Development of a Downhole Tri-axial magnetic field measuring system and its application to Subsurface Fracture Characterization

该项目分三个阶段实施，具体如下：(1)进行了井下三轴磁场测量系统的硬件研制。该系统的基础是日本MITT的NEDO开发的井下仪器。在东北大学东八满台地质勘探田的实验场进行了空气和钻孔中的基础实验，以了解工具的特性。计算了理论工具响应，并检查了对周围材料电导率的方向性。(2)实验表明，同相和正交相分量的独立响应。研究了电导率和磁导率同时测井方法的理论探讨。该方法对薄层探测和地质边界的精确确定具有较高的灵敏度。(3)利用低频井下仪器对井间层析成像技术进行了试验。该系统采用2 MHz-32 MHz常规钻孔雷达和感应技术的中频。通过实验和理论分析，证明了井下三轴磁场测量系统可以同时进行电导率和磁导率测井，并可用于地下裂缝分类的表征。

This project was carried out in the three phases described below.(1) Hardware development of the downhole tri-axial magnetic field measuring system was carried out. The basis of the system is the downhole tool developed by the NEDO under MITT, Japan. Fundamental experiment in air and in a borehole at Tohoku University experiment site at Higashi Hachimantai Geothermal field was achieved for understanding the tool characteristics. Theoretical tool response was calculated and directivity to electrical conductivity of the surround material was examined.(2) Independent response of in-phase and quadrate - phase components was experimentally shown. Theoretical approach to the method of simultaneous electrical conductivity and magnetic permeability logging was studied. Also, we found this technique is sensitive to the thin layer detection and precise determination of geological boundary.(3) A Cross-hole tomography technique was tested by using a low frequency downhole tool. The system use 2MHz-32MHz, which are intermediate frequency of conventional borehole radar and induction technique. We could show that this technique can be used in relatively conducting material.Throughout this project, we showed experimentally and theoretically that the dowohole tri-axial magnetic field measuring system can be used for simultaneous electrical conductivity ad magnetic permeability logging, and that it is useful for characterization of subsurface fracture classification.