HR: 1340h
AN: T43C-1348    [Abstracts]
TI: AHC (Active Heave Compensation) - 800 Drilling on the Atlantic (New Jersey) Margin
AU: * Austin, J A
EM: jamie@ig.utexas.edu
AF: UT-Austin Institute for Geophysics, 4412 Spicewood Springs Rd. #600, Austin, TX 78759 United States
AB: The New Jersey continental shelf, an old, stable passive margin, has been a focus of latest Pleistocene-Holocene sea-level studies for decades, because eustasy is a major driving force in the production of the surficial stratigraphic record there. This margin is also geographically proximal to diverse oceanographic resources - laboratories, ports and ships - so hypothesis-testing using a "natural laboratory" approach has been suitable for data acquisition, analysis and interpretation. The Office of Naval Research has taken advantage of this shelf's characteristics to support collection and interpretation of a huge and diverse suite of geophysical data off New Jersey since the late 1980's - MCS profiles at multiple frequencies, deep-towed boomer and chirp profiles, multibeam bathymetry/ backscatter control, and most recently sediment samples using a lake-drilling system owned and operated by Drilling, Observation and Sampling of the Earth's Continental Crust (DOSECC), Inc., modified with active heave compensation for deployment off the Woods Hole Oceanographic Institution research vessel Knorr. The goal is to understand how the diverse interaction of depositional and erosional processes culminates in the preserved stratigraphic record. A first test of the AHC-800 drilling system took place in November 2001 in Block Island Sound off southern New England, with the following results: 1) some success was achieved in sampling mud, 2) fall weather was a limiting factor (heave compensation limits of 2.44 m in 8 s were often exceeded), 3) the vessel's dynamic positioning (DP) system was not always capable of maintaining station in shifting winds; precise navigation (beyond differential GPS) was required, and 4) the need for automated drillpipe handling to increase efficiency was recognized. A second test took place on the New Jersey shelf in September-October 2002, using updated software, automated pipe handling, and differential GPS navigation supplemented by a POS/MV 320 (inertial navigation) installation; this test included extensive calibration and tuning of the Knorr's DP system. Three sites were occupied: Site 1 in 129 m of water to a sub-seafloor depth of 5 m, Site 2 in 79 m of water to a sub-seafloor depth of 13 m, and Site 3 in 75 m of water to a sub-seafloor depth of 8 m. Sediment recovery at all three sites was excellent. Deep-towed chirp sonar profiles were correlated to recovered lithologies, and to gamma ray (multi-sensor track, MST) logs, using a derived reference compressional wave velocity of 1750 m/s. Lessons learned from the 2002 test included the following: 1) weather remains a factor, 2) despite improvements in navigation and DP, open shelf sampling is at the limit of the Knorr's capabilities, and 3) hydraulic piston coring, advancing by half (approximately 1.5 m) strokes, appears to be the most successful sampling tool in the intercalated unconsolidated muds and sands of the latest Pleistocene-Holocene section. The need for ground truth of the extensive geophysical control on the New Jersey shelf remains as strong as ever. We remain convinced that the AHC-800 is the right drilling system to use, given optimal weather, navigation and dynamic positioning. We envision further sampling off New Jersey, with support from a variety of sources, in either 2006 or 2007.
DE: 4219 Continental shelf processes
DE: 3022 Marine sediments--processes and transport
DE: 3094 Instruments and techniques
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting