Structure of the crystalline basement in the West Bengal basin, India, as determined from DSS studies

Abstract

S U M M A R YDeep seismic sounding data were acquired in the West Bengal basin, India, along two profiles: (i) Bishnupur-Palashi-Kandi, along a line about 227 km long in the northsouth direction and (ii) Taki-Arambagh, along a line about 120 km long in the east-west direction. Seismic refraction and wide-angle reflection data were recorded by continuous profiling using two 60-channel digital seismic units (DFS-V) with an 80 m geophone group interval and 4 ms sampling rate. These data were interpreted in order to delineate the basement configuration. The 2-D models of the seismic data both indicate a five-layer velocity structure above the Archaean crystalline basement (5.9-6.2 km s-I). A low-velocity layer (4.0 km s-') is inferred immediately above the basement in the shelf region of the basin corresponding to the Gondwana sediments (Upper Carboniferous to Lower Triassic) below the Rajmahal Traps (Upper Jurassic to Lower Cretaceous) of 4.6 to 4.8 km s-' velocity, which is also confirmed from the nearby well data. The results along the Taki-Arambagh profile and the drilling results at the Jaguli (J-1) well are used to investigate whether Gondwana sediments and the Rajmahal Traps exist in the deep part of the Bengal basin. An additional layer of velocity 5.2-5.3 km s-', delineated in the Palashi-Kandi profile overlying the basement, may correspond to the Singhbhum group of rocks of the Proterozoic.A structural contour map of the basement prepared from the present results indicates a south-easterly dip of the basement in general. The depth of the basement on the stable shelf of the basin gently increases to about 8 km and dips steeply, plunging to a maximum depth of 14 km in the deep basin. No structural high that can be related to the 'Calcutta gravity high' is found in the basement around the Hooghly River.

Velocity Structure of the West-Bengal Sedimentary Basin, India along the Palashi-Kandi Profile Using a Travel-time Inversion of Wide-angle Seismic data and Gravity Modeling – An Update

“…The present results obtained along the Palashi-Kandi profile in the Bengal basin provide an update of the results published earlier by KAILA et al (1996). KAILA et al (1996) derived the velocity model mainly using the first arrival seismic data.…”

“…The basement starts rising again to a value of 4.9 km near the northern end of the profile. A comparison of the present model with that derived earlier by KAILA et al (1996) shows that the alluvium (first layer) *700 m thick at 25-35 km profile distance, whereas the previous model depicted only 300 m alluvium at about the same distance of the profile. The Rajmahal trap (fourth layer) shows the velocity variation of 4.5-4.7 kms -1 unlike an average velocity of 4.6 km/s in the earlier model.…”

“…The magnitude of travel-time 'skip' and the travel times of the wideangle reflected phases from the top and bottom of the LVZ, and the refraction segment of the overlying high-velocity trap layer facilitate resolving this problem. Hence the present model is better resolved than the model derived earlier by KAILA et al (1996). We have also provided a measure of model parameter resolution and uncertainty of estimated model parameters.…”

“…KAILA et al (1996) derived the velocity model mainly using the first arrival seismic data. Here we derive the velocity model, which is better constrained from the inversion of both refraction and wideangle reflection data.…”

“…The data were recorded during the night to avoid cultural noise. KAILA et al (1996) derived the basement configuration and the overlying sedimentary formations mainly from the first arrival seismic data, and the sub-basement crustal structure from the wide-angle reflection data. The data were analyzed by the 2-D forward modeling technique (CERVENY and PSENCIK, 1981).…”