| Summary: | The presence of fractures, joints and karst features within rock strongly influence
the hydraulic and mechanical behavior of a rock mass, and there is a strong desire to
characterize these features in a noninvasive manner, such as by using ground penetrating
radar (GPR). These features can alter the incident waveform and polarization of the
GPR signal depending on the aperture, fill and orientation of the features. The GPR
methods developed here focus on changes in waveform, polarization or texture that can
improve the detection and discrimination of these features within rock bodies. These
new methods are utilized to better understand the interaction of an invasive shrub,
Juniperus ashei, with subsurface flow conduits at an ecohydrologic experimentation plot
situated on the limestone of the Edwards Aquifer, central Texas.
First, a coherency algorithm is developed for polarimetric GPR that uses the largest
eigenvalue of a scattering matrix in the calculation of coherence. This coherency is
sensitive to waveshape and unbiased by the polarization of the GPR antennas, and it
shows improvement over scalar coherency in detection of possible conduits in the plot
data. Second, a method is described for full-waveform inversion of transmission data to
quantitatively determine fracture aperture and electromagnetic properties of the fill,
based on a thin-layer model. This inversion method is validated on synthetic data, and
the results from field data at the experimentation plot show consistency with the
reflection data. Finally, growing hierarchical self-organizing maps (GHSOM) are
applied to the GPR data to discover new patterns indicative of subsurface features, without representative examples. The GHSOMs are able to distinguish patterns
indicating soil filled cavities within the limestone.
Using these methods, locations of soil filled cavities and the dominant flow
conduits were indentified. This information helps to reconcile previous hydrologic
experiments conducted at the site. Additionally, the GPR and hydrologic experiments
suggests that Juniperus ashei significantly impacts infiltration by redirecting flow
towards its roots occupying conduits and soil bodies within the rock. This research
demonstrates that GPR provides a noninvasive tool that can improve future subsurface
experimentation.
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