Full Waveform Inversion of DAS Crosswell Data to Monitor HT-ATES: A Synthetic Feasibility Study

Abstract Monitoring underground reservoirs is essential to ensure their efficient and sustainable operation. This study investigates the detectability and imaging, using seismic methods, of modifications in elastic properties caused by cyclic injection and production of geothermal fluid in high-temperature aquifer thermal energy storage (HT-ATES) systems. In such systems, the induced variations in elastic properties are expected to be very small, requiring high-sensitivity methods for reliable detection. The work is conceived as a feasibility study in a setting where field data are not yet available. Synthetic modelling is used to evaluate the effectiveness and sensitivity of Full Waveform Inversion (FWI) to reservoir variations. FWI is selected for its ability to exploit the full recorded wavefield, which is critical for resolving subtle elastic changes in thin, low-contrast reservoirs. The DeepStor HT-ATES research project in Karlsruhe, Germany, where layers thinner than 10 m at 1.3 km depth are targeted, serves as a case study. Two elastic models were developed: one unperturbed and one incorporating anomalies expected after five years of cyclic operations. Due to the thickness and depth of the targets, surface seismic methods were excluded in favour of cross-well configurations using Distributed Acoustic Sensing (DAS). Active sources are adopted to ensure controlled and repeatable illumination required for time-lapse monitoring. Results indicate that geometries with receivers intersecting the reservoirs significantly improve imaging and parameter estimation. This study evaluates the applicability of established FWI approaches in thin, stratified geothermal systems and highlights the importance of survey design for successful HT-ATES monitoring.