Proposed Research

We propose a research cruise aboard the R/V Ewing to acquire two related data sets: a three-dimensional, multichannel seismic reflection image of the hydrate/gas system on the Blake Ridge, and two profiles of three- component, ocean-bottom seismometer data. Below we describe the specific goals and experimental design of these data sets.

Three-Dimensional, Multichannel Seismic Reflection Data

• Provide basic 3D geometries of stratigraphy, structures, hydrate concentration, and gas accumulations. A 3D MCS data set is required to image the complex hydrate/gas system in sufficient detail to understand hydrate formation, gas accumulation, and migration pathways. We would like to understand the roles of stratigraphy, topography, and faulting in controlling gas migration and accumulation.

• Estimate the base of free gas from amplitude anomalies and seismic velocities. Existing single-channel data suggest that the base of free gas is marked by a sharp decrease in reflection amplitude (See this figure... or this figure). We would like to verify this by acquiring stacked multichannel data, which will have a higher signal/noise ratio. We can then use amplitudes and seismic velocities to map the free gas zone over the entire 3D region.

• Investigate evidence for gas escape pathways into the hydrate stability zone. The anomalous "stringer" reflection (or package of reflections), which seems to follow a stratigraphic unconformity, may represent free gas within the HSZ, perhaps fed by gas migration along a system of faults that appear to terminate near the stringer.

Ocean-Bottom Seismometer Data

• Estimate the large-scale shear-wave velocity structure of the Blake Ridge. A determination of Vs in the hydrate zone based on traveltimes of converted shear wave arrivals will allow us to address whether hydrate cements sediment and whether the free gas zone is overpressured or underconsolidated. We will assess whether there are anomalously high S-velocities above the BSR and/or low S-velocities beneath the BSR. This will tell us whether shear wave velocity is a potential indicator for hydrates in sediments. Pilot data from the Blake Ridge suggest that the concentrated zone of hydrates at ~220 mbsf constitutes a P-to-S converter (Figure).

• Investigate the cause of P- to S-conversion. The waveforms of converted shear wave arrivals reflect the fine-scale velocity structure and hence the geologic nature of converting boundaries. We will test whether (a) Vs shows a step increase at the top of the hydrate zone, which would indicate cementation; (b) a thin layer of high Vs exists at independently interpreted zones of high hydrate concentration; (c) a thin layer of low Vs exists at the BSR, which would indicate overpressuring within a gas-charged layer; or (d) a broad zone of low Vs occurs beneath the BSR, which would suggest underconsolidated sediments.

• Investigate whether the "stringer" represents free gas in the hydrate stability zone. A thin layer of sufficiently low P-wave velocity would indicate that free gas causes the stringer reflection, as previously demonstrated for BSRs. This would support Dillon et al.'s hypothesis and prove that under certain circumstances free gas can exist in the HSZ.

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Last Revised: January 27, 1998