Land CSEM Simulations and Experimental Test Using Metallic Casing in a Geothermal Exploration Context: Vallès Basin (NE Spain) Case Study

Abstract

Controlled-source electromagnetic (CSEM) measurements are complementary data for magnetotelluric (MT) characterization, although its methodology on land is not sufficiently developed and tested as in marine environments. Acquiring expertise in CSEM is crucial for surveys in places where MT cannot be performed due to high-levels of cultural noise. To acquire that expertise, we perform CSEM experiments in the Vallès fault (Northeast (NE), Spain) where MT results have been satisfactory and allow us to verify the CSEM results. The Vallès basin is relevant for potential heat generation because of the presence of several geothermal anomalies, and its nearby location to urban areas. In this paper, we present the experimental setup for that region, a 2-D joint MT+CSEM inverse model, several 3-D CSEM simulations in the presence of metallic casing, and its comparison with real data measurements. We employ a parallel and high-order vector finite element algorithm to discretize the governing equations. By using an adapted meshing strategy, different scenarios are simulated to study the influence of the source position/direction and the conductivity model in a metallic casing presence. An excellent agreement between simulated data and analytical/real field data demonstrates the feasibility of study metallic structures in realistic configurations. Our numerical results confirm that metallic casing strongly influences electromagnetic responses, making surface measurements more sensitive to resistivity variations near the metallic structure. It could be beneficial getting higher signal-to-noise ratios and sensitivity to deep targets. However, such casing effect depends on the input model (e.g., conductivity contrasts, frequency, and geometry).

HPC Geophysical Electromagnetics: A Synthetic VTI Model with Complex Bathymetry

“…Because we employ PETSc solver implementations, no particular work was undertaken to reduce run-time, as this is an completely different goal. However, these numerical results indicate a remarkable benefit when parallel executions are employed (similar conclusions to that described in [31,32]).…”

“…We point out that the significant differences in the electric field components imply that the vertical EM field is more susceptible to the perturbations of the resistivity model and is more informative than the in-line EM field for laterally heterogeneous targets. Then, this feature could be employed to get better signal-to-noise trade-offs on stations near to the vicinity of the resistivity unit [32]. Nonetheless, we acknowledge that this EM field pattern is reduced and only effective close to the reservoir location (x = 730.5 km to x = 735 km).…”

“…Nonetheless, we acknowledge that the electric field pattern depends on the input model setup (e.g., source type, fundamental frequency, resistivity, depth of station where EM fields are measured). For a comprehensive analysis of the impact of these factors on EM field behaviour we refer to [32]. We compute the electric field amplitude around the reservoir location to present a full analysis of the 3D EM behavior.…”

“…Thus, its effect depends on where the reservoir unit is located with respect to the source. Nevertheless, the effect on EM measurements should be considered to avoid misunderstanding interpretations [32].…”

Tailored meshing for parallel 3D electromagnetic modeling using high-order edge elements

“…This section only provides a brief outline for understanding the HPC implementation. For a comprehensive introduction to the PETGEM code and its computational performance, we refer to (Castillo-Reyes et al 2018Castillo-Reyes et al 2021;Castillo-Reyes et al 2022).…”