Volume 4, Number 1, Spring 1996
Texas A&M Deep Tow
Wayne A. Dunlap
Anne K. Rutledge
With recent cruises of the R/V Gyre in which the new Texas A&M University (TAMU) Deep Tow and [TAMU]2 seismic systems were engaged, Texas A&M has emerged as the only marine science center to possess the state-of-the-art equipment for conducting swath bathymetric surveys, long-range side-scan sonar surveys (up to 40 kilometers wide), high-resolution subbottom profiling surveys, and high-resolution side-scan sonar surveys at any water depth.
Swath bathymetry is the process of determining the depth of water over tracks up to 40 kilometers wide. Side-scan sonar produces images of the seafloor, while subbottom profiles reveal the internal structure of the sediment up to 100 meters below the seafloor.
Dr. Thomas Hilde described the operation of the [TAMU]2 seismic system in "[TAMU]2 and related geophysical studies of the East Pacific Rise," which appeared in the Winter 1995 Quarterdeck.
TAMU Deep Tow is a newly refurbished version of the EDO Deep Tow, given to Texas A&M by Shell Offshore Inc. (Houston Texas) via the TAMU Offshore Technology Research Center (OTRC). The TAMU Deep Tow system includes a DYNACON traction winch system, a large ocean-depth instrument package called the "Fish," and the topside electronics.
Gone "fishing"-back in six days
The "Fish" houses a subbottom profiler capable of operating at 3.5 or 7 kilohertz, a 100 kilohertz side-scan sonar system, a depth sensor, and a direction-locating pinger. The positively buoyant "Fish" is towed at a constant height (optimally 30 meters) above the seafloor.
This unique bottom-tracking characteristic of the Deep Tow "Fish" is the result of using a shot (90') of anchor chain dragged along the seafloor as a depressor weight. The chain attaches to the tow cable with a clamping device known as a fairlead. The position of the fairlead with respect to the "Fish" (ideally 35 meters up-cable from the "Fish"), along with the length and weight of the chain and the speed of the ship determine the height of the "Fish" above the seafloor. The resulting high-resolution subbottom profiles and side-scan sonographs are outstanding in their clarity, resolution and consistency in any water depth.
Other deep tow seismic systems, such as SeaMARC, never purposefully contact the seafloor but rather fly the instrument package above the seafloor by altering the ship speed and tow-cable length, a very difficult procedure in areas of high topographic relief.
TAMU researchers first used the TAMU Deep Tow for a survey of Pigmy Basin on the Louisiana Continental Slope aboard the R/V Gyre (96-G-2) during February 2127, 1996. Drs. William Bryant of the Department of Oceanography and Wayne Dunlap, Associate Director of OTRC, were co-chief scientists of this research cruise in which five nautical archeology graduate students, two geology-geophysics graduate students, and one marine science undergraduate participated.
Pigmy Basin, a intraslope/intralobal basin formed by coalescing salt canopies, lies in water depths of 17002400 meters. It is one of three theme areas in the Gulf of Mexico chosen for study by the National Science Foundation (NSF)/OTRC research project "Seafloor structures interaction." The other two areas are Vaca Basin and Alaminos Canyon. Drs. Aubrey Anderson, William Bryant and Anne Rutledge of the Department of Oceanography and Dr. Wayne Dunlap of OTRC carry out the oceanographic research portion of this project.
The survey of Pigmy Basin was undertaken to determine the stability characteristics of the basin's very steep walls. Bathymetric surveys revealed wall slopes averaging 20 degrees with a maximum of 50 degrees. The data obtained during the Deep Tow survey indicate that the walls of the basins are subject to slumping and sediment creep processes.
These data are necessary to determine the best and safest locations for the placement of seabed structures such as equipment related to the exploitation of hydrocarbons, including templates, piles, and pipelines.
The continental slope off Texas and Louisiana is known to contain vast hydrocarbon reserves which, with the recent advances made in deepwater drilling and production technology, are now recoverable. To date, exploratory wells have been drilled in areas with water depths in excess of 3,000 meters, while oil and gas wells in areas with depths in excess of 1000 meters are currently in production.
One unexpected outcome of the Pigmy Basin survey was the discovery of the highly faulted nature of the seafloor between the intraslope basins. This faulting is the result of halokinesis (i.e. the movement of salt) associated with the thick salt deposits known to underlie almost all of the sediments on the Texas and Louisiana Continental Slope. The character of the faults visible in both the subbottom and side-scan sonar records attest to the active nature of the salt movement in this area.
The next TAMU Deep Tow survey will take place in Alaminos Canyon, a submarine canyon off Texas with water depths up to 3000 meters. The canyon was formed jointly by coalescing salt canopies and erosional processes. The six-day cruise will take place in early April 1996.
The authors would like to extend their grateful appreciation to Desmond Rolf, Billy Green, Eddie Webb, and Sandy Green for their hard work in fielding the February cruise.
Dr. William Bryant stands next to the take-up reel and traction winch of the Texas A&M Deep Tow system. The apparatus consists of 25,000 feet of new 0.680-inch conductor cable, a hydraulically controlled levelwind, the large traction unit, and a diesel-hydraulic power supply.
The Texas A&M Deep Tow "Fish" with its forehead missing after being submerged for four days. This instrument package is lowered to the seafloor where it closely follows the bottom while being towed by the ship. It contains a side-scan sonar and a high-resolution subbottom profiler.
A high-resolution subbottom profile.
Oceanography can be fun, but also very wet! The Gyre's deck is awash as we retrieve the Deep Two "Fish" with its anchor-chain depressor weight.
Comments to: firstname.lastname@example.org
Last updated February 25, 1997