On the difficulty of detecting streaming potentials generated at depth
Résumé
In order to investigate how a streaming potential coefficient measured in the laboratory, at a typical scale of 10cm, can be incorporated into a field model, with a typical scale of 1 to 10 km, we measured the electric field induced by water flows forced at 150m depth through a 10-m wide granite fractured zone. The water flows were obtained by pumping cyclically 10m of water from a borehole that cut the fractured zone at depth, and contemporaneously reinjecting i t into another borehole located 50m away. After one day a steady-state fluid flow regime was reached, with pumping cycles lasting 45 minutes, indicating a hydraulic conductivity of 105 ms 1 and a specific storage coefficient of 3:25106m1. The expected self-potential at the surface was ananomaly with two maxima of opposite signa nd 2lV amplitude each, both located 160m away from the middle of the borehole heads, the signal being divided by two 500m away from the middle of the borehole heads (inagreementwithWURMSTICHandMORGAN,1994). Instead, we observed an electrical signal of 8mV midway between the borehole heads, and smaller than 5mV, 33m away from the borehole heads. The discrepancy observed between the data and the model can be explained by fluid flow leakages that occurred close to the water-table head, represented about 20% of the total water flow, and activated smaller but closer electric sources. This experiment thus illustrates the practical difficulty of detecting streaming potentials generated at depth. It shows in particular that in fractured zones, and hence in the vicinity of a major active fault small water flows located distantly from an energetic targeted source,but close to some of the electrode s of the network,can sometimes drastically distort the shape of the expected anomaly.Models of possible electricale arthquake precursors therefore turn out to be more speculative than expected.