Hydrogeological mapping using the Seismoelectric Method

Abstract

Seismoelectric methods are based upon physical properties of the earth that produce electrical signals from seismic waves. Electrokinetic sounding (EKS) is one such method that has great potential for hydrogeological studies as the signal arises from the movement of pore fluids under seismic excitation. In principle, the method should be able to directly map changes in hydraulic permeability, rock porosity and fluid-chemistry. A number of researchers have recently tried to exploit the phenomenon in groundwater problems where conventional methods worked poorly. However, publications of successful case histories to support the theory are rare. This may be due to the very weak amplitudes of the electrical signals generated from the seismic wave, which are millivolts 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 palaeochannel and the other is over a freshwater aquifer. The seismic waves were generated from a sledgehammer source and acoustic and electrical data were recorded by a seismic acquisition system. Seismic refraction and reflection data provide seismic velocity information for depth conversion and support the seismoelectric data. The data 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 to at least 100 m deep in freshwater aquifers.