Near surface seismoelectrics in comparative field studies

In porous saturated media, seismic compressional waves can cause measurable electric signals via electrokinetic coupling. Despite increasing interest in the seismoelectric method, little material has been published on actual field measurements. In order to address this deficiency, several sites in Northern Germany were visited at different times to investigate the repeatability and variability of these seismoelectric signals. In general, there is a strong correlation between the overall quality of seismoelectric data and the quality of the associated seismic data. Factors like the electrical resistivity or electrode coupling are less significant. A strong dependence of the seismoelectric signals on the pH value could be shown with borehole seismoelectric measurements. A hodogram analysis of three-component seismoelectric data reveals that the polarization mainly follows theoretical predictions. The measurements, interpretations and comparisons with other methods presented in this work will help to assess the chances of successful application of seismoelectric measurements in near surface geophysical studies

Dependence of seismoelectric amplitudes on water content

International audienceThe expectation behind seismoelectric field measurements is to achieve a combination of the sensitivity of electrical properties to water content and permeability and the high spatial resolution of seismic surveys. A better understanding of the physical processes and a reliable quantification of the conversion between seismic energy and electric energy are necessary and need to take into account the effect of water content, especially for shallow subsurface investigations. We performed a field survey to quantify the seismoelectric signals as the water content changed. We measured seismoelectric signals induced by seismic wave propagation, by repeating the observations on the same two profiles during several months. The electrical resistivity was monitored to take into account the water content variations. We show that the horizontal component of the seismoelectric field, normalized with respect to the horizontal component of the seismic acceleration is inversely proportional to the electrical resistivity ρ0.42±0.25 . Assuming that the observed resistivity changes depend only on the water content, this result implies that the electrokinetic coefficient should increase with increasing water saturation. Taking into account the water saturation and combining our results with the Archie law for the resistivity in non-saturated conditions, the normalized seismoelectric field is a power-law of the effective saturation with the exponent (0.42 ± 0.25)n where n is Archie's saturation exponent. Hydrogeophysics; electrokinetics; streaming potential; seismoelectric conversion; field observation; water conten

Influence of water pressure dynamics and fluid flow on the streaming-potential response for unsaturated conditions

International audienceA B S T R A C T Streaming-potentials are produced by electrokinetic effects in relation to fluid flow and are used for geophysical prospecting. The aim of this study is to model streaming potential measurements for unsaturated conditions using an empirical approach. A conceptual model is applied to streaming potential measurements obtained from two drainage experiments in sand. The streaming potential data presented here show a non-monotonous behaviour with increasing water saturation, following a pattern that cannot be predicted by existing models. A model involving quasi-static and dynamic components is proposed to reproduce the streaming potential measurements. The dynamic component is based on the first time derivative of the driving pore pressure. The influence of this component is investigated with respect to fluid velocity, which is very different between the two experiments. The results demonstrate that the dynamic component is predominant at the onset of drainage in experiments with the slowest water flow. On the other hand, its influence appears to vanish with increasing drainage velocity. Our results suggest that fluid flow and water distribution at the pore scale have an important influence on the streaming potential response for unsat-urated conditions. We propose to explain this specific streaming potential response in terms of the behaviour of both rock/water interface and water/air interfaces created during desaturation processes. The water/air interfaces are negatively charged, as also observed in the case of water/rock interfaces. Both the surface area and the flow velocity across these interfaces are thought to contribute to the non-monotonous behaviour of the streaming potential coefficient as well as the variations in its amplitude. The non-monotonous behaviour of air/water interfaces created during the flow was highlighted as it was measured and modelled by studies published in the literature. The streaming potential coefficient can increase to about 10 to 40 when water saturation decreases. Such an increase is possible if the amount of water/air interfaces is increased in sufficient amount, which can be the case