GPR and the effects of conductivity

Abstract

Recent research has shown that polarisation currents are significant in transient electromagnetic surveys, despite the high loss tangents of geological materials at the frequencies used. In a similar way, the presence of conductivity can significantly affect the results of Ground Penetrating Radar (GPR) surveys, despite the low loss tangents of rocks at radar frequencies. It has long been recognised that the effective range of GPR is strongly dependent on conductivity and that conductivity increases with frequency. However, the effect of conductivity on other aspects of GPR theory have often been overlooked to allow simpler lossless models to be used. In many instances lossless models are inadequate to describe observed behaviour. This paper shows examples of how conductivity, and in particular frequency-dependent conductivity, affects antenna radiation patterns, wavelet propagation and reflection, and illustrates how knowledge of these effects can be used to improve data acquisition, processing and interpretation. The strong lobal structure of the subsurface radiation pattern of dipole antennas over a lossless ground largely disappears when conductivity is introduced. The null which is present along the surface is also strongly diminished. In contrast to the lossless theory, this explains why dipole antennas can be so sensitive to objects on the surface and are particularly sensitive over conductive ground. The increase in conductivity with frequency not only affects radar range as a function of frequency, but also causes the shape of a GPR pulse to change as it propagates through the ground. This change causes a deterioration in the resolution of radar images with depth and can make the detection of individual reflections difficult. The distortion of GPR pulses can be well described by a model similar to the constant-Q model used in seismic studies but the distortion is usually far more severe in GPR. It is possible to remove the distortion by applying a propagation deconvolution which repairs the effects of frequency-dependent attenuation. The reflection of GPR waves is also strongly dependent on the conductive properties of the materials on either side of the reflecting interface. Even at normal incidence, substantial phase shifts can occur on reflection where either of the materials has finite conductivity. This phase shift causes further distortion of the GPR pulse which, if not recognised, can lead to mis-interpretations of the data. More importantly though, if the phase shift can be recognised, it can be used to provide an indication of the nature of the target as well as its geometry. This information can be further expanded by observing the amplitude-with-offset and phase-with-offset behaviour of the reflections.