Orthogonality in CSAMT and MT measurements

Abstract

The electric and magnetic fields from a single plane‐wave source on a one dimensional (1-D) earth, or a plane‐wave source polarized parallel or perpendicular to strike on a two-dimensional (2-D) earth, are orthogonal. On a layered earth and in the far‐field of a controlled source, the electric and magnetic fields are also orthogonal. Therefore, orthogonality of E and H data is a necessary condition to justify the application of 1-D or 2-D modeling algorithms having a plane wave source. A strict criterion to prove orthogonality, and thus provide a rationale for the choice of interpretation methods, can be defined directly in terms of field data. However, field data acquired in the intermediate and near‐field of any electromagnetic (EM) source are generally not orthogonal, even on a plane‐layered earth. Representing these nonorthogonal data in an orthogonal coordinate system can be misleading, particularly for the minor axis components of the polarization ellipses. Nonorthogonality also arises because of 3-D scattering, with one common example being the electric field response of near surface structure. An example of field data illustrates the nonorthogonality in CSAMT measurements caused by the response of surficial geology. In these EM data, the angle between E and H is a sensitive indicator of geological contacts and faults. Quantitative analysis of these data can be performed with the assumptions of a “bulk” 1-D earth (i.e., orthogonal E and H in the far‐field) and purely galvanic scattering of the EM fields.