ACCEPTED FOR PUBLICATION ON IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING 1 Land CSEM simulations and experimental test using metallic casing in a geothermal exploration context: Vall` es Basin (NE Spain) case study Octavio Castillo-Reyes, Pilar Queralt, Alex Marcuello, Juanjo Ledo Abstract—Controlled-source electromagnetic (CSEM) mea- surements are complementary data for magnetotelluric (MT) characterization, although its methodology on land is not suf- ficiently developed and tested as in marine environments. Ac- quiring 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 Vall` es fault (Northeast (NE), Spain) where MT results have been satisfactory and allow us to verify the CSEM results. The Vall` es 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 electro- magnetic 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). Index Terms—Controlled-source electromagnetic (CSEM), nu- merical modeling, metallic casing effects, high-performance com- puting, geothermal exploration. I. I NTRODUCTION O VER the last 20 years, electromagnetic (EM) methods have become invaluable research tools in geophysics with rapidly increasing applications in both industry and academia. Both active and passive EM methods can improve the characterization and interpretation of geophysical datasets by mapping conductivity variations, and reducing ambiguities during exploration surveys. In this sense, the CSEM has become more and more popular in many different application scenarios due to its ability to display conductivity contrasts with respect to their surrounding sediments (e.g., detecting resistive zones in a conductive background). As a result, Octavio Castillo-Reyes is with Barcelona Supercomputing Center (BSC), Nexus II Building c/Jordi Girona 29, 08034, Barcelona (Spain), Tel.: +34 934 137 992 octavio.castillo@bsc.es (corresponding author) Pilar Queralt, Alex Marcuello and Juanjo Ledo are with Institut Geomodels, Departament de Din` amica de la Terra i de l’Oce` a Universitat de Barcelona, 08028 Barcelona, Spain nowadays CSEM has real application in many contexts such as hydrocarbon exploration [1]–[10], reservoir monitoring [11], [12], CO 2 storage characterization [13]–[18], and geothermal reservoir imaging [19]–[22], among others. In marine exploration contexts, CSEM plays a fundamental role in the geological characterization of faults and reservoirs, and allow monitoring the latter due to its high sensitivity to fluid movements and to useful buried resources. In these scenarios, CSEM is a well established supplementing tech- nique to MT [23]–[25]. Also, marine CSEM has provided valuable complementary information to seismic imaging, and has been also used for setting up seismic inversions. However, in land-based exploration environments, CSEM applications for imaging conductivity contrasts among targets and neigh- bouring materials are scarce, since CSEM needs very different data acquisition and processing strategies [26], [27]. Usually, regions of interest are urbanized and industrialized areas (e.g., railways, power grid, telephone networks, and indus- trial facilities). Consequently, human-generated noise prevents passive methods such as MT, which is a strong limitation for relevant land-based applications, such as CO 2 storage and geothermal exploration. Numerical simulation tools allow us to reproduce different materials’ responses to external excitation to analyze observed data and infer models of the subsurface as correct as possible. These simulations have been proven to validate geological models by direct comparison between data and synthetics in different application fields. In the case of land CSEM, these modeling codes are essential to understand the effect of metallic infrastructures and to study their use to improving the signal-to-noise ratio. The analysis of these effects has gained traction recently, and many different approaches have been evaluated on differ- ent application contexts. Out of these applications, studies in the area of energy reservoir modeling [28]–[33], water flooding [34], [35], geological storage [36]–[38], geothermal exploration [39], [40], and fractures and fault zones [41], [42], stand out. Regardless of numerical methodology or application area, these works stress out the significant effects on EM responses generated by the presence of steel-cased wells and other metallic infrastructure. Also, authors affirm that CSEM modeling in the presence of metallic structures present a numerically challenging problem because of two main reasons. Firstly, the considerable conductivity contrast between surrounding media and metallic casing. Secondly, the large-scale variation of target bodies requires different resolution levels of discretization. Consequently, simulation “© 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.”