by Dan King, State University of New York at Oneonta
The Other Seismic Bandits:
Lizzie Kenny, Bowdoin College; Julian Alwakeel, Florida International University; Josh Ivie, Tarleton State University; and Mike Staron, Keene State College
Most research groups here at Camp 10 have now prepared, or already begun, their various projects. The seismic group, or the “Seismic Bandits” as we’ve come to call ourselves, has completed our research because all of our fieldwork needed to be done on the Taku Glacier.
Our research became a race against the clock upon our arrival at Camp 10. The delayed departure of the latter trail parties from Camp 17 certainly didn’t help our cause. Our faculty research advisers, Don Voigt and Kiya Riverman (both from Penn State University) were set to fly out of camp just under two weeks after the arrival of the first members of our research group ( Lizzie and I) and our fieldwork needed to be concluded prior to their departure. Don and Kiya were also kind enough to provide the group with all of the essential equipment needed to conduct our fieldwork. Just a few days after arriving at Camp 10, our group began testing the field equipment in Icy Basin, just a couple hundred meters Southeast of Camp 10. Collection of the critical data for our research project did not begin until all of the Bandits had arrived at Camp 10 several days later.
In the past, Don and Kiya have typically used explosives for their seismic work. However, since we didn’t have permits for seismics, we used a sledge hammer, and occasionally an instrument known as “Betsy,” for a sound making device. Betsy is a surprisingly light piece of equipment that fires blank 12 gauge shotgun rounds into the glacier.
Before I get too specific, I’ll take a step backward… Our original goals were to use standard methods of seismic reflection to determine the depth of the Taku Glacier, and also to determine the underlying material (bedrock, sediment, water, etc.). We also planned to do seismic refraction surveys in areas of both low and high strain on the Taku Glacier in order to create firn density profiles. Firn is snow that has survived at least one entire melt season. It is denser than fresh snow, but not as dense as glacial ice. Using the firn density profiles that we create from our surveys, we hope to better understand how regional strain can affect the rate at which firn densifies into glacial ice. Firn densification is important to consider when trying to understand ice flow dynamics – this is where our work becomes valuable.
After getting into our fieldwork, we decided unanimously that we would prefer to devote our time to the refraction surveys, and to drop reflection from our work entirely. We did this, in part, because we were crunched for time, but mainly for other reasons: While seismic reflection had been used on the Taku Glacier before, refraction surveys to examine properties of firn densification have not. The Bandits agreed that it would be best to devote our time to a single research project that was unique to the area, and our advisers supported that decision.
We conducted 4 refraction surveys: two in areas of relatively low strain, and two in areas of relatively high strain. Each individual survey, however, was conducted in the same manner. We would start by laying out 500 meters of cable in a line, with nodes at 20 meter intervals. At each node, we dug a hole, buried a geophone, and connected it to the node. The cable was connected to a magical box called the “geode”. Also connected to the geode, was the trigger switch, which we connected to the end of the sledge hammer, or to the mallet used to trigger Betsy. We ended up using the sledge hammer much more than Betsy because it was faster and worked just as well, if not better. The geode was connected to a battery and our laptop. These connections made up our temporary seismic station for each survey.
Starting at the closest geophone, and gradually moving to a distance of 20 meters, we would place the “shot” then take several recordings. The “shot” is the term for each sledge hammer hit. When ready, the Bandit manning the computer would say, “Quiet on the line,” to eliminate noise interference from the group, and the person manning the sledge hammer would strike a metal pipe on the surface of the glacier. The impact would trigger the switch and start a timer. The computer then recorded the magnitude and time of arrival of the sound wave(s) produced by the shot at each individual geophone, which we analyzed back at camp. Shots were usually taken 0-20 meters away from the first geophone, at an interval of 2 meters each.
Although the fieldwork was repetitive, we never got bored. Each of the Bandits learned the various team roles, and became masters of using the equipment. After a while, I became the designated sledge hammer-er… The team even started calling me “Thor.” Mike’s ski ballet was also a brilliant source of entertainment during our brief moments of down time. In the end, we were all able to walk away with great data and close bonds from our memorable moments in the field. All that’s left now is to return to camp and crunch our data.
I’d like to thank Don Voigt and Kiya Riverman, not only for their instruction and the use of their equipment, but also for the enthusiasm and patience they expressed while working with the Bandits. I speak for us all when I say that it was a pleasure to work with you two. We couldn’t have done it without you. Perhaps the Bandits will someday reunite.