Analysis of magnetotelluric data along the Lithoprobe seismic line 21 in the Blake River Group, Abitibi, Canada

Abstract

Magnetotelluric sounding data have been collected along the Lithoprobe seismic line 21 as a complement to seismic reflection in order to better constrain the structure of the Blake River Group and of the underlying crust. The presence of shallow conductive heterogeneities in an otherwise highly resistive environment typical of the Canadian Shield generates large galvanic distortion that affects the estimates of the impedance tensor. Distortion analysis and tensor decomposition techniques show that MT data support a model represented by a 2D regional model over a 2D deep structure with two different strikes direction and distorted by shallow heterogeneities. The residual static shift affecting each mode for each sounding site is determined using a combination of techniques: shifting the apparent resistivity curves to a reference curve obtained by the authors in the region using 100 km long dipoles, a shallow resistivity estimate using TDEM sounding, 2D minimum structure inversion with free static shift factors. The resulting model does not satisfy both TE and TM modes. A better fit is found by trial and error using a 2D anisotropic model. The Blake River Group is represented as an highly anisotropic resistive layer with a possible decrease in thickness from northwest to southeast. The middle crust is moderately conductive and isotropic. The lower crust and upper mantle is found to be anisotropic and conductive. The geometry of the Blake River Group obtained from the MT data is in agreement with previous gravity and seismic studies.

Modeling temporal variations of electrical resistivity associated with pore pressure change in a kilometer‐scale natural system

“…In particular, galvanic distortion of crustal origin can affect long‐period MT data, and thus their interpretation in terms of the Earth's mantle conductivity [ Menvielle and Tarits , 1994]. In this context, galvanic distortion can be particularly strong when the lower crust is very resistive [ Madden et al , 1993; Tournerie and Chouteau , 2002], and for sea‐floor MT data because of the very conductive ocean [ White et al , 1997; Nolasco et al , 1998].…”