Ambient noise tomography of three-dimensional near-surface shear-wave velocity structure around the hydraulic fracturing site using surface microseismic monitoring array

Surface microseismic monitoring array has been widely used to monitor hydraulic fracturing process for developing unconventional oil/gas reservoirs. Here we take advantage of relatively dense surface seismic stations for monitoring the fracturing process of a shale gas reservoir in southwest China, to determine the shallow Vs structure around the fracking site using the ambient noise tomography. This is because having a detailed three-dimensional (3D) Vs model is important for 3D seismic structure inversion. For ambient noise tomography, we use 21 days of continuous ambient noise data to extract Rayleigh wave group and phase dispersion data in the period band of 0.1-0.5 s. The 3D Vs structure is inverted by the direct surface wave tomographic method that directly inverts surface wave dispersion data for 3D Vs model. With the spatial coverage of 4 km by 4 km for this surface seismic array, we can determine the Vs structure down to 400 m. Our inversion results show that shear wave velocity in this region can be up to about 3 km/s above 400 m depth, which is consistent with the lithology of drilling core along the well. The obtained model is further validated by checkerboard test and the close relationship between waveform amplitude and the local velocity structure around stations. It is found that large waveform amplitudes are associated with stations located at shallow low velocity anomalies due to surface reverberations. This application shows that ambient noise tomography using local dense seismic array can be used to reliably determine high-resolution 3D shear velocity structure in the near surface.