Bering Sea Velocity-amplitude Anomalies: Exploring the Distribution of Natural Gas and Gas-hydrate Indicators
Ginger A. Barth, David W. Scholl, Jonathan R. Childs, 2009. "Bering Sea Velocity-amplitude Anomalies: Exploring the Distribution of Natural Gas and Gas-hydrate Indicators", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
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Velocity-amplitude anomalies (VAMPs), comprising coincident seismic traveltime anomalies and gas bright spots, are features widely identified in seismic reflection images from the deep-water Bering Sea basins. Interval traveltime anomalies are used to develop a method for the objective detection and quantification of these features. The approach selected uses relative traveltime variation in the sedimentary intervals above and below the gas-hydrate bottom-simulating reflector (BSR) as a diagnostic, measuring pull-up in the hydrate stability zone and push-down in the underlying gas zone, relative to a 400-common-depth-point (CDP) running-average interval reference. The method is used to explore the distribution of gas and hydrate indicators within a 120-km (74-mi) reflection profile segment in the central Aleutian Basin. This study segment includes 17 detected VAMPs, only 6 of which appear to contain significant quantities of stored hydrate. The total estimated volume of natural gas stored within the hydrate caps of these VAMPs is about 4 tcf (0.1 tcm). The largest three VAMP features contain greater than 85% of that total. Not all of the most visually obvious VAMPs are important hydrate contributors. We suggest that VAMPs are fluid-expulsion features that have become involved in the transport of natural gas. As such, the VAMP systems should have been more active in the past. The VAMPs with significant hydrate present are most likely to be active today. The largest VAMPs, including all of those associated with hydrate indicators, are located above prominent basement highs. This association suggests that fluid-migration patterns in these undeformed deep-water basins were originally established in response to sedimentation and compaction over basement topography, and that those ancient patterns have never been superceded. It also suggests that to make an informed estimate of gas-hydrate total volumes for the deep-water Bering Sea, the regional relationship between VAMP hydrate concentrations and basement topographic highs needs to be considered.