High-resolution Shallow Structure Revealed with Ambient Noise Tomography on a Dense Array

Abstract

In March 2016, the PoroTomo Team deployed a dense seismic array to image the structure of the Brady Hot Springs geothermal reservoir in Nevada. At this site, a 4 km by 1.5 km elliptical subsidence area was observed by analyzing InSAR images (Ali et al., 2016). The array was composed of 244 short-period, three-component geophones and ~8,600 m of distributed acoustic sensing (DAS) fiber-optic cable installed in surface trenches plus ~350 m installed in a borehole. The geophone array provided about 60 m spatial sampling whereas sampling of the surface DAS was nominally 1 m. The acquisition system provided 15 days of continuous records that were used to calculate noise cross-correlation functions (NCFs). NCF cans be treated as empirical Green’s functions and used in ambient noise tomography. The surface wave dispersion curves were obtained from the NCFs between geophones pairs and pairs of DAS channels. The dispersion curves were inverted for shear wave velocity profiles in different locations of the array. A low velocity zone is associated with the area of subsidence obtained from InSAR.