We will drill and instrument to varying degrees six boreholes extending from the surface of Khumbu Glacier to its bed. These will be located at four sites to provide gradients in ice dynamics and ice thickness. They closely coincide with known supplies of supraglacial meltwater, required for the drilling. We will begin in the active ice zone around Everest Base Camp. There is no debris cover at this elevation and persistent supraglacial streams exist from which we will siphon meltwater. Site 2 will be located 1 km up-glacier of the lowermost confluence, coinciding with where a major surface meltwater channel enters the englacial system and ice is again exposed. Site 3 will be located approximately 2 km up-glacier of our base camp at Lobuche, where supraglacial ponds with extensive clean ice exposures are in abundance. Site 4 is located adjacent to our base camp and 1.5 km from the glacier terminus, where supraglacial ponds are fewer in number but larger in size.
Previous estimates indicate ice thicknesses decreasing from ~300 m at Site 1 to ~60 m at Site 4 (Fig. 2). We plan to drill boreholes to a depth of ~175 m at Site 1 and to the bed at Sites 2 – 4. This strategy avoids a step increase in logistics required to drill beyond ~175 m, requiring an up-scaled drill, borehole winch and hoses and thereby restricting manoeuvrability across the glacier surface. Even at Site 1 this approach will measure englacial temperature and tilt data for over half of the ice column, which is more than eight times the depth of any currently available data.
To achieve this, the team will use a hot water drilling system based around a Kärcher pressure/heater unit (PHU), capable of producing 15 litres of hot water per minute at a pressure of 100 bar, powered by a petrol generator adjusted for high-altitude. The PHU will be fed by filtered supraglacial meltwater stored in a 200 litre settling tank, and water will be delivered to the ice surface via a 2.4 m-long drill stem. We anticipate a single hole taking less than one day in the field to drill, but will allocate 6 days field time per Site to allow for multiple holes and sensor installation, as well as contingency e.g., in case of delays in helicopter support.