Polarimetric borehole radar application for characterizing subsurface structure

Polarimetric borehole radar experiment was carried out in 2000 in Korea. Two boreholes separated by 20m were used. The host rock is granite. The cavity is located at about 80m depth. Single-hole and cross-hole radar profiles were acquired. We could clearly detect a subsurface cavity filled with air in the raw data. They have shown that cross-hole signal shows "double-dip" attenuation caused by scattering from an air-filled cavity. Although it is a simple technique, we found that it is suitable for detection of subsurface anomaly. Then we checked the attenuation between two boreholes, and showed that we can detect anomalous zone by a ray-based technique. In order to have vertical 2-D image between the boreholes, we developed a reverse time migration technique. In this analysis, we could assume two horizontal layers having different velocities, and we could image the cavity. The location of the cavity could clearly be determined by these signal interpretation.