Urban Near‐Surface Seismic Monitoring Using Distributed Acoustic Sensing

16 Urban subsurface monitoring requires a system with high temporal-spatial resolution, 17 low maintenance cost, and minimal intrusion to urban life. Distributed acoustic sens18 ing (DAS), in contrast to conventional station-based sensing technology, has the poten19 tial to provide a passive seismic solution to urban monitoring requirements. Based on 20 data recorded by the Stanford Fiber Optic Seismic Observatory, we demonstrate that 21 near-surface velocity change induced by the excavation of basement construction can be 22 monitored using existing fiber optic infrastructure in a noisy urban environment. To achieve 23 the superior results, careful signal processing with noise removal and source signature 24 normalization are applied to raw DAS recordings. The repeated blast signals from quarry 25 sites provide free, unidirectional, and near impulsive sources for periodic urban seismic 26 monitoring, which are essential for increasing the temporal resolution of passive seismic 27 methods. Our study suggests that DAS will likely play an important role in urban sub28 surface monitoring. 29 1 Plain Language Summary 30 Seismic monitoring can provide crucial information about near-surface changes due 31 to natural or manmade activities. However, the high cost and the ”after-effect” nature 32 of conventional station-based monitoring methods limit their application in urban en33 vironments where near real-time and meter-scale resolution are required. Distributed acous34 tic sensing (DAS) has the potential to achieve all requirements utilizing existing com35 munication infrastructure. Using Stanford Fiber Optic Seismic Observatory, we demon36 strate that its recordings of quarry blasts 13.3 km away carry important subsurface ve37 locity information within the footprint of the array. These short bursts of quarry blast 38 signals provide us free, unidirectional, and repetitive sources that sample the urban sub39 surface at an interval frequent enough for monitoring. We observe large velocity decrease 40 from the recordings close to the excavation site. Our study suggests that telecommuni41 cations fiber repurposed for DAS will potentially play an important role in many urban 42 subsurface monitoring applications. 43 2 Introduction 44 Characterizing and monitoring changes in the top tens of meters of the Earth’s sub45 surface will play a significant role to satisfy the increasing need of urban sustainability 46 and resilience (Díaz et al., 2017). Near-surface changes due to natural or man-made events 47 may lead to hazards including ground subsidence (Tran & Sperry, 2018), sinkholes (Dahm 48 et al., 2011; Gutiérrez et al., 2014), and landslides (Renalier et al., 2010; Schenato et al., 49 2017), which may result in direct casualties of urban residents and damages to existing 50 infrastructure (Douglas, 2004). Many such subsurface changes manifest themselves as 51 temporal variations in geophysical properties (such as velocity, attenuation, electric con52 ductivity, gravity, etc.) before catastrophic hazards occur, which can be monitored and 53 predicted by geophysical prospecting using seismic, electric, electromagnetic and grav54 itational methods. 55 Compared to conventional geophysical exploration for resources, near-surface mon56 itoring in urban environments has the unique acquisition requirements of: 1) high spa57 tial resolution towards meter-scale; 2) high temporal resolution towards real-time data 58 collection and daily warning; 3) low maintenance for long term monitoring; and 4) min59 imal intrusion to urban life. These requirements are met by a densely distributed, fre60 quently recording, easy to maintain, passive system that we present in this paper: a pas61 sive seismic monitoring system enabled by distributed acoustic sensing (DAS). A DAS 62 array measures strain along kilometers of optical fiber, producing large datasets with kilo63 hertz time sampling and at sub-meter channel spacing (Parker et al., 2014). Over the 64 past dozen years, DAS has been a rapidly evolving technology for vertical seismic pro65