A deep geoelectric survey of the Carnmenellis granite

Abstract

SUMMARY A magnetotelluric survey across the Carnmenellis granite, part of the Cornubian batholith of SW England, is described. The granite is being investigated with reference to the extraction of Hot Dry Rock geothermal energy. The sounding data are found to be influenced by near-surface static distortion which can be minimized by the construction of a granite-average sounding tensor. The anisotropy in the average tensor can be attributed to the spatial form of the granite. This is verified by 2-D modelling and the model results are used as control in the assessment of the vertical resistivity profile. The resistivity values obtained by the survey are found to correspond to a saturated ‘wet’ granite down to at least 10km. Despite the high geothermal gradient the anticipated decrease of resistivity with increasing temperature does not take place. Below a depth of just over 1 km, resistivity values are found to increase to a depth of about 6km. The results are compared with the laboratory-observed response of granite to increasing applied pressure and stress. The resistivity/depth section appears consistent with the completion of joint and crack closure by a depth of 7 km and a transfer to a pore-dominated resistivity mechanism below this depth. An electrical base to the granite is resolved at a depth of 14 km in accord with the depth to base adopted by regional gravity models.