The northwest-southeast trending Pranhita-Godavari graben, in which Gondwana sediments are deposited, is an important tectonic feature of peninsular India. However, its extension under the northeast-southwest trending coastal sedimentary Godavari Basin is not very well established in seismic exploration, mainly because the older low velocity Gondwana sediments which are exposed at the Bapatla basement ridge are overlain by younger higher veracity Rajahmundry sandstone/basaIt. With the recent find of Gondwana sediments in the deep wells of ONGC in the east Godavari Basin and the importance being attached to the Barakar and Kamthi formations for their oil potential, the extension of the Gondwanas under the entire coastal sedimentary cover assumes more importance. During the recording of Deep Seismic Sounding (DSS) profiles in the Godavari region, extensive refraction surveys were also carried out. One of the DSS profiles recorded along the axis of the Godavari graben, crossed the Bapatla ridge and ended in the Godavari coastal sedimentary basin near Narsapur. The first arrival refraction data, recorded along this profile has shown large skips in the first arrival travel times on six shot points towards southeast of the Bapatla ridge. The skip, which is of the order of 290-800 m, occurs on both the forward and reverse branches of the travel time curves. Such phenomenon is normally associated with the presence of a thick low velocity layer under a thin high velocity overburden. The P-wave velocity in the exposed Rajahmundry sandstones/basalt is about 4.0 km/s. The veracity in die exposed Upper Gondwanas is about 2.8 km/s. The velocity in the older sediments (Lower Gondwanas?) is comparable to that of the Rajahmundry sandstones/basalt. Analysis of the first arrival refraction data from eight shot points southeast of Bapatla ridge, by kinematic forward modelling with two dimensional ray tracing technique; yielded a thickness of 1000-1200 m of velocity 2.8 km/s, under a 200-400 m thick layer of velocity 4.0 km/s. Another layer of velocity 4.2 km/s, with a thickness of 200-2400 m underlies the formation of 2.8 km/s velocity. The delineation of these layers, at least up to Narsapur indicates the extension of the Godavari graben below the coastal basin. The 4.2 km/s-layer is followed by the basement rocks of velocity 5.4 km/s. The maximum depth to the basement is about 4700 m near Narsapur and about 4400 m between the Tanuku and the Bapatla ridges.
