Seismic Surveys Help Pinpoint
Chemosynthetic Communities on Gulf of Mexico Slope
(Originally Published in People, Land & Water, Dec., 2000)
A team of MMS scientists is investigating the strong correlation between the location
of unique seafloor features in the Gulf of Mexico and the presence of chemosynthetic
communities. The findings can improve the study and monitoring of marine organisms that
live off chemical energy as well as the safety of deepwater oil and gas exploration in the
gulf.
Using the
Johnson Sea Link II research submersible and seismic survey maps of deepwater sites,
members of the team made numerous observations of the seafloor on the gulf’s Outer
Continental Shelf slope. Data from the surveys can help to pinpoint the location of
communities of worms, mussel, and other organisms that derive their energy from methane
gas and hydrogen sulfide seeping from the seafloor.
Some of these communities live on exposed mounds of gas hydrates—natural
methane-water ices that form under high pressure and low temperature near seafloor vents.
MMS is responsible for protecting high-density chemosynthetic communities from potential
impacts of oil and gas activities.
The scientists from the MMS Gulf of Mexico Region who took part in the research cruise
aboard the R/V Edwin Link were Mary Boatman, Greg Boland, Jesse Hunt, Bill Shedd,
and Mike Smith.
The project aimed at "ground truthing" (confirming by actual on-site
observations) what appeared to be a strong correlation between data from 3-D seismic
surveys and the locations of chemosynthetic communities. For the last two years, team
members Hunt and Shedd have participated in a special project to map the seafloor
reflection data from all deepwater 3-D seismic surveys.
These surveys obtain images of seafloor features by
transmitting sound waves through the earth and analyzing the energy that comes back to the
surface. When integrated and processed, the reflection data produce detailed images that
resemble a cube cut from the earth’s surface, providing a dense grid of information
that enables scientists to locate geologic features on and below the seabed, including oil
and gas deposits and other formations.
To date, the seafloor reflection data from 110 of the 3-D seismic surveys have been
mapped, covering about 80,000 square miles of the deepwater gulf from the shelf break to
the abyssal plain. Early in this study, it was discovered that the amplitude (strength) of
the reflections—an indication of hardness—showed an interesting pattern where
large, deep faults cut the seafloor.
By researching known gas hydrate sites, locations where core samples had indicated
hydrocarbons, and the occurrence of chemosynthetic communities from previous research,
scientists determined that there appeared to be a strong correlation with the high
amplitudes on the maps.
The MMS group then teamed up with Dr. Harry Roberts, of the Coastal Studies Institute
at Louisiana State University, to "ground truth" the amplitude maps and try to
improve the predictive capability of using 3-D seismic surveys to locate chemosynthetic
communities. Roberts, who had an ongoing MMS research program to study seafloor features
associated with hydrocarbon seeps on the slope, hired the Johnson Sea Link II to expand
his research through the inclusion of data from MMS.
At the MMS Gulf of Mexico Regional Office, Roberts and members of the MMS team selected
about 10 sites with the best looking amplitudes. The group planned the transects across
the sites and plotted latitude and longitude coordinates for waypoints along each
transect. Roberts was chief scientist on the research cruise, which was carried out in two
legs.
During the first three days of Leg 1, instruments were placed on Bush Hill, a site in
the Green Canyon area that has a well-studied chemosynthetic community where gas hydrates
are exposed on the seafloor. The instruments, which also were placed at adjacent vent
sites, measured the flow of natural gas from gas seeps around the mound, the flow of
current, and temperatures in the gas seeps. The last two days of Leg 1 were used to survey
transects across amplitudes mapped in other Green Canyon sites. On the second leg of the
cruise, scientists logged two dives a day for eight days, survey-ing seafloor features at
six other sites mapped with 3-D seismic.
Every site showed indications of the venting of hydrocarbons, mud, or brine, and
several had carbonate rocks, tube worms, clam or mussel beds, and mud flows. After the
first few sites, the amplitude maps proved so accurate that the scientific crew began
predicting what the crewmen on the sub would see and when they would see it.
However, the next couple of sites brought everyone back to reality. Despite high
amplitudes on the maps, the seafloor proved to be mostly soft, burrowed deepsea mud. The
plausible explanation is that the venting in those areas had been dormant for a long time,
and the carbonate or hydrate hardgrounds that were present were covered by a layer of soft
deepsea mud. Small patches of chemosynthetic animals were seen at many of the sites, but
only two were considered significant and added to the list of lease blocks with known
chemosynthetic communities.
A notable discovery was made at a site in the Garden Banks where two pinnacles (mud
volcanoes) rise about 300 feet off the seafloor. Samples taken at the top of each
consisted of medium to fine grain sand. Some of the quartz grains were well rounded, and
others were highly angular. The suite of minerals observed in the samples indicates a
metamorphic source (formed from heat and pressure in a mountainous area) for the sands.
The only logical explanation for the occurrence of such sand at this deepwater site is
that they were very old sands that were deeply buried, then fluidized, and were brought up
to the seafloor from below with oil, gas, or brine along a major fault. This is a rare
instance where it can be shown that sands can be deposited on the top of topographic
highs.
One of the questions that scientists considered on this cruise was whether 3-D
exploration seismic data are sufficient, by themselves, to evaluate shallow geohazards
before drilling at a site. It appears that seafloor areas with deepwater mud can be easily
identified and separated from anomalous areas that have carbonate hardground and
gas-charged sediment associated with gas vents—areas that are avoided as drilling and
anchor sites. During future cruises, piston cores in anomalous areas, where a layer of mud
covers shallow carbonate or hydrate, will provide additional ground truth for evaluating
deepwater geohazards.
For more information, contact Keith Good.
