Kalachand Sain

Gas-hydrates in Krishna-Godavari and Mahanadi basins: New data

KALACHAND SAIN, MAHESWAR OJHA, NITTALA SATYAVANI, G.A. RAMADASS, T. RAMPRASAD, S. K. DAS and HARSH GUPTA CSIR National Geophysical Research Institute, Uppal Road, Hyderabad 500 007 National Institute of Ocean Technology, Velachery-Tambaram Main Road, Chennai 600 100 CSIR-National Institute of Oceanography, Dona Paula, Goa 403 004 Ministry of Earth Sciences, Prithvi Bhavan, Lodhi Road, New Delhi 110 003 Email: kalachandsain@yahoo.com

Inversion of wide-angle seismic reflection times with damped least squares

The wide‐angle seismic reflection times appearing in the postcritical range are used extensively to image the crustal structure in deep seismic sounding investigations. The most commonly used method to calculate interval velocities and thicknesses of a stack of horizontal layers is based on Dix’s hyperbolic equation that requires traveltimes at zero offset and a prior estimate of root‐mean‐square (rms) velocity. Since the wide‐angle reflection times are represented by the nonhyperbolic Taner and Koehler series, a forced fit of such a data set by a hyperbolic equation causes large errors in the estimation of interval velocities. We propose a fast and simple method to determine the interval velocities from wide‐angle reflection times by minimizing the squared errors between the observed traveltimes and the forward response using a damped least‐squares technique. The forward response is calculated, including higher order terms of the reflection series, through Chebychev (orthogonal) polynomial approximations...

Geothermal modeling for the base of gas hydrate stability zone and saturation of gas hydrate in the Krishna-Godavari basin, eastern Indian margin

The passive eastern Indian margin is rich in gas hydrates, as inferred from the wide-spread occurrences of bottom-simulating reflectors (BSRs) and recovery of gas hydrate samples from various sites in the Krishna Godavari (KG) and Mahanadi (MN) basins drilled by the Expedition 01 of the Indian National Gas Hydrate Program (NGHP). The BSRs are often interpreted to mark the thermally controlled base of gas hydrate stability zone (BGHSZ). Most of the BSRs exhibit moderate to typically higher amplitudes than those from other seismic reflectors. We estimate the average geothermal gradient of ∼40°C/km and heat flow varying from 23 to 62 mW/m2 in the study area utilizing the BSR’s observed on seismic sections. Further we provide the BGHSZ where the BSR is not continuous or disturbed by local tectonics or hidden by sedimentation patterns parallel to the seafloor with a view to understand the nature of BSR.Since, gas hydrate bearing sediment has higher electrical resistivities than that of the host sediment, we estimate two levels of gas hydrates saturations up to 25% in the depth interval between 70 to 82, and less than 20% in the depth interval between 90 to 104 meter below the seafloor using the resistivity log data at site 15 of NGHP-01.

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.

Interpretation of first arrival travel times in seismic refraction work

The necessary condition for the seismic refraction method to succeed is that the refracted first arrivals from each layer in a multilayered earth system should be detected on a seismogram as first arrivals, and this is possible only when velocities of all underlying layers are successively greater. The usual procedure to interpret the refraction travel times is to fit such a data set with several intersecting straight lines by employing a visual technique which may lead to errors of subjective judgment, as the velocity model depends on the selection of various line segments through the data. To remove the visual fit we propose here a layer stripping method based on minimum intercept time, apparent velocity, rms residual, and maximum data points by least-squares fitting to yield several intersecting straight lines. Once data are segmented out, the conventional equations can be used to determine the velocity structure.

Seismic Insights into Bottom Simulating Reflection (BSR) in the Krishna-Godavari Basin, Eastern Margin of India

The multichannel seismic data along one long-offset survey line from Krishna-Godavari (K-G) basin in the eastern margin of India were analyzed to define the seismic character of the gas hydrate/free gas bearing sediments. The discontinuous nature of bottom simulating reflection (BSR) was carefully examined. The presence of active faults and possible upward fluid circulation explain the discontinuous nature and low amplitude of the BSR. The study reveals free gas below gas hydrates, which is also indicated by enhancement of seismic amplitudes with offsets from BSR. These findings were characterized by computing seismic attributes such as the reflection strength and instantaneous frequency along the line. Geothermal gradients were computed for 18°C and 20°C temperature at the depth of BSR to understand the geothermal anomaly that can explain the dispersed nature of BSR. The estimated geothermal gradient shows an increase from 32°C/km in the slope region to 41°C/km in the deeper part, where free gas is present. The ray-based travel time inversion of identifiable reflected phases was also carried out along the line. The result of velocity tomography delineates the high-velocity (1.85–2.0 km/s) gas hydrate bearing sediments and low-velocity (1.45–1.5 km/s) free gas bearing sediments across the BSR.

Use of postcritical reflections in solving the hidden‐layer problem of seismic refraction work

In a multilayered earth system, when the thickness of a layer compared to the overlying layer is small, refraction signal from that layer may not appear as a first arrival. In such a case, the analysis of first-arrival refraction data cannot detect the layer and this leads to errors-overestimation of the thickness of the overlying layer and underestimation of depths to all underlying layers. This is known as the hidden-layer problem. In a field situation, hidden layer(s) can be identified with the help of high-energy postcritical reflections, which appear as strong later arrivals. In this paper, we describe an approach to calculate the thickness of the overlying layer and the thickness and velocity of the hidden layer based on the traveltime inversion of postcritical reflections from the top and bottom of the hidden layer. The blind-zone thickness is also calculated using the estimated velocity of the hidden layer and the thickness of the overlying layer. The applicability of the method is illustrated with the help of both synthetic and field data.

2-D velocity structure in Kerala-Konkan basin using traveltime inversion of seismic data

The existence of gas-hydrates in marine sediments increases the seismic velocity, whereas even a small amount of underlying free-gas reduces the velocity considerably. The change in velocities against the background (without gas-hydrates and free-gas) velocity can be used for identification and assessment of gas-hydrates. Traveltime inversion of identifiable reflections from large offset multi channel seismic (MCS) experiment is an effective method to derive the 2-D velocity structure in an area. We apply this method along a seismic line in the Kerala-Konkan (KK) offshore basin for delineating the gas-hydrates and free-gas bearing sediments across a bottom simulating reflector (BSR). The result reveals a four layer 2-D shallow velocity model with the topmost sedimentary layer having velocity of 1,680–1,740 m/s and thickness of 140–190 m. The velocity of the second layer of uniform thickness (110 m) varies from 1,890 to 1,950 m/s. The third layer, exhibiting higher velocity of 2,100–2,180 m/s, is interpreted as the gas-hydrates bearing sediment, the thickness of which is estimated as 100 to 150 m. The underlying sedimentary layer shows a reduction in seismic velocity between 1,620 to 1,720 m/s. This low-velocity layer with 160–200 m thickness may be due to the presence of free-gas below the gas-hydrates layer.

Velocity-porosity and velocity-density relationship for shallow sediments in the Kerala-Konkan basin of western Indian margin

During the expedition 01 of the National Gas Hydrate Program (NGHP), drilling and coring were carried out at one site in the Kerala-Konkan basin on the west cost of India to validate gas hydrate. Drilling/coring results show a homogenous sequence of oozes and explains that the reflector, which was identified as a bottom-simulating reflector (BSR) on seismic section, is mainly due to changes in formation density because of less clay content in carbonate-rich sediment. Downhole logs collected at this site are of good quality and have been used to establish empirical relationship between the P-wave velocity (VP), S-wave velocity (VS), density (ρ) and porosity (ϕ). The established relations show very good fit with high R2 value (>0.73), and can be used for further studies in this region. Well known existing empirical formulas between VP, VS, ρ and ϕ deviate significantly from our established relations.

Direct calculation of thicknesses for high-velocity and underlying low-velocity layers using post-critical reflection times in a seismic refraction experiment

Abstract In a multi-layered earth system, when the velocity of a layer is lower than that of the overlying layer, the former cannot be recognized on the time-distance plot resulting in an overestimation of the thickness of the overlying layer and the depths of all subsequent deeper layers. This low-velocity layer ( lvl problem in seismic refraction work cannot be solved using traveltimes of first arrivals alone. Use of post-critical reflections (observed strongly after first arrivals on a seismogram) from the bottom of the lvl provides valuable information regarding the solution to the lvl problem. Here, we propose a layer-stripping method applied to the strongly observable post-critical reflection times from the bottom of the lvl to calculate the thickness of the overlying high-velocity layer ( hvl ) and that of the underlying lvl directly. A-priori information for the velocity of the lvl from other seismic evidence is utilized. We show in this paper that even if we use traveltimes of both first arrivals and wideangle reflections from the bottom of the lvl , we cannot calculate three parameters (i.e. the thickness of hvl and, the thickness and velocity of ( lvl ) unequivocally.