The use of invariant impedances in magnetotelluric interpretation

Abstract

S U M M A R YIt is suggested that the use of 'invariant' impedances may avoid the possible erroneous interpretation of magnetotelluric data which can occur when 1-D modelling of the E-polarization response is carried out. The possibility of erroneous interpretation occurs in the vicinity of near-surface 3-D conductivity anomalies. The suggestion is tested on a numerical 3-D model first published by Wannamaker et al. (1984). A correct interpretation of the conductivity structure is obtained. However, over a shallow highly-conducting anomaly it is not possible to resolve deep structure.

Preliminary report on the Sanyi-Puli seismic zone conductivity anomaly and its correlation with velocity structure and seismicity in the Northwestern Taiwan

“…It can be shown numerically that, in the case where the large-scale conductivity structure is one-dimensional, the determinant response in the presence of near-surface heterogeneities has a phase unaffected by the distortions and an apparent resistivity that is purely statically shifted (e.g. figure 2 of Ingham, 1988; figures 2 and 3 of Pellerin and Hohmann, 1990).…”

“…3) used in this modeling are those calculated from the determinant impedance of Ranganayaki (1984). This parameter is invariant with rotation of the axial system and may be less sensitive to local three-dimensional structure (Ingham, 1988). It can be shown numerically that, in the case where the large-scale conductivity structure is one-dimensional, the determinant response in the presence of near-surface heterogeneities has a phase unaffected by the distortions and an apparent resistivity that is purely statically shifted (e.g.…”

Feasibility of using the magnetotelluric method for sub-basalt imaging at Kachchh, India

“…An average rotationally invariant impedance tensor ZGAV (Ranganayaki, 1984) was used for further 1-D modeling and inversion of the MT data. Similar approaches for 1-D inversions using rotational invariant tensors have been performed (Ingham, 1988;Harinarayana et al 2006). Using rotationally invariant M T d a t a , i n i t i a l m o d e l s w e r e constructed by the Occam linear inversion scheme (Constable et al, 1987).…”

Sub-basalt sediment imaging — The efficacy of magnetotellurics

“…2), a 1D modeling approach is adopted to derive the subsurface resistivity section. The rotationally invariant determinant average of the observed MT impedance tensor represents the subsurface resistivity variations in 1D earth situations and also works reasonably well in near 1D conditions (Ranganayaki, 1984;Ingham, 1988). In other situations also the determinant estimates offer useful results provided that the 2D/3D heterogeneities, if any, are sufficiently away from the site of investigation (Park and Livelybrooks, 1989).…”