Permafrost Beneath The Beaufort Sea: Near Prudhoe Bay, Alaska

Abstract

ABSTRACT The occurrence and properties of subsea permafrost near Prudhoe Bay, Alaska, were investigated by drilling and probing. Nine holes were drilled and 27 sites were probed with a cone penetrometer. The deepest drill hole was 65.1 m below the seabed, while a depth of 14.1 m was reached with the cone penetrometer. Engineering and chemical properties were determined from core samples and point penetration resistance data were obtained with the penetrometer. Thermal profiles were acquired at both the drill and probe sites. ABSTRACT Temperatures below 0°C were observed in all the drill and penetrometer holes logged, although frozen sediments were encountered only occasionally. Seasonally frozen sediments were observed near the seabed at each site. The degree of ice bonding, or strength, could be related to seabed temperature and was greatest in shallow water (<2 m). The penetrometer resistance and thermal data indicated that deeper ice-bonded sediments occur, for example approximately 12.7 m below the seabed in 2 m of water off the Sagavanirktok delta. Of eight holes drilled offshore, it appeared that four encountered bonded permafrost. In general, the position of the ice-bonded permafrost interface was extremely irregular. The depth below the seabed to this interface at various distances from shore along the line studied was 28.8 m at 1 km, 65.1 m at 3.5 km. 44.1 m at 6.8 km, and 29.5 m at 17.2 km. Shallow, overconsolidated marine sediments were found in the upper fine-grained section at all of the drill sites investigated; the degree of overconsolidation varied considerably among the sites. This fine-grained section was up to 10 m thick and covered sands and coarse gravels. INTRODUCTION Scientific and engineering investigations conducted by several nations bordering the Arctic Basin have documented the existence of subsea permafrost. Many questions concerning distribution, and particularly the properties, of these perennially frozen sediments still remain. The earliest quantitative studies commenced around 1953 in the USSR. The Soviet investigations have continued, with emphasis on field observation and theoretical studies.1 Few studies were conducted in North America prior to the extensive activity stimulated by petroleum exploration in the arctic regions. Early direct observations and theoretical models of subsea permafrost by Lachenbruch and others2,3 were based on studies made near Barrow, Alaska. Lewellen4,5 followed these investigations with drilling and probe studies in the same region. In the Canadian Beaufort Sea, probably the first evidence of permafrost and ice-bonded sediment was obtained during an Arctic Petroleum Operators Association drilling program.6 Shearer et al.7 observed the occurrence of bingo-like features on the outer shelf. Hunter et al. followed these studies with an extensive investigation of the distribution of ice-bonded permafrost. These studies, which were largely based on examination of seismic records, showed extensive subsea permafrost.