Back to Bransfield Strait OBS Experiment 2.

 

Gail's Cruise Reports You may write to Gail at christga@nbp.polar.org

November 13 We have now left Bransfield Strait and are transiting back to Punta Arenas. Our last profile went well - no equipment problems, all OBSs came back, and good data. We did have one OBS come up at night without the strobes working. The bridge was able to get a heading on it through the radio transmitter, and using the spotlight (which is very bright - they use it to work in ice at night) were able to find it and get it back on board. There has been a revolt among the watchstanders and logging has ended. When we started our transit 8 of us each took a 3 hour watch, but all we were logging were navigation, gravity, magnetics, and seabeam. When we reached the 200 mile territorial limit the revolt took place and watches ended (once inside the territorial limit you are not supposed to collect data). Jamie and Ian found out about the revolt about 3 hours after it happened, but besides from having to hear about how watches took place when Jamie was a graduate student there were no repercussions. Overall the data collected on the 4 profiles looks great. As for preliminary results, it looks like crustal thickness near the Shetland Islands is ~23 km, with thickness within the Strait of 12-22 km. The thinnest crust is along the interpreted rifting axis. The crust appears to be thinner to the ne than to the sw, which supports Dan's model of rifting being more developed to the ne. There is also segmentation within the Strait, with some regions of fast and slow velocities correlating over lateral distances of 30 km. The data also seem to indicate upper mantle velocities beneath Bransfield Strait of 7.5 km/s. We are all very pleased with the success of the experiment, but are looking forward to our return. Regards, Gail November 9, 2000 We are currently deploying OBSs along Line 10, which will be our last profile before heading back to Punta Arenas. The data along Line 9 look great - arrivals out to 100 km, and PmP arrivals (which we use to make crustal thickness calculations) on almost every instrument. To compare, Line 9 was shot between Line 2 and Line 4, and those lines had PmP arrivals on about 1/3 of the instruments. I'm not sure if the difference is geology or the fantastic weather we've been having. Seas have been very calm ever since we've started shooting. Line 9 was shot within Bransfield Strait, with Bridgeman Island near one end and Deception Island near the other. We had great views of both. Bridgeman Island is a very inhospitable looking place - the sides are very shear and it is rapidly eroding away. The top is covered by flows which formed in a lava lake. Deception Island is a historically active volcano with the last eruptions about 30 years ago. It has hot springs and is a location that some special cruise ships visit. In fact, one of the computer technicians on the ship took a cruise to Deception Island and afterwards quit her job and joined the Antarctic program. Ian has visited all the islands in the area to look at the geology. It is very surreal to be away from all the election hype. We get our news in dribs and drabs - there is e-mail twice a day, plus they did an extra run the morning after Election Day. Cheers, Gail November 6 Yesterday we picked up our OBSs from Line 7, and today we are deploying them along Line 9. Line 7 was a reshoot of Line 1 from the April cruise - Line 1 had numerous gun problems, and half the profile was shot without the largest gun. This time around we only had 6 hours worth of gun problems and were able to use the full array on the entire profile. The weather was also much improved from April - very calm for most of the line. The OBSs were deployed in the same positions for Line 7 as for Line 1, but this time around we are seeing arrivals out to twice the offsets on most instruments as compared to Line 1. The weather has really been fantastic. Yesterday started out very foggy, and there were small icicles hanging from the railings. Then suddenly it cleared up and got sunny and the visible was incredible. Later it got foggy again. Cary went up to the bow to enjoy the weather and somehow knocked the glasses off her head into the water. Not only did she lose her glasses but she got a shiner as well. Yosio loaned her a spare pair of glasses that he had - the prescription was pretty close, but they were bifocals which are hard to get used to. Now she is wearing Jim Dolan's glasses - he has been wearing contacts so can spare them for the cruise. We have 2 more lines to shoot and then will head back for Punta Arenas. It took us 4 days to get here, but with better weather and a shorter distance (we had to go around the southern end of the Shetland Islands for our first profile) we should make it back in 2.5 days and be in on the evening of November 16. Cheers, Gail November 3, 2000 I just got back from the bridge where about 10 of us had gathered to watch penguins on an iceberg. With binoculars we had a great view of penguins leaping out of the water onto the berg - it seemed to take about 4 or 5 tries before they were successful. The other times they would slide back in, or would be knocked over by another penguin leaping out of the water. We were all laughing pretty hard. We have finished deploying OBSs along Line 7 and are now just playing tourist ship for a few hours since the OBSs won't start recording until 11 tonight. Yesterday we spent the day recovering OBSs along Line 6. The first 10 came back right on time, but the last one took about an hour longer than expected. After it was late coming up we started driving the ship around so that we could send the acoustic release signal from different directions in case the instrument was somehow in an acoustic shadow. Finally most of us had left the bridge and had gathered in the lab making a plan for the next day since the backup timer was not going to release for another 22 hours. Just as we had resigned ourselves to a big detour we got a call from the bridge that the OBS had surfaced. A big cheer went up in the lab. When we got the OBS on board we discovered a large starfish plastered to the glass sphere. We still do not know why the OBS was released late. Recovering OBSs from the Palmer is relatively easy. First a release signal is sent through the hull transducer to the OBS when we are within a few hundred meters of the deployment location. Then several of us head to the bridge to look for the OBS to surface. Each OBS has a radio transmitter, strobes, and an orange flag. Usually the first indication that the OBS has reached the surface iis that the bridge picks up the radio transmission. At night sometimes you see the strobes first. We then maneuver over to the OBS and pick it up with a net on a long pole. One OBS was picked up within 50 m of an iceberg which added plenty of excitement - especially since the wind was pushing the OBS away from the ship towards the iceberg. I've made record sections for all the Line 6 OBSs. The data is highly variable - at some locations the OBSs are only getting arrivals for 20-40 km, at others you see arrivals out to 140 km with beautiful PmP reflections (this is energy that reflects off the Moho - the base of the crust). Our next line will be Line 7, which is a re-shoot of Line 1 from the April cruise. Line 1 had gun problems - half the line was shot without the 1000 cu in gun and the data do not look very good. We are hoping that the data quality improves with the full array in the water. Cheers, Gail November 1, 2000 Today we will finish shooting Line 6. So far the profile has gone off without a hitch. We deployed 11 OBSs along the line. At the northeast end of the profile we started getting icebergs - one iceberg was just about where we had planned to put an OBS, so we placed the OBS about 100 m on the upwind side of the iceberg. After deploying all the OBSs we put the guns in the water and started shooting back along the line. There were a few spots where we had to detour around the icebergs. The bridge has a nice radar display that shows all the icebergs, and you can click on them and see how far away each berg is. Some of the icebergs have penguins on them. Last evening I went up to the bow to see the sunset. The weather was fantastic - blue skies, calm water, little wind, and a temperature of about 25 degrees. The sun set around 8:45 p.m. It is nice having the long days. This morning I woke to even nicer weather - the wind had almost disappeared, and I could see Livingston Island out my porthole. After breakfast I went to the bow for about 30 minutes and watched the scenery. Every now and then I would see a pool of penguins in the water - about 20 of them swimming together and all leaping out of the water at the same time. After dinner we should be done shooting Line 6 and will start recovering the OBSs. Then my real work will begin - looking at the data we collected! Cheers, Gail

October 30 We have spent the last 3.5 days transiting from Punta Arenas and are now in Bransfield Strait. We had winds up to 40-50 knots and the ship was really rocking at times. Now the wind is down to 20 knots and the ship is mostly stable. At 4:00 this morning we started deploying OBSs along Line 6. The days are long down here at this time of year, so at 4 AM.the sun was just rising. Line 6 runs by the Shetland Islands so we are getting some nice views of snow-capped mountainous islands. We also went by Deception Island which is an active volcano. We will continue deploying a total of 11 OBSs along the profile. If you don't know, an OBS is an ocean bottom seismograph which we use to record the movement of the earth in order to build a model of the underlying crustal structure. After we finish deploying the OBSs we will put the air guns in the water. Hopefully the winds will have died down by then. We will then shoot the guns along Line 6, recover the gun array, and then move back along the line picking up all the OBSs. The entire process will take several days. Cheers, Gail

October 26, 2000 We left port at 9 a.m. and are now underway. I can see whitecaps out on the water, but the ship rides so smooth that I can hardly tell we're underway. At 10 a.m. we had an orientation meeting and each got to put on our survival suits. These are bright orange suits that will keep you alive in very cold water, but are not easy to put on or walk in. We then wore the suits out to a life boat and each strapped ourself in. The ship has 2 life boats each of which could carry the entire ship's complement. There are also 6 life rafts. After the orientation meeting we had a deck meeting where we all put on bright orange float coats and learned the rules about working on deck. After lunch we had a watchstanders meeting to learn about the lab. I think we have now gotten all the meetings out of the way. Later today Ian is going to give a presentation about Bransfield Strait and how it fits into the work he does in the Andes. There are 8 people here from UTIG - Jamie Austin, Gail Christeson, Ian  Dalziel, Dan Barker, Jim Dolan, Yosio Nakamura, Steffen Saustrup, and Ben Yates. Also along are Yann Hello, an OBS technician from  Orstom, France, Carrie Olson who will be a watchstander and was along on the last Bransfield Cruise (The story about how she got involved is convoluted, but involves being on the same plane from McMurdo as Steffen last December. She just finished her undergraduate degree and lives in Chicago.), and Susan Herr who will also be a watchstander (she is Jamie's sister and has been on a few cruises before). I haven't had a chance to explore all of the ship yet, but am sure I will  get to know it quite well. There is a large exercise room on the same deck as my room, and nearby is a sauna. There is also a lounge that I haven't discovered yet, and a large conference room. The mess hall is large enough that we can all eat at once instead of in shifts. Unlilke other ships I've been on the Palmer serves 4 meals a day - besides breakfast, lunch, and dinner they also serve a meal around midnight. Cheers, Gail

October 25, 2000 Today I went to the warehouse to get my cold weather gear. Ian, an old hand at Antarctic work, suggested I take everything that was offered since we could store it all in our cabins on the ship. This is what I got: 2 pairs long underwear (tops and bottoms), fleece pants, fleece jacket, wind/water resistant pants and jacket, down vest, flannel shirt, waterproof bib pants and jacket, fleece hat, 4 pairs gloves and 1 pair mittens, 4 pairs socks, and a pair of boots with 2 liners. I tried it all to make sure it fit. I spent the rest of the day touring the town and unpacking for the cruise. Crab is a specialty in Punta Arenas, so I had some for both lunch and dinner.

October 24, 2000 Today I arrived in Punta Arenas. I flew from Austin to Dallas,Dallas to Santiago, Santiago to Punta Arenas. The flight from Santiago was spectacular - out the left-side window you could see snow-capped mountains poking through the cloud cover along most of the journey. An agent met us at Santiago and whisked us through customs and quickly got us checked in for our flight to Punta Arenas. In Punta Arenas another agent met us and transported us to either the ship or a hotel for the night.