Characteristics of near-surface electrokinetic coupling

Abstract

Naturally occurring electric potentials at the Earth’s surface are traditionally studied using self‐potential geophysics. Recent theoretical and experimental work has reinvestigated the manner in which the measurement can be made dynamically using a pressure source. The methodology, often referred to as seismoelectric, relies on electrokinetic coupling at interfaces in the streaming potential coefficient. The ultimate aim of the developing methodologies lies in the detection of zones of high fluid mobility (permeability) and fluid geochemical contrasts within the subsurface. As yet there are no standard methods of recording and interpretation: the technique remains experimental. Field measurements are made using a seismic source and by recording electric voltage across arrays of surface dipoles. This study presents observational characteristics of electrokinetic coupling based on experiments carried out in a wide range of environments. Theory concerning the coupled elastic and electromagnetic wave equations in a saturated porous medium is discussed. It is predicted that coupling will produce electromagnetic radiation patterns from vertical electric dipoles generated at interfaces. Surface‐ and body‐wave coupling mechanisms should provide different time–distance patterns. Vertical electric dipole radiation sources are modelled and their spatial characteristics presented. A variety of experimental configurations have been used, and geometries that exploit phase asymmetry to enhance the separation of signal and noise are emphasized. The main experimental results presented are detailed observations in the immediate vicinity of the source. Simultaneous arrivals across arrays of surface dipoles are not common. The majority of such experiments have indicated that shot‐symmetric voltages which display low‐velocity moveout are the dominant received waveforms.

Laboratory measurements and theoretical modeling of seismoelectric interface response and coseismic wave fields

“…[3][4][5][6][7] Furthermore, a wide range of field and laboratory validations of the coseismic field and the interface response was presented in the literature. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] Several of these works compare either field measurements 15,17 or laboratory measurements 20 with a seismoelectric wave propagation model, corroborating that the coseismic and interface response fields are predicted by the theory.…”

Spectral peaks in electric field at resonance frequencies for seismically excited motion of ions in the Earth’s magnetic field

“…Even for electric field variations associated with the passage of seismic wave, there are many examples (e.g. Thompson and Gist, 1993;Butler et al, 1996;Mikhailov et al, 1997;Beamish, 1999;Garambois and Dietrich, 2001;Haines et al, 2007) as well as theoretical studies (e.g. Pride and Morgan, 1991;Gershenzon, 1992;Pride, 1994;Pride and Haartsen, 1996;Haartsen and Pride, 1997;Garambois and Dietrich, 2002;Huang, 2002;Ren et al, 2010a, b).…”

A review on electrokinetically induced seismo-electrics, electro-seismics, and seismo-magnetics for Earth sciences

“…The mechanisms involved in the subsurface electrokinetic coupling have been summarized by Beamish (1999) in Fig. 6.…”

“…The SE signals obtained with polarizable (stainless-steel, lead rods) or non-polarizable (Cu/CuSO 4 ) electrodes do not differ significantly from each other (Beamish, 1999). Electrodes are often stainless steel tubings of 30-50 cm length.…”

“…Some authors suggest to water the electrodes with a mixture of clay and water. The dipole length is usually 1-2 m. The effect of the dipole length between 1 and 10 m has been tested: the amplitude results showed that the received voltages are independent of dipole length when the position of the inner (nearest the shot point) electrode remains at a fixed offset, the inner electrode controlling the amplitude and character of the received voltage (Beamish, 1999), as already mentioned by Martner and Sparks (1959). Data collected with and without geophones present between two electrodes are similar, so that the geophones do not affect the SE signal (Haines, 2004).…”