2D SEISMOELECTRIC MODELING OF COUPLED ELASTIC AND POROELASTIC MEDIA - AN APPLICATION TO GLACIAL SYSTEMS PROSPECTING

BUCHER, FEDERICO; ZYSERMAN, FABIO I.; MONACHESI, LEONARDO B.; CASTROMÁN, GABRIEL A.

Congreso; 2nd SEG Latin America Virtual Student Conference; 2022

Due to the existence of the so-called electric double layer (EDL) in the grain boundaries offluid-saturated porous media, an electrokinetic phenomenon takes place when a relative motionbetween the solid matrix and the pore fluid occurs. A seismic wave propagating through this type ofmedia induces a streaming electric current, which acts as a source of electromagnetic fields. Thecoseismic field is spatially supported by the seismic wave and in turn, travels at seismic velocity. This field, together with the interface response (IR) generated when the seismic wave arrives at an interface between two media with different electrical and/or mechanical properties, are both the main seismoelectric conversions. They strongly differ in their propagation velocity, since the IR is anindependent field that propagates with the electromagnetic velocity of the medium. Another difference lies in the spatial support, since the IR propagates through the whole space with a dipolar behaviour. In virtue of its generation mechanism, the IR contains detailed information about the subsurface heterogeneities. When these electric fields are caused by a seismic source deployment, they can be recorded at the Earth’s surface and in nearby wells, thus providing a very useful prospective method. This technique was successfully applied in the study of hydrocarbon reservoirs [1] and glacial systems [2], among others.In this work we study the seismoelectric response of a geological model composed of anon-porous elastic layer on top of a poroelastic half-space. Due to the absence of the EDL in theelastic layer, the necessary support for the generation of the coseismic field is not provided. Therefore, receivers located within this layer will only detect the IR conversion generated at theelastic-poroelastic boundary. Conversely, within the poroelastic half-space, the coseismic field willmask the IR because of its higher amplitude. As a hypothetical scenario, we proposed a glacial system composed of an ice body (modeled as a 150 m thick elastic layer) laying over a water-saturated bedrock (modeled as a poroelastic half-space). As commonly assumed, both media are in welded contact. We employed a compressional seismic source with the time signature of a Ricker wavelet. Pride’s equations governing the seismoelectric phenomenon were solved through numerical algorithms based on a finite element procedure [3]. The solutions obtained for the spatial and temporal distribution of the aforementioned electric fields were analyzed based on snapshots such as those shown in Figure 1. In addition, the sensitivity to variations in electrical and poromechanical properties of the half-space was tested. The results showed that IR amplitude is strongly dependent on these variations, such as in porosity and fluid salinity. We conclude that the seismoelectric method can serve as a useful complementary tool for the hydrogeophysical characterization of a porous glacier bedrock.