Subsurface fracture measurement with polarimetric borehole radar

Abstract

A full-polarimetric borehole radar system is presented with combinations of dipole antennas and axial slot antennas and is applied to subsurface fracture measurement. First, to determine a scattering matrix from measurements with antennas having different antenna transfer functions between orthogonal polarizations, the authors present an antenna compensation algorithm that is achieved by an inverse filtering method with the antenna transfer functions experimentally determined by crosshole measurements. The results of crosshole and single-hole measurements carried out in granite at the Kamaishi Mine are shown to verify the proposed method. In the crosshole measurement, the authors find that the used dipole and slot antennas have almost the same radiation pattern and frequency dependency from 50 to 120 MHz. The transmission matrices through the host rock are the unit matrix for most paths, while depolarization is observed in some sections. Reflections from subsurface fractures show significant depolarization in the polarimetric single-hole reflection measurements. Using the scattering matrix, it is evaluated that subsurface fractures have a randomly isotropic rough surface for wavelengths between 1 and 2 m on the rotation of polarization basis.