Direct inversion of velocity interfaces

Seismic tomography traditionally inverts for velocity field on a regularly spaced and fixed-in-space model grid. Such an approach is not able to adequately describe pinchouts or wedge-shaped velocities as commonly seen at basin boundaries, faulted rock beds, and back-arc mantle wedges. A regularly spaced model grid also requires the use of a large number of model variables. This paper shows a different approach called deformable layer tomography (DLT) to directly invert for velocity interfaces. The use of thickness-varying layers allows a much smaller number of model variables for the DLT than the regularly spaced model grid. In the DLT the geologic framework and known velocity range are adopted into the initial reference model, and the best-data-fitting geometry of the velocity interfaces is determined, as illustrated by synthetic and field data examples. Introduction Higher resolution of the depths of velocity discontinuities and morphology of velocity anomalies such as basins, cratons and subducted slabs is necessary for seismic tomography to assist the studies of the Earth’s interior. The current practice of seismic tomography inverts for velocity values on a regular grid of nodes or cells that are fixed-in-space (Aki et al., 1976; Nolet, 1987). As shown in Figure 1a, the tomographic calculations using fixed-in-space grid cannot directly resolve pinchout features, such as basin edges, and cannot reliably estimate the depths of important velocity discontinuities, such as crystalline basement and the Moho. It is also difficult for conventional tomography to explicitly incorporate geologic data and other geophysical data, such as the geometry of the Moho, because the fixed-in-place model parameterization does not allow for the direct data-driven adjustment of the spatial position of velocity boundaries.