Characterization of a fractured granite using radio magnetotelluric (RMT) data

Abstract

We applied tensor radio magnetotellurics (RMT) in the 10–250 kHz frequency range to study major fracture zones on Ävrö, a small island (1.6 × 1.2 km2) in southeastern Sweden with bedrock dominated by highly resistive granite. The interpretation of a 950‐m RMT profile was facilitated by seismic reflection and borehole data but was complicated (1) by possible 3D effects of the surrounding sea and (2) because the quasi‐static assumption is violated. Inversions based on the quasi‐static assumption give severely distorted models in this type of environment. Inversion codes that include displacement currents are restricted to 1D structures. Therefore, 2D inversions were applied to lower frequencies only. The central part of the inverted profile showed a 30–40‐m‐thick weathered layer over an almost intact bedrock down to a depth of at least 200 m, where higher salinity and/or fracturing yielded higher conductivities. The first 200 m of the profile revealed a major fracture zone, which coincided with a seismic reflector. We used 3D forward modeling to understand the sea effect and to model the conductor in three dimensions. We believe that 3D forward modeling is a highly valuable tool to distinguish known 3D effects (i.e., the sea) from regional 2D features of interest. We suggest that water flow at Ävrö is dominated by a few major fracture zones.