We propose a new three-dimensional anisotropic inversion scheme for magnetotelluric (MT) data. In this method, the earth is discretized into unstructured tetrahedral grids that can fit complex structures well, such as the earth topography and coastline. We use a 3 × 3 tensor to describe the anisotropic conductivity in the governing equation for MT inversions that can be simplified to three principal conductivities and three rotation angles. The edge-based finite-element (FE) method is adopted to do the forward and adjoint modeling to compute full impedance and sensitivity. To make the inversion flexible in the restoration of anisotropic and isotropic structures tendentiously, we add an additional regularization in the objective function, with its weight controlled by a distribution function. Before solving the inverse problem using L-BFGS optimization, we add upper-bound and lower-bound constraints to the parameters to suppress the solution space. Four synthetic experiments are designed to test the resolvability and resolution for the six anisotropic parameters in the inversion. The results show that our inversion method can well recover the resistivity in the principal x and y directions and the strike angle γ. Finally, we apply our inversion algorithm to the USArray MT data with the aim to reveal anisotropic structure under Oregon State.

中文翻译：

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使用非结构化四面体离散化的任意各向异性地球三维大地电磁反演

我们为大地电磁（MT）数据提出了一种新的三维各向异性反演方案。在这种方法中，地球被离散成非结构化的四面体网格，可以很好地拟合复杂的结构，例如地球地形和海岸线。我们使用 3 × 3 张量来描述 MT 反演的控制方程中的各向异性电导率，可以简化为三个主要电导率和三个旋转角。采用基于边缘的有限元（FE）方法进行正向和伴随建模以计算全阻抗和灵敏度。为了使反演在各向异性和各向同性结构的恢复中趋于灵活，我们在目标函数中添加了一个额外的正则化，其权重由分布函数控制。在使用 L-BFGS 优化求解逆问题之前，我们对参数添加上限和下限约束以抑制解空间。设计了四个综合实验来测试反演中六个各向异性参数的可分辨性和分辨率。结果表明，我们的反演方法可以很好地恢复主体电阻率。x和y方向以及冲击角γ。最后，我们将我们的反演算法应用于 USArray MT 数据，旨在揭示俄勒冈州下的各向异性结构。