On conductive ground: Analysis of “Bistatic sounding of the deep subsurface with ground penetrating radar − experimental validation” by V. Ciarletti et al.

A prototype ground-penetrating radar (GPR) for planetary exploration was tested in the Western Desert of Egypt by Ciarletti et al. (Planet. Space Sci., 117, 177, 2015), and penetration depths of 200-300 m were reported. However, geoelectrical and electromagnetic soundings performed during the same campaign indicate very low resistivities (<10 Ω m) between depths of several meters to a few hundred meters. The local near-surface geology is limestone intercalated with clays. Such conductive ground is impenetrable to GPR, even with very long signal integrations. Indeed, high-conductivity ground causes most of the GPR energy to be radiated above the ground surface. Events in the single GPR time series published by Ciarletti et al. (2015) are most likely echoes from nearby topography and not reflections from subsurface interfaces. The principal lesson of this experiment is that Earth's hot deserts, which may be good geological analogs to desiccated planetary surfaces, are poor GPR analogs due to the ubiquity of clays, salts, and trace amounts of water.