Three Dimensional Electromagnetic Prospecting System for Deep Crust Structure

本研究的目的是建立一个适用于大地电磁法三维地下电阻率结构成像的测量系统。提出了基于“M参数”估计的鲁棒叠加法、基于数据矩阵奇异值分解的阻抗估计法和基于相位与阻抗之间的Hirbert变换的阻抗平滑法等数据处理方案。我们已经验证了这些技术的有效性。我们还开发了一种基于交错网格有限差分法的高精度三维电磁建模方案。我们试图改善大尺寸系数矩阵的条件，使其易于求解，并克服了该方案中高电阻率对比模型和地形模型的问题。对基于ABIC约束光滑最小二乘法的三维反演方案进行了修改，使其适用于实际的油田数据。我们在北海道北部Nakatonbetsu地区以网格形式分布的25个站点进行了MT调查，作为本研究开发的调查系统的示范研究。利用MTU5型宽带大地电磁测量机，获得了380 ~ 0.00055 Hz范围内40个频率的电场和磁场数据。为了获得低频数据，我们使用磁通门式磁强计进行了为期两个月的MT观测。虽然获得了较为合理的三维电阻率结构，但仍需进一步研究以提高深部分辨率

The purpose of this study was constructing an appropriate survey system to image three dimensional (3D) subsurface resisitivity structure using the magnetotelluric (MT) method.We developed the data processing scheme involving the robust stacking method based on the "M parameter" estimation, the impedance estimation method using Singular Value Decomposition of the data matrix and impedance smoothing method based on the Hirbert transformation between phase and impedance. We have verified the effectiveness of these techniques.We also developed a high accuracy 3D EM modeling scheme based on the staggered grid finite difference method. We tried to improve the condition of a large size coefficient matrix to easily solve and have overcome the problem of high resistivity contrast model and topography model in this scheme. The 3D inversion scheme based on the ABIC constrained smoothness least square method was modifying to apply a actual field data.We carried out MT survey at 25 sites distributed like a grid in the Nakatonbetsu area, Northern Hokkaido as a demonstrative research for the survey system developed in this research. We used MTU5 wide band MT survey machine and obtained electric and magnetic field data at 40 frequencies in the range of 380-0.00055 Hz. To obtain lower frequency data, we carried out MT observation using the fluxgate type magnetometer over two months. A reasonable 3D resistivity structure was imaged but the result suggested that we need further study to improve the resolution at deeper part