On electric and magnetic galvanic distortion tensor decompositions

Abstract

The physics governing galvanic distortion of natural source electromagnetic induction measurements is reexamined beginning from first principles. The conditions under which a decomposition of measured magnetotelluric response tensors and magnetic transfer functions is applicable are described, and the form of the decomposition describing distortion of the electric and magnetic fields is derived directly from the integral equation defining the scattering of electric and magnetic fields by surface heterogeneities. The inclusion of magnetic field galvanic distortion leads to indeterminacy of the regional magnetotelluric response in the form of scaling by frequency‐dependent, complex factors controlled by two unknown real constants. This is a generalization of the well‐known static shift effect from electric field galvanic distortion and can in principal be removed if the magnitude and phase of the regional response are known at some frequency. Distortion of the magnetic transfer function is shown to be even more indeterminate, containing a term proportional to one of the regional magnetotelluric responses which is inseparably additive to the regional magnetic transfer function, as well as the complex scaling seen for magnetotellurics. A set of simultaneous nonlinear equations describing the full electric and magnetic field galvanic distortion decomposition of the magnetotelluric response tensor and magnetic transfer function is derived, and methods for their solution are described, including implementation of jackknife error estimates. The full magnetotelluric decomposition is applied to severely distorted data from the Canadian shield and seafloor data from the EMSLAB experiment. In both cases, magnetic field galvanic distortion is important at periods under a few thousand seconds. This suggests that greater attention to galvanic distortion of the magnetic field is needed during magnetotelluric surveys.

The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study

“…There are both magnetic and electric effects of the charges (see e.g. Chave & Smith 1994;Chave & Jones 1997;) but the magnetic effects drop off very quickly with increasing period for isolated anomalies.…”

“…It should be noted that the magnetic effects caused by galvanic distortion charges do affect Z, H and T (e.g. Chave & Smith 1994;Chave & Jones 1997), however, these effects rapidly decrease with increasing period due to the square root dependence of their sensitivity on frequency, and can effectively be ignored over broad frequency bands. This rapid decrease is true for pure galvanic distortion but not in other situations as for example in the case of presence of current channelling caused by thin-long structures (e.g.…”

Magnetotelluric imaging of the Society Islands hotspot

“…In particular, it is unlikely that the re-gional electric field remains uniform across the large heterogeneities observed in the Tahiti area or that the electric field at the inhomogeneity can be approximated by its value at the observation point. As shown by Chave and Smith [1994], these assumptions are implicit in the use of a tensor decomposition, and their violation will lead to poor tensor decomposition fits. In addition, tensor decomposition methods implicitly assume that self-induction is negligible, and that condition is not appropriate for much of the Tahiti data.…”

Electrical conductivity images of Quaternary faults and Tertiary detachments in the California Basin and Range

“…[60] Impedance transfer functions often exhibit distortions due to local heterogeneities, and there are several schemes to estimate and remove these effects [e.g, Groom and Bailey, 1989;Chave and Smith, 1994]. Several of our sites exhibit distortions, identified by the offset of apparent resistivities at the highest frequencies ( Figure A6).…”