Strong correlation of borehole optical backscattering and volcanic sulfate signatures at Siple Dome

A borehole logging instrument measuring optical backscattering was deployed at Siple Dome (SDMA) in January 2014. The logger body was a completely passive instrument consisting of a lightweight aluminum structure supporting two optical fiber cables and associated collimating connectors. One cable guided light from an 808 nm laser on the ice surface while the second collected backscattered light and guided it to a silicon detector, also on the surface. Both laser and detector were positioned within a few cm of one another for high resolution. An image of the device is shown in Figure 1. The laser wavelength was chosen so that the scattering length for light within the ice would be comparable to the absorption length. In this way it was possible to guarantee that most of the light received had been directly backscattered from sites within the ice rather than traveling for many meters through many scattering events, which would reduce the resolution of the layer measurement. The resolution of the measurement was estimated to be on the order of a centimeter to a few centimeters, the lower limit given by the size of the collimated laser beam and the other by the geometry of the laser-detector unit and the nature of the optical backscattering at 808 nm. The ice could not be sampled directly to determine the physical source of the scattering but, based on prior work at Siple Dome and other boreholes, the background signal was assumed to be produced by bubbles with the observed layer structure primarily due to volcanic ash. Useful data was measured to a depth of approximately 175 m, with the resulting signal given in Figure 2, minus the background scattering due to bubbles. 20 of the 27 statistically significant deviations from the background average matched closely with the volcanic sulfate record published by Kurbatov, et. al. in 2006 using the chronology of Taylor, et. al. in 2004. The correlation between optical backscattering, presumably from ash, and the presence of volcanic sulfate is perhaps the highest resolution logging record to date and enables one to resolve individual closely spaced eruptions.