Invariant TE and TM magnetotelluric impedances: application to the BC87 dataset

Abstract

Abstract Invariants under rotation of the magnetotelluric impedance tensor relax the directional requirements when fitting data using two-dimensional (2D) models. This happens, for instance, when using the familiar impedance derived from the determinant of the tensor. In this work, we use two particular invariants that reduce in 2D to the traditional transverse electric (TE) and transverse magnetic (TM) impedances. They are obtained as the missing first link of an infinite chain of invariants that are computed iteratively without any reference to a strike direction. We challenge the current view that the TE and TM impedances are not invariant. We argue in favor of the invariance. We apply our approach to the well-known and difficult BC87 dataset. These data present (1) electro-galvanic distortions, (2) static shifts and (3) inconsistent strikes. To remove the electro-galvanic distortions, we use recent developments that combine the phase tensor with the quadratic equation. To handle statics, we use a mimic of the electromagnetic array profiling (EMAP) method; the property of the TE mode of acting as a natural filter for static effects, as reported recently in the literature. Finally, the multiple strikes are handled by the invariant character of the TE and TM impedances. The processed TE responses are interpreted as one-dimensional models to construct a 2D resistivity image of the subsurface, which is accompanied by the complementary image of the multiple strikes. Our model correlates very well with an independent EMAP survey over the Nelson Batholith and with seismic reflection and refraction lines.

Keywords

• Invariant TE and TM
• MT strike
• BC87 dataset