On the benefit of collecting the seismic divergence using Distributed Acoustic Sensing with coiled fiber

Distributed Acoustic Sensing (DAS) captures the longitudinal strain fluctuations along fiber optic cables. With locally straight cables, the measurement is closely related to the horizontal gradient of the horizontal velocity fields ∂_xV_x which could alternatively be obtained by differencing closely spaced conventional point sensors such as geophones and seismometers. The latter approach however often suffers from instrument and deployment perturbations as well as finite-difference bias and we discuss the advantage of using DAS to obtain higher fidelity gradients over a larger operating bandwidth, both spatially and temporally. We then introduce the potential of DAS to extract the divergence (∂_xV_x+∂_yV_y) of the seismic wavefield by interrogating horizontally coiled fiber. This results in an omni-directional measurement that is closely related to near-surface pressure fluctuations which, we demonstrate, is insensitive to Love waves but closely related the horizontal acceleration of particle motion H induced by Rayleigh waves. Such a wavefield separation is attractive for local ground-roll attenuation and reflection imaging with reduced field effort. We finally show that the H/D spectral ratio provides a local estimate of the Rayleigh wave dispersion curve(s). The proposed method does not rely on travel time analysis and applies to waves originating from any directions, therefore it is particularly suitable to process Rayleigh wave dominated ambient noise, as illustrated with a real data example collected in urban environment (Zurich, Switzerland). In brief, we propose a novel land acquisition and processing strategy that does not require dense sensor arrays nor active sources for cost-effective near-surface characterization.