Seismic Methods for Recognition and Evaluation of Gas-Hydrates

Gas-hydrates are crystalline form of water and methane that form at high pressure and low temperature. They have attracted the global attention due to their wide-spread occurrences in the permafrost and outer continental margins; potential as major energy resources; role in climate change etc. It is estimated that the carbon in gas-hydrates is two times the carbon content in fossil fuel reserves, and hence gas-hydrates are assumed to be the future potential energy resource. Presence of gas-hydrates makes the sediments impervious and hence trap free-gas underneath. Therefore, recognition and quantification of gas-hydrates are very essential for evaluating the resource potential and assessing the impact of environmental hazard. Gas-hydrates are mostly recognized by identifying an anomalous reflector, known as the bottom simulating reflectors or BSR on seismic data based on its characteristic features. With a view to make quantitative assessment of gas-hydrates, we have developed some approaches based on seismic traveltime tomography; waveform inversion; Amplitude versus offset (AVO) inversion; AVO attributes coupled with rock-physics or effective medium modeling. We have also demonstrated that some attributes like reflection strength, blanking, instantaneous frequency and attenuation can be used to detect if we can not identify BSR on seismic section. We have also shown that studies of other proxies such as pockmark and gas escape features such as faulting or gas-chimney offer indirect evidences for the identification of gas-hydrates in absence of BSR. As pure gas-hydrates have much higher seismic velocities than that of host sediments, presence of gas-hydrates increases the seismic velocities, whereas free-gas below the hydrates-bearing sediments decreases the velocities. Thus estimating seismic velocities using AVO modeling coupled with rock physics theory or delineating accurate velocities using full-waveform inversion followed by effective medium modeling provides an excellent tool for evaluating gas-hydrates and free-gas across a BSR. We will present all these approaches and show their application to marine multichannel seismic data.