Antarctic report – drilling the hydro holes

By Guest Author 26/01/2012

by Dr Craig Stevens, Physical Oceanographer at the National Institute of Water and Atmospheric Research (NIWA)

We got the all-clear for heading out to the site in the morning so we could set up our sea ice camp site.

It was a stunning day and we had all day to drill a hole, set up the tent and do some sampling — easy! Step one was delivery of ourselves and our gear in helicopter sling-loads to the field site some 30 minutes helicopter flying south of the Italian station. These sling loads wrap up our boxes in big nets and chain them to the underside of the helicopter. When the load is on the ground at its destination point it is gently released. Hundreds of kilograms of gear can be moved in this way.

The setup proceeded slowly as it always does with the usual sequence of breakages and challenges that is essential in fieldwork and never more so than polar oceanography where everything takes three times as long as you expect. For these shallow ’hydro-holes’ in remote locations we use a combination of drilling techniques that gives us a not very pretty, but functional, hydro-hole. The ice was very soft and wet and almost immediately the holes we were drilling filled with slush which maked it hard going. Right as we broke through the last little piece the drill fell apart so that was good timing!

Setting up the field camp on 2 m of sea ice
Setting up the field camp on 2 m of sea ice

We then moved our tent over the hole and setup the gear inside. The first thing we did was get the acoustic current profiler in place — this is called ADCP for short-hand with the D standing for Doppler. It sends out pings like a fish finder but instead of simply listening to the amount of return signal as it bounces off small particles in the water it listens to the Doppler-shift in the signal. This way, after some geometry, we arrive at the water speed over a hundred or so metres of water depth. This unit will stay in for the duration hopefully giving a nice picture of the flow beneath the ice. Next we used a depth sounder to measure the depth. The charts put the depth in the vicinity of 600 m here but with very little certainty. However, the sounder struggled to find something to lock on to so the results were inconclusive unfortunately. This meant we didn’t have the depth and would have to do the first profile slowly — essentially by feel.

Finally after all day setting up and with time getting away from us before pickup. I turned on the turbulence profiler – our reason for being here. And it didn’t work! Your worst nightmare! It was clear no power was getting to the unit so I bypassed the power conditioner and it worked but not very well as spikes from the generator corrupt the signal. The power was sufficient for us to profile slowly to get the depth and basic properties of the water column. At the same time the winch was putting out a funny burning smell which is never good. Either it’s a problem with the fan in which case we’re probably ok as the air will help cool the unit. Or it’s something more sinister in which case there could be challenges ahead. The trouble is the winch is 130 kg and not easily moved back to base for repair. Of course it’s just gone 100 years since Scott and his team reached the pole… second. So we’re not complaining.

And the data! Even the one profile recorded so far is interesting and probably the first such data from this complicated piece of the ocean where ice shelf water meets polynya water. In other words two radically different waters – but both associated with the birth of sea ice. The profile captured a warm (well… -1.9 oC) fresh layer just beneath the ice, then deep down there is a clear layer that is a little bit colder and saltier than the water around it. We hope to get a few more profiles over the next few days, especially in high resolution mode where we can really begin to unravel things relating to the sources and rates of mixing of these waters.