Decomposition and Modelling of the BC87 Dataset

Abstract

The BC87 data exhibit distortions due to three-dimensional structures at almost all scale sizes from that of the electrode array (<100 m) to that of the 150×50 km plutonic Nelson batholith. These distortion effects must be identified and removed, as far as possible, prior to interpretation. A galvanic model of these distortions is shown to be valid for most of the frequency range of observation, but 3D induction is significant at certain sites for some bands. A first-order regression was fit to the decomposition recovered long period E-polarization apparent resistivities to correct for the remnant local site “static shifts”. We model the distortion- and level-corrected data using both 2D forward trial-and-error fitting and Occam2 smooth inversion. The crustal section of the resulting models correlates well with some geological features. However, the long period phase difference between the E- and B-polarization lasting for more than a decade is difficult to interpret without recourse to introduction of a layer with differing electrical conductivity in the two orthogonal horizontal directions, i.e., an anisotropic layer. There is greater than an order of magnitude difference in the conductivities, with the higher value in conductivity being in a direction which is commensurate with the Juan de Fuca plate-push direction. © 1993, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.