Urban Near-Surface Seismic Monitoring Using Distributed Acoustic Sensing

Urban subsurface monitoring requires high temporal-spatial resolution, low maintenance cost, and minimal intrusion to nearby life. Distributed acoustic sensing (DAS), in contrast to conventional station-based sensing technology, has the potential to provide a passive seismic solution to urban monitoring requirements. Based on data recorded by the Stanford Fiber Optic Seismic Observatory, we demonstrate that near-surface velocity changes induced by the excavation of a basement construction can be monitored using existing fiber optic infrastructure in a noisy urban environment. To achieve satisfactory results, careful signal processing comprising of noise removal and source signature normalization are applied to raw DAS recordings. Repeated blast signals from quarry sites provide free, unidirectional, and near-impulsive sources for periodic urban seismic monitoring, which are essential for increasing the temporal resolution of passive seismic methods. Our study suggests that DAS will likely play an important role in urban subsurface monitoring. Plain Language Summary Seismic monitoring can provide crucial information about near-surface changes due to natural or manmade activities. However, the high cost and the "after-effect" nature of conventional station-based monitoring methods limit their application in urban environments where near real-time and meter-scale resolution are required. Distributed acoustic sensing has the potential to achieve all requirements utilizing existing communication infrastructure. Using Stanford Fiber Optic Seismic Observatory, we demonstrate that its recordings of quarry blasts 13.3 km away carry important subsurface velocity information within the footprint of the array. These short bursts of quarry blast signals provide us free, unidirectional, and repetitive sources that sample the urban subsurface at an interval frequent enough for monitoring. We observe large velocity decrease from the recordings close to the excavation site. Our study suggests that telecommunications fiber repurposed for distributed acoustic sensing will potentially play an important role in many urban subsurface monitoring applications.