Orthogonality in CSAMT and MT measurements

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 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. 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. -from Authors