Characterization of Subsurface Cracks with Polarimetric Borehole Radar

(极化钻孔雷达系统)我们开发了一种能够在钻孔中进行雷达极化测量的雷达系统。该系统采用偶极子天线和缝隙天线相结合的方式来控制发射机和接收机的极化。极化雷达测量的目的包括表征异常的地质性质，如地下裂缝的粗糙度和地质边界。(野外测量)讨论了1995年神户地震天然断层调查时在Awaji岛的一个钻孔中进行的现场实验结果。在该测量中，用两种不同的天线组合在同一深度测量了共极化反射和交叉极化反射。该材料为花岗岩，具有频繁的破碎带。引起神户地震的野岛断层在这个井下1500米处被观测到。极化雷达测量是在1000-1052米之间进行的。雷达剖面与电测井、超声波成像(BHTV)等数据吻合较好。(雷达偏振仪)还讨论了反射率随偏振的变化。单个裂缝的反射信号是单向极化的，而裂隙带的反射信号是随机极化的。这些信息不能用传统的雷达测量获得，将成为表征亚表面裂纹的重要工具。

(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.