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Artificial seismic source for ice boreholes

Wednesday, 19 October 2016

Dave Prior

NZARI funding to Dave Prior, Christina Hulbe (Surveying), Jennifer Eccles (Auckland) and other collaborators (including Christian Ohneiser and Andrew Gorman) aims to test ways of using seismic data to measure remotely the ice crystal alignments and ice temperature in ice sheets and glaciers. In an early stage of this project we have been designing a new artificial seismic source for use in experiments on the McMurdo Ice Shelf (testing field season in late Nov 2016) and the Ross Ice Shelf (late 2017). The source works by the same basic idea as a hammer and plate: bang the ground hard. The problem in the Antarctic field areas is that hard ground at the surface is hard to find. So the idea is to drill a hole (up to 10m) to harder ice and then poke a stick down the hole and bang that. One sophistication is that we want to generate a strong and controlled component of shear wave energy: different ice crystal alignments affect shear waves much more than compression waves, so shear wave information will give us more information about ice crystal alignment. So, the tool we have clamps into the borehole - much like the expansion bolt you get from DIY stores to clamp into a hole in brickwork to hold up your shelves and pictures. With this tool we can bang both down and up, reversing the polarity of the shear waves generated but (hopefully) keeping the p-wave signal the same. We can then filter out the p-wave data and look at the shear waves in isolation.

In order to test the system we have built an artificial ice borehole in a bucket. Lisa Craw, Harriet Love, Brent Pooley, Jim Woods (who builds stuff for us) and I had the glamorous task of spending a couple of hours in a freezer full of squid guts last Friday afternoon. The test was very useful- it highlighted some things that don’t work well at 25 below zero. The source is now undergoing some final modifications before being taken up onto the Tasman Glacier (this week) by Dave, Jennifer, Lisa and Heather Purdie (Canterbury) for more realistic field testing - most specifically to see how far away from source can the signal can be detected. We’ll give an update of how this goes next week.