Drone-mounted ground-penetrating radar surveying: Flight-height considerations for diffraction-based velocity analysis

Recent studies highlight the potential of the drone platform for ground-penetrating radar (GPR) surveying. Most guidance for optimizing drone flight heights is based on maximizing the image quality of target responses, but no study yet considers the impact on diffraction traveltimes. Strong GPR velocity contrasts across the air-ground interface introduce significant refraction effects that distort diffraction hyperbolas and introduce errors into diffraction-based velocity analysis. The severity of these errors is explored with synthetic GPR responses, using ray- and finite-difference approaches, and a field GPR data set acquired over a sequence of diffracting features buried up to 1 m depth. Throughout, GPR antennas with 1000 MHz center frequency are raised from the ground to heights <0.9 m (i.e., 0–3 times the wavelength in air). Velocity estimates are within +10% of modeled values (spanning from 0.07 to 0.13 m/ns) if the antenna height is within 1/2 wavelength in air above the ground surface. Greater heights reduce diffraction curvature, damaging velocity precision, and masking diffractions against a background of subhorizontal reflectivity. Field data highlight further problems of the drone-based platform, with data dominated by reverberations in the air gap and reduced spatial resolution of wavelets at target depth. We suggest that a drone-based platform is unsuitable for diffraction-based velocity analysis, and any future drone surveys are benchmarked against ground-coupled data sets.