Comparative analysis of surface-DAS and collocated 3-component geophones for surface-wave studies

Fiber-optic distributed acoustic sensing (DAS) has become a standard tool in borehole monitoring and earthquake seismology, with its applications expanding into other areas. Recent studies demonstrate the use of surface-DAS (S-DAS) arrays for reflection seismic imaging and both passive and active source surface-wave analysis and inversion. S-DAS offers high sensitivity in low frequencies and denser channel sampling than conventional geophone arrays. However, it also presents challenges of directional sensitivity along the fiber and difficult, nonuniform fiber-ground coupling. Understanding these challenges is crucial for the further development of S-DAS applications.

In June 2022, a seismic field campaign was conducted at a hardrock mineral exploration site in Blötberget, central Sweden. The campaign utilized a broadband vibroseis source with a 2-200 Hz linear sweep and diverse collocated receivers, including S-DAS, 3-component (3C) 10 Hz geophone arrays, and vertical component broadband MEMS accelerometers. The receivers were arranged in a 2D profile with 5 m receiver and source spacing, except for the 3C geophones, which were spaced at 10 m intervals. The approximately 2000 m long line was primarily deployed for active source reflection seismics, but seismic data were also recorded passively for ambient noise surface-wave analysis.

This study focuses on the fiber's directional sensitivity and the application of S-DAS for surface-wave analysis and inversion. We compare our results to the active-source 3C geophone data and the vertical accelerometer data (both active and passive). We attempt to quantify S-DAS's response compared to other recording systems and evaluate its applicability for retrieving dispersion curves of surface waves. Results suggest phase consistency between different arrays and an increased resolution of the surface-wave phase-shift array in S-DAS data.

Acknowledgements: This work is partly supported by the Smart Exploration Research Centre. The center has received funding from the Swedish Foundation for Strategic Research (SSF) under grant agreement no. CMM22-0003. This is publication SE25-002.