Wednesday, October 26, 2011

Crevasse Detection: what to do when you don't have ice

The end of summer and start of fall has proved a tumultuous time for Yeti. The shortage of funds and lack of ice breakers means Yeti will not be allowed to go to Antarctica for the Traverse, though he has picked up a moonlighting gig at South Pole Station. The old, buried station seems to be giving tractors a rough time, and Yeti will do a few grid searches with his GPR to locate pipes, empty spaces, and the like before new construction begins.

In the meantime, what does a Polar robot do without snow and ice? The answer, surprisingly enough, is dry, hot sand! It turns out that very dry sand has similar electrical properties as snow and ice, and it's possible to build a scale model of a crevasse in the lab.

So, all Yeti needs is a nice big space to build a huge sandbox. The crevasse itself can be modeled with good old fashioned polystyrene, which has the same electrical properties as air! We'll make an inverse "mold" of a crevasse with polystyrene in a wedge shape and place it in a large container (dumpster, swimming pool, pit, etc), and pour in the sand. To create striations we can intersperse some ply-wood or drywall layers.

The problem with scale models for GPR is a need for different radar frequencies. Lower frequency radio waves emit less power, but penetrate much deeper. High frequency pulses give off lots of power, but their depth penetration is shallow. If our model is smaller than a real crevasse (it better be; some crevasses penetrate deeper than 50 meters!), we don't want the GPR energy radiating deeper than our model, because then we will get signals from the floor, the building foundation, and anything in-between! We currently use 400 MHz antennas with a SIR-3000, which can image 25 meters deep in ice! We'll need to switch to a much higher frequency antenna, preferably in the giga-Hertz range. Common frequencies are 2.1 and 2.6 GHz. Then, we can scale a crevasse by at least a factor of 5. A 12.5 meter deep crevasse then could be modeled with a 2.5 meter deep polystyrene wedge buried in dry sand.

Yeti and I will continue to be resilient in the face of adversity. Stay tuned.