BOS (Borehole Optical
Stratigraphy) uses a camera with a built-in light source to measure layers
in the sides of boreholes in firn. Bob Hawley developed BOS at
the University of Washington between 2000 and 2005, and has used it in the
Arctic and Antarctic. Making a BOS survey has two parts: 1. Drilling the hole: We will drill our holes with an electric
ice-core drill that makes a 5 inch hole. The drill is very simple-
it's a fiberglass pole with a cutter on the bottom and a motor on
top. The drill breaks into sections so that we can pull it out of the
hole in pieces, and a hollow section above the cutter holds pieces of core
and shavings. photo credit: Jay Kyne Here’s a picture of a
drilling project in We will weigh and
measure each segment of core to determine its density, then abandon most of
it in the snow- we’re more interested in the hole than we are in core! 2. Logging the hole. We
lower our camera on a cable that runs over a wheel connected to a
computer. We measure the time at which each frame of video is
collected by the computer, and count how many times the wheel has
turned. This lets us work out how bright the wall looked at any depth
in the hole. photo credit: Bob Hawley This is the kind of
image we see in our holes- the circles you see here are melt layers, where
the surface temperature reached zero degrees and a crust of hard snow
formed at the surface. We
don’t expect to see layers this sharp at WAIS, where the temperature almost
never reaches melting, but more subtle layers can result from temperature
differences between winter and summer. Analyzing the data: We analyze our data by
plotting the brightness of a ring of pixels in a camera image against
depth. Plots like this from
Borehole Optical Stratigraphy at the WAIS divide
