Repeated Electrical Resistivity Acquisitions with Buried Permanent Equipment – Estimation of the Noise

In order to monitor the water saturation of an unstable cliff formed by weathered rock in the region of the Cote Basque (France), a permanent electrical array has been installed. This array is composed of forty-eight steel electrodes embedded in a trench since January 2006. In order to determine variations in electrical resistivity values due to noise and those that could be associated to water content variations, repeatability tests have been realised with the same array in the shortest time. The statistical analyses of the results show that the first measurement of a day is significantly different from the other. This result could be explained by a polarisation of the system array-clay at the time of the first acquisition. Furthermore, despite the removal of the first acquisition of the day, a residual noise is still recorded at the foot of the cliff. This area, also characterised by rapid and high variations of spontaneous polarisation, seems to be affected by rapid water flows. These repeatability tests performed thanks to a rapid resistivity-meter, allows, firstly, removing the cause of the major noise, and, secondly, allows locating areas where electrical resistivity values cannot be stabilized because of water flow.