Principles of spatial surface electric field filtering in magnetotellurics: Electromagnetic array profiling (EMAP)

Abstract

Electromagnetic Array Profiling (EMAP) is an adaptation of magnetotellurics to overcome spatial aliasing effects associated with the sampling of the surface electric field. Undersampling lateral electric field variations can result in misleading geoelectric interpretations of the subsurface, particularly under the common presence of static distortion. In the EMAP field procedure, electric dipoles are positioned end‐to‐end along a continuous survey path; this configuration, in addition to reducing aliasing effects, lends itself to low‐pass filtering of the lateral electric field variations. We show that lengthening an electric dipole can reduce the static effect due to confined resistivity anomalies smaller than a dipole length. This modification of the sensor characteristics involves a spatial filtering process in which the cutoff wavenumber is inversely proportional to the length of the dipole. However, excessively long dipoles may not prove appropriate at high frequencies where the objective is to sense geoelectric features smaller than a dipole length. Thus, an array of adjacent dipoles along the survey path allows us to control the characteristics of the low‐pass filter employed to reduce static effects. A standard dipole length should be comparable with the shallowest depth of penetration at the highest frequency. The survey path, on the other hand, should be long enough to allow the suppression of static effects due to geoelectric structure of size comparable to the depth of penetration at the lowest frequency. Previously, we showed that a profile of the subsurface resistivity distribution can be estimated by inverting the inductive part of the surface electric field response derived from spatial filtering. For this purpose, the suggested wavenumber filter is one for which the cutoff wavenumber decreases with decreasing values of frequency, just in the same way the subsurface resistivity distribution is naturally averaged at depth to produce its inductive surface electric field response. Hence, the EMAP field procedure for the measurement of lateral electric field variations is suitable for an inversion of this type. We introduce a data‐adaptive spatial filtering technique that responds to changes in local average resistivity at a given frequency to estimate both effective depth of response and resistivity along the survey path. Application of this filtering procedure shows encouraging results in the interpretation of data simulated numerically from two‐dimensional models possessing different degrees of structural complexity. We also present results from a field study carried out in the northern basin and range geological province of Nevada.