DECOMPOSITION AND MODELING OF THE BC87 DATASET

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 x 50 km plutonic Nelson batholith. These distortion e ffects must be identified and removed, as far as possible, prior to in terpretation. A galvanic model of these distortions is shown to be val id 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 app arent resistivities to correct for the remnant local site ''static shi fts''. We model the distortion- and level-corrected data using both 2D forward trial-and-error fitting and Occam2 smooth inversion. The crus tal section of the resulting models correlates well with some geologic al features. However, the long period phase difference between the E- and B-polarization lasting for more than a decade is difficult to inte rpret without recourse to introduction of a layer with differing elect rical conductivity in the two orthogonal horizontal directions, i.e., an anisotropic layer. There is greater than an order of magnitude diff erence in the conductivities, with the higher value in conductivity be ing in a direction which is commensurate with the Juan de Fuca plate-p ush direction.