Geotechnical Site Characterization with 3D Ambient Noise Tomography: Field Data Applications

A new 3D ambient noise tomography (3D ANT) method is presented for geotechnical site characterization. It requires recording ambient noise wavefields using a 2D surface array of geophones, from which cross-correlation functions (CCF) are then extracted and directly inverted to obtain S-wave velocity (Vs) structure. The method consists of a forward simulation using 3D P-SV elastic wave equations to compute the synthetic CCF and an adjoint-state inversion to match synthetic and field CCFs for extraction of Vs. Compared to conventional passive seismic methods using characteristics of Green’s function (GF), the main advantage of the presented method is that it does not require the energy balance at both sides of each receiver pair to retrieve the true GF. Instead, the source power spectrum density is inverted during the analysis and incorporated into the forward simulation to account for source energy distribution for accurate extraction of Vs profiles. The presented 3D ANT method was applied to 3 h of noise recordings from an array of 196 geophones placed on a grid with 5 m spacing at the Garner Valley Downhole Array (GVDA) site in California. The inverted 3D Vs model is found to be consistent with previous invasive and non-invasive geotechnical characterization efforts at the GVDA site.