CSEM and MT Inversion at the Hontomín CO₂ Storage Site in Burgos, Spain

In this article, we present the results of a controlled source electromagnetic method (CSEM) and magnetotelluric (MT) inversion research conducted at the Hontomín (Burgos, Spain) CO2 storage site. We evaluate the ability of the joint inversion of surface-to-surface CSEM and MT to identify the deep structure of the study area by inverting both experimental and numerical data. Two different inversions with different initial models were considered. After that, we further studied borehole-to-surface CSEM inversion (without metal casing). A new E electric field data type was put forward, which was more convenient for conducting inversion with vertical line sources at depth. A series of synthetic E data inversions using borehole-to-surface CSEM were performed to evaluate resistivity changes arising from CO2 injection in deep saline aquifers. Our forward modeling method was based on the staggered-grid finite-difference method, and the inversion process was realized using the limited-memory Broyden–Fletcher–Goldfarb–Shanno (LBFGS) method. The results of this inversion study of the Hontomín CO2 storage site showed that adopting borehole-to-surface CSEM was sensitive to resistivity changes in deep saline aquifers.