Electroseismic and Seismoelectric Measurements of Rock Samples in a Water Tank

An electromagnetic wave or a seismic wave can induce seismic or electric waves due to the electrokinetic conversion based on the electric double layer in a fluidsaturated porous medium. In this paper, we observe the acoustic fields generated around the electrodes excited by an electric pulse in a water tank. The electroseismic or seismoelectric waves are measured in the water tank system to confirm that the recorded seismic or electric signals are induced in porous samples due to the electrokinetic conversions. The electroseismic and seismoelectric frequency-responses in Berea sandstone and Westerly granite samples are measured at frequency range of 15 kHz to 150 kHz. The experimental measurements show that the effects of the electric source, background noises, and electroseismic or seismoelectric conversions are separated very clearly. We calculate the electroseismic and seismoelectric normalized coupling coefficients in the rock samples and compare them with the theoretical calculation. The variation trends of the normalized coupling coefficients are in agreement with the theoretical predictions. The measurement method in this paper could be used to investigate other electroseismic and seismoelectric properties for petroleum exploration applications. INTRODUCTION There is an electric double layer on the interface between solid and fluid in a fluid-saturated porous rock (Morgan et al. (1989); Pride (1994); Pride and Haartsen (1996)). When the fluid is an electrolyte, there are also movable free charges. When a seismic wave propagates in a fluidsaturated porous rock, the seismic wave generates relative movement between the fluid and rock matrix. Consequently, the moving charges induce an electric field. The electric field will be an electromagnetic wave when the seismic wave propagates across an interface which is not continuous in acoustical, electrical, or mechanical properties (Mikhailov et al. (2000); Zhu and Toksoz (2003, 2005)). This process is called a seismoelectric conversion. On the other hand, if there is an electric field in the water or in the fluid-saturated porous rock, the moving charges in the fluid cause fluid movement. This relative movement between solid and fluid generates a seismic wave. This process is called an electroseismic conversion. The seismoelectric and 2 electroseismic conversions are related to the fluid conductivity, rock porosity, permeability, pore size, etc. Seismoelectric and electroseismic measurements made on Earth’s surface or in a borehole may provide information about the subsurface properties (Mikhailov et al. (1997); Zhu and Toksoz (2005)). Electroseismic laboratory experiments (Zhu et al. (1994, 1999); Reppert and Morgan (2002)) with scaled layered and borehole models showed that electroseismic waves are induced with electrodes buried in a fluid-saturated formation or in a borehole. Deckman et al. (2005) determined the electroseismic coupling coefficients with a simple measurement cell in the laboratory. Field-scale electroseismic measurements (Thompson (2005); Thompson et al. (2005)) detected possible accumulations of natural gas targets. The results demonstrated discrimination of gas sands at a depth of less than 1000 meters. Reppert and Morgan (2002) studied the theoretical frequency dependency of electroseismic conversion in a porous medium and derived the frequency-dependent electroseismic voltage coupling coefficient: !! " # $$ % & ' !! " # $$ % & = ( ( 1 ) ( ) ( 2 ) ( ) ( 2 ) ( ) ( 1 1 a J a J a a J a J a V P o o ) ) ) ) ) *+) , , (1) where