Multiscale traveltime tomography

Abstract

Nonuniqueness in traveltime tomographic velocity analysis is largely the result of insufficiency in model parameterization. Traditional tomography parameterizes the model with nonoverlapping cells. Such single‐scale tomography (SST) from inverting traveltimes only may produce underdeterminacy at places of insufficient ray coverage. To cope with the poor and uneven ray coverage, a multiscale tomography (MST) method is devised, which is a simultaneous application of many overlapping SSTs with different cell sizes. The MST decomposes velocity anomalies into components in a set of submodels of different cell sizes. At each model location the cells containing higher consistency between data contributions gain more from the inversion. The final MST model is a postinversion superposition of all submodel solutions. The MST method is applicable to the determination of interval velocity and interface geometry using turning rays or reflection rays. In comparison with the SST, superior results of the MST are seen in synthetic examples. The MST method is used to construct the 3D crustal velocities of southern California using the first arrivals from local earthquakes. While the SST and MST models may fit the data equally well, the MST model is smoother and geologically more plausible than the SST model.