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Magnetotellurics - Measurements and Modeling of Topography and Galvanic Distortion

机译：大地电磁学-地形和电流失真的测量和建模

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In typical volcanic-hosted geothermal settings, geophysicists have long noted that the measured magnetotelluric (MT) data include natural responses from topography and other "distortion" from near-surface resistivity variations, complicating the interpretation of MT responses from deeper features of exploration interest. Following earlier work considering the 3D response from rugged terrain we have analyzed galvanic distortion and the problem of separating the effects from geological structure in 3D inversion modeling. Direct inversion for distortion, which is mathematically represented by frequency-independent real-valued 2×2 matrices multiplying the undistorted impedances, reduces the structural impact from distorting structures in 3D inversion. The effectiveness of the method is demonstrated with synthetic examples and on a real dataset from the Darajat field. These also show that the recovery of the input distortion parameters depends significantly on the chosen model regularization controlling shallow model smoothness. Furthermore, a distortion term that is added to the objective function quantifies the estimates' deviation from a neutral identity matrix via a multiplicative parameter. This turns out to be a useful means to control the overall absolute level of estimates in real data inversion. We conclude that inversion for galvanic distortion is a valuable tool in addressing observed distortion effects in MT data, but is no golden bullet to fully isolate it, which remains impossible without further information. In a related exercise we quantified the consideration of finite electric dipole lengths from the point measurement assumptions in MT inversion modeling. Electric fields are measured across dipoles of typically 50m to 200m at MT soundings. Modeling algorithms, however, normally use point electric field values at the surface of single cells to calculate MT transfer functions. This is perfectly reasonable for the majority of cases, but there are situations with strong shallow variability of resistivity, where available detailed information might not be used optimally. We explore the consequences or this omission by quantifying the difference between point solutions and electric field integrations across dipoles in 3D forward calculations for selected cases. The topic ties closely with galvanic distortion and inversion for related parameters, lateral magnetic field variations, and the benefit of providing shallower constraints for the imaging of deeper targets. As a side product, the analysis led us to focus on the fields output from the 3D modeling, and we illustrate electric current systems through the cases analyzed. We observe that in the presence of strong topography and outcropping inhomogeneities, finite dipole solutions can differ considerably from point solutions, while over a variable regolith case the effect appears more contained.
机译：在典型的火山成因地热环境中，地球物理学家早已注意到，测得的大地电磁（MT）数据包括地形的自然响应和近地表电阻率变化的其他“失真”，这使得对MT响应的更深层次的勘探兴趣解释变得复杂。在较早的工作中考虑了崎terrain地形的3D响应之后，我们分析了电畸变以及在3D反演建模中将影响与地质结构分离的问题。失真的直接反演在数学上表示为与频率无关的实值2×2矩阵乘以未失真的阻抗，从而减少了3D反演中因扭曲结构而产生的结构影响。该方法的有效性通过综合示例和来自Darajat领域的真实数据集得到了证明。这些还表明，输入失真参数的恢复在很大程度上取决于控制浅模型平滑度的所选模型正则化。此外，添加到目标函数的失真项会通过乘法参数量化估计值与中性恒等矩阵的偏差。事实证明，这是控制实际数据反演中估算的总体绝对水平的有用方法。我们得出的结论是，电变形的反演是解决MT数据中观察到的变形影响的有价值的工具，但并不是完全隔离它的金钥匙，如果没有进一步的信息，这仍然是不可能的。在相关的练习中，我们从MT反演模型中的点测量假设中量化了有限电偶极子长度的考虑。在MT测深处跨50m至200m的偶极子测量电场。但是，建模算法通常使用单个单元格表面的点电场值来计算MT传递函数。在大多数情况下，这是完全合理的，但在某些情况下，电阻率的浅层波动性很强，因此可能无法最佳地利用可用的详细信息。我们通过在选定情况下的3D正向计算中量化点解与跨偶极子之间的电场积分之间的差异来探索后果或这种遗漏。该主题与相关参数的电流失真和反演，横向磁场变化以及为较深目标的成像提供较浅约束条件的好处紧密相关。作为副产品，分析使我们专注于3D建模输出的场，并通过分析的案例说明了电流系统。我们观察到，在强地形和露头不均匀性的存在下，有限的偶极子解可能与点解有很大的不同，而在可变重灰石的情况下，这种影响似乎更为明显。

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