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Extraction of Subsurface Features from InSAR-derived Digital Elevation Models

! Remote sensing at radar wavelengths has the potential to be a beneficial tool to detect subsurface features in desert areas. Penetration depth varies from different wavelengths of different sensors with the greater the penetration the longer the wavelength and conversely the shorter the frequency. It has been shown that microwave imaging at low frequency (e.g. 1.25 GHz/24 cm, L-band) can penetrate down to about 2 m depth under sand cover in Libyan Desert [1, 2, 3] with extremely low surface and bulk humidity. Thus, subsurface features, such as paleo-channels and buried craters, have been unveiled. On the other hand, global Digital Elevation Models (DEMs) are now being produced from spaceborne earth observation sensors, such as at microwave frequencies from SRTM, TanDEM-X and ALOS-PALSAR as well as stereo-optical, such ASTER or PRISM or echo waveform lidar such as ICESat, which can be useful data resources to detect subsurface features over desert regions. By using such DEMs, surface geomorphological features can be detected [4], while subsurface feature detection in desert regions can sometimes have surface expression but mostly not. ! In order to detect subsurface features under dry sand cover, we investigate the penetration ability of different DEMs acquired by SRTM (Xand C-band), ALOS-PALSAR (L-band), ASTER (stereo-optical) and ICESat (1064nm lidar) over three study sites in Africa/Middle East, which includes Gilf Kebir Plateau, Kufrah River and Kuwait. Gilf Kebir Plateau is located in southwest Egypt, where 1,300 craterlike features over 4,000 km2 were found by Paillou [3]. This study site covers a 23,000 km2 region between latitude 23°N-24°N and longitude 26°E-28°E. Kufrah River is located in southeast Libya, the study site covers an 11,000 km2 region between latitude 23°N-24°N and longitude 23°E-24°E. There is a paleodrainage system which passes through this region, which has been discussed in former pulications [5, 6]. The last study site is located in Kuwait, covering Kuwait city between 28°N-30°N and longitude 46°E-48°E. Since elevation data obtained from different sources have different vertical datums, they were brought into the same reference frame. SRTM-X DEM and ICESat/GLA14 data were transformed into the EGM96 datum, thus, the elevation used in this study is orthometric height referenced to EGM96. Horizontal offsets between ASTER and SRTM DEM were checked, which appear to be within one pixel. Although the ICESat/GLA 14 data are referenced to a different ellipsoid cf. SRTM-C DEM and ASTER DEM, the differences in geodetic latitude and longitude only produce a horizontal displacement of less than a metre, which can be ignored[7]. ! Our preliminary analysis shows that, in terms of subsurface features detection, SAR (SRTM at C-band and PALSAR at L-band) and lidar (ICESat) are superior to stereo-photogrammetric (ASTER), which is affected by severe noise due to the lack of contrast in the stereo image pairs and possible atmospheric sand particles obscuring the surface. This is consistent with previous studies [3] that indicate that electromagnetic waves of lower frequency can penetrate deeper in sandy layers. Surprisingly some subsurface features are obvious on the SRTM-C DEM, which also conforms well with elevations measured by ICESat. Besides that, The elevations extracted from SRTM-X DEM are also consistent with elevations measured by ICESat with a slight deviation at the edge of SRTM-X DEM’s strip. This appears to be due to sand filling-up previous water channels. However, these are absent in the ASTER DEMs probably because of the high noise content of these DEMs. By analysing topographic features of the DEMs such as those detected using bi-quadric modelling of the DEM height points using a surface patch model, a preliminary automated detection of subsurface Kufrah river has been realised. But the extraction of Kufrah river is affected by refilled sand and dunes, which change the slope of riverbed. Further work will be shown on how to deal with this problem and the adopted method will be applied to the other two study sites. Results will be shown from different areas using this method to auto-detect paleochannels and buried craters from recently released SRTM DEM. ! REFERENCES: ! [1] McCauley J F, Schaber G G, Breed C S, et al., Subsurface valleys and geoarcheology of the eastern Sahara revealed by shuttle radar [J]. Science, 1982, 218(4576): 1004-1020. [2] Elachi C, Roth L E, Schaber G G., Spaceborne radar subsurface imaging in hyperarid regions [R]. IEEE Trans. On Geoscience and Remotes Sensing, 1984, GE-22(4): 383-388. [3] Paillou P, Reynard B, Malezieux J M, et al., An extended field of crater-shaped structures in the Gilf Kebir region, Egypt: Observations and hypotheses about their origin [J]. Journal of African Earth Sciences, 2006, 46(3): 281-299. [4] Tarboton D G, Bras R L, Rodriguez Iturbe I., On the extraction of channel networks from digital elevation data [J]. Hydrological processes, 1991, 5(1): 81-100. [5] Paillou P, Schuster M, Tooth S, et al., Mapping of a major paleodrainage system in eastern Libya using orbital imaging radar: The Kufrah River[J]. Earth and Planetary Science Letter, 2009, 43(2):327-333. [6] Robinson C A, El-Baz F, Al-Saud T S M, et al., Use of radar data to delineate palaeodrainage leading to the Kufra Oasis in the eastern Sahara[J]. Journal of African Earth Sciences, 2006, 44(1): 229-240. [7] Website of NASA Distributed Active Archive Center (DAAC) at NSIDC, http://nsidc.org/data/icesat/