Groundwater Division, Western Cape Conference: Tales of a hidden treasure; Somerset West, 16 Sept. 2002 Information on aquifer elastic parameters and aquifer deformation from the analyses of borehole electroseismic data Information on aquifer elastic parameters and aquifer deformation from the analyses of borehole electroseismic data F.D FOURIE, J.F. BOTHA, G.J. VAN TONDER The Institute for Groundwater Studies, The University of the Free State e-mail: francois@igs-nt.uovs.ac.za Abstract Rock deformation caused by stresses can lead to significant decreases in the aperture widths of fractures in the rock. The amount of fracture deformation (closing) is dependent on the strength of the rock hosting the fracture. Electroseismic (ES) energy conversion in fluid-saturated porous rocks depends on the properties of both the fluid and the solid matrix. Matrix properties that play a role in ES phenomena include the bulk modulus of deformation and the shear modulus of deformation. These elastic parameters may be used to quantify the strength of the porous rock. ES techniques therefore offer the possibility of obtaining information on the elastic properties of the rock matrix and, consequently, on the deformability of fractures in the rock. The electroseismic and magnetoseismic transfer functions for real earth materials at seismic frequencies depend on various physical and chemical parameters of the fluid- saturated porous system and the aquifer elastic parameters can therefore not be obtained unambiguously without prior knowledge of the other parameters. Porosity changes due to aquifer deformation may lead to measurable changes in the transfer functions. The electroseismic transfer function associated with the Biot fast wave is very insensitive to porosity changes. The electroseismic transfer function associated with the Biot slow wave, on the other hand, is extremely sensitive to porosity changes. These waves are, however, very difficult to measure due to their dispersive nature. The magnetoseismic transfer function associated with shear waves is sensitive to porosity changes and correctly predicts zero ES energy conversion for porosities of zero and unity. The magnetoseismic transfer function is therefore likely to yield the most useful information on aquifer deformation. Keywords: aquifer, deformation, elastic parameters, electroseismics 1. INTRODUCTION Studies by Makurat (2001) showed that rock deformation caused by stresses can lead to significant decreases in the aperture widths of fractures in the rock. His results also indicated that the amount of fracture deformation (closing) is dependent on the strength of the rock hosting the fracture. With electroseismic (ES) techniques the earth is seismically excited and an electromagnetic (EM) response is measured. Since the solid grains of earth materials generally display net electric charge, charge separation occurs in the fluid in the vicinity of the solid grains. A mechanical disturbance that propagates through a fluid-saturated porous medium causes a displacement of the solid phase relative to the fluid phase, and an electric current is thus produced in the medium. The resulting macroscopic electric current is called the streaming current. In ES phenomena mechanical and EM energies are coupled through the electrokinetic coupling coefficient (Pride, 1994). This coefficient is dependent on the properties of both the fluid and the solid. The propagation of a disturbance in a porous medium saturated with a viscous fluid