The problem of velocity inversion in refraction seismics: some observations from modelling results

The applicability of seismic refraction profiling for the detection of velo city inversion, which is also known as a low-velocity layer (LVL), is inves tigated with the aid of synthetic seismogram computations for a range of mo dels. Our computational models focus on the inherent ambiguities in the int erpretation of first-arrival time delays or 'skips' in terms of LVL model p arameters. The present modelling results reveal that neither the measure no r even the existence of a shadow zone and/or a time shift (skip) in first a rrivals is necessarily indicative of an LVL. Besides attenuation effects, t he caplayer velocity gradient is a critical parameter, determining the term ination point of the cap-layer diving wave and thus the time skip. We suggest that shallow LVLs can be delineated more reliably by traveltime and amplitude modelling of coherent phases reflected from their top and bot tom boundaries, often clearly observed in the pre- and near-critical ranges in seismogram sections of refraction profiling experiments with a close re ceiver spacing. We demonstrate the applicability of this approach for a fie ld data set of a refraction profile in the West Bengal Basin, India. The in ferred LVL corresponds to the Gondwana sediments underlying the higher-velo city layer of the Rajmahal Traps. This interpretation is consistent with th e data from a nearby well in the region.