Improving Borehole Optical Stratigraphy (BOS) T.J. Fudge Ben Smith Ed Waddington
What is BOS? Hawley, 2008 BOS measures brightness of borehole wall with depth
Annual Layers are Visible Allows rapid determination of past accumulation rate Ability to observe seasonal firn compaction Hawley, 2005
Why is getting in situ grain size and density important? Densification of Firn Trapping of Air in Firn What causes the variations in brightness? Can the physical characteristics of the firn, such as grain size and density, be inferred from the brightness variations?
Modeling the cause of brightness variations -Photons enter the firn and move by scattering. -Keeps track of scattering events and position photon exits firn Examine the effects of changes in grain size and density by using a Monte-Carlo model that tracks photons the mean distance a photon travels between scattering events Scattering Length: snow grain photon track Firn surface
Scattering length radius of snow grain Density of scatters (grains) Scattering efficiency, ~2
Scattering length radius of snow grain Density of scatters (grains) Scattering efficiency, ~2 Brightness Variations can be caused by either grain size or density changes
Fine Grain Coarse Grain Spreading of Light due to Scattering Length Change
Grainsize also affects albedo, but does not dominate scattering length brightness effects. Multiple bounces are required to get large enough brightness changes One limitation is that measurements cannot be made on cores more absorption less absorption
New Device Emphasis on measuring grain size Currently designed for cores
New Device Two light sources of different wavelengths Measures spreading of light Yellow (590nm) Near Infrared (950nm)
Intensity (arbitrary units) Scattering Lengths Single-scatter coalbedo geometric mean grain size, m Returned Intensity at meters Distance from source (mm) Non-dimensionalized Intensity Modeling Results 590nm 950nm
Preliminary Results from Summit Greenland Firn Core Light Sources are located closest to pixel 62
Preliminary Results from Summit Greenland Firn Core Layer counting suggests annual layer thicknesses of 17-40cm at this depth, with 3-4 years per meter Light Sources are located closest to pixel 62
Conclusions BOS brightness variations can be caused by either changes in density or grains size. The measurement are insufficient to determine physical properties of the firn. A new device using two separate wavelengths of light and simplifying the geometry of the measurement yields promising results for inferring grain size.