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REFERENCES
1. "Quarterly Progress Report for the Period 2 December
1980 to 31 March 1981," The Analytic Sciences Corpora-
tion, Report No. PR-1325-1, 7 May 1981.
2. Brammer, R.F., Sailor, R.V., and Lazarewiez, A.R., "MAGSAT
Investigation of Lithospheric Magnetic Anomalies in the
Eastern Indian Ocean," EOS Transactions of the American
Geophysical Union, Vol. 62, No. 17, p. 270, 28 April
1981.
3. Brammer, R.F., Sailor, R.V., and Lazarewicz, A.R., "MAGSAT
Investigation of Lithospheric Magnetic Anomalies in the
Eastern Indian Ocean," Presented at the Fourth Scientific
Assembly of IAGA, Edinburgh, Scotland, 4 August 1981. 	
A
4. Brammer, R.F., and Sailor, R.V., "Preliminary Estimates
of the Resolution Capability of the SEASAT Radar Alti
meter," Geo h sical Research Letters, Vol. 7, No. 3,
pp. 193-19 , March 1980.
5. "Satellite Magnetic and Gravity Investigation of the
Eastern Indian Ocean," The Analytic Sciences Corpora-
tion, Technical Proposal No. TP-1325, 6 February 1979.
6. Lowes, F.J., "Spatial Power Spectrum of the Main Geo-
magnetic Field and Extrapolation to the Core," Geo h_^s.
J. Royal Astronomical Soc., Vol. 36, p. 717-730, 1974.
18


MAGSAT anomaly profiles of the eastern Indian Ocean

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