UTIG RESEARCH PROJECTS ARCHIVE

Hydrate Ridge Flyer

UTIG Gas Hydrates Projects

Collaborative Research
Shear Wave Studies of Hydrate Ridge,
Oregon Continental Margin,
During Planned ODP Leg 204

Ingo Pecher, Nathan Bangs, University of Texas
Anne Trehu, Oregon State University

Check out the data from the cruise!

Our understanding of the process of gas hydrate formation and dissociation in marine sediments and its interaction with the environment hinges on characterizing gas hydrates and their gas sources. Drilling of ODP Leg 164 on the Blake Ridge has vastly improved our knowledge of gas hydrate systems in the low fluid-flux environment on passive margins. The study of gas hydrates and free gas in an accretionary wedge with high fluid-flux and strong small-scale lateral variability is the focus of planned ODP Leg 204 on Hydrate Ridge, offshore Oregon (fig. 1). Data from logging and coring will provide critical constraints on hydrate and free gas occurrences, but will be limited to the boreholes. We propose a series of seismic experiments to extend the borehole results into the surrounding region for a study of the gas hydrate system beneath Hydrate Ridge.

Fig. 1: Seismic section over Hydrate Ridge with proposed ODP sites. Offset VSPs are planned for HR-1 and HR-2. After Trehu et al. (1999)

Our primary focus is on shear wave studies because of their success during ODP Leg 164, when compressional-to-shear (P-to-S) converted waves through gas-hydrate-bearing layers were documented for the first time in data from a four-component (4-C) walkaway-vertical seismic profile (VSP) and an ocean bottom seismometer (OBS) study. Recent laboratory and borehole studies confirm that gas hydrates lead to a significant increase of shear wave velocity (Vs), which emphasizes that Vs is a critical parameter for gas hydrate characterization. We now propose to calibrate Vs of gas-hydrate-bearing sediments at typical frequencies for seismic surveys and to use shear waves to study gas hydrate distribution away from the boreholes. For shear wave measurements, we need to excite P-to-S converted waves with shots at offsets that are only possible with a second ship. We therefore propose a piggy-back two-ship experiment to exploit the unique opportunity of ODP Leg 204 for a study of shear waves through gas-hydrate-bearing sediments.

We propose the following experiments (fig. 2):
(1) Constant-offset VSPs, with a fixed source-receiver offset, will be coordinated with planned zero-offset VSPs. As a result, we will obtain vertically continuous Vs profiles.
(2) Walkaway VSPs will be acquired by shooting over the borehole to a receiver at fixed depth. We will analyze amplitudes and waveforms as a function of offset to obtain high-resolution Vs across P-to-S conversion horizons. High-resolution Vs together with Vp and gas hydrate saturation will form the basis to calibrate rock physics models of gas-hydrate-bearing sediments.
(3) Four-component OBSs will be deployed in a dense array from one of the boreholes to a bright spot on seafloor swathmapping records. Vs from converted waves will answer if the bright spot is caused by a cap of gas hydrates and carbonates indicative of highly focused fluid- flux and how the cap is related to gas hydrates in the subsurface.
(4) In a novel experiment, we plan to use the Advanced Piston Corer as a seismic source in the borehole. Signals will be recorded on OBSs close to the borehole reversing VSP geometry.

Fig. 2: Proposed experiments. c-VSP: constant-offset VSP, w-VSP: walkaway VSP, OBS: ocean bottom seismometer, APC: Advanced Piston Corere. VSP design is based on experience from walkaway-VSPs durling Leg 164 on the Blake Ridge (Pecher et al., 1997)

We plan to tie these Vs results into the 3-D/4-C survey of Hydrate Ridge scheduled for May 2000, in order to investigate the regional gas hydrate distribution. These complementary studies will be the first comprehensive seismic investigation of a gas hydrate and free gas system. Together with drilling, these experiments are designed to constrain relationships between hydrate formation and gas sources and to characterize and analyze the development of a gas hydrate reservoir in the high fluid-flux environment of a convergent margin.

Related sites:

• Hydrate Ridge Site Survey, Nathan Bangs

References:
Pecher, I.A., Holbrook, W.A., Stephen, R.A., Hoskins, H., Lizarralde, D., Hutchinson, D.R., Wood, W.T. 1997: Offset-vertical seismic profiling for marine gas hydrate exploration - is it a suitable technique? First results from ODP Leg 164. In: Proceedings 29th Offshore Technology Conference, May 5-8, 1997, Houston, TX, 193-200.
Trehu, A.M., Torres, M.E., Moore, G.F., Suess, E., Bohrmann, G., 1999. Temporal and spatial evolution of a gas hydrate-bearing accretionary ridge on the Oregon continental margin. Geology, 27, 939-942.