Abstract: Many synthetic model studies suggested that the best way to obtain good 3D interpretation results is to distribute the MT sites at a 2D grid array with regular site spacing over the target area. However, MT 3D inversion was very difficult about 10 years ago. A lot of MT data were collected along one profile and then interpreted with 2D inversion. How to apply the state-of-the-art 3D inversion technique to interpret the accumulated mass MT profiles data is an important topic. Some studies on 3D inversion of measured MT profile data suggested that 2D inversions usually had higher resolution for the subsurface than 3D inversions. Meanwhile, they often made their interpretation based on 2D inversion results, and 3D inversion results were only used to evaluate whether the overall resistivity structures were correct. Some researchers thought that 3D inversions could not resolute the local structure well, while 2D inversion results could agree with the surface geologic features much well and interpret the geologic structures easily. But in the present paper, we find that the result of 3D inversion is better than that of 2D inversion in identifying the location of the two local faults, the Shade Fault(SDF)and the Yunongxi Fault(YNXF), and the deep structures.
In this paper, we first studied the electrical structure of SDF and YNXF based on a measured magnetotelluric(MT) profile data. Besides, from the point of identifying active faults, we compared the capacity of identifying deep existing faults between 2D inversion models and 3D models with different inversion parameters. The results show that both 2D and 3D inversion of the single-profile data could obtain reasonable and reliable electrical structures on a regional scale. Combining 2D and 3D models, and according to our present data, we find that both SDF and YNXF probably have cut completely the high resistivity layer in the upper crust and extended to the high conductivity layer in the middle crust. In terms of the deep geometry of the faults, at the profile's location, the SDF dips nearly vertically or dips southeast with high dip angle, and the YNXF dips southeast at depth. In addition, according to the results from our measured MT profile, we find that the 3D inversion of single-profile MT data has the capacity of identifying the location and deep geometry of local faults under present computing ability. Finally, this research suggests that appropriate cell size and reasonable smoothing parameters are important factors for the 3D inversion of single-profile MT data, more specifically, too coarse meshes or too large smoothing parameters on horizontal direction of 3D inversion may result in low resolution of 3D inversions that cannot identify the structure of faults. While, for vertical mesh size and data error thresholds, they have limited effect on identifying shallow tectonics as long as their changes are within a reasonable range. 3D inversion results also indicate that, to some extent, adding tippers to the 3D inversion of a MT profile can improve the model's constraint on the deep geometry of the outcropped faults.

Key words: single-profile magnetotelluric data, 3D inversion, faults identification, Yunongxi Fault, Shade Fault

摘要： 文中基于实测的单剖面大地电磁数据，研究了沙德和玉农希断裂的深部电性结构，并从识别浅部活动断裂的角度出发，对比了二维模型及不同反演参数下单剖面三维反演模型对已有断裂构造深部结构的分辨能力。研究表明，单剖面数据的二维反演模型与三维模型均能获得合理可靠的区域深部结构特征。综合二维和三维解释模型，在现有的观测数据条件下，文中分析发现沙德断裂和玉农希断裂可能均切割上地壳高阻层，并向深部延伸到中地壳高导层。玉农希断裂在剖面位置近自立（或高角度倾向SE）从地表向深部延伸，沙德断裂的深部倾向为SE。此外，文中的实测资料结果表明，在现有计算能力条件下，三维单剖面反演模型也具有分辨局部断裂构造位置和深部几何结构的能力。最后，文中的工作反映了合适的水平网格间距和模型光滑参数对三维单剖面反演模型结果具有重要影响，过粗的水平网格尺寸和过大的模型水平光滑参数可能会导致三维单剖面反演模型无法正确分辨出浅部断裂构造。而在合理区间调整垂向网格的大小和数据误差门槛，可能对三维单剖面反演模型分辨浅部断裂构造的影响较小。带入倾子的三维单剖面反演能够进一步改善模型对出露地表的断裂构造深部几何结构的约束。

关键词: 大地电磁单剖面数据, 三维反演, 断裂构造识别, 玉农希断裂, 沙德断裂