Woods Hole Science Center
Although gas hydrate has been recognized in drilled cores, its presence over large areas can be detected much more efficiently by acoustical methods, using seismic-reflection profiles. Hydrate has a very strong effect on acoustic reflections because it has a high acoustic velocity (approximately 3.3 km/s - about twice that of sea-floor sediments), and thus grains cemented with hydrate produce a high-velocity deposit due to the mixing of hydrate with the sediment.
The contrasts in velocity created by the hydrate-cemented zone produces a strong reflection called the "bottom simulating reflection" (BSR). Lower velocities below the hydrates occur because underlying water saturated sediments have lower velocities(water velocity is about 1.5 km/s) and often contain gas trapped by the overlying, less porous hydrate-cemented sediments. This contrast produces a strong reflection. Because the base of the gas-hydrate stable zone occurs at an approximately uniform sub-bottom depth throughout any small area, the well-defined seismic reflection from the base of the zone roughly parallels the sea floor(hence "bottom simulating").
Over large areas the thermal field may be distorted and even the pressure field and the chemistry of pore water may vary unpredictably; all are factors that affect the phase stability of hydrate. Therefore the variation in depth to the base of the gas-hydrate stable zone can indicate a great deal about sub-sea-floor conditions.
Laboratory studies are necessary to understand the processes by which gas hydrate may be concentrated on the continental margin and to develop tools (such as acoustic models) that facilitate recognition and resource analysis of these offshore hydrate deposits. The USGS has begun using a computer-controlled test system in which gas hydrate is formed and decomposed in sediment samples under simulated in-situ (deep-sea or arctic) conditions.
Possible mechanisms of natural concentration are also simulated, including: 1. an accumulating sediment column, 2. cyclic sea-level change, 3. faulting and fluid migration, and 4. trapping of free gas beneath a hydrate seal. Experiments are being conducted to assess the impact of gas hydrate on sediment behavior, particularly with respect to slope failure and other potential geohazards, and to estimate the amounts and rates of release of methane into the atmosphere in response to geological phenomena.
U.S. Department of the Interior |
U.S. Geological Survey
URL: woodshole.er.usgs.gov/project-pages/hydrates/how.html
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Page Last Modified: Monday, 25-Sep-2006 13:51:27 EDT
