Exploring Subsurface Geology
Imagine being in a remote field camp in Antarctica, set up on sea ice just two meters thick. What could scientists possibly be doing there?
Well, a group of researchers just returned from such a project: Mackay Sea Valley Seismic Survey. We were in a location called Granite Harbor, around 100 miles north of McMurdo Station, to conduct a seismic survey of the seafloor and the rocks below the seafloor which accumulated in this large trough carved by the Mackay Glacier. Under the leadership of Dr. Ross Powell, of Northern Illinois University, we are working to get a picture of the rocks below the seabed where thick accumulations of Holocene (the past 7,000 years) sediments are located. These sediments are of interest because they can tell us information about what the climate has been like in this part of Antarctica for the past 7,000 years, and how it compares to the climate seen in atmospheric records and marine geological records for other parts of Antarctica during the same time period. The seismic survey will not drill for rocks, but will provide information for future drilling projects about where the sediments are located and where a future drill hole should be placed.
Seismic surveys operate using a sound blast, made by an airgun, which is set off below the ice. We drill a hole through the sea ice so that we can place the airgun, which is on a cable, down 8 meters in the water. We have a 1.5 kilometer long streamer that is dragged on the ice, behind a vehicle called a pisten bully, that carries the laboratory with all of the seismic recording devices. The streamer contains 60 geophones that record the sound waves after they have blasted out of their air gun, traveled down through the different layers of rock on and below the sea floor, and been reflected back up. Different types of rocks give off different types of reflections, and require differing amounts of time to travel back up to the geophones. Computers in the mobile lab record the sounds as they bounce back to the geophones, and based on their time delay, they give an image of the different layers of rock in the subsurface.
This type of work is typically done by ship with the geophones towed behind the ship. We are using a very new technology to conduct this survey over sea ice, and getting excellent results. The survey was very successful in identifying where the pockets of sediment are. The seismic data was all collected digitally and will require continued processing back at Montana Tech University where Dr. Marvin Speece, the geophysicist on the site survey, teaches. Once processed, the seismic records will tell us in much greater detail, about the types and thicknesses of sediments – and allow scientists to determine the optimal site for future drilling.
Postcards from the Field: ANDRILL