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Geology
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Gravity field mapping in the Malaysian exclusive economic zone (EEZ) from a combined GEOS-3 and seasat radar altimeter data base
Abd. Majid, A. Kadir
Faculty of Surveying. University Tech. Malaysia.
Locked Bag 791, 80990 Johor Bahru, Johor, Malaysia
Abstract
This work has clearly demonstrated the ability of the satellite radar altimeter data to hold a detailed mapping of the gravity field n the Malaysian Exclusive Economic Zone (EEZ). Gravity anomalies and mean sea surface heights were derived farm a combined Geos-3/SSeasat altimeter data abase. The method of gravity field recovery was based on the theory of the squares collocation. The computations were carried out using OSU81 reference gravity33 model defined by a set of potential coefficients complete to degree and order 180 and th scaled scheming/Rasd covariance model. Satellite track bas was estimated in small 4° x 4° religious to create a bias-free altimeter data set prior to the gravity and production rest.
Fairly good agreement was observed between the predicted point gravity animates and the observed 'ground truth" defined by point (ship) gravity measurements when a sufficiently dense altimeter data set was used. Comparisons along a 300 km gravity profile (free air) at latitudes 6°.2 with the one derived from radar altimetry indicate root-mean-square differences of 5.3 mgal.
Combined GEOS-3 Seasat RADAR altimeter data base
Until recently, the main sources of the altimeter data have been the Geos-3 and Sea sat altimeter satellites. The Geos-3 data was acquired in the time period 1975-1978. While he Sea sat data was limited to the time period June through October 1978. The results to be presented in this study will be based n a combined Geos-3/Sea sat database developed through a series of studies conducted at the Ohio State University, USA. A brief review of these studies can be found in Majid (1987). Figure 1 shows satellite track of Geos-3 and Sea sat in the study area.
The use of altimeter data for determinations of the gravity field in local areas has widely been done in two steps: 1) a cross-over adjustment of the altimeter data. and 2) estimations of gravity field related quantities from the cross-over adjusted altimeter data. The second step will be discussed in section 2.0. The purpose of the adjustment f the altimeter data is to remove blazes due to orbit errors or oceanographic phenomena using the crossing are technique. Details on the global adjustments and editing of Geos-3 and Sea sat data carried out the Ohio State University can be found in Liang (1983). However, further adjustment in local areas are needed to diminish the remaining track errors and to remove any unedited spikes.
For the present study, the Malayalam EEZ is partitioned into ten 3° x 3° equiangular blocks so that cross-over adjustments can be performed in each of these areas. However , in area #5 which is located in the narrowest part of the Straits of Melaka, altimeter tracks are too sparse for any meaningful adjustment. Hence, excluding area #5cross-over adjustments were performed in all the areas shown in Figure 3 to create a bias-free altimeter data set prior to the prediction runs. Although the specific prediction area has a 3° x 3° block size. , the adjustment was carried out in a 4° x 4° block size in order to account for a 0°.5 border area. The statistics resulted from the above adjustment are summarized in Table 1.
Figure.1: Satellite track of Geos-3 (low inclination)and Seasat (high inclination). The 3° X 3° areas numbered # 1,2,3.........10 are the prediction blocks.
Table.1 Altimeter Cross-Over Adjustment Statistics
| |
PREDICTION AREA (4° x 4°) |
| #1 #1 |
#2 |
#3 |
#4 |
#6 |
#7 |
#8 |
#9 |
#10 |
The words are illiegible in the source
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1262
33
63
27
9 |
1297
31
58
24
16 |
1386
31
82
26
16 |
1620
96
193
28
20 |
1135
27
63
26
15 |
906
16
28
28
12 |
1219
21
60
39
22 |
648
18
22
24
4 |
1230
21
61
29
16 |
It is clear from Table 1 that the reot-mean squares (rms) of cross-over differences improved quite substantially after he local adjustments.
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