High resolution seismic reflection data recorded offshore from Prudhoe Bay, Alaska, were processed digitally to determine the reflectivity structure of the uppermost layers of the seafloor. A prominent reflector found at 27 m below the mudline (water depths 7-9 m) has a negative reflection coefficient greater than 0.5. The large acoustic impedance contrast, coupled with a report of gas encountered at a corresponding depth in a nearby drillhole, shows that the reflector is the upper boundary of a zone containing gas. The gas exists in sandy gravel capped by stiff, silty clay. Analysis of unprocessed conventional high-resolution records from the region indicates that the gas-bearing layer may extend over an area of at least 50 km at depths of 19-35 m below the mudline. Similar-appearing reflectors, previously unexplained, occur in patches over wide regions of the shelf where offshore oil development is beginning at a rapid pace. This suggests the exercise of caution with respect to possible hazards from shallow gas pockets.
|Title||Seismic evidence for an extensive gas-bearing layer at shallow depth, offshore from Prudhoe Bay, Alaska|
|Authors||Gary Wynn Boucher, Erk Reimnitz, Edward W. Kempema|
|Publication Subtype||USGS Numbered Series|
|Series Title||Open-File Report|
|Record Source||USGS Publications Warehouse|