MULTIPLE DIVING WAVES AND HIGH-VELOCITY GRADIENTS IN THE BENGAL SEDIMENTARY BASIN

Seismic refraction and wide-angle reflection studies were carried out in the Bengal basin, India. Trace normalized record sections were prod uced from the digital data recorded in this sedimentary basin. The mos t significant feature observed in these seismograms is the presence of high-energy later arrivals which appear in arcuate shapes with their curvatures concave downwards. These arrivals are identified as free-su rface reflected refractions (or multiple diving waves), which belong t o the family of free-surface multiples. The nature and generation of t hese waves are demonstrated with the help of synthetic examples. It is found that the presence of high-velocity gradients in the top layers is a necessary prerequisite for the generation of these high-energy mu ltiple diving waves. These later arrival multiples, along with the oth er primary phases present on the observed record section from the Beng al basin, have been used to prepare a well-constrained velocity model. The results bring out the presence of very high-velocity gradients (s imilar to 0.3-1.0 km s(-1) km(-1)) in the sedimentary layers of the Be ngal basin, which are an order of magnitude higher than those obtained in other sedimentary basins of India.