A three-dimensional Distributed Acoustic Sensing array to monitor local seismicity

Summary Distributed Acoustic Sensing (DAS) has emerged as a valuable complement to conventional seismic monitoring techniques. By converting fiber-optic cables into dense arrays of virtual sensors, DAS enables the application of standard large-array processing methods. However, its directional sensitivity—limited to strain measurements along the fiber axis—may restrict its potential for full wavefield analysis. To address this limitation, we investigate the capabilities of DAS on a fiber-optic cable installed both horizontally, near the surface, and vertically, in a borehole, thereby creating a so-called 3D-DAS array. The survey was carried out in the southern Munich region (Germany) to monitor local seismicity associated with nearby deep geothermal operations. In this study, we present the data acquisition setup and describe a processing workflow developed to characterize source and wavefield parameters of seismic events from DAS recordings. The workflow is illustrated using a nearby {Mw} = 0. 48 seismic event. Taking advantage of the configuration of the fiber optic cable, we demonstrate that the 3D-DAS array enables estimation of the wavefield back-azimuth, incidence angle and slowness, and compare these results with those provided by a local network of seismometers. In addition, seismic source parameters, including seismic moment and stress drop, are estimated from DAS data acquired in the 250-meter-deep vertical well. These parameters are derived after converting strain-rate to ground motion, a process quantitatively validated using a co-located three-component broadband seismometer. The results and waveform evaluation demonstrate that the 3D-DAS array provides reliable and comprehensive measurements, independently of the existing local seismic network.