THE POLARIMETRIC CHARACTERISTICS OF BOTTOM TOPOGRAPHY RELATED FEATURES ON SAR IMAGES

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1 THE POLARIMETRIC CHARACTERISTICS OF BOTTOM TOPOGRAPHY RELATED FEATURES ON SAR IMAGES Taerim Kim Professor, Ocean System Eng. Dept. Kunsan University Miryong Dong San 68, Kunsan, Jeonbuk, Korea, ABSTRACT This is a preliminary study on bottom topographic related features on SAR images observed by different frequencies and polarizations. X-SAR images taken on the coastal waters of Hwanghe province in Korea during SIR-C/X-SAR campaign in 1994 show strong patterns associated with bottom topography. ENVISAT ASAR images also show strong patterns indicating the boundary of sandbank in the water. All seem to be produced by strong interactions between topography in the coastal waters and tidal currents and its intensity is dependent on the tidal phase, depth, wind, and frequency and polarization of the sensor. Bottom features on different frequency and polarization SAR images and different satellite images are introduced, such as RADARSAT, ERS2 and LANDSAT5 1 INTRODUCTION Synthetic aperture radar(sar) remote sensing of the sea surface has shown that the modulation of surface roughness by the changes of current velocity over shallow bottom can produce bottom features on SAR images caused by interactions between currents and bottom topography. Reference [1] identified that linear features on SEASAT images of the English Channel and Thames Estuary Approaches is linked to the steep edge of sand banks. Reference [2] described the bottom topography-current interaction using the continuity equation and showed the relationship between radar image intensity, sea surface roughness, tidal flow, and underwater bottom topography by schematic plot. They assumed that the velocity component normal to the direction of the ridge or sand wave obeys the continuity equation and the velocity component parallel to it remains constant. Reference [3] explained the relationship between the SEASAT SAR signatures and the bottom topography of the ocean in the Southern Bight of the North Sea and Nantucket Shoals, and found these SAR-observed features are only present when a tidal current(0.4 m/s or greater) is flowing over the bottom and wind conditions are moderate(less than 10 m/s). The relation between radar imagery and bathymetry is also studied using a Side Looking Airborne Radar(SLAR)[4]. They concluded that the extremes in the radar backscatter are located right above regions with maximum bottom slopes, within the positional accuracy of 30 m. In order to deepen the theoretical foundations of the patterns on SAR images caused by underwater topography, Reference [5] approached with large-eddy simulations using the full Navier-Stokes Equations other than simple continuity equations. The bottom features also often appear on SAR images by intensified bragg scattering at the coastal current front lines caused by sudden depth changes such as shelf break and edges of estuarine channels[6].

2 This preliminary study examines depth contours on X-SAR images and L-SAR images taken during SIR- C/X-SAR campaign, and the same depth contours shown on different and polarization SAR images are also to be checked. Demilitarized Zone(DMZ) between South Korea and North Korea and access to this area for in-situ measurements is prohibited and not much in-situ data are available. Fig. 1 shows the map of study area in the Yellow Sea and its bathymetry chart. 2. DESCRIPTION OF STUDY AREA X-SAR images taken on the coastal waters of Hwanghe province in Korea during SIR-C/X- SAR(Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar) campaign in April and October 1994 are analyzed. The space shuttle loaded with SIR-C/X- SAR acquired radar signatures at three different frequencies and made measurements for different polarizations at two of those wavelengths. The shuttle flied three times on April 15, 16 and October 7 on the coastal waters of Hwanghe province and KyungGi Bay in the Yellow sea from the northwest to the southeast. The swath was 15 km ~ 40 km and the incidence angle was 15 ~ 55. X-SAR provides single frequency(9.6 GHz), single polarization(vv) and SIR-C provides multi-frequency, multi-polarization radar data, such as C-band(5.3 GHz ) and L-band(1.25 GHz ) and dualpolarization(hh, VV, HV, VH). At the beginning of this study, all frequency and polarization SAR images were to be compared but L-band and X-band SAR images with VV polarization were only available because of the problems of archiving system. The study area is characterized with tide dominated coastal waters having complicated coastal lines and bathymetry with many sand banks. The depth is less than 40 meters and the tidal range is about 8 meters with strong tidal currents. It is anticipated that the interaction between tidal currents and bottom topography is very strong causing complicated flow regime. Unfortunately, this area is very near to the Fig. 1 Study area map DEPTH CONTOURS ON SAR IMAGES Fig. 2~ Fig. 4 show very interesting features related tidal current interaction with topography of coastal waters such as bottom contours. (a) image is an original X-SAR(VV) image, (b) shows rectified X-SAR image of (a) overlaid on coastal bathymetry map, (c) is L- SAR(VV) image taken simultaneously with (a) during SIR-C/X-SAR campaign and (d) is a time series of tidal elevation measured near the imaging location. The images in (b) and (c) are not rectified in order to prevent deteriorating of the radar signals of bottom features. Coincidently, all three images are taken near low tide when tidal current velocity is relatively slow compared to the other phase of tidal cycles. Specially, both images on April 15 and 16 indicate very precisely the boundary of sand banks along the 10 m depth contour line. In case of the image on October 7 the depth contour line is not clearly appeared but the bank area is blurred overall with high intensity backscattering. The curved line features indicating 10 m depth contour seem to be

3 Fig. 2 (a)x-sar image, (b) its rectified image overlaid on bathymetry map, (c)l-sar image on April, 15, 1994 and (d) time series of tidal elevation. Fig. 3 (a)x-sar image, (b) its rectified image overlaid on bathymetry map, (c)l-sar image on April, 16, 1994 and (d) time series of tidal elevation.

4 Fig. 4 (a) X-SAR image, (b) its rectified image overlaid on bathymetry map, (c)l-sar image on October, 7, 1994 and (d) time series of tidal elevation. originated by current gradient generated by strong bottom slope. The average depth of the sand bank is about 7 m and the outside the bank is about 35 m and the tidal current during the ebb flows along the 10 m depth contour to the northwest. The current gradient is caused by the effect of bottom friction. The tidal flow over the sand bank is more slowed down by the friction compared to the flows at the channel and the strong gradient of the flow is appeared on the SAR image as high intensity backscattering. Compared to the X-band SAR images, the bottom features on L-band images are not clear as the one on the X-band image. Shorter wave length of radar seems to express better the surface roughness caused by shears between two different velocity currents. Since SIR-C/X-SAR images at the different phase of tidal cycles are not acquired, it is difficult to conclude that which current velocity is the best to express the depth contours on SAR images. But it seems that X-SAR is good at detecting current gradients caused by bottom slope specially when the current velocity is not so strong. In case of ENVISAT ASAR image(fig. 5) and ERS2 SAR image(fig. 7 (a)), the depth contours are also appeared on SAR images at various phase of tidal cycles but they are not sharp and distinct like the features on X-SAR images. RADARSAT SAR(HH) images(fig. 6) also show the bottom features but they are difficult to tell from other features and seem to be poor to present surface current shears. At the present stage, it is too early to conclude that which frequency and polarization SAR image show the best bottom features at different tidal cycles, tidal velocity and water depths. For further study, different band and polarization SAR images at different tidal conditions needs to be examined. Those different SAR characteristics will provide more clearly the interaction mechanism between tidal currents and bottom topography in the coastal waters. Fig. 7 (b) is a

5 Fig. 5 ENVISAT ASAR image on (a) October 19, 2005 and (b) May 25, 2005 Fig. 6 RADARSAT SAR image on (a) July 5, 2002 and (b) May 28, 2002 Fig. 7 (a) ERS2 SAR image on July 8, 1998 and (b)landsat5 image on April 25, 1999 LANDSAT-5 image taken at the same region. The curved lines on SAR images are also appeared on LANDSAT image as distinct line features with blue color meaning less sediment suspension. This property will be able to provide another reason why bottom information is shown on SAR images. REFERENCES 1. Lodge, D. W. S., 1983, Surface expressions of bathymetry on SEASAT synthetic aperture radar images. Int. J. Remote Sensing, 4(3), Alpers, W. and I. Hennings, 1984, A theory of the imaging mechanism of underwater bottom topography by real and synthetic aperture radar, Journal of Geophysical Research, 89, pp: Shuchman, R.A., Lyzenga, D. R., and Meadows, G. A., 1985, Synthetic aperture radar imaging of oceanbottom topography via tidal-current interactions: theory and observations, Int. J. Remote Sensing, 6(7),

6 4. Vogelzang, J., Wensink, G.L., De Loor, G.P., Peters, H.C., and Pouwels, H., 1992, Sea bottom topography with X-band SLAR: the relation between radar imagery and bathymetry, Int. J. Remote Sensing, 13(10), Calhoun, R.J. and Street R.L.,2002, Patterns on a free surface caused by underwater topography: a laboratoryscale study. Int. J. Remote Sensing, 23(8), Valle-Levinson, A., L. Chunyan, K. C. Wong, and K. M. M. Lwiza, 2000: Convergence of lateral flow along a coastal plain estuary. J. Geophys. Res., 105,