Well, the good news is that our radar kit has arrived! We managed to get the kit out and working and set it up for a reflection survey. As we had taken the inclinometer survey in the borehole we figured it was about time we had some data to go with it!
As background information, for those who don’t know, a reflection survey uses two antennas: one that transmits energy, and one that receives and measures its strength. For radar, this energy is electromagnetic, like light. In this case, the antennae are fixed at a set distance apart and placed into a borehole. We have a borehole wheel which communicates with the computer so we know how far down the borehole we have lowered the antenna. The energy is sent out and reflected off surfaces between two materials with strongly contrasting electrical properties, for example ice and water. This information is recorded on the computer to be recorded later. In this case, energy will reflect strongly from the borehole walls, crevasses (especially those containing water) and even microscopic pockets of water contained between ice crystals.
Now for a bit about my science. My project is focused on measuring the amount of water stored at the microscopic level, found in ice that has been warmed almost to melting point, either through direct heating or through compression. Although we cannot resolve the individual pockets of water (and air) stored in the ice with these types of survey, their presence will still have a clear effect on the geophysical data. To do this, I will be using two types of geophysical equipment, seismics and ground-penetrating radar (GPR) using acoustic and electromagnetic waves respectively.
The survey type that I hope to do involves drilling two adjacent boreholes, putting the transmitter in one borehole and the receiver in the other. Starting at the surface, these will be lowered by equal amounts; the energy should travel in a straight, horizontal line between the two boreholes, measuring the properties of the volume of ice in between them. This is known as a zero-offset profile (ZOP).
While attempting to drill a second hole downstream from the first borehole (BH1), I completed the radar reflection survey in the borehole, along with supporting inclinometer and borehole camera surveys. We cannot perform a seismic survey while drilling as this creates a huge amount of background noise with the generator, pump and diesel heater rattling around on the glacier surface. The drill was again being particularly awkward and very inefficient, and after drilling a mere 15m into the ice for the second hole, it stop working. So began the days of frustration in trying to figure out what was wrong.
With the drill being off, we took the opportunity to complete some practice surveys with the boreholes we had drilled; this enabled us to check that the equipment was working. The only equipment we could not test was the seismic borehole source, as this required a wider and water-filled borehole. To test this, we tried a vertical seismic profile (VSP) and a radar zero-offset profile (ZOP). The geophone (seismic microphone) used for the VSP is a 3-component (measures vertically and in 2 horizontal directions) borehole geophone that clamps to the sides of the borehole (kindly lent to us by Queen’s University, Belfast). The geophone is only ~5cm wide, however as we had drilled BH1 a few days ago, we found that the geophone would not go past 3m depth in the borehole. This was also the case for BH2, producing two very short practice surveys! But we have at least had experience now.
Now, back to the drill. Up until a few days ago, we had found countless issues with the drill, that I shall not bore you with now. I shall simple state that with the timely arrival of a new car battery (via helicopter) and the expert and frankly invaluable assistance from Erich and Bernard Heucke, who conveniently happened to be in Tarfala precisely when needed(!), we are now able to use a working hot water drill! They have even pieced together a new drill bit for us to attach to our existing drill stem. This will hopefully produce a hole that is both straight, and wide enough for the seismic source to be used.
We have started drilling further towards the centre of the glacier now in an attempt to produce a non-draining borehole. One borehole has been completed to a depth of 67m, even with the occasional issues with water supply and drill failure. The full depth we wanted to reach was rejected as we are running short on time. The second, larger borehole has been drilled to 22m so far, although the drilling speed is far slower in order to obtain the required width. Luckily the sun and rain over the last few days have created a great deal of stored water for us to use, so the lack of water is no longer an issue. And I’m sure Yoann is quite grateful not to have to dig anymore water pits!
At some point soon we hope to put the seismic and radar instruments down the borehole. Lets hope that the new drill bit works so we can finish the borehole and get some usable data for my PhD!
This blog was created a few days ago and due to limited internet connectivity it has not been published until today. You will be able to read an update on our progress on the next installment very soon!
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