OS2B-5:P- And S-Wave Velocity Structure In A Gas Hydrate Reservoir In The Krishna-Godavari Basin, Eastern Indian Margin

N. SATYAVANI*1, Kalachand SAIN1, Harsh.K.GUPTA2
1. CSIR-National Geophysical Research Institute, India; 2. National Disaster Management Authority, Govt of India, India

    Presence of gas hydrate in marine sediments can be indicated by identifying an anomalous seismic reflector, known as the bottom simulating reflector or BSR that has the reverse polarity, simulates the seafloor, and cuts across inclined sedimentary strata. The BSR is a physical boundary between gas-hydrate bearing sediments above and free gas bearing sediments below. The distribution and quantification of gas hydrates / free gas rely heavily on accurate estimation of seismic velocity. Mostly, gas-hydrates are estimated by applying rock physics modeling to P-wave (Vp) velocity derived from the large-offset (with 5 km streamer) multi-channel seismic (MCS) data. Such an estimate may be inaccurate, if we derive the S-wave velocity (Vs) from Vp using a standard Vp - Vs relation. Both the Vp and Vs depend on the shear modulus of the sediment that varies with the gas hydrate / free gas concentration. In this paper, we derive both the  Vp and Vs in the Krishna-Godavari (KG) basin in the eastern Indian margin using the MCS and ocean bottom seismic (OBS) data that were acquired in 2010 by CSIR-National Geophysical Research Institute (Sain et al. 2012) to delineate the gas-hydrates reservoir and quantify the resource potential. We have derived the two-dimensional Vp model from travel time inversion of identifiable phases from closely spaced CDP gathers of MCS data and the H-component of the OBS data along a seismic line. We have also derived the two-dimensional Vs model from the travel time inversion of corresponding phases of the X-component of the OBS data along the same line.

    Travel times of various phases for 5 OBSs spaced at 1.5 km interval and recorded by shots at 50 m intervals, and travel times of shots (with 250 m intervals) from MCS data along the 10 km long line were picked for modeling. To constrain the velocity model, travel times of P to S mode converted phases on the radial (horizontal) component of the OBS data were picked. Finally, we derived six layered Vp, Vs and Poisson’s ratio models that show undulating structures, indicative of intense activity. However, we notice that the BSR topography approximates the seafloor accompanied by velocity inversion. Both the P- and S-wave velocities above the BSR shows a strong lateral variation with Vp varying between 1.83 to 2.0 km/s. The average thickness of this high velocity hydrate bearing sediments is ~ 40 m, underlain by an undulating low velocity layer with laterally varying velocity of 1.45 to 1.52 km/s. The Poisson’s ratio decreases from 0.49 at the seafloor to about 0.38 at the BSR.  The S-wave velocity increases with depth attaining a velocity of ~550 m/s just above the BSR. It shows a small decrease as compared to the P-wave below the BSR. The rock physics modeling shows that the saturation of gas-hydrates varies between 10 to 25%.