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Occam2: Regularized inversion to generate smooth, two-dimensional resistivity models from magnetotelluric data.

机译：Occam2：正则化反演，可从大地电磁数据生成平滑的二维电阻率模型。

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摘要

The magnetotelluric inverse problem is a nonlinear problem which consists of finding a resistivity (or conductivity) model to fit a discrete set of noisy observations of the ratios of the electric and magnetic fields at the Earth's surface.;The major focus of this dissertation concerns the development of a regularized inversion method (Occam2) to solve for smooth two-dimensional resistivity models fitting magnetotelluric data. The inversion algorithm, which optimizes the step size and direction at each iteration, proves to be a robust, efficient means of model construction. It is shown that the regularization technique is flexible in allowing a priori information in the model.;Smooth regularized inversion is applied to the problem of determining the static shifts at each measurement site, by solving jointly for static shifts and resistivities. Static shift is defined as a uniform bias in the apparent resistivities over a broad frequency band. A regularization term penalizing roughness is applied to the resistivities, while the static shift parameters act upon the data. The solution yields the static shifts corresponding to the smoothest possible resistivity structure. Recovered static shifts agree well with the known values.;The algorithms are applied to magnetotelluric observations over the North American Central Plains (NACP) anomaly in Saskatchewan, Canada. The flexibility of smooth regularization is exploited to deal with special situations presented by the data, such as the approximate one-dimensionality of the near-surface resistivity structure. It is found that the NACP structure is not truly 2-D and that the data are affected by either off-axis structure or the effects of anisotropy.;Finally, it is noted that the determination of the electromagnetic fields necessary to form the model responses does not constitute an independent problem from that of determining the model conductivities. Therefore, an alternative method of inversion, in which one solves simultaneously for the EM fields and conductivities satisfying the observations and the EM induction equations, is presented and compared with the Occam2 inversion method. It is found the Occam inversion method, which interleaves forward calculations and changes to the conductivity model, yields a solution more quickly.
机译：大地电磁反问题是一个非线性问题，它包括找到一个电阻率（或电导率）模型来拟合一组离散的对地球表面电场和磁场比率的嘈杂观测结果。正则化反演方法（Occam2）的开发，以解决适合大地电磁数据的平滑二维电阻率模型。可以在每次迭代中优化步长和方向的反演算法被证明是一种强大，有效的模型构建方法。结果表明，该正则化技术在允许模型中具有先验信息方面具有灵活性。平滑正则化反演应用于通过共同求解静态位移和电阻率来确定每个测量位置的静态位移的问题。静态位移定义为在宽频带内视电阻率的均匀偏差。将惩罚粗糙度的正则化项应用于电阻率，而静态位移参数作用于数据。该解产生对应于最平滑的电阻率结构的静态位移。恢复的静态位移与已知值非常吻合。该算法被应用于加拿大萨斯喀彻温省的北美中原（NACP）异常的大地电磁观测。利用平滑正则化的灵活性来处理数据所呈现的特殊情况，例如近表面电阻率结构的近似一维。发现NACP结构不是真正的二维，并且数据受轴外结构或各向异性的影响。最后，应注意，确定形成模型响应所必需的电磁场与确定模型电导率无关。因此，提出了另一种反演方法，其中同时求解满足观测值和EM感应方程的EM场和电导率，并将其与Occam2反演方法进行了比较。发现Occam反演方法可以交叉进行正向计算并更改为电导率模型，从而可以更快地得出结果。

著录项

  • 作者

    deGroot-Hedlin, Catherine Dorothy.;

  • 作者单位

    University of California, San Diego.;

  • 授予单位 University of California, San Diego.;
  • 学科 Geophysics.
  • 学位 Ph.D.
  • 年度 1991
  • 页码 126 p.
  • 总页数 126
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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