Measuring seasonal velocity variations on an urban DAS array

Abstract

Distributed acoustic sensing (DAS) converts a deployed optical fiber into a dense seismic array. Owing to low-cost maintenance, fixed infrastructure, and non-intrusiveness, DAS-based seismic acquisition provides a cost-effective near-surface monitoring network, especially when using existing telecommunication fiber in urban settings. Applying seismic interferometry methods, one can convert the ambient seismic signals into crosscorrelated virtual shot gathers containing subsurface information. We present a case study using a 10-month ambient DAS data set acquired on a fiber array deployed in the metropolitan area of Perth, Australia. We investigate whether long-term seasonal variations can be recovered from ambient urban waveforms on such a DAS array. Weekly stacked interferometric virtual shot gathers reveal bulk 3-4% variations in surface-wave velocities between winter and spring months, which are likely related to variations in near-surface groundwater content. This observation suggests that urban DAS fiber networks may provide an opportunity for urban-subsurface monitoring of subsurface groundwater variations.