Assessing Subwavelength VHF Radar Scattering Losses in Hyperarid Carbonate Formations

Subsurface scattering losses associated with very high frequency (VHF) subwavelength heterogeneities are crucial for assessing the total radar signal attenuation in fractured and heterogeneous environments such as carbonate formations, which represent one of the most predominant aquifer types in hyperarid areas. Yet, the resulting signal losses are largely unquantified due to the difficulties in constraining their sources, amplitude, and frequency dependence on the VHF spectrum, hence, compromising the ability to perform large-scale characterization of shallow aquifers using radar probing in these areas. To address this deficiency, we present an experimental model quantifying volumetric scattering losses in heterogeneous carbonate formations accounting for wavelength-sized structural elements and subwavelength-sized heterogeneities ranging from 1 mm to 2 cm for the VHF radar signals. In particular, we use an analytical model that estimates the overall scattering effects produced by the interaction of an electromagnetic plane wave with randomly distributed vugs scatterers. We then compare our analysis with field radar measurements collected in the karst limestone aquiferous formation in the Qatar Peninsula at 80 MHz (10-dB bandwidth (BW): 30-150 MHz). Our results estimate that the total losses in karstic environments range between ~0.6 and 1.4 dB/m, of which 20%-65% is due to volume scattering. Furthermore, we find that despite being usually underestimated, subwavelength volumetric scattering accounts for a considerable portion (~40%) of the overall radar attenuation: between ~0.045 and 0.35 dB/m. The results of our analysis can constrain the degree of karstification associated with vertical artesian movements in fossil aquifer systems in hyperarid environments.