Mapping Methane-Rich Fluid Systems Within Southern Hydrate Ridge With High-Resolution 3-D Seismic Reflection Data

In June and July, 2000 we acquired a high-resolution 3-D seismic reflection data volume to examine the "plumbing" system of methane-rich fluids within Hydrate Ridge, offshore Oregon. Eighty-one lines were acquired with a 15 m shot and 50 m line spacing covering an area of 4 x 11 km across the crest of southern Hydrate Ridge. The vertical resolution is approximately 5 m. These data have now been binned, stacked, and migrated as a full 3-D data volume. Clues to the regions of methane-rich fluids within Hydrate Ridge come from several observations made from the seismic data. First, the pattern of heat flow inferred from the depth to the bottom simulating reflection (BSR) maps regions of locally high advective heat flow with estimates as high as 140 mW/m². A major methane conduit and active vent on the western flank of Hydrate Ridge (described by Trehu et al. this session) lies at a local high on our heat flow map confirming a correlation between a known methane conduit and inferred heat flow. However, a broader region of high heat flow is observed on the eastern flank of the ridge crest just north of the active vent site. Underlying this segment of the ridge are a series of eastward dipping normal faults that intersect a series of strata that are also eastward dipping. These faults may have high fracture permeability and thus enhance fluid and methane flux. Further evidence for the flux of methane beneath the local heat flow highs are phase-reversed seismic reflections that are "bright" beneath the BSR and terminate or nearly vanish at the intersection with the BSR. Several of these reflections appear to switch from reversed to normal polarity across the BSR and may indicate a methane conduit that produces a reversed polarity from the low-velocity free gas below the BSR to one containing high-velocity gas hydrate above it. Seismic modeling and inversion for acoustic impedance of these reflections will be conducted to provide further quantitative details for analysis of free gas and gas hydrate distribution with in Hydrate Ridge, which will be further constrained by drilling during Ocean Drilling Program Leg 204 scheduled for July-September, 2002.