A controlled experiment to investigate the correlation between early-time signal attributes of ground-coupled radar and soil dielectric properties

Ground-coupled radar has been used in the literature to estimate shallow subsoil permittivity using groundwave velocity measurements. It has also been shown that the electromagnetic (EM) properties of the soil significantly affect antenna performance,modifying in particular the amplitude, shape, and duration of the ‘early-time’ Ground Penetrating Radar (GPR) signals. To quantitatively evaluate these effectswe built a test site consisting of a 4 × 7 × 1.2 m volume filled primarily with sand; this volume is hydraulically isolated from the surroundings and contains buried pipes in which water can be introduced or removed to control the level of the water table. On a regular grid of 28 points we measured the soil dielectric properties at depth intervals of 0–10 and 0–20 cm using Time Domain Reflectometry (TDR) probes, and collected GPR data using both 250 and 500 MHz bistatic antennas. The measurements were performed with the water table at different depths to systematically change the shallow-soil dielectric properties. Relative permittivity and conductivity values were calculated from the TDR data, and the average envelopes of the first half cycle of the early-time GPR signals were computed. Data analysis shows a high degree of linear correlation (r ≥ 0.8) between the early-time signal attributes for both antenna frequencies and the EM properties obtained using both TDR probe lengths. The highest correlation (r = 0.9) was found between the 500 MHz data and the permittivity measured along the 0–20 cm depth interval; this relationship is explained in terms of ground wave penetration. The results of our investigation confirm previous field observations and are in full agreement with theoretical predictions and related numerical simulations, highlighting the potential for alternative convenient approaches to predict EM properties of the shallow subsoil.