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

The applicability of seismic refraction profiling for the detection of velocity inversion, which is also known as a low-velocity layer (LVL), is investigated with the aid of synthetic seismogram computations for a range of models. Our computational models focus on the inherent ambiguities in the interpretation of first-arrival time delays or 'skips' in terms of LVL model parameters. The present modelling results reveal that neither the measure nor even the existence of a shadow zone and/or a time shift (skip) in first arrivals is necessarily indicative of an LVL. Besides attenuation effects, the cap-layer velocity gradient is a critical parameter, determining the termination 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 bottom boundaries, often clearly observed in the pre- and near-critical ranges in seismogram sections of refraction profiling experiments with a close receiver spacing. We demonstrate the applicability of this approach for a field data set of a refraction profile in the West Bengal Basin, India. The inferred LVL corresponds to the Gondwana sediments underlying the higher-velocity layer of the Rajmahal Traps. This interpretation is consistent with the data from a nearby well in the region.