Characteristics of near-surface electrokinetic coupling

Naturally occurring electric potentials at the Earth's surface are traditio nally studied using self-potential geophysics. Recent theoretical and exper imental work has reinvestigated the manner in which the measurement can be made dynamically using a pressure source. The methodology, often referred t o as seismoelectric, relies on electrokinetic coupling at interfaces in the streaming potential coefficient. The ultimate aim of the developing method ologies lies in the detection of zones of high fluid mobility (permeability ) and fluid geochemical contrasts within the subsurface. As yet there are n o standard methods of recording and interpretation: the technique remains e xperimental. Field measurements are made using a seismic source and by reco rding electric voltage across arrays of surface dipoles. This study present s observational characteristics of electrokinetic coupling based on experim ents carried out in a wide range of environments. Theory concerning the cou pled elastic and electromagnetic wave equations in a saturated porous mediu m 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-di stance patterns. Vertical electric dipole radiation sources are modelled an d their spatial characteristics presented. A variety of experimental config urations have been used, and geometries that exploit phase asymmetry to enh ance the separation of signal and noise are emphasized. The main experiment al 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-symmetri c voltages which display low-velocity moveout are the dominant received wav eforms.