| Summary: | 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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