Validation of ALOS/PALSAR subsurface penetration depth in arid regions based on field ground-penetrating radar measurements

Abstract

In the last few years, ALOS/PALSAR (L-band) (HH, HV, VH, and VV) images have been widely used due to their ability to penetrate the surface in certain conditions, such as low moisture or dry friable sandy soil. Images from the ALOS-1 sensor were used to delineate subsurface structures, and optical images such as Landsat-7 ETM + data were used to discriminate between scatterings from the Earth’s surface and subsurface materials. Thus, the Farafra Desert is an optimal geological environment for L-band microwave penetration, as its geology is characterized by friable sand sheets covering limestone (Tarawan Formation). Speckle noise is found in radar images for many reasons, such as when an object strongly reflected between itself and the spacecraft causes noise. Refined LEE filter (RLF) is applied for speckle noise reduction; moreover, full polarimetric ALOS/PALSAR images (PLR) are transformed into circular polarization by changing both angles into orientation angle ψ = 0° and elliptical angle χ = 45°. The validation of ALOS/PALSAR outputs was carried out using ground penetrating radar (GPR) measurements. Three GPR long profiles using a 200 MHz antenna were scanned along with areas that were annotated according to ALOS/PALSAR results (high backscattering coefficient). The GPR system operated by a low-frequency antenna with a frequency of 200 MHz was capable of detecting the annotated geological structures beneath the sand sheets. Furthermore, statistical comparison of L-band SAR and GPR data illustrated a correlation that can reveal identical regions to delineate subsurface structures. These results prove that the integration of synthetic aperture radar SAR (L-band) and on-site low-frequency radar systems can be vital to detect soil structures down to several meters, ultimately innovating Earth observation systems for geological and hydrogeological mapping in arid regions.

Detection of Ring Structures and Their Surrounding Tectonic Pattern in South-Algeria, North-Mali and North- Niger based on Satellite Data

“…SNAP from ESA provided the tools as well for the processing of radar data. Synthetic Aperture Radar (SAR) space-borne satellites operate by using microwave radiation with a frequency in a range of wavelengths, for example, the L-band (23 cm-1.30 GHz and 24 cm-1.25 GHz), C-band (6 cm-5.0 GHz), (Mansour et al, 2022). L-band radar coverage was acquired by the JERS-1 satellite of the Japanese space agency (JAXA), C-Band coverage by the Sentinel 1 mission of ESA.…”

“…Larger circular structures appear on the different satellite images as demonstrated in the next figures. The Advanced Land Observing Satellite (ALOS) L-band Synthetic Aperture Radar (PALSAR, Japanese Space Agency-JAXA) complex single look (SLC) images are characterized by the optimal conditions of penetration in these desert areas depending on the grain size, rare moisture, surface roughness, radar backscattering mechanism and radar illumination geometry (Sieber & Theilen-Willige 1984;Theilen-Willige, 1986;Mansour et al, 2022) As mineral deposits are often linked to these types of ring structures (Reimold & Koeberl, 2005;Osinski et al, 2012), more detailed investigations would make sense. The evaluation of different satellite data helps to focus cost-and time-intensive field research.…”

“…The smoothness, roughness, and density of a surface affect the return signal. Unlike optical instruments, which can only see the surface, radar can penetrate loose, dry materials such as sand sheets.ALOS/PALSAR L-Band data are characterized by the optimal conditions of penetration in desert areas depending on the grain size, moisture, roughness of the surrounding geology and radar backscattering mechanism(Mansour et al, 2022). In very dry soils, L-Band SAR (1.25 GHz) can intrude into the subsurface down to several meters.…”