Analysis of OBS and MCS data offshore Oregon: Estimation of elastic properties of gas hydrates

Abstract

The inversion of seismic data offers a tool for the remote extraction of physical properties of the sediments from surface-recorded data. Physical properties of the gas hydrate-bearing sediments derived from inversion of seismic help us identify the presence of gas hydrates. They can also be used to study their character, formation and distribution, and to estimate the amount of gas hydrate and/or free gas that may be present in the sediments. A 3-D multi-channel seismic (MCS) and ocean bottom seismometer (OBS) survey in the Hydrate Ridge, offshore Oregon was conducted to image structures controlling the migration of methane-rich fluid and free gas and to map the gas-hydrate distribution. We first obatined preliminary Vp and Vs profiles from OBS data by interactive analysis and then used them as a starting model to estimate Vp from the streamer data by waveform inversion. The results from our inversion and interpretation study in Hydrate Ridge reveal that: (1) both 3-D streamer and OBS data show a strong BSR indicating the presence of gas hydrate above and free gas below; (2) interactive P- and S-wave velocity analysis of OBS data allows us to identify the presence of a “conversion surface” in the gas hydrate-bearing sediments; (3) inverted velocity profiles show a lowvelocity layer existing below the sea floor and above the normal gas hydrate, suggesting a new geological model of gas hydrates; (4) two types of hydrate fabrics, massive and porous hydrates, observed by deep-towed video survey, were identified in the P-wave velocity profiles, and three main layers of gas hydrate-sediments separated by the conversion surface and BSR are distinguished; (5) the profiles reflecting the physical properties of sediments, such as the P-wave velocity, acoustic impedance and Poisson’s ratio profiles, are able to map the distribution of gas hydrates and show very similar trends of lateral variation of the main layers; (6) a series of faults in the accretionary complex under the ridge not only offer pathways for methane and fluid ascending from deeper layers but also control the distribution of the porous hydrates with low velocity below the seafloor, and (7) Hornbach et al. (2003) suggest their results using velocity analysis of seismic reflection data on the Blake Ridge is the first direct seismic detection of concentrated hydrate confirmed by velocity analysis. Our results of direct inversion of seismic data extend these results to greater resolution of the entire seismic data set. Further, our results may be the first seismic indication of visually observed porous hydrate zone.