Overview:
As part of an MMS Coastal Marine Institute (CMI) project with Dr. Harry Roberts of LSU,
groundtruth data were collected using the Johnson Sea Link submersible (submarine). The
research cruise took place in 2 legs between August 16 and 31, 2000. Leg 1 (August 16 to
22) was spent installing instrumentation on Bush Hill (Green Canyon 185) and at adjacent
mud vents, as part of another of Dr. Roberts’ projects, and surveying traverses on
MMS amplitude maps. Leg 2 (8/22 to 8/31) was spent surveying traverses on MMS amplitude
maps. Observers in the submersible made visual observations. Video and still pictures were
collected along every transect and grab samples were collected from a few locations of
interest. A calm sea allowed for the successful completion of all planned transects.
One of the questions considered on this cruise was whether 3D exploration seismic data,
particularly if reprocessed at a 2 ms sample interval, are sufficient by themselves for
pre-drill shallow geohazards evaluation. This issue, along with identification and the
hazard potential of gas hydrates, will be addressed in the MMS Notice to
Lessees and Operators on shallow geohazards in the Gulf of Mexico, currently under
revision. It appears that seafloor areas with hemipelagic mud can be easily identified and
separated from anomalous areas with carbonate hardground and gas-charged sediment
associated with gas vents. Another principle objective of this project is the ground
truthing of the association of chemosynthetic communities with geophysical characteristics
represented in surficial amplitude anomalies. Although there is no certainty that
chemosynthetic communities are in fact present at these anomalous features, their
association with these features has been strongly supported. These anomalies are avoided
at drilling and anchor sites. The new MMS requirement (ROV NTL) to conduct ROV surveys
around the drill site of the first 5 to 10 exploration wells in each of 17 deepwater areas
(grids) will supplement the cruise results with observations of non-anomalous drilling
areas. During future cruises, 20-foot piston cores in anomalous areas where mud drape
covers shallow carbonate will provide additional ground truth for deepwater geohazards
evaluation.
Detailed Report of Sea Link Dives
