## Electromagnetic images of a volcanic zone

### Abstract

Data from twenty MT soundings in the Coast belt of southwestern British Columbia, Canada, in the vicinity of volcanic Mounts Meager and Cayley (approximately 150 km north of Vancouver), are analysed to determine both the internal electrical conductivity structure of the Garibaldi volcanic belt and the regional conductivity structure. Galvanic distortion effects on the data are removed using Groom-Bailey tensor decomposition, and the regional 2D responses are determined in a survey-consistent reference coordinate frame for 1D and 2D modelling and inversion. The remaining unknowns, the site gain at each location, are estimated by requiring the long period asymptotes of the E-polarization $\rho_a$ curves to be the same, and alternatively are derived as part of the 2D inversion procedure.

Positive correlations are observed between an increased temperature gradient at a depth of about 200 m, a zone of enhanced electrical conductivity, and drilling results. The zone of enhanced conductivity is interpreted to be due to montmorillonite-dominated clay alteration minerals which form the cap rock of the geothermal reservoir. Below this cap, the zone is electrically resistive due to the chlorite and sericite which form the reservoir itself.

The volcanic belt is underlaid by highly resistive (>10,000 Ohm.m) crystalline rock, beneath which is a deep conductive region (<70 Ohm.m) beginning at a depth of 12 km. The cause of this crustal zone of enhanced conductivity is thought to be free saline water possibly released by metamorphic devolitization from the downgoing Juan de Fuca slab. Within this conductive layer is a region of high conductivity directly beneath the volcanic belt, which we interpret as the magma source body for the belt. Sensitivity studies imply that the upper crustal resistive zone underlies the whole complex, although a weakly-conductive, thin, vertical channel, representative of a magma conduit from the deep source body to the near-surface, cannot be excluded.

### Source

Physics of the Earth and Planetary Interiors, 81, 289-314, 1993.