Delineation of shallow structure and the Gondwana graben in the Mahanadi delta, India, using forward modeling of first arrival seismic data

We derived 2-D shallow velocity structure, in order to delineate low-velocity Gondwana sediments underlain by high-velocity volcanics in the north, to determine sediment thickness in the south and to map the basement configuration along the N-S trending Konark-Mukundpur profile, Mahanadi delta, India. We applied the ray-based 2-D forward modeling technique to the first arrival seismic refraction traveltime data. The shallow velocity structure has been derived to a depth of 4 km. The main features of the model are the Konark depression (0-15 km), the Bhubaneswar ridge (15-50 km), the Cuttack depression or the Gondwana graben (50-100 km) and the Chandikhol ridge (100-115 km) along the profile. The overall structures represent alternate graben and horst features. In the south, the Konark depression is composed of three sedimentary formations with velocities of 1.75, 2.4 and 4.0 km/s and attains a maximum depth of 2.9 km at 9 km profile distance. To the north, a low velocity (4.0 km/s) layer of basinal shape, believed to be the Gondwana sediments, is delineated in the Gondwana graben using the `skipʹ phenomenon of travel time data. This layer with a maximum thickness of 1.75 km near Cuttack lies between a thin (~100-300 m) cover of high-velocity volcanic (5.25 km/s) and underlying basement (6.0 km/s) rocks. The model indicates upwarping of basement on either side of the Gondwana graben.