Field Update: Dec. 10, 2010

The plan at Site #3 is to run a series of oceanographic instruments and sampling devices down the hole through the ice shelf for short duration deployments. Craig Stewart and Mike Williams (NIWA) have just recovered an acoustic doppler current profiler (ADCP). This instrument measures water current velocity for a range of depths in the water column under the ice shelf. Prior to this they took water samples and ran a CTD (conductivity, temperature and depth) sensor up and down through the water column for 28 hours to measure changes during this part of the tidal cycle. The plan is to make measurements during both the spring and neap tides.

During all of these deployments it is important to keep careful notes about what was done and how it was done so that we can evaluate the data that are collected in a suitable context. Richard Levy takes notes about the deployment of various instruments for the report that will be written about the outcomes of these surveys. The responsibility for keeping accurate records is shared among the scientists and students taking part in this work. They record the date, time, instrument type and depth that measurements are made along with other information needed to understand the data collected.

The DOCTOR (Downhole Optical Caliper Tool for Observing Roundness) is moved over to the hole by Rich and J.R., with Craig Stewart looking on.

Each person plays a different role in the deployment. Mike Williams prepares the CTD, which will hang below the DOCTOR. He has been working on these deployments all day and is ready for a break. Richard waits to prepare the video camera that will be placed in a pressure housing.

The video camera will ride at the top, with the lighting system below, followed by the CTD. This sensor geometry has been shown to work well.

Richard places the video camera in a Styrofoam ring that holds it in position and then pushes the camera into the pressure vessel, ready to be turned on.

Bob Detrick looks at the O-ring and plate that seals the pressure vessel, preventing water from getting inside as it goes down to the seafloor. The video camera and the lights are turned on and the system is deployed.

Mike Williams guides the CTD and the DOCTOR into the hatch and down through the ice, while J.R. provides assistance. A series of plastic “whiskers” keep the instrument centered in the ice hole, and measure the hole diameter.

As the system is lowered, J.R. watches for any problems while Mike records the time and other information about the deployment. The video camera has only a certain amount of battery life and data storage capabilities, so both of these factors must be taken into account when planning the deployment. How fast the winch is lowered determines how much time to get to the base of the ice shelf and then to the seafloor. We need to have enough time for the camera to make observations at each of these locations, and the camera is always running once it is turned on; therefore, careful planning is essential.

This DOCTOR run will take about an hour, and then it will be time to shift gears and collect some gravity cores to sample the sediments below the seafloor. The last core was over a meter long, so this has been working well. Each measurement or sample adds to our understanding of this area and the environment within and below the ice shelf, which is why we’ve come here. This work will continue for another 10 days and then we’ll bring the SCINI remotely operated vehicle (ROV) to camp to conduct dives through the ice to look further away from the ice hole at the bottom of the ice shelf. Bob Zook and the rest of the SCINI team are back in McMurdo waiting for their turn and preparing the vehicle for it’s first dive through the ice shelf.

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