A major part of MIDAS’ field programme involves investigating the internal structure and physical properties of the Larsen C Ice Shelf, and in particular how these properties are influenced by the presence of intermittent surface melt ponds. In order to measure the shelf’s 3D temperature, via thermistor strings, and density, via optical televiewer logging, we first need to gain access to its interior.
To do this we are drilling a series of boreholes, each about a hundred meters deep, using pressurized hot water. Water is fed through a pump, pressurizing it to about a hundred times atmospheric pressure, before it is heated by an oil-fired burner and delivered as a fine jet to the ice via a long, high-pressure hose and straight, steel stem. The stem is lowered slowly down the borehole, acting as a pendulum to keep the hole vertical, melting the ice in front of it as it progresses. The hot water delivered along the hose, along with that generated by ice melting itself, passes back up the borehole enlargening it until the water is lost into the permeable near-surface firn.
Although drilling a borehole by using water in Antarctica sounds rather counter intuitive, it is actually highly effective: allowing a hundred-meter-long borehole to be drilled in a couple of hours. That said, the water used to drill the hole does itself need to be melted from snow using a melting tank heated by an oil-fired domestic central-heating unit. All being well, which frankly it often isn’t in Antarctica, a day and a half is needed to melt the two thousand litres of water typically used up in drilling a hundred meter borehole. It takes a minimum of three days to drill one of MIDAS’ boreholes (two days to set-up and melt snow, and one day to drill the hole, log it and install temperature sensors). In reality, it typically takes a week per borehole allowing for poor weather (Larsen C is notoriously changeable in this regard) and troubleshooting the innumerable and somewhat inevitable issues that arise from mechanical and electrical glitches—particularly following the hours-long shaking that accompanies a camp move.
Indeed, in the event of a delay, one of the challenges we face is keeping a large water reservoir liquid in sub-freezing conditions. Although our water tank is insulated, ice may still need to be carefully removed from it and replaced by liquid water each morning no matter how bad the weather—often a somewhat unpleasant task.
Another issue arising from the perpetual cold is that the drill’s main hose also needs to be warmed, before water can be passed along it without risk of freezng. We currently overcome this by preheating the hose and its spool with hot water delivered through a short section of hose, warmed to just above zero degrees Celsius by submerging it within the main water tank. Given that we have field generators, a better way to heat the main hose would be to warm the entire spool with some form of weatherproof electric blanket (a tyre warmer?). Alternatively, it may be possible to blow hot air along it—but such air flow would not be possible if the hose were already blocked by a small plug of ice, which is highly likely. Any suggestions for overcoming this problem would be gratefully received.
We are pleased to report that MIDAS 2015’s drilling has so far met with one hundred percent success, having completed and logged three boreholes to date.
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