Ground Penetrating Radar
Abstract: Using a combination of laboratory measurements and modeling results, we demonstrate the potential of distinguishing two dispersive materials by estimating quality factor (Q*) using radar signals at two different frequencies. Here, we report on new complex dielectric permittivity measurements of a pulp sample mainly composed of pyrite (25%) and quartz (55%) from a massive sulphide mine, which shows frequency-dependent permittivity, and of a calcium-rich montmorillonite sample (STx-1b) for comparison. We made these measurements using the coaxial transmission line technique. To understand the dispersion observed in both samples, we fitted the measured complex permittivities using the Cole-Cole model to obtain the relaxation times that best represent the dielectric losses. We chose montmorillonite as the “control” material because it readily absorbs water, which has well-known dielectric relaxation mechanisms, thus providing a means of testing whether the pulp sample relaxations could be distinguished from those caused by adsorbed water. Our inverted montmorillonite relaxation times show one interlayer-water relaxation and one free water relaxation, as expected for this clay structure. By contrast, the pyrite-quartz sample shows intrinsic dispersion that is independent of the influence of water. The measurements show that the two materials have opposite concavity in the attenuation v.s. frequency plot, which can be detected using Q* in principle. Using these results, we conducted a series of 3D Finite-Difference-Time-Domain (FDTD) simulations in a cross-hole setup to explore the effects of the observed dispersion on material detectability. We show that it is possible to distinguish intrinsically dispersive materials from those that are simply wet.
Keywords: Ground Penetrating Radar (GPR); Dispersion; Complex permittivity measurement; Spectral decomposition; Quality factor.
Ground penetrating radar (GPR) is a nondestructive measurement technique which uses the transmission or reflection of electromagnetic (EM) waves to locate targets, anomalies or interfaces beneath or within natural or artificial surfaces. One basic assumption of GPR surveys is that subsurface features return reflections that are replicas of the transmitted signal with lower amplitudes. This implies that electrical properties of materials are independent of frequency within the frequency range of GPR, which is often referred to as the “GPR plateau” . While this assumption holds true for most materials in the frequency band of GPR operation, some materials, especially materials that contain water, have frequency-dependent dielectric permittivities. As the complex permittivity varies with frequency, both the velocity and attenuation of the EM waves also change. This type of dispersion is categorized as physical property dispersion . Scattering from heterogeneities in the subsurface can cause frequency-dependent attenuation as well. The attenuation measured in the field is mainly the combination of intrinsic dispersion and scattering dispersion, and it is difficult to isolate the two. There are several parameters used to characterize frequency-dependent attenuation. Turner and Siggins  show that, similar to seismic wave analysis , we can use a constant Q* parameter to characterize materials with frequency-dependent attenuation in GPR surveys. Bradford  defines a more general dispersion parameter D that includes all frequency-dependent attenuation. One of the methods to extract Q* from radar signals is the spectral shift method . As the signal propagates, the peak frequency shifts lower from the original source value, and the difference can be used to estimate Q* of the material.
In previous studies, water content was reported to be the major cause of dispersion. Therefore materials containing variable amounts of water, such as clay [7–10] and concrete [11–13] are of major interest in the study of frequency-dependent attenuation of radar waves. Our goal for this project was to identify a dispersion behaviour that is independent of the influence of water and determine whether the difference can be identified from radar signals . Since clay is a typical soil material that can absorb a large amount of water, we chose a montmorillonite sample as a reference material to characterize the influence of water on its dielectric properties. We measured the complex permittivity of the montmorillonite at varying moisture levels, then fitted the data with a multi-pole Cole-Cole model to obtain the dielectric relaxations responsible for the dispersion. We also fitted the measurements of a pulp sample that is mainly composed of pyrite and quartz from the LaRonde massive sulphide mine that shows a different dispersive behaviour from the montmorillonite sample. Then, we show with numerical simulations that it is possible to distinguish between these two types of dispersive behaviours by comparing the Q* values at two frequencies. We believe that this technique expands the potential application of radar signals in material characterization.
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