GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.1 GNGTS 2023 Modeling and analysis of Distributed Acoustic Sensing (DAS) data in Geothermal environments D. Pecci 1 , J.L. Porras 2 , M. De Solda 2 , F. Grigoli 2 , E.M. Stucchi 2 , R. Iannelli 1 1 DESTeC, University of Pisa, Italy 2 Department of Earth Science, University of Pisa, Italy Since the second half of the 2000s, Distributed Acoustic Sensing (DAS) has seen a rapid increase in microseismic monitoring both for research and industrial applications. A DAS acquisition system consists of a fiber optic cable and an interrogator that sends a laser pulse down to the fiber optic cable. Due to small fluctuations (i.e., defects) within the fiber, a portion of the light pulse that travels through the cable is back-scattered (Rayleigh Backscattering). The back-scattered pulse travels back along the fiber to the source device that, in analogy with monostatic radar systems, also act as a receiver. The phase difference between the input pulse and the backscattered one allows to detect the dynamic strain along the fiber. If, due to an external perturbation (e.g., seismic waves), the fiber length changes and produces a corresponding phase delay in the backscattered pulse, this is recorded and converted by the interrogator into strain or strain rate. In this sense a DAS system can be seen as dense array of geophones able to samples seismic wavefields with extremely high spatial and temporal sampling. Typical DAS acquisition consist of fiber cables of several kilometers that samples the wavefield at about 1 m spacing and at sampling frequencies of 1 kHz or higher. For a fiber of 10 km length this results in about 5 TB of data collected per day, highlighting the lack of current data analysis procedures to deal with such large datasets. DAS technology is particularly suitable for microseismic monitoring application in geothermal environments. This instrumentation can resists to high temperatures (up to about 100°C or more) higher than the operational temperature of standard acquisition instruments (e.g., geophones), allowing the fiber to be located very close to the reservoir (or co-located with the injection/production well). For this reason, DAS is particularly useful for induced seismicity monitoring of Enhanced Geothermal System (EGS). EGSs are artificial geothermal reservoirs in which the rock formations in the subsurface are at naturally high temperatures but are not saturated with fluids. To exploit deep geothermal resources, EGS operations involve the injection of large volumes of fluids at high pressure in the subsurface (hydraulic stimulations). Such operations generate induce seismicity that need to be adequately monitored, hence the possibility of having the acquisition instrument very close to the injection well greatly increases the efficiency of the monitoring operations.