Characterization of Subsurface Cracks with Polarimetric Borehole Radar

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(Polarimetric Borehole Radar Sstem)We developed borehole a radar system which is capable of radar polarimetric measurement in boreholes. The system uses the combination of dipole antennas and slot antennas to control the transmitter and the receiver polarimetry. The aim of the polarimetric radar measurement includes characterization of geological properties of anomalies such as the roughness of the subsurface cracks and geological boundaries.(Field Measurement)Field experimental results in a borehole in Awaji Island, which was drilled to investigate the natural fault caused the Kobe earthquake in 1995 was discussed. In this measurement, co-polarization reflection and cross-polarization reflection were measured at the same depth with two different combinations of antennas. The material is granite and has frequent fractured zones. The Nojima fault, which caused the Kobe earth quake was observed below 1500m in this borehole. The polarimetric radar measurement was carried out in the section between 1000 and 1052m. The radar profile corresponds well to the other data such as electrical logging and BHTV (Ultra sonic imaging).(Radar Polarimetry)The difference of reflectivity depending on the polarization was also observed in this measurement was discussed. Reflected signal from single fractures is polarized in one direction, however, those from fractured zones are randomly polarized. Theses information could not be obtained with conventional radar measurement and will be a important tool for characterization of subsurface cracks.