Development of a New Electromagnetic Tool for Evaluation of Ground-water Environment

电磁法是目前土木工程测量中用来补充地震法的一种方法。用电磁法测量基岩的电导率，可以在地表不接触电的情况下进行。电磁法测量电阻率的理论和实验研究在现有文献中还没有很好的报道，本文阐述了聚焦技术的原理，并介绍了一种新的电磁法测量电阻率的方法，即双发射线圈聚焦法。聚焦技术最初是为感应测井而开发的，其中测量必须在高导电环境中进行，并且对周围环境的探测对传感器周围填充的水和泥浆不太敏感。利用双线圈或多线圈技术可以大大改善这种情况，这种技术不仅在井下，而且在地面，特别是在民用建筑的测量中得到了应用。本文通过数值试验研究了响应与地下电导率之间关系的主要特征。数值试验结果表明，该方法可以消除近地表非均匀性的影响，并开发了电阻率测量的比例尺模拟系统。物理模拟的结果验证了数值实验的正确性。此外，研制了电阻率原位测量测试系统，并用于电阻率原位测量。结果表明，与模拟结果相比，测量结果在其合理范围内。但由于制度建设的不完善，还存在一些问题，有待进一步完善。

The Electromagnatic (EM) method is presently applied in civil engineering surverys to compliment the seismic method. With the EM method, we can measure the conductivity of base rock without electrical contact at the surface. Theoretical and exprimental studies of EM tools for in-situ resistivity measurement have not been well covered in available literature.This study explains the principles of focusing techniques, which have been applied in the method, and shows the effectiveness of a new EM tool with double transmitter loops for focussing. Focussing techniques were originally developed for induction logging where the measurements have to be carried out in a high conductive environment and the detective of the surroundings are less sensitive by the water and mud filled around the sensor. With the usage of two or more coil technique this situation can be greatly improved.Now, this technique has found its application in the surveys not only under borehole but also on the surface, especially in the surveys for civil construction.Several numerical experiments were carried out to examine the main features of the relation between the response and the underground conductivity. The results of several numerical experiments have shown the effect of near surface inhomogeneity can be canceled by using this new tool.And the scale modeling system for resistivity measurement has been developed. The results of physical modeling proved the truth of numerical experiments.Furthermore, the test system for in-situ resistivity measurement has been developed, and used for in-situ resistivity measurements. The results show that the measurements are in their resonable range compared to the simulation results. But problems still remain due to imperfect system's construction and further improvement is expected.