P diving wave tomography using ambient noise recorded at Long Beach

Thursday, 18 December 2014
Nori Nakata1, Jason P Chang1 and Jesse Fisher Lawrence2, (1)Stanford University, Stanford, CA, United States, (2)Stanford University, Los Altos Hills, CA, United States
We retrieve P diving waves by applying seismic interferometry to ambient noise records observed at Long Beach, California, and invert travel times of these waves to estimate 3D P-wave velocity structure. The ambient noise is recorded by a 2D dense and large network (2500 receivers with 100-m spacing) at Long Beach, CA. In contrast from surface-wave extraction, body-wave extraction is much harder for surface acquisition because body-wave energy is smaller than surface waves. The crosscorrelation functions at a receiver pair obtained from ambient-noise data does not show clear body waves. Although we can reconstruct body waves when we stack correlation functions over all receiver pairs, we need to extract body waves at each receiver pair separately for imaging spatial heterogeneity of subsurface structure. Therefore, we employ two filters, which isolate body-wave energy and enhance signal-to-noise ratio, after correlation to seek body waves between single receiver pairs. After these steps, we can reconstruct clear body waves from each virtual source. As an application of using extracted body waves, we estimate 3D P-wave velocities with travel-time tomography. The velocities estimated from body waves are much higher resolution than those from surface waves.