Seismic imaging of complex structures by non-linear traveltime inversion of dense wide-angle data: application to a thrust belt

A dense wide-angle data set is used to test a two-step procedure for the separate inversion of first-arrival and reflection traveltimes. Data were collected in a complex thrust belt environment (southern Italy) along a 14 km line, with closely spaced sources (60 m) and receivers (90 m). A fully non-linear tomographic technique, that is specially designed to image complex structures, is applied to over 6400 first-arrival traveltimes in order to determine a detailed velocity model. A bi-cubic spline velocity model parameterization is used. The inversion strategy follows a multiscale approach and employs a non-linear velocity optimization scheme. The tomographic velocity model is adopted as the background reference medium for a subsequent interface inversion, which is aimed at imaging a target upper-crust reflector. The interface inversion method is also based on a multiscale approach and uses a non-linear technique for model parameters (interface position nodes) optimization. It is applied to over 1600 reflection traveltimes of a target event picked both in the near- and in the wide-angle offset range. The retrieved interface is well resolved in central part of the model, where ray coverage mainly includes clear post-critical reflections and the background velocity model is accurate in depth thanks to large offset deep turning rays. The determined velocity and interface models are consistent with VSP data and correlate well with the geometry of known geologic structures. This study shows that the used inversion approach is efficient for target-orientated investigations in complex geologic environment