High‐Resolution Shallow Structure at Brady Hot Springs Using Ambient Noise Tomography (ANT) on a Trenched Distributed Acoustic Sensing (DAS) Array

An ~8700 m fiber-optic cable is installed in surface trenches at the Brady Hot Springs geothermal site in Nevada, USA. A distributed acoustic sensing (DAS) system is applied to use this fiber-optic cable to record seismic ambient noise. Noise cross-correlation functions (NCFs) of channel pairs on one linear DAS segment and on two in-line DAS segments are obtained, from which the Rayleigh wave signal emerges. The dispersive group and phase velocities in a high-frequency band of 2–18 Hz are measured with two different methods (multichannel analysis of surface waves (MASW) and frequency-time analysis (FTAN)) and then used to invert shear wave velocities at shallow depths. The obtained velocity model successfully reveals low-velocity zones (LVZs) in warm ground areas. Our results demonstrate that using a DAS array with ambient noise tomography (ANT) to image near-surface seismic structure is feasible.