Ambient Noise Interferometry from DAS Array in Underground Telecommunications Conduits

Ambient noise interferometry allows us to characterize the near surface without the cost and regulatory constraints of controlled seismic sources. Geophone arrays can be expensive to install and maintain, but trenched distributed acoustic sensing (DAS) arrays have shown promise for reducing the maintenance cost of ambient noise acquisition. In this paper we test a novel, easily installed acquisition system using fibre in existing underground telecommunications conduits for ambient noise collection on the Stanford University campus. This installation only uses gravity to hold the fibre in place in the air-filled conduits, and the conduit installation method varies some throughout the site. We outline the basic theory of ambient noise interferometry for DAS measurements, present a method of using nearby earthquake response as a sensor coupling proxy, and present signal processing methods for coping with a variety of noise sources throughout the array. We show cross-correlations and dispersion calculations along a subset of the array expected to primarily yield Rayleigh waves.