Field Update: Jan. 6, 2011

The seismic refraction experiment is ready to be shot. The explosives are down in the 55 shot holes and the 100 geophones are buried in the snow on the opposite side of the proposed drill site location from the shot holes. The MECC and hot water drill are in place and the hydrophone is on the seafloor hanging from a cable deployed through a hole melted in the ice shelf.

The MECC is located over the hole in the ice where the hydrophone has been deployed, which is between receivers #19 and #20, offset 25 meters south of the line. We have been using a precision GPS system to locate each of the shot holes and receivers, as well as the hydrophone location, so we understand and can describe the geometry of the experiment and are able to analyze the results. The key thing during the experiment is to remove all other sources of ambient noise by turning off vehicles and generators, and also by requesting that people avoid opening and closing doors. We only want to record sound waves generated by the experiment itself.

The hydrophone is a small sensor attached to a thin cable – it is wrapped in a mesh bag. We attached a weight to the end of the cable to anchor it to the seafloor and attached small floats to the hydrophone to help it float free of any vibration coming from the cable. Then we lowered both down the hole.

The geophones and cables were prepared earlier and coupled to the battery-powered Texan recording devices, which were all deployed in the snow.

The lithium batteries are capable of powering the recording devices for 36 hours, so once they were installed, the clock started to complete the test.

The batteries and recorders were installed together with the geophones in the snow every 25 meters, at a meter or less from the flags marking each spot.

Bob Greschke and Ethan Marcoux were the firing and recording team. They ensured that the signal was being recorded when each shot was fired.

They moved quickly from shot to shot, firing one after another in a line.

Ethan would connect the ends of the wires coming from the charge to the firing controller and then call “fire in the hole” while Bob looked on.

During the shooting, the rest of the team took a well-earned break and enjoyed the outcomes of their hole melting labor – knowing that more work lies ahead. It’s time to appreciate the sun on the ice shelf and good company!

For the seismic team, the completion of this experiment means that their time at the ANDRILL camp is at an end and they will be going home soon.

Chris and Doug are pretty happy thinking about this outcome! Bob Greschke is happy too, but he will be staying for a few more days to make sure that all of the data from the experiment are downloaded and backed up, and that all of the seismic equipment is packed for shipment back to McMurdo.

With the completion of the seismic survey, the pace of life at the camp slows down a bit. The team decides to run the gravity core and the CTD sensor on the wireline down into the hole to get some additional data. This can continue until we’re ready to move back to Site #1 and begin to pick up the oceanographic moorings. We’re back to being a week or more ahead of schedule because the oceanographers don’t arrive until January 15th. With Doug, Chris, Ethan, and eventually Bob heading back to McMurdo, the camp population will be down to 7 people, which is the lowest number for the whole season – plus me, but I’m still in McMurdo looking after paperwork and coordinating with the various work centers around town to get the supplies we need to the camp. We’re closing another chapter in the story of the Coulman High site surveys and getting closer to the day when we all traverse the equipment and containers back to McMurdo Station and Scott Base to conclude our season. Stay tuned….more to come!

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