Field Update: Jan. 3, 2011

Bob Greschke from IRIS/PASSCAL made it out to the ANDRILL camp on December 30th to start preparing for the geophysical experiment. The camp moved from Site #3 to Site #4 on New Year’s Eve, so the first thing he did was help everyone pack up the containers and relocate everything to Site #4. The plan was to make a hole through the ice at Site #4 that would allow us to deploy a hydrophone to the seafloor to listen for the sound waves coming from the surface of the ice during the experiment and to measure the time for the sound to penetrate down through the sedimentary layers and reflect back to the surface (two-way travel time).

The hydrophone was tested by lowering it to the seafloor on a cable to determine if this acoustic sensor could hear the camp generator and other ambient noise generated at the surface of the ice. Some high-frequency noise was detected and it was determined that this was probably vibration of the cable due to the tidal currents in the water column. The solution for mitigating this noise was the re-deployment of the cable and hydrophone with a larger weight that would put more tension on the main cable and pull it tight, while allowing the hydrophone to lay undisturbed near the bottom.

The weather cleared enough to get additional people out to the ANDRILL camp by helicopter, so we sent the rest of the geophysical team. Bruce Luyendyk and Doug Wilson (University of California, Santa Barbara) planned the experiment using geophones that could be placed into the ice.

Doug rode in the Pisten Bulley with Nate Bowker to deploy the geophones.

The geophones were placed in holes dug into the snow and then buried.

The 100 geophones (listening devices or receivers) were placed every 25 meters for 2.5 kilometers along the line to the east of the center point of the experiment, which was the proposed Coulman High drillsite. The seismic sources for the experiment were the shotholes that were drilled earlier. The MECC and the hot water drill system were placed between receivers #19 and #20 to make a hole through the ice shelf to deploy the hydrophone. The combination of the 100 geophones (receivers) buried in the ice at the surface and the hydrophone (acoustic listening device) deployed to the seafloor gives us two different ways to listen to the energy (sound waves) coming from the shotholes during the experiment.

As explained in earlier blogs, the shotholes were small diameter holes down to 35 meters into the ice, which were filled with explosive charges. These charges provide the energy to create sound waves that penetrate downward through the ice and the water and into the sediment below. The waves reflect and refract off interfaces (e.g., the bottom of the ice, the seafloor, and layers of rock below the seafloor) and are recorded by the hydrophone and the geophones listening for these sound waves. The shotholes were spaced 50 meters apart over 2.5 kilometers to the west of the proposed Coulman High drillsite to mirror the 100 geophone locations placed to the east of this location. Each individual shot (explosion) was recorded by all of the geophones as well as the hydrophone to create the spatial geometry of the experiment, with sound waves going down and back up to these receivers.

There are 8 explosives, or “boosters” taped together for each individual shot. These were lowered into each shothole with a length of detonating cord (conducting wire) that connects the charge to the surface of the ice. When the time comes to fire the shots, the two ends of the conducting wire will be trimmed and attached to the firing switch by Ethan Marcoux, the explosives handler who is working with our project. Test shots located to the north and south of the line of receivers will be used to triangulate and establish the position of the hydrophone on the seafloor as well as ensure that all of the geophones are recording data correctly. When everything is finalized, Ethan and Bob will go down the line firing the shots and recording the data while everyone else tries to be as quiet as possible.

Thanks to Chris for providing these pictures. Things are going well.

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