Measuring the Electrical Properties of Soil Using a Calibrated Ground‐Coupled GPR System

Abstract

Traditional methods for estimating vadose zone soil properties using ground penetrating radar (GPR) include measuring travel time, fitting diffraction hyperbolae, and other methods exploiting geometry. Additional processing techniques for estimating soil properties are possible with properly calibrated GPR systems. Such calibration using ground‐coupled antennas must account for the effects of the shallow soil on the antenna's response, because changing soil properties result in a changing antenna response. A prototype GPR system using ground‐coupled antennas was calibrated using laboratory measurements and numerical simulations of the GPR components. Two methods for estimating subsurface properties that utilize the calibrated response were developed. First, a new nonlinear inversion algorithm to estimate shallow soil properties under ground‐coupled antennas was evaluated. Tests with synthetic data showed that the inversion algorithm is well behaved across the allowed range of soil properties. A preliminary field test gave encouraging results, with estimated soil property uncertainties (±σ) of ±1.9 and ±4.4 mS/m for the relative dielectric permittivity and the electrical conductivity, respectively. Next, a deconvolution method for estimating the properties of subsurface reflectors with known shapes (e.g., pipes or planar interfaces) was developed. This method uses scattering matrices to account for the response of subsurface reflectors. The deconvolution method was evaluated for use with noisy data using synthetic data. Results indicate that the deconvolution method requires reflected waves with a signal/noise ratio of about 10:1 or greater. When applied to field data with a signal/noise ratio of 2:1, the method was able to estimate the reflection coefficient and relative permittivity, but the large uncertainty in this estimate precluded inversion for conductivity.

Water content evaluation in unsaturated soil using GPR signal analysis in the frequency domain

“…These estimated values fit very well both TDR measurements and soil-specific empirical model predictions (similar to Topp's equation). Oden et al (2008) have calibrated and validated a new model for measuring the electrical properties of soil. The algorithm estimates the shallow soil properties using the early-time arrivals, i.e.…”

Measuring soil surface water content in irrigated areas of southern Tunisia using full‐waveform inversion of proximal GPR data

“…Its main drawback for realtime mapping applications is the requirement, in addition to the measurements above the soil surface, of corresponding measurements above a smooth perfect electric conductor (PEC, e.g., metal sheet) at exactly the same height. Recent studies have shown that soil surface electromagnetic properties can also be obtained from early‐time GPR wavelet analysis (Pettinelli et al ; Oden et al ). Whether for the ground wave, the surface reflection coefficient methods, or early time analyses, a major concern with these approaches is that they are usually applied with traditional, commercial GPR systems, for which the measured quantities are not fully physically described and rely on strongly simplifying assumptions with respect to electromagnetic wave propagation phenomena.…”

Uncertainty in ground penetrating radar models

“…In some models operational calibration measurements are carried out to account for antenna effects, [38,39]. The model assumptions that lie at the base of such calibration techniques are not evaluated for their accuracy and validity in general.…”