Toward improved distributed acoustic sensing sensitivity for surface-based reflection seismics: Configuration tests at the Aquistore CO2 storage site

Abstract

Alternative fiber configurations have been tested in an attempt to improve the sensitivity of surface-deployed distributed acoustic sensing (DAS) fiber cables for the purpose of recording steep-angle P-wave reflections. Four alternative fiber configurations were deployed at the Aquistore CO2 storage site to record 401 dynamite shots during a 3D vertical seismic profiling survey. The test cable comprised horizontal configurations (straight fiber, helixes, and asymmetric helixes) buried in a shallow trench and vertical configurations (straight fiber and helixes) deployed in 3.5 m drillholes. Evaluation focused on deep reflections with two-way traveltimes of 0.8–1.8 s. All of the alternative fiber configurations increased the sensitivity relative to the horizontal straight fiber. Sensitivity was highest for the vertical straight fiber configurations and the asymmetric helixes with sensitivity increases of more than 10 and 5 dB, respectively, and amplitude-variation-with-offset behavior similar to that of a vertical-component geophone for reflections with incidence angles of 0°–15° at the surface and 0°–34° at the reflector. Modeling of the DAS responses explains the general pattern of sensitivity variability among the different configurations, but it does not explain the large range of observed sensitivities.