Laboratory studies of electrical potential during rock failure

We have investigated electrical potential and acoustic emissions signals associated with rock deformation. Five rock types were studied; Clashach, Bentheim and Darley Dale sandstones (all quartz-rich) and a Seljadur basalt and Portland limestone (both quartz-free), both air dry and the rocks were tested in distilled water. Shallow crustal conditions were simulated in a triaxial rock deformation cell with a confining pressure simulating depth of 40 MPa, pore pressures ranging 5-35 MPa, and strain rates 10−7-10−4 s−1. Precursory electric potential signals prior to failure were observed in both saturated and dry samples of the quartz-rich sandstones, but only observed in the water saturated quartz-free rocks. Co-seismic electrical signals were obtained in all tests, providing strong evidence that two of the main sources for precursory and co-seismic signals are the piezoelectric and electrokinetic phenomena. Lowering the strain rate resulted in an increase in the number of acoustic emissions. The pore volume changes during compaction and dilatancy remained approximately constant for all strain rates. Streaming potential generated by fluid flow across the sample was also measured at different stages of deformation. The potential signals increased with the pore pressure gradient.