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Preliminary evaluation of radar images for geomorphological investigation in SW Libya



Background: the sand penetration capability of radar signal.

Images acquired in north-eastern Sahara in 1981 during the first NASA Shuttle Image Radar mission (SIR-A) demonstrated the capability of L-band (wavelength 24.5 cm) to penetrate 1 or 2 m of loose sand and return information about geologic and geomorphologic features covered by sand (McCauley et al., 1982; Schaber et al., 1986). Subsequently radar images in L-band by Seasat, SIR-A and SIR-B acquired in the Baden-Jaran Desert of China (Guo et al., 1986), in Saudi Arabia (Berlin et al., 1986; Avery and Berlin, 1992) and in the Mojave Desert of California (Blom et al., 1984; Farr et al. 1986) confirmed the benefit of radar images to study bedrock feature beneath few meters of loose sand.
Ancient drainage pattern cut in the bedrock, not visible in the optical images because of the sand cover, most of times appears as dark network in radar images. From here the term "radar-rivers" used for the buried palaeochannels. The reflection of radar images waves from smoother channel filling and their attenuation by the same material cause the dark aspect of the buried palaeodrainage contrasting the brightness of hard substrate (Breed et al., 1983; Davis et al., 1993; Elachi et al., 1984; McCauley et al., 1982; 1986; Schaber et al., 1986).
In the early 1990s data from AIRSAR and SIR-C/X-SAR permitted to study the sand penetration capability of different wavelengths in different polarization modes. Image analysis and field investigations conducted in Egypt (Schaber et a., 1997), Arabia (Dabbagh et al., 1997), Arizona (Schaber, 1999; Schaber and Breed, 1999) and Israel (Blumberg et al., 2004) demonstrated that longer wavelengths penetrate deeper in the sand. HH and VV polarization modes have similar penetration properties while cross-polarization modes (HV and VH) have slight higher penetration capability than co-polarization modes. Co-polarized data are more responsive to surface roughness or volume scatterers on the scale of band's wavelength while cross-polarized data are most responsive to the geometry or texture of surface or volume scatterers (Schaber et al., 1997).
Schaber et al. (1986) determined the radar imaging depth through loose dry sand is equivalent to 0.25 times the attenuation distance (or skin depth) that is the distance at which the E-field decay to 1/e = 0.368 of its original strength (also see Schaber et al., 1997; Schaber and Breed, 1999). They calculated maximum radar imaging depth for X, C, L and P-band is equal to 0.25 m, 0.52 m, 2.07m and 5.87 m respectively. These values was confirmed by field investigation in Egypt and Arizona, while Dabbagh et al. (1997) asserted L-band signal penetrated sand thickness up to 4 m in Arabia. This higher value may be due to the low content of ferric iron. In fact Matzler (1998) suggested pure sand quartz has higher penetration depth than oxidated sand. Also clay absorbs or attenuate radar signal (Schaber et al., 1986; 1997).
The return of subsurface images needs a relative flat topography, such as sand sheets with low eolian ripple, with radar smooth surfaces and buried features with strong backscatter. Best radar penetration occurs with high look angles (> 30°) as this allows more refraction at the air-sand interface (Elachi and Granger, 1982) and the penetrated material should be extremely dry and fine grained with thickness of few meters (McCauley et al., 1982; Blom et al., 1984.; Schaber et al., 1986; Schaber and Breed, 1998; Schaber, 1999). Nevertheless Farr et al. (1986) and Blumberg et al. (2004) observed radar penetration in areas where soil moisture is higher than in hyperarid regions. Laboratory measurements by Williams and Greely (2001) showed radar attenuation by sand for wavelengths from 5 to 60 cm with three different moisture contents: with 10.7% of moisture volume the attenuation for L-band is less than 10 dB/m while X-band and C-band are very strong (about 320 and 130 dB/m respectively).
Recently with the SAHARASAR project the whole East Sahara has been observed with JERS-1 radar imagery (L-band) (Paillou and Rosenqvist, 2003a,b). Important results have already been obtained such as the discovering of buried palaeochannels, tectonic features and impact craters (Paillou et al., 2003; Paillou et al., 2004).



SIR-C/X-SAR images of Wadi Tanezzuft and Erg Uan Kasa (SW Libya)

In order to evaluate the use of radar data in SW Libya three previews of images acquired during SIR-C/X-SAR mission have been observed. They are not the effective images but previews (survey images) free of charge with lesser quality. However they show interesting elements not or poorly visible in optical images such as Landsat scenes.

X-SAR and SIR-C coverage
Fig. 1

SIR-C images in the C and L bands cover part of the Erg Uan Kasa (fig. 1). The brighter areas mostly represent bedrock or cemented sediments while dunes appear dark. In C-band the dunes, much darker regarding the surrounding areas, are evident while in the L-band their edges often become poorly detectable. This is an effect of the greater ability of L-band to penetrate through the sand and make lower dunes transparent at this wavelength.

Landsat and SIR-C
Fig. 2

This property permits us to better recognize hydrographical elements in radar images than in Landsat ones. An example of palaeofluvial cut filled by sand is showed in fig. 3. In fig. 4 the arrows indicate many fluvial elements not visible in the Landsat image. Their number is greater in the L-band thanks to his higher sand penetration capability.

palaeofluvial cut
Fig. 3

palaeofluvial traces
Fig. 4

L-band in fig. 5 show dark elements aligned in north-south direction; they are poorly visible in C-band and not visible in Landsat image. Probably they are evidences of a fracture or fault hidden by sand.

fault
Fig. 5

The X-SAR image covers part of the Wadi Tanezzuft (fig. 1). As X-band has weaker sand penetration capability than C and L bands, eventual buried structures are not detectable, however many features on the surface have higher contrast than in optical images. In fig. 6 the artificial cultivated oasis is well visible in radar image with many bright spot where vegetation grows. Toward north radar images show with good contrast the meandering palaeochannels poorly visible in Landsat image.

meandering palaeochennales
Fig. 6

These brief observations confirm the utility of radar images for geomorphological investigation and demonstrate the studied area in SW Libya has good environmental conditions for sand penetration by radar signal.



References

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Berlin, G. L., Tarabzouni, M. A., Al-Naser, A., Sheikha, K. M., Larson, R. W. (1986), SIR-B subsurface imaging of a sand-buried landscape: Al Labbah Plateau, Saudi Arabia. IEEE Trans. Geosci. Remote Sens. GE-24:595-602.

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by Alessandro Perego, September 2007

 

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Settembre 2007 - modificato Agosto 2015
Alessandro Perego