We applied tensor radio magnerotellurics (RMT) in the 10-250 kHz
frequency range to study major fracture zones on Avro, a small island
(1.6 x 1.2 km(2)) 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 30-40m-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 Avro is
dominated by a few major fracture zones.