Polarimetric Borehole Radar System for Fracture Measurement

Radar polarimetry is a technology that overcomes the limitation between the radar resolution and the penetration depth of borehole radar. We have developed a stepped-frequency polarimetric borehole radar system. This is a polarimetric borehole radar system which measures the full-radar polarimetry in a borehole by changing the antenna arrangements. By using a network analyzer and an optical analog signal link, this system has a frequency bandwidth of 2–500MHz, which is suitable for two different antennas for radar polarimetry. We also propose a technique for polarimetric antenna calibration. In order to understand the potential of polarimetric borehole radar, field measurement were carried out at the Mirror Lake fractured-rock research site (NH, USA). The radar penetration depth from the borehole in the reflection measurement was over 10m, at the frequency range of 2–402MHz. We observed many clear reflections from fractures in each polarization status. Even in the raw data, we found the differences in the radar profile for different polarization status. We believe the polarimetric feature is closely related to the roughness of each fracture, and it is also related to the physical properties of the fracture such as water permeability.