Quarterdeck 4.1
Cruise News
TAMU Deep Tow
by William R. Bryant, 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 .i.Winter, the 1995;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.
Plumbing the abyss
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
21p;27, 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 1700p;2400 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.
Salty bottom
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.
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Oceanography, Texas A&M University
rshatto@ocean.tamu.edu
URL=http://oceanography.tamu.edu/Quarterdeck/QD4.1/Bryant/bryant.html
Updated May 27, 1996