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

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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