Electromagnetic phenomena associated with rock deformation have been detected using various electrical measurement methods. Studies demonstrate that this includes electric field. A recent approach to this is enabled through a new, low cost sensor technology, the Electric Potential (EP) Sensor. The EP Sensor has previously measured complementary electric field signals extant with stress-induced currents in marble and granite. This paper presents preliminary research, measuring stress-induced electric field changes in uniaxial compressive stress tests on sandstone and granite until failure. The result is a unique data-set that reveals stress induced electric field, which increases in amplitude and frequency simultaneously, up to the point of failure. It is seen that the electric field is sensitive to two factors controlled in these tests, specifically lithology and drying condition (moisture content). The findings are important since they provide information regarding stress state and fracture processes occurring. This technology has potential for application to rock at the field-scale.
Since the 1970's, evidence for pre-earthquake electromagnetic phenomena has been growing and many of these attribute this to rock deformation (Mizutani et al. 1976) (Hayakawa et al. 1994) (Tzanis et al. 2001). This viewpoint is supported by laboratory-based studies, which have found the flow of electric currents and the detection of electric potential in rock are associated with their deformation and the application of stress (Brady et al. 1986) (Aydan et al. 2001) (Freund 2002) (Hadjicontis et al. 2004) (Eccles et al. 2005) (Triantis et al. 2006) (Takeuchi et al. 2011). All have made measurments with laboratory based electrometers that are expensive and draw current from the rock specimen.
While these studies have demonstrated the existence of pressure stimulated currents (Anastasiadis et al. 2004), the precise mechanism/mechanisms responsible are still debated. There are also factors that may inhibit application of these techniques to in situ field studies. Inherit problems include low level currents (pico Amps level) and the closed loop system configuration requiring ends to be some arbitrary width apart for the measurement.
