Laxmidhar Behera

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

Abstract 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 ‘skip’ 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.

Migration Velocity Analysis And Imaging For Tilted TI Media

Tilted transversely isotropic (TTI) formations cause serious imaging distortions in active tectonic areas (e.g., fold-and-thrust belts) and in subsalt exploration. Here, we introduce a methodology for P-wave prestack depth imaging in TTI media that properly accounts for the tilt of the symmetry axis as well as for spatial velocity variations. For purposes of migration velocity analysis (MVA), the model is divided into blocks with constant values of the anisotropy parameters ǫ and δ and linearly varying symmetry-direction velocity VP0 controlled by the vertical (kz) and lateral (kx) gradients. Since estimation of tilt from P-wave data is generally unstable, the symmetry axis is kept orthogonal to the reflectors in all trial velocity models. It is also assumed that the velocity VP0 is either known at the top of each block or remains continuous in the vertical direction. The MVA algorithm estimates the velocity gradients kz and kx and the anisotropy parameters ǫ and δ in the layer-stripping mode using a generalized version of the method introduced by Sarkar and Tsvankin for factorized VTI media. Synthetic tests for several TTI models typical in exploration (a syncline, uptilted shale layers near a salt dome, and a bending shale layer) confirm that if the symmetry-axis direction is fixed, the parameters kz , kx, ǫ, and δ can be resolved from reflection data. It should be emphasized that estimation of ǫ (with known VP0) in TTI media requires using nonhyperbolic moveout for long offsets reaching at least twice the reflector depth. We also demonstrate that application of VTI processing algorithms to data from TTI media may lead to significant image distortions and errors in parameter estimation, even when tilt is moderate (e.g., 20-30◦). The ability of our MVA algorithm to separate the anisotropy parameters from the velocity gradients can be also used in lithology discrimination and geologic interpretation of seismic data in complex areas.

Sub-basalt Imaging of Hydrocarbon-Bearing Mesozoic Sediments Using Ray-Trace Inversion of First-Arrival Seismic Data and Elastic Finite-Difference Full-Wave Modeling Along Sinor–Valod Profile of Deccan Syneclise, India

Imaging below the basalt for hydrocarbon exploration is a global problem because of poor penetration and significant loss of seismic energy due to scattering, attenuation, absorption and mode-conversion when the seismic waves encounter a highly heterogeneous and rugose basalt layer. The conventional (short offset) seismic data acquisition, processing and modeling techniques adopted by the oil industry generally fails to image hydrocarbon-bearing sub-trappean Mesozoic sediments hidden below the basalt and is considered as a serious problem for hydrocarbon exploration in the world. To overcome this difficulty of sub-basalt imaging, we have generated dense synthetic seismic data with the help of elastic finite-difference full-wave modeling using staggered-grid scheme for the model derived from ray-trace inversion using sparse wide-angle seismic data acquired along Sinor–Valod profile in the Deccan Volcanic Province of India. The full-wave synthetic seismic data generated have been processed and imaged using conventional seismic data processing technique with Kirchhoff pre-stack time and depth migrations. The seismic image obtained correlates with all the structural features of the model obtained through ray-trace inversion of wide-angle seismic data, validating the effectiveness of robust elastic finite-difference full-wave modeling approach for imaging below thick basalts. Using the full-wave modeling also allows us to decipher small-scale heterogeneities imposed in the model as a measure of the rugose basalt interfaces, which could not be dealt with ray-trace inversion. Furthermore, we were able to accurately image thin low-velocity hydrocarbon-bearing Mesozoic sediments sandwiched between and hidden below two thick sequences of high-velocity basalt layers lying above the basement.