An approach for monitoring resistivity variations using surface magnetotelluric data and its application to CO2 storage site

Abstract:

In this work we present an approach to perform electromagnetic (EM) monitoring using surface magnetotelluric (MT) data. The proposed methodology, called layer stripping, is based on the analytical solution of the one-dimensional MT problem and the fact that resolution to resistivity changes produced at a given depth increase when increasing the depth at which data are acquired. Thus, giving a well-known geoelectrical baseline model of a reservoir site, the layer stripping approach aims to remove the effect of the upper, unchanging, structures in order to obtain the time-varying MT responses at the target depth.

In this paper the proposed method is tested and validated using the 3D geoelectrical baseline model of the Hontomín site (Spain) for CO₂ geological storage in a deep saline aquifer. The 3D resistivity model of Hontomín defines the subsurface in the pre-injection state, showing the dome-like structure of the saline aquifer and imaging the principal set of faults. For monitoring purposes, the model allows obtaining the MT responses at the main reservoir depth using the layer stripping approach to remove the upper structures not affected by the injection of the gas.

The results obtained in this work suggest that the layer stripping method improves the resolution of surface MT responses being able to detect smaller resistivity changes.