Hydrogeological mapping using the seismoelectric method

Abstract

Seismoelectric methods are based upon physical properties of the earth that produce electrical signals from seismic waves. The electrokinetic sounding (EKS) is one such method that has great potential for hydrogeological studies as it arises from the movement of pore fluids under seismic excitation. In theory, the method should be able to directly map changes in hydraulic permeability, rock porosity, or fluid-chemistry. A number of researchers have recently tried to exploit the phenomenon in groundwater problems where the conventional methods worked poorly. However, publications of successful case histories to support the theory are rare. This may be blamed upon the very weak amplitudes of the electrical signals generated from the seismic wave which are milivolts to nanovolts in magnitude, and the presence of cultural noise which is usually much greater in magnitude. We demonstrate electrokinetic responses from formations more than 50 metres deep in two test areas in Western Australia. One is over a saline paleochannel and the other over a freshwater aquifer. The data was generated from a sledgehammer source and recorded by a seismic acquisition system. Seismic refraction and reflection data provide seismic velocity information for depth conversion and support the seismoelectric data. The signals were then compared to borehole logs to find what physical contrasts were detected. Significant hydrogeological boundaries were detected up to 50 m deep in saline groundwater conditions, and at least 80 m deep in freshwater aquifers. In addition, we examine some pitfalls in the method and our approach to overcoming these problems.