Effect of heterogeneity of hydraulic conductivity on streaming potential

Self-potential(SP) is electrical potential mainly generated by thermoelectric, chemical and streaming potentials in the subsurface. The flow of groundwater is often recognized as a bigger source of SP. Using this feature, there are many attempts to infer and quantify flows of liquid in the soil, including groundwater. In case that underground structure is homogeneous, electrical current density according to ground-water flow becomes uniform. Therefore SP on the surface increases monotonically from upstream to downstream of groundwater flow. As a basic interpretation of SP, the direction of increase in SP corresponds with the direction of groundwater flow. However, such anomalous fluctuations of SP start to appear for subsurface inhomogeneous groundwater flow due to the non-uniform conveyance of changes. Our simulation shows that the anomalies of permeability and coupling-coefficient of electrokinetic effect in the subsurface are predominant parameters to effectively estimate the distribution of surface SP in the existence of inhomogeneous underground flow. The amount of conveyance of charge depends on these parameters, and the extra charges occur on the boundary of these parameters. As a result, localized minimum or maximum SP anomalies in the survey profile are generated just above the boundaries. In the previous studies, the effect of coupling coefficient on the SP is often simulated on the underground flow scale. Some anomalies of SP can be explained by the inhomogeneity of coupling coefficient. Also, simple model simulations, for example the well-pumping model, show the effect of hydraulic conductivity on the SP. However, SP anomalies that are generated by inhomogeneity of hydraulic conductivity (or excess electric charges on the boundaries of hydraulic conductivity) are not simulated on the larger scale such as an underground flow scale, and has not been so much discussed. In this study, we simulate the anomalies of SP generated by inhomogeneous of hydraulic conductivity and coupling coefficient. We compare the difference of these anomalies and study on the difference of two SP anomalies; one generated by hydraulic conductivity and the other by coupling coefficient. As a result, we found that the fluctuations of SP for the inhomogeneity of hydraulic conductivity are larger than for the coupling coefficient. There are differences between the predicted fluctuations of SP that are influenced by hydraulic conductivity and by coupling coefficient. In addition to the numerical studies, we measured the SP along a profile in a field where the groundwater flow patterns will be governed by a heterogeneous distribution of hydraulic conductivity. The real SP data was measured in the Saijo plain, Ehime, southwestern Japan, in August, 2010. On the profile, a large active fault exists and plays a role as a semi-vertical boundary of hydraulic conductivity between the basement and sedimentary rocks. We compare the observed SP data along the profile with that of the numerically simulation. It gives us a conclusion that the observed SP data is probably influenced by the difference in the hydraulic conductivity of the rocks.