The Experimental platform of Tournemire (Aveyron, France) developed
by IRSN (French Institute for Radiological Protection and Nuclear
Safety) is located in a tunnel excavated in a clay-rock formation
interbedded between two limestone formations. A well-identified regional
fault crosscuts this subhorizontal sedimentary succession, and a
subvertical secondary fault zone is intercepted in the clay-rock
by drifts and boreholes in the tunnel at a depth of about 250 m.
A 2D electrical resistivity survey was carried out along a 2.5 km
baseline, and a takeout of 40 m was used to assess the potential
of this method to detect faults from the ground surface. In the 300
m-thick zone investigated by the survey, electrical resistivity images
reveal several subvertical low-resistivity discontinuities. One of
these discontinuities corresponds to the position of the Cernon fault,
a major regional fault. One of the subvertical conductive discontinuities
crossing the upper limestone formation is consistent with the prolongation
towards the ground surface of the secondary fault zone identified
in the clay-rock formation from the tunnel. Moreover, this secondary
fault zone corresponds to the upward prolongation of a subvertical
fault identified in the lower limestone using a 3D high-resolution
seismic reflection survey. This type of large-scale electrical resistivity
survey is therefore a useful tool for identifying faults in superficial
layers from the ground surface and is complementary to 3D seismic
reflection surveys.