Shear-wave crosswell tomography using distributed acoustic sensing: A breakthrough in geotechnical site characterization

Abstract This study presents a novel approach to perform high-resolution S-wave crosswell tomography by combining distributed acoustic sensing (DAS) with a borehole-coupled SV-wave source. Linear DAS fibers, which exhibit optimal sensitivity to vertically polarized shear waves, were used to acquire dense seismic data sets with significantly improved spatial resolution compared to conventional geophones. The SV-wave source (BIS-SV) provides efficient signal generation and coupling, allowing accurate traveltime picking and stable tomographic inversion results. Field experiments conducted at the ECCSEL Svelvik CO₂ Field Lab demonstrated that DAS-based S-wave tomography can be performed with acquisition times of 3–4 min per shot depth and a twofold increase in spatial resolution compared to conventional methods. In contrast to P-wave tomography, S-wave results revealed distinct subsurface structures and allowed direct derivation of dynamic shear stiffness, a key parameter in geotechnical engineering. A low correlation (R² = 0.196) between P- and S-wave traveltimes confirmed that both methods provide complementary insights into subsurface properties. In addition, the DAS system eliminated the need for water-filled boreholes and mechanical geophone coupling, providing significant cost and logistical advantages. DAS seismic tomography adds significant value by providing continuous, high-resolution, cost-effective subsurface monitoring with minimal field effort, making it a transformative tool for modern geotechnical and environmental applications.