The problem of velocity inversion in refraction seismics: some observations from modelling results 1 V.G. Krishna, 2 N.M. Rao 2 and D. Sarkar 2 Abstract 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. Introduction Seismic record sections from refraction profiles are routinely interpreted in terms of velocity–depth models primarily by modelling refractions and also post-critical reflections. This routine is, as a rule, applicable to ‘normal’ situations when velocity increases with depth. A deviation from this normal situation, where velocity decreases with depth (velocity inversion), is also not uncommon. In exploration seismics some of the potentially important targets are shallow low- velocity (often sedimentary) layers. Such layers are sandwiched between an overlying higher-velocity layer (such as a volcanic flow) and an underlying crystalline basement  1999 European Association of Geoscientists & Engineers 341 Geophysical Prospecting, 1999, 47, 341–357 1 Received July 1997, revision accepted August 1998. 2 National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India.