Three-dimensional electrical conductivity structure around the Clery fault, Vercors

Audio-magnetotelluric data have been acquired at 15 sites around the Clery fault, in southern Vercors. A low noise level ensured good quality data reinforced by robust processing of all recorded time series. Impedance data show a nearly constant resistivity for TM mode and a decrease for the TE mode with decreasing frequency starting at about 100 Hz. Parkinson arrows undergo a change in direction at about the same frequency: high-frequency arrows point toward the E or SE before turning toward the SW at low frequencies. Three-dimensional forward modelling was used to test possible hypotheses to explain the observed data. A model including a thin conducting (100 ohm.m) layer in the SE edge of the Clery plain and a general increase in conductance SW of the plain - achieved by thickening of a <100 ohm.m layer - agrees well with the data. Shallow conductors are interpreted as meteoric water concentrations while the increased conductance can be interpreted as either a thickening of alluvial sediments or as a deepening of the Clery fault gouge. We prefer the former interpretation compatible with an increase of alluvial deposit thickness with the observed decreasing topography.