Three Dimensional Electromagnetic Prospecting System for Deep Crust Structure

为构建适合于大地电磁成像的地下三维电阻率结构测量系统，提出了基于M参数估计的稳健叠加方法、基于数据矩阵奇异值分解的阻抗估计方法和基于相位与阻抗Hirbert变换的阻抗平滑方法的数据处理方案。验证了这些技术的有效性，并提出了一种基于交错网格有限差分法的高精度三维有限元建模方法。我们试图改进系数矩阵的大尺寸条件，使之易于求解，克服了该方案中高阻比模型和地形模型的问题。对基于ABIC约束光滑度最小二乘法的三维反演方案进行了改进，使之适用于实际的野外数据，并在北海道北部Nakatonbetsu地区的25个网格状分布的站点进行了大地电磁测深，作为本研究开发的测量系统的示范研究。使用MTU5型宽带大地电磁测绘机，获得了380-0.00055赫兹范围内40个频率的电场和磁场数据。为了获得更低的频率数据，我们使用磁通门磁力仪进行了两个多月的大地电磁观测。成像得到了合理的三维电阻率结构，但结果表明，需要进一步研究以提高更深层次的分辨率。

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