Micro Polarimetric Borehole Radar for Subsurface Fracture Measurement

极化钻孔雷达在探测和评价雷达目标方面具有很好的潜力，其探测和评价的目标超出了由波长决定的雷达分辨率。这对于钻孔雷达来说是一个很大的优势，钻孔雷达通常用于有损耗的地下材料中。本课题研制了一种工作频率在200 MHz以下、可安装在直径小于5 cm的钻孔中的极化钻孔雷达系统，并研制了一种安装在直径为2 cm的圆柱形套管中的光学模拟信号链路，以实现井下探头的薄型化。研制完成的极化钻孔雷达系统直径为41 mm，通过现场测试，证明该系统能够采集到极化雷达信号。同时，我们发现，由于钻孔直径远小于钻孔直径，钻孔雷达接收到的信号不稳定，需要额外的设备将井下探头放置在钻孔的中心位置，我们在美国的镜湖、德国的Asse和日本的Hirabayashi等地进行了极化钻孔雷达测量。我们表明，全极化钻孔雷达剖面可用于地下裂缝的分类。在一定条件下，地下裂隙的表面粗糙度与地下裂隙的透水性有关，极化钻孔雷达可以评价地下裂隙的透水性，利用跨孔钻孔雷达测量，提出了一种基于偏移的成像技术。该技术可以实现比常规层析成像分析更高的分辨率。

Polarimetric borehole radar has a good potential for detecting and evaluating radar targets beyond the radar resolution determined by the wavelength. This is a great advantage for borehole radar, which is often used I lossy subsurface material. In this research project, we have developed a polarimetric borehole radar system which operates under 200MHz, and can be installed in boreholes having a diameter less than 5cm.In order to have a thin down hole sonde, we have developed a optical analog signal link which is equipped in a cylindrical casing of 2cm in diameter. The completed polarimetric borehole radar system has a diameter of 41 mm. This system was evaluated in field test and could show it can acquire polarimetric radar signal. At the same time, we found that the signal acquired by borehole radar, whose diameter is much smaller than that of a borehole is unstable, and we need additional equipment to set the down hole sonde at the center of the borehole.Polarimetric borehole radar measurements were carried out in several test sites including Mirror Lake, USA, Asse, Germany and Hirabayashi, Japan. We showed that he full-polarimetric borehole radar profiles can be used for classification of subsurface fractures. Under certain conditions, we found that the water permeability is related to the surface roughness of the subsurface fractures, and the polarimetric borehole radar can evaluate them.By using the cross-hole borehole radar measurement, we proposed a imaging technique based on migration. This technique can achieve higher resolution that the conventional tomography analysis.