Baseline DAS observations for active-source time-lapse monitoring at the TU Delft geothermal site

At TU Delft, a combined geothermal and high-temperature aquifer thermal energy storage (HT-ATES) infrastructure is being developed, linking a deep geothermal doublet to a shallow storage system that buffers the seasonal mismatch between heat supply and demand. Geothermal heat production and seasonal thermal energy storage both require reliable subsurface monitoring to assess reservoir behaviour, system efficiency, and the evolution of injected heat in both space and time.

The site includes injector-producer wells, deep and shallow monitoring boreholes, and a planned set of five storage wells arranged in hot and warm groups for direct use and reheating, respectively. These boreholes are instrumented with fibre-optic systems for distributed strain, temperature, and acoustic sensing, providing an experimental setup for evaluating the role of distributed acoustic sensing (DAS) in geothermal monitoring.

In this study, we present an overview of baseline DAS measurements acquired to support future monitoring of the geothermal site operation. The dataset includes observations from fibre installations deployed inside and outside the casing and enables an initial comparison of acquisition parameters, including pulse width, gauge length, and fibre type. Baseline active-source measurements were acquired using an electric vibrator operating over a 2–180 Hz sweep band, with repeated sweeps to improve signal-to-noise ratio through stacking.

The analysis aims to identify acquisition configurations that provide robust repeatability and sufficient sensitivity for active-source time-lapse monitoring. The work forms the foundation for repeated seismic surveys to target thermal-plume evolution and reservoir response during future operation of the TU Delft system. In the long term, these baseline observations will support the development of 4D full-waveform inversion to track changes in elastic properties resulting from temperature and fluid injection, with the broader goal of improving monitoring and maintenance of geothermal energy systems.

Acknowledgements: This work was supported by the European Union under the Horizon Europe PUSH-IT project (grant no. 1011096566) and by CETP Q-Fibre (proposal code Cetp-FP-2023-00079). CETP Q-Fibre is co-funded by the European Commission (GA no. 101069750), the Netherlands Enterprise Agency (RVO), the Research Council of Norway (RCN), and the U.S. Department of Energy (DOE).