HR: 0800h
AN: C11A-1066    [Abstracts]
TI: Evidence for discontinuous ice-bearing permafrost beneath the Beaufort Sea continental shelf
AU: * Herman, B M
EM: bruce.herman@mms.gov
AF: Minerals Management Service, 3801 Centerpoint, Suite 500, Anchorage, AK 99503 United States
AU: Cranswick, M S
EM: scott.cranswick@mms.gov
AF: Minerals Management Service, 3801 Centerpoint, Suite 500, Anchorage, AK 99503 United States
AB: Simple models for the thermal evolution of the upper several km of the Beaufort Sea continental shelf predict that the entire shelf should be underlain by a remnant layer of permafrost that formed during the last glacial period when the shelf was subaerially exposed to air temperatures of approximately -12° C. It is expected to be present only at depth beneath the continental shelf because the contemporary bottom water temperature is roughly 0° C. Direct observation of frozen rock beneath the continental shelf seafloor is limited to one hydrocarbon exploration well. Elsewhere, permafrost is indicated by the presence of ice-bearing strata, referred to as ice-bearing permafrost (IBPF). IBPF is inferred from either locally high resistivity values measured by wireline logging in hydrocarbon exploration wells or from reflection seismic velocity data. Nearly all hydrocarbon exploration wells drilled on the barrier islands in the Beaufort Sea or landward of them appear to encounter IBPF. The inferred IBPF layer is roughly 300 m below sea level. Where both the top and base of the IBPF layer are indicated, it is up to approximately 200 m thick. Seaward of the barrier islands, resistivity values indicative of an IBPF are found in only about half of the more than 20 hydrocarbon exploration wells that have log measurements at depths at which it would have been observed were it present. Reflection seismic velocity profiles beneath the continental shelf are made in the process of stacking multi-channel seismic reflection (MCS) data. These profiles of stacking velocity do not resolve an IBPF layer as such. Rather, the presence of IBPF increases the average velocity between the surface and any given depth relative to that in an area with no IBPF. Mapped average velocity at 1 sec two-way travel time (TWT) shows relatively high velocity on shore trending to lower velocity with increasing distance from the coastline. The barrier islands are surrounded by halos of relatively high average velocity at 1 sec TWT. Beyond the barrier islands the horizontal velocity gradient is negligible. Reflection velocity profiles themselves also indicate the presence of IBPF because of the anomalously high stacking velocities detected in the upper half second of TWT. Average velocities of approximately 2.7 km/sec at 0.25 sec TWT decrease to 2.3 km/sec at roughly 1 sec TWT where an IBPF is suspected. Elsewhere, the respective typical average velocities are 1.8 km/sec and 2.0 km/sec. Although interpretation of the profiles is somewhat subjective most profiles landward of the barrier islands clearly exhibit inversions in the upper second of TWT. Outboard of the barrier islands, far fewer profiles exhibit inversions and they tend to be smaller in degree. These various lines of evidence suggest that IBPF is relatively common beneath the Beaufort Sea continental shelf between the coast and the barrier islands. Beyond the barrier islands, it appears to be discontinuous. Areas lacking apparent IBPF may be predominated by fine grained rocks that would have little free water available to form ice. Alternatively, lateral variations in heat transport due to advection with subsurface water flow may have melted the ice.
DE: 0475 Permafrost, cryosphere, and high-latitude processes (0702, 0716)
DE: 0702 Permafrost (0475)
DE: 0772 Distribution
DE: 0915 Downhole methods
DE: 0935 Seismic methods (3025, 7294)
SC: Cryosphere [C]
MN: Fall Meeting 2005