Chapter 16 Polarimetric borehole radar approach to fracture classification

In order to improve the ability of borehole radar, we propose a use of radar polarimetry technology. Scattering from subsurface fractures having a rough surface causes cross-polarized components of the radar field. A 3-D FDTD algorithm was used to simulate the scattering measurement by borehole radar and showed that the degree of depolarization is related to the surface roughness. In order to demonstrate the potential of polarimetric borehole radar, field measurement was carried out at the Mirror Lake fractured-rock research site (NH, USA). We observed many clear reflections from fractures in each polarization state. Even in the raw data, we could find the difference in the radar profile for different polarizations. Polarimetric features of the acquired radar signal were analyzed and it was found that the depolarization effect is independent in each fracture. We propose a technique of classifying fractures by using the energy of the scattering matrix measured by polarimetric borehole radar and show that this information could be used to evaluate water permeability of subsurface fractures.