Laboratory electrical studies on the thermo-chemo-mechanics of faults and fault slip
In nature, electrical signals have been recorded contemporaneously with volcanic and seismic activity, and have been proposed as precursors to earthquakes and volcanic eruptions. In the hydrocarbon industry, streaming potentials are used to investigate steam fronts, thus aiding enhanced oil recovery. There is therefore considerable current interest in electrical signals emanating from the Earth's crust and the mechanisms which give rise to them. Two of the theories that have been proposed to explain electrical signal generation are: The piezoelectric effect, caused by stress changes on piezoelectric minerals, such as quartz, which is found in many crustal rocks. The electrokinetic phenomenon, produced at a solid-liquid interface, where an electrokinetic current such as the streaming potential can be induced through a pressure, chemical or temperature gradient, resulting in electrical charge transport within the moving fluid. In order to investigate the possible mechanisms responsible for the generation of electrical signals in the Earth's crust, carefully controlled laboratory rock deformation and rock physics experiments have been performed under simulated shallow crustal conditions, where both electrical potential signals and acoustic emissions were measured. The deformation strain rate, confining pressure, pore fluid pressure, pore fluid chemistry and temperature were all varied systematically during conventional triaxial rock deformation tests on a range of rock types. Confining pressures were varied from 20 MPa to 100 MPa, pore fluid pressures from 5 MPa to 40 MPa, strain rates from 1.5 x 10"4 s"1 to 1.5 x 10"7 s"1 and temperatures from room temperature (25 C) up to 125 C. Over thirty five experiments were completed at room temperature on rock samples Clashach, Bentheim and Darley Dale sandstones and Portland limestone. More than ten experiments were done at elevated temperature on both dry and saturated samples of Clashach sandstone using a range of pore fluid chemistries. Significant developments in experimental apparatus were necessary for these latter experiments, including the design and construction of an electrical internal heater for the triaxial deformation cell. I identify that, for the temperature range between 25 and 125 C, that the primary sources of electrical potential signal generation are (i) piezoelectric in dry quartz-rich sandstone and (ii) electrokinetic in saturated samples of both sandstones and limestone. Factors that are found to influence the electrical potential signals during deformation include effective pressure, temperature, strain rate, pore fluid type and fluid flow. As failure is approached, both pre-seismic and co-seismic signals are observed with the magnitude of the signals varying with rock type. These observations can be explained by differences in the rock composition and variation in hydraulic and electrical pathways available for electric current flow during rock deformation. Variation in electrical potential difference can be seen during both the compactive and dilatant stages of deformation. At slower strain rates, local rock variation can be seen through changes in electrical potential signals which appear to be obscured at higher strain rates. The change in electrical and streaming potential signals during deformation reflect both the accumulating and accelerating damage identified by acoustic emission prior to fracture and the localisation of damage at dynamic fracture. After failure the potential decreases to a background value where it remains during constant frictional sliding at essentially constant stress. The presence of a crack or fault was identified to affect the electrical and streaming potential signals depending on their relative position with respect to the fault suggesting that electrical potential could be used as a method for fault location. An increase in temperature was found not to affect the mechanical properties within the range of experimental conditions explored. The effect of increased temperature on the electrical potential signals depends on the conditions applied to the rock such as thermal equilibrium times, deformation and ionic species within the pore fluid. If the rock is allowed to reach thermal equilibrium, the electrokinetic reactions between the solid-liquid interface are increased with an average electrical potential increase of 38 mV per 25 C. However, the new surfaces formed during rapid deformation cannot reach equilibrium, so that temperature has no effect on the electrical potential signals during compaction, dilatancy & failure. With this, the results as a whole suggest that in shallow crustal rocks, the change in electrical potential signal with temperature is below the background electrical noise level.