November 13, 1995
Location: Approximately 60 02í South Latitude x 56 26í West Longitude
Doing Seismic Survey in the Southwest Scotia Sea
Update:From midnight until 1:30 PM we continued to do seafloor mapping of the Southwest Scotia Sea. At 1:30 PM, we changed course heading into the wind and slowed our speed to 2 knots and began to deploy the streamer to prepare to do the first official seismic survey of the cruise.
Deploying the streamer is a three hour job, and requires about five people to be done safely. Jim Lundy a graduate student from Texas, and I helped Barney, Bruce and Rhonda, the marine technicians on board, deploy the streamer and air guns. When we started the seas were high. Some waves were over fifteen feet, and plenty of white caps were generated by the wind. The barometric pressure had begun to increase slightly and the winds were changing to the east. The consensus was that the seas would calm, so we started deployment of the seismic equipment.
Barney and Rhonda worked on the aft deck making sure that the streamer came off of its winch smoothly. The streamer has a total length of 1400 m, nearly a mile, when it is towed behind the ship. It is stored on a large winch which measures 3 meters in circumference around the main spool. The winch is hydraulically controlled. At regular intervals leveling devices called ìbirdsî are attached to the streamer. My job was to control the winch, stopping and starting the winch to attach the birds. This took nearly an hour and a half. Mark Wiederspahn, seismic systems analyst from Texas, helped with the birds and made the final electrical connections for the streamer to be connected to the ships computers.
The streamer looks like a large yellow garden hose. It is filled with 48 pairs of thin wires which are attached to special microphones called hydrophones. The hydrophones are placed at 5 meter intervals along the length of the streamer. The hydrophones detect sound reflected from the earthís crust from a large sound source. For this cruise, the sound source is six air guns which are towed in the water behind the ship.
The air guns are large stainless steel cylinders about three feet long, and look very much like cannon barrels. There is a piston which moves in the gun releasing 2000 pounds of air pressure. The rapid release of air creates and explosion which produces sound waves. The sound waves travel through the water and are reflected off of the crust up to the hydrophones. Some of the sound penetrates through the upper layers of the crust and is reflected a split second later to the hydrophones by deeper layers of rocks in the crust. A sound explosion is produced every 12 seconds by the guns. The computers on the ship record the reflected sound data, and using mathematical software programs, convert the data into a picture of the vertical layers of the earths crust in the area.
The guns weigh about 200 pounds each and there are three guns per array. The guns are connected to a 15 foot steel beam by chains. Three large plastic buoys are attached to the beam to help position the guns at about 10 feet below the surface. This whole system of beams, buoys, and guns, called a gun array, must be lowered into the ocean and towed behind the ship. Rhonda handles the winch for this part of the deployment and Barney is in charge on deck. Anyone near the back 10 feet of the deck has a safety line attached. The gun array has to be hoisted up off the deck using a winch, and then lowered behind the ship. It is a twisted bundle of chains, ropes, as well as air hoses and electrical lines that are connected to the guns to control their operation. Getting the arrays into the water is a dangerous and challenging task in these seas. While putting in the first array we took two large waves over the aft deck. We all got a bit wet, Barney took the worst of the wave.
By the time we were ready to put in the second gun array the seas had calmed considerably. They were put in the water with only one minor problem. They had to be attached to a cable to the starboard side. Until this was done, there was a good chance that the array could cross over the streamer and the cables and ropes could tangle in the streamer and cause damage to it. Jim and I attached safety lines, and worked off the back deck pulling the leads attached to the streamer to prevent the guns from getting tangled. After about 20 minutes, the second array was secure. We were done, tired, and a little wet, but happy that everything was finally deployed.
It was up to Mark to make the final connections and adjustments to the computer systems. At 4:05 PM we started to collect our first seismic data of the cruise. Within a half hour there was a problem with gun 5. Air was turned off to that gun, and we continued on using only five rather than the six which were planned. At about 4:45 rough unedited data was being plotted from the ships seismic computers. It looked really good. To be honest, it looked better than expected considering that this was the first time the streamer had ever been used. Mark explained that it takes time to get everything calibrated properly, usually a few hours. Fortunately for the Texas team, he was able to make the corrections in considerably less time.
Larry informed the bridge that all was in order, and we set a course due north over some of the ridge formations that had been identified by Sea Beam mapping. We traveled at a constant speed of 5 knots for about 5 miles. Then we slowly made a turn to the west at a rate of about 5 degrees per minute. All the time that the turn is being made the bridge must be aware that the streamer is being towed behind the ship and is nearly a mile long. If the ship turns too quickly it can cross over the stream and catch it in the props. Our first turn was negotiated successfully. We then traveled due west for a couple of miles to allow the streamer to stretch out, then turned slowly south for a 20 mile survey. This snake like survey route continued until about 3:00 Am. The guns and streamer were brought in and we began our 100 mile trip back to Bransfield Strait for more hydrothermal vent study.
It was a very productive seismic survey. Some interesting ridge and cliff structures were identified from the rough data, as well as some unique features of the crust which will have significant impact on understanding the interaction of the tectonic plates in this area.
lhs1-manthey: When we return to the Bransfield Strait the general plan is to locate the previously identified plumes using the global positioning system on the ship. We have dredging equipment on board as well as a submersible still photograph camera. We plan to dredge these areas for evidence of the vents. This evidence would ideally be a piece of the vent chimney. We will also attempt to take photographs of the area using the camera. A mini sub is not on board, but plans are already in the works to bring the mini sub Alvin, to this area in the next two years to look for the vents.
J Hilger: Greeting English class. The researchers that left the ship had started with us in Chile. Special arrangements had been made well in advance so that a sea rendezvous could be made with another NSF ship returning to port. This is an unusual activity. Transfers are typically made only in emergencies. Although we have a helicopter pad on the ship, the only time a helicopter would be sent would be in the event of a serious life threatening emergency. I am not sure whether the US has any helicopters within 300 miles of our position. I think we would have to make a special request from the Argentinean station on King George Island if such an emergency arose.
We do not have a physician on board. We have two EMTís who would handle any medical emergency. We have a fully stocked hospital room on board. Radio links with a doctor would be made so that special treatment could be given by the EMTís.
It is hard to gauge the size of waves, but to give you an idea of how the waves affect the ship let me give a few examples. The back of the aft deck is about 15 feet above the waterline. We regularly have waves that pitch the entire 300 foot long ship so that the aft section is flooded by waves. The portholes on the deck level are about 20 feet above the water line and are nearly constantly splashed by waves in the seas in the Drake. There are TV cameras all over the ship. The highest most camera is on the ice bridge 60 feet in the air. In rough seas, we take waves that pound over the bow of the ship and crash into the ice bridge camera.
Wis Rapids/Jmanthey: Thanks for the words of encouragement. i am pleased that so many teachers and students are finding my updates interesting. They are very important to me. It is very easy to lose track of days and time here. Writing them helps keep me connected, and it makes me feel as if I am able to share my daily activities with a friend.
ljuech: Glad to hear that more WASDI people have found the page.
sluther: I hope that the connection problems to the page have been corrected. Bob Manthey, at Lincoln High School, has been working to keep things on the web page updated and connected. We have experience a few technical difficulties with our Internet Service, but that now seems to be corrected. He has done a great job under some very trying circumstances.