Triple-difference surface-wave traveltime adjoint tomography

SUMMARY Structural boundaries are often the features of most interest geologically, but imaging them can be difficult due to wavefield scattering and interference caused by the sharp velocity contrasts. One example of this is the apparent Rayleigh-wave anisotropy (1-psi anisotropy) that has been observed near major structural boundaries using seismic arrays. The cause of the apparent anisotropy is the interference between the incident surface wave and waves scattered from velocity discontinuities. In this study, we first investigate the sensitivity of apparent anisotropy measurements to lateral boundary sharpness through 2-D full waveform simulations. We demonstrate that 1-psi anisotropy can vary based on boundary sharpness, station spacing and period of surface waves. We show that a misfit defined using triple-difference traveltimes, that is, the difference in double-difference traveltimes between station pairs with opposite propagation directions, well characterizes the apparent anisotropy. The sensitivity kernel for this triple-difference misfit can be constructed using the adjoint method. We show that triple-difference traveltimes are mainly sensitive to velocity contrasts rather than absolute velocities, in contrast to double-difference traveltimes. With sensitivity kernels constructed, we demonstrate how triple-difference traveltimes can be combined with double-difference traveltimes into a tomography inversion. We show that by including triple-difference traveltimes, seismic inversions converge faster and resolve boundary and average structure better in early iterations, compared to using double-difference traveltimes alone. Recent advancements in dense array experiments could facilitate the application of this method to better delineate tectonic and basin structural boundaries.