--> 3-D Velocity Model of Hydrocarbon Ventsite in Cascadia Region Offshore Vancouver Island, by Mikhail M. Zykov and Ross Chapman; #90035 (2004)

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3-D VELOCITY MODEL OF HYDROCARBON VENTSITE IN CASCADIA REGION OFFSHORE VANCOUVER ISLAND

Mikhail M. Zykov and Ross Chapman
School of Earth and Ocean Sciences, University of Victoria, BC, Canada. E-mail: [email protected]

The Cascadia region offshore Vancouver Island represents one of the provinces in the world with an extensive presence of gas hydrates in the sediments. The first indications of gas hydrate occurrences were observed as far back as 1985 when Bottom Simulating Reflectors (BSRs) were recognized in the seismic sections from the surveys in the area. Since then extensive geophysical and geological survey have been carried out with the overall goals being to describe the distribution of hydrates, estimate the amount of hydrate and the quantity of free gas, and investigate the conditions for hydrate formation and dissociation. The long list of methods consists of numerous seismic surveys (including single- and multichannel; high resolution deep towed (DTAGS); Ocean Bottom Seismometers (OBS)), drilling, piston coring to obtain samples of gas hydrates near the sea floor, heat flow measurements, and sea floor visual with remote operating submersible vehicles. With the increasing amount of new information, some questions were answered while new problems arose. “Blank” zones, observed in the seismic sections, became one of such unexplainable feature. Technically, blank zones are portions of seismic sections with reduced amplitudes of the signal. Their geometry may vary from a chimney shape with the strict vertical or slightly inclined orientation, to a bell shape forms. Such phenomena occur at the other hydrate sites worldwide. By this date there are several reasonable explanations for blanking, but none of them has proved to be fully consistent with all the available geophysical and geological data. Moreover there is a suggestion that the nature of blanking may vary from site to site.

In 1999 the COAMS experiment was carried out in the area near ODP 889/890 drill site and was focused on a hydrocarbon vent site and the associated blank zones. The survey included multichannel the COAMS streamer, a single channel Teledyne streamer, and 5 OBS's. The OBS's were placed in the corners and in the center of a square with the diagonals 2 km in length. The central seismometer was placed just outside of the vent, so that it was enclosed by the OBS array. Twenty two main grid lines with the distance 200 m between lines and the three cross lines were recorded by OBSs using a 40 qu.in. single airgun as a controlled source. The overall target of the survey was to provide new information that can lead to an explanation of the blanking phenomenon.

The OBS and single channel seismic datasets were used to carry out a 3-D travel time tomography inversion in order to obtain the distribution of the velocities in the sediment section above the BSR around vent site. The inversion was done using Jive3D code by James Hobro. This code implements a linearized iterative inversion approach with regularization. It uses the ray tracing theory to obtain the velocity and interface model with a desired smoothness that best fits the picked travel times. The parametrization of the model is implemented using cells in a 3-D grid for the velocity and 2-D grid for interfaces. The model can consist of several layers and all parameters are inverted simultaneously.

For this study the model was constructed with dimensions 2.3 x 2.6 x 0.25 km and a cell size of 50 x 50 x 20 m. The model consists of 4 layers and 4 interfaces the deepest corresponding to the BSR and 3 others representing the most prominent reflectors in the seismic sections. About 8000 normal incidence ray time picks from single channel survey and 100,000 picks from the OBS recordings were used as input data for the inversion.

The results show that the overall velocity structure is quite uniform laterally with the sediment velocity immediately below the sea bottom being 1500-1520 km/s and increasing downwards to the BSR to values around 1750 km/s. These values correspond well with the previous 1-D velocity studies (vertical seismic profiling and multichannel interval velocities from Yuan et al., 1999, Chapman et al., 2002, etc. ).Vertical velocity anomalies of limited spatial extend were revealed the most prominent being an increase of 10-20 m/s and corresponding spatially with the blank zone associated with the ventsite. These anomalies may indicate localized portions of sediments with increased amount of gas hydrates in the pore space.

Two principal models for the distribution of hydrates and free gas in the sediments have been proposed to explain the observed blanking (Wood et al., 2002 and Riedel et al. 2004). These models will be tested using the results of the tomographic inversion.

References:

Chapman, N.R., J.F. Gettrust, R. Walia, D. Hannay, G.D. Spence, W.T. Wood, and R.D. Hyndman, High-resolution, deep-towed, Multichannel seismic survey of deep-sea gas hydrates off western Canada, Geophysics, vol. 67, No. 4, 1038-1047, 2002.

Riedel, M., I. Novosel, G.D. Spence, R.D. Hyndman, R.N. Chapman, R.C. Solem, T. Lewis, and L. Zuelsdorff, Geophysical and Geochemical Signatures Associated With Gas Hydrate Related Venting at the North Cascadia Margin, GSA Bulletin, submitted, 2004.

Wood, W.T., J.F. Gettrust, N.R. Chapman, G.D. Spence, and R.D. Hyndman, Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness, Nature, vol. 420,656-660, 2002.

Yuan, T., G.D. Spence, R.D. Hyndman, T.A. Minshull, and S.C. Singh, Seismic velocity studies of a gas hydrate bottom-simulating reflector on the northern Cascadia continental margin: Amplitude modeling and full waveform inversion, J. Geophys. Res., 104, 1179-1191, 1999.