Use of ERS-1 SAR Data for Some Marine Applcations in Malaysia
Ocean Wave Spectra
ERS-1 data has been demonstrated to provide ocean wave information in terms of wavelengths and directions. Ergo Vista software was used to calculate wave spectra from PRI images. Single SAR images frames comprising of 512 x 512 image pixels was extracted. Since each pixels represents a 12.5 m x 12.5 m area, the entire image frame corresponds to a 6.4 km x 6.4 km path on the ocean surface. The frame size provides a sufficiently large area that at least 10 cycles of very long surface waves, up to 640 m in length, can be included in a single image frame. They are also small enough that the ocean can be reasonably assumed homogeneous within a frame (Monaldo, 1991). The wave spectra were calculated based on a 2-dimensional Fast Fourier Transform, and the results are as shown in figure 3. This process was repeated for other frames in the image.
Figure 3: Wave spectra derived from single frame of ERS-1 data acquired on 5 Mar 93.
Circle and arrow represent ocean wavelength and direction respectively.
Visual Image Interpretation
ERS-1 images also used to detect oil slick, ship and ship wakes and water fronts. Figure 4(a) illustrates the presence of oil slicks/spills which appear as dark thin lines in the image for both study areas. Figure 4(b) shows the variation of the dB along the azimuth direction around the slicked area. The backscatter from the slick area is about 10 dB lower than the surrounding areas. Figure 5 illustrates the use of ERS-1 data for locating ship traffic in the Kuala Terengganu study area. Figure 6 shows the presence of ocean eddies caused by natural-occurring surface slick patterns (Gower, 1994). Figure 7 illustrates the possible presence of an oceanic phenomena, a lone internal wave, in the Kuala Terengganu study area. Figure 8 shows a water front in the Kuala Terengganu study area due to the meeting of two water masses, one from the sea and one from the river. Figure 9 illustrates a storm-like echo in the South China Sea near the Kuala Terengganu study area seen by ERS-1 SAR.
Figure 4(a) : Oil slicks on ERS-1 Frost filtered image of 12 October 1993 in
Sarawak study area.
Figure 4(b) : Variation of the Averaged Backscattered Power along the Azimuth
Direction around the slick area (difference about 10 dB).
Figure 5 : Ship traffic and ship wake in the Terengganu study area
Figure 6: Water eddies formed by natural occurring surfactants on ERS-1
SAR image of 2 September 1993 in Terengganu study area.
Figure 7: Lone internal wave on ERS-1 SAR image of 12 September 1993 in
Terengganu study area.
Figure 8: Water front on ERS-1 SaR image of 12 September 1993
In Terengganu study area.
Figure 9: Strong footprint on ERS-1 SAR image of 12 September 1993
in Terengganu study area.
