Comparison of GPR and unilateral NMR for water content measurements in a laboratory scale experiment

Abstract

Several factors affect antenna‐soil coupling in a Ground Penetrating Radar (GPR) survey, like surface roughness, lithology, lateral heterogeneities, vegetation, antenna height from the surface and water content. Among them, lithology and water content have a direct effect on the bulk electromagnetic properties of the material under investigation. It has been recently pointed out that the wavelet of the early‐time portion of a radar signal is correlated to the shallow subsurface dielectric properties of a material. This result indicates that some information on such properties can be directly extracted from the analysis of GPR early‐time traces. In the present paper, we use the early‐time GPR signal, in terms of average envelope amplitude computed on the first half‐cycle, to map the near‐surface (few centimetres) lateral distribution of dielectric parameters, induced by changing the shallow water content on a concrete slab. This controlled experiment was specifically designed to study the effect of water content variations on antenna‐material coupling, minimizing the influence of both surface roughness and heterogeneity. The quantitative control of the water in the shallow portion of the slab is performed by using a portable unilateral Nuclear Magnetic Resonance (NMR) sensor, which is able to determine the water content in the material on the basis of the measured proton density. The results show a matching pattern of the physical parameters measured with the two different techniques and a very high degree of linear correlation (r = 0.97) between the radar early‐time signal average amplitude and the intensity of the NMR signal, which is proportional to the proton density, i.e., to the water content. This experiment suggests that the early‐time approach could be used as a fast and high‐ spatial resolution tool for qualitatively mapping water content lateral variations in a porous material at shallow depth, using a ground‐coupled single‐offset antenna configuration and that a quantitative evaluation of the moisture content would require a calibration procedure.

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“…The waveform amplitude has an inverse linear dependence on the permittivity. In particular, [14] and [15] confirmed that because the first arrival of the GPR signals is strongly dependent on variations in the shallow subsurface permittivity, the signal to noise ratio can be maximized by minimizing the interference from reflections caused by shallow interfaces.…”

An evaluation of the early-time GPR amplitude technique for electrical conductivity monitoring

“…2). The choice of analysing the first half-cycle comes from previous experimental and theoretical studies [1,2], where it was found that the first half-cycle is the part of the signal which exhibits the best correlation with the soil dielectric properties, maximizing the signal-to-noise ratio and minimizing interference from reflections caused by shallow interfaces. To determine the uncertainty associated with the GPR envelope amplitudes, we acquired a total of 1000 traces for each measurement.…”

“…The TDR-derived average permittivity was also compared with the GPR-derived average permittivity, which was determined from the two-way travel (TWT) time according to standard procedures. Specifically, to extract the permittivity information from the GPR data, we have calculated the cross-correlation function between the direct and reflected wavelet envelopes [2,4], determining a permittivity average value of ε r_GPR =29±2.…”

“…It has recently been demonstrated [1,2] that the 'earlytime' portion of the GPR (Ground Penetrating Radar) signal, consisting of complex superposition of the direct air and ground wave events, is dependent on the bulk electromagnetic properties of the shallow subsurface material. The early-time technique [3] is a new radar approach, which correlates the radar signal amplitude with shallow-soil dielectric properties.…”

“…So far, however, the early-time technique has been tested on GPR data acquired in a laboratory and in natural test sites [2,3] with the aim of evaluating the effect of permittivity, while the influence of conductivity has never been analysed.…”