Probing Shallow Aquifers in Hyperarid Dune Fields Using VHF Sounding Radar

Large-scale characterization of water table depth in shallow aquifers in hyperarid areas provides crucial insights into groundwater dynamics under increasing anthropogenic discharge and climatic fluctuations. Due to their penetration capabilities into arid soils, airborne very-high-frequency (VHF) sounding radars can achieve this objective under specific system design, topographic and geophysical constraints, superseding sporadic well logs, and ground-based surveys that provide compromised assessments of the distribution and depth of these water bodies. One of the least constrained ambiguities limiting the design of such systems, however, is the maximum penetration depth in desiccated sandy soils, which covers a sizeable fraction of desert landscapes. To constrain the latter, we perform a ground survey using 50- and 80-MHz GPRs with effective dynamic ranges of ~80-dB at the surface to probe the unconfined aquifer under desiccated linear dunes in the Wahiba Sands in Oman. Our survey resolves the water table down to at least 69 m depth, the deepest achieved at VHF frequencies in hyperarid terrains. We observe the average two-way plane-wave subsurface radar attenuation, accounting for both dielectric and scattering losses, to range from 0.1 to 1.4 dB/m through these sandy formations. Dielectric and scattering losses can be of equal magnitude depending on the sounding frequency and stratigraphic setting of the subsurface. Penetration depths to the water table are validated with time-domain electromagnetic (TDEM) measurements and well-log data. In addition, we identify shallow paleochannels from L-band synthetic aperture radar (SAR) observations that suggest modern meteoritic recharge of the probed aquifer, creating shallow localized anomalous losses in the radar signal in the first few meters. We conclude that the minimum requirements for an airborne VHF sounding radar to probe shallow aquifers at depths of tens of meters in sandy formations in hyperarid areas are a signal-to-noise ratio (SNR) of 55 dB at the surface, a bandwidth of 10 MHz, and a surface $h_{\mathrm {rms}}$ not exceeding 2 m.