Students participate in Alpine Fault field work
For the last two weeks of January, the Whataroa Valley of Westland was awash with strange equipment, coloured flags and foreign accents. A team of scientists from Vic, Otago and University of Aukland joined with scientists from University of Calgary and University of Alberta (in Calgary and Edmonton, Alberta, Canada respectively), University of Freiburg, Germany and a representative from French oilfield services company Schlumberger. The aim of the Marsden-supported collaboration was to image the shallow Alpine Fault in the immediate area surrounding the Deep Fault Drilling Project-2B borehole completed last year.
There were a number of techniques being used throughout the study. Two sets of instruments were deployed in the DFDP-2B borehole; one hDVS system from Schlumberger, and a string of borehole seismometers from the Univerisity of Alberta with the intent to create a VSP (vertical seismic profile) surrounding the drill site. Away from the drill pad, the team from Germany deployed hundreds of short period seismometers (germanically named 'Cubes') while the University of Calgary installed multiple lines of geophones (the longest being roughly 2km in length) as part of their Aries II acquisition system. The University of Aukland also deployed a number of passive source seismometers throughout the study area for the duration of the experiment.
Two main seismic sources were shared by all of the systems described above. The first, and most extensively used source, was a vibroseis truck shipped over from the University of Calgary. The 'Vibe' is essentially a vehicle which carries a heavy mass. The process of lowering the mass to the ground and vibrating it through a continuum of known frequencies (what's called a 'sweep') creates seismic waves which are used as a source of energy for seismic surveys. The second source was a well traveled 'thumper' from Otago, which generates seismic waves by dropping a mass from a given height. Both sources were used daily for shooting either into the various arrays of 'Cubes' and Aries lines or the downhole instruments from different locations throughout the valley. The vibe truck alone did roughly 9000 sweeps during the two weeks of shooting.
Vic students Danielle Lindsay (summer student with newly coronated head of school John Townend) and Chet Hopp (PhD, Geophysics) were on hand to provide much needed muscle and moral support. Thier responsibilities were varied but mostly consisted of deployment, maintenance and retrieval of the various above-ground seismic systems. In particular, the Aries II system from Calgary required multiple days to lay out the kilometers of cable, batteries and geophones as well as regular daily maintenance and troubleshooting.
A day-in-the-life of the 'line crew' went something like this:
Morning began with a 'toolbox' safety meeting outlining the day's potential hazardous activities. This was followed by a morale-boosting drive from the Harihari field station to the Whataroa Valley (normally accompanied by Dire Straits). Upon arrival, PPE (Personal Protective Equipment) was applied and the day's adventures of battery changing, gate opening, singing, dancing and puddle jumping began. Provided the vibe truck was in a good mood, the line crew were given the task of driving the multi-ton vehicle. In addition, the German cube array, consisting of ~160 cubes, was moved twice daily. This colossal undertaking required the help of most of the field crew but was managed efficiently by our fearless leader Vera. At mid-morning all personnel were treated to Lisa's (our cook/geophysicist) supreme sustenance in the form of freshly baked scones, cinnamon scrolls, home made lamb shank pies and ginger slice. The rest of the day continued as it began with more batteries, bugs and banter. Evenings were spent feasting and exchanging ideas with only mild bickering. After dinner entertainment usually consisted of watching cricket and playing Israeli card games (John avoided participation in the fun, but was always keen to provide colour commentary).
In all, our participation in the experiment was both enjoyable and highly educational. It was a great way to gain a deeper understanding of active source seismic techniques while helping in collecting what will hopefully be an extremely useful dataset. Keep your eyes peeled for finished results as they come in!
-Dani and Chet