The telluric‐magnetotelluric method in two‐ and three‐dimensional environments

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Abstract
The assumption of spatial uniformity of the horizontal magnetic field, which is an implicit assumption made in straightforward applications of the telluric‐magnetotelluric (T-MT) method, is not always valid near conductivity inhomogeneities. For a two‐dimensional (2-D) case, the transverse electric mode horizontal magnetic field may vary more than a factor of three. The spatial variation of the horizontal magnetic field is not as great over three‐dimensional (3-D) inhomogeneities, but it may still contribute significantly to impedance magnitude and phase over shallow inhomogeneities at higher frequencies. Spatial variation of the horizontal magnetic field can cause T-MT impedances to differ significantly from magnetotelluric (MT) impedances. Consequently, MT modeling of T-MT field data could result in a misleading interpretation of conductivity structure. To avoid erroneous interpretation, numerical modeling programs should calculate actual T-MT responses, rather than MT responses, for comparison with T-MT data. Resolution of a conductivity structure which produces significant spatial variation of the magnetic field is less with T-MT data than with an equal number of MT data. However, the increased number of data obtained from a T-MT survey may offset this shortcoming if the data are properly interpreted.

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