Irregular interface parametrization in 3-D wide-angle seismic traveltime tomography

We investigate the potential advantages and pitfalls of using an irregular interface parametrization in 3-D wide-angle seismic traveltime tomography. Several synthetic tests are performed using an interface surface consisting of a mosaic of cubic B-spline surface patches and a source-receiver array designed to produce a highly variable distribution of refracted and reflected ray paths. In such circumstances, an irregular parametrization can be adapted to suit the data coverage, resulting in fewer parameters being needed to describe the solution model, faster computation time and a better determined inverse problem. We demonstrate that a judicious parameter distribution can also result in a solution that is well constrained everywhere, satisfies the data to an acceptable level and extracts more information from the data than a regular parametrization. However, introducing an irregular parametrization means that the minimum wavelength of structure permitted in the model will vary both spatially and directionally. Careful consideration of surface patch size and shape, in addition to resolution estimates and data fit, is therefore required to meaningfully interpret this class of solution. An application of the irregular parametrization method to 3-D wide-angle data collected in Tasmania is also presented, and reveals several features consistent with the known geology that were not recovered with a regular parametrization.