SUMMARY

The magnetotelluric (MT) impedance tensor has a nil diagonal when one of the axes of the coordinate system coincides with the strike of a 2-D structure. In general, real data are full tensors either because of 3-D effects or measurements not aligned to the geological strike. The usual practice to adapt the field tensor to the 2-D assumption is to rotate to a new system of coordinates. In most cases, there is no single angle of rotation that warranties that the diagonal elements become zeros as in the ideal 2-D case. Even maximizing the off-diagonal elements does not necessarily produce a nil diagonal. Consequently, the 2-D inversions proceed by neglecting whatever there is left in the diagonals. In this work, we explore an alternative that places no constraints on direction but assures a nil diagonal. We use two rotational invariants that compact the four elements of the tensor into only two and reduce in 2-D to the TE and TM impedances. These are obtained readily by solving a quadratic equation. We explore four different scenarios: (1) using the invariants, (2) rotating the tensor perpendicular to the profile, (3) rotating to the average maximum orientation for each station and (4) maximizing the off-diagonal elements of the tensor for each site, frequency to frequency. These approaches were applied to 3-D synthetic and field data. The field data correspond to two marine magnetotelluric surveys in the Gulf of California. In one of them, there is no information on the instrument orientation because the compasses failed. In this case, the rotational invariants come handy to overcome the problem. In the other survey, there was orientation information and the 2-D inversions illustrate the better performance of the invariants relative to the traditional approaches.

中文翻译：

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海洋大地电磁法中不变的TE和TM阻抗

概要

当坐标系的轴之一与2-D结构的走向重合时，大地电磁（MT）阻抗张量为零对角线。通常，由于3-D效应或测量结果与地质走向不一致，真实数据为全张量。使场张量适应二维假设的通常做法是旋转到新的坐标系。在大多数情况下，没有一个旋转角度可以保证对角线元素像理想的2D情况一样变为零。即使最大化非对角线元素也不一定会产生零对角线。因此，通过忽略对角线中剩下的任何东西来进行二维反演。在这项工作中，我们探索了一种替代方案，该方案对方向没有任何限制，但可以确保对角线为零。我们使用两个旋转不变量将张量的四个元素压缩为两个，并将二维阻抗减小到TE和TM阻抗。这些可以通过求解二次方程轻松获得。我们探索了四种不同的情况：（1）使用不变式；（2）垂直于轮廓旋转张量；（3）旋转到每个测站的平均最大方向；（4）最大化张量的非对角线元素。每个站点，频率到频率。这些方法已应用于3D合成和现场数据。现场数据对应于加利福尼亚湾的两次海洋大地电磁测量。在其中一种中，由于罗盘失效，因此没有关于仪器方向的信息。在这种情况下，旋转不变量变得很容易克服该问题。在另一项调查中