POTENTIAL FOR VHF SOUNDING RADAR TO PROBE SHALLOW FOSSIL AQUIFERS IN HYPER-ARID ENVIRONMENTS

Fossil aquifers are the largest freshwater bodies in the North African Sahara and the Arabian Peninsula; their groundwater dynamic and response to climatic variability and anthropogenic discharges remain largely unquantified due to the absence of high resolution large-scale monitoring methods. Today the largest portion of observations measuring the variations in the depth of the different water tables and its salinity are made from sporadic well logs that hardly cover few percent of the geographical extension of these aquifer systems. To address these deficiencies, we develop the use of low Frequency Airborne Sounding Radar to characterize the depth and shape of the water tables over large areas. In a first phase, we use 40 and 70 MHz Air-Launched wide-band antennas (bandwidth at 3 dB is about half of the central frequency) connected to a Ground Penetrating Radar (GPR) to identify the measurement requirements in term of dielectric and scattering attenuations and the radiometric accuracy to assess the reflection on the water table. Our study is performed in the aeolian Wahiba Sands, in Northern Oman, which is an optimal area for understanding the potential and requirements for radar deep subsurface sounding of aquifers due to its high aridity resulting in high ground resistivity, well characterized shallow aquifers, well-known chronostratigraphy and mountains shielding the site from electromagnetic interferences from large urban areas. Along a four kilometers GPR profile, we detect a flat water table at 35 to 37 meters deep in agreement with well logs observations along the profile. The radar reflection from the water table reflection is respectively 15dB and 10dB above the noise level for the 40 and 70 MHz frequency bands with a total two-way signal attenuation of 0.2 dB/m. We validate our radar sounding observations with the vertical electrical resistivity of the shallow subsurface and Time-Domain Electromagnetic (TDEM) profiles to assess the deep subsurface resistivity. Our study determine the set of geophysical and radar parameters under which it is possible to perform future airborne radar sounding of fossil aquifers in hyper-arid areas.