We consider the case of two wells in parallel to each other. A seismic source is located in the first well and electrodes are located in a second well. For times comprised between the time of the seismic source and the time of the first arrival of the seismic waves in the second borehole, all the recorded electrical disturbances are associated with seimoelectric conversion of the seismic energy when the seismic P wave reaches heterogeneities located between the two wells. These heterogeneities need to correspond to change in electrical conductivity and electrokinetic properties between the materials. We use a finite element method to solve both for seismic wave equation and the associated seismoelectric disturbances in a piecewise homogeneous porous material. We consider the case corresponding to a vertical contact between two homogeneous materials. The seismoelectric data recorded in the second well can be inverted using a deterministic algorithm to retrieve the position of the vertical heterogeneity. This is a part of an ongoing study
The seismo-electric (seismic-to-electric) conversion corresponds to the generation of electromagnetic disturbances associated with the passage of seismic waves in a heterogeneous rock. This coupling is due to the existence of an electrical double layer at the solid/water interface. Recent works have demonstrated that the seismoelectric method may be useful to characterize heavy oil reservoirs (Revil and Jardani, 2010). Furthermore, a stochastic inversion approach to jointly invert seismic and seismoelectric data to retrieve the material properties of a set of sedimentary units was developed by (Jardani et al. 2010). When seismic waves propagate in a heterogeneous porous material, three types of electrical disturbances can be observed. The propagation of seismic waves in a porous material generates an electrical current associated with the displacement of the electrical diffuse layer in a Lagrangian framework attached to the solid phase. This co-seismic electrical signal travels at the same speed as the seismic waves in a linear poroelastic body (Pride, 1994). The amplitudes of the co-seismic electromagnetic signals are controlled by the properties of the porous material, i.e. the formation factor, and by the properties of the pore fluid/solid interface; the excess charge per unit pore volume in the formulation given by (Revil and Jardani, 2009). In addition to the co-seismic signals, another phenomenon occurs when a seismic wave moves through a sharp interface characterized by a change in the textural properties or a change in salinity or clay content/mineralogy. In this situation, a fraction of the mechanical energy is converted into electromagnetic energy and a dipolar electromagnetic excitation is produced. In the present paper, we show how the seismoelectric signals produced by seismic waves propagating between two wells can be potentially used to image heterogeneities between two wells. We first present the forward modelling of the seismoelectric response in a simple case using the finite element code Comsol Multiphysics 3.4. We explain how the distribution of the electrical signals can be inverted to retrieve the position of the source.