ELECTROMAGNETIC IMAGES OF A VOLCANIC ZONE

Data from 20 magnetotelluric (MT) soundings in the Coast belt of south western British Columbia, Canada, in the vicinity of volcanic Mounts M eager and Cayley (approximately 150 km north of Vancouver), are analys ed 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 tenser decomposition, and the regional 2D responses are determined in a survey-consistent reference coordinate frame for 1D and 2D modellin g and inversion. The remaining unknowns, the site gain at each locatio n, are estimated by requiring the long-period asymptotes of the E-pola rization rho(a) curves to be the same, and alternatively are derived a s part of the 2D inversion procedure. Positive correlations are observ ed 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 montmorillo nite-dominated clay alteration minerals which form the cap rock of the geothermal reservoir. Below this cap, the zone is electrically resist ive owing to the chlorite and sericite which form the reservoir itself . The volcanic belt is underlaid by highly resistive (more than 10 000 Omega m) crystalline rock, beneath which is a deep conductive region (less than 70 Omega m) beginning at a depth of 12 km. The cause of thi s crustal zone of enhanced conductivity is thought to be free saline w ater possibly released by metamorphic devolatilization from the downgo ing Juan de Fuca slab. Within this conductive layer is a region of hig h 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 magm a conduit from the deep source body to the near surface, cannot be exc luded.