Processing and modelling of short-offset seismic refraction-coincident deep seismic reflection data sets in sedimentary basins: an approach for exploring the underlying deep crustal structures

Abstract

SUMMARY We present a new approach to extract deep crustal velocity structure from short-offset seismic refraction sections acquired over sedimentary basins. A coincident deep seismic near-vertical (NV) reflection stack section is used to constrain the derived crustal structure. The high-amplitude free-surface multiples, often found on refraction sections due to high velocity gradients in shallow sedimentary layers, are routinely modelled for velocity and Q structure of the sedimentary strata. These multiples almost completely mask most of the arrivals, including reflected phases from crustal interfaces. By application of velocity filtering with a rejection band that includes the apparent velocity of the free-surface multiples, they can, however, be significantly attenuated. The relatively weak signals, notably the deep crustal reflections in the subcritical (SC) range, can thus be well developed. This approach is demonstrated here by application to a short-offset refraction section in two steps: initially, the free-surface multiples are modelled for obtaining the sedimentary basin velocity structure, later they are substantially attenuated by velocity filtering to enhance the weak SC reflections, further modelled for the velocity structure of the deep crust underlying the west Bengal sedimentary basin, India. The stack section obtained by processing the deep seismic NV reflection data set, coincident with the short-offset refraction section, is consistent with and well substantiates the derived model of the crustal velocity structure in the region.