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Estimation and correction of wavelet dispersion in ground penetrating radar data Irving, James D.

Abstract

The attenuation of electromagnetic (EM) waves in many geological materials is strongly dependent upon frequency in the ground penetrating radar (GPR) range; high frequencies are attenuated much more quickly than lower ones during propagation. For this reason, the GPR wavelet often undergoes a significant change in shape as it travels through the subsurface, and reflections received at later times contain less high frequency information than those received at earlier times. This phenomenon is known as wavelet dispersion. In the GPR image, it is displayed as a characteristic "blurriness" that increases with depth. Correcting for wavelet dispersion in GPR data is an important signal processing step that should be performed before either qualitative interpretation or quantitative determination of subsurface electrical properties are attempted. Previous work by other researchers has shown that the EM wave attenuation parameter for many geological materials is approximately linear with frequency over the bandwidth of a GPR wavelet. Thus, the change in shape of a GPR pulse as it propagates can often be well described using one parameter, Q*, which is related to the slope of the linear region. In this thesis, we confirm and build on these results. Assuming that all subsurface materials can be characterized by some Q* value, the problem of estimating and correcting for wavelet dispersion in GPR data becomes one of determining Q* in the subsurface and deconvolving its effects through the use of an inverse Q filter. A method for the estimation of subsurface Q* from GPR data based on a technique developed for seismic attenuation tomography is presented. Essentially, Q* is determined from the downshift in the dominant frequency of the GPR wavelet with time down a trace. Once Q* has been obtained, an inverse Q filtering technique based on a causal, linear, model for constant Q wave propagation is proposed as a means of removing wavelet dispersion. Tests on field data collected near Langley, British Columbia indicate that these methods are very effective at enhancing the resolution of the GPR image.

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