Improving Borehole Optical Stratigraphy (BOS): Modeling, laboratory calibration, and design

Borehole Optical Stratigraphy (BOS) uses a video camera with a white light source lowered into a borehole to record variations in returned brightness. The variations have been shown to correspond to annual layers, though the cause of the brightness changes is not well understood. A combination of radiative transfer modeling and ray-tracing was used to model both the reflectivity of the borehole wall and the multiple reflections within the borehole that produce the received brightness signal. Radiative transfer modeling showed reflectivities between 75% and 95% for most polar firn conditions, with small (< cm) scale lateral energy transfer within the borehole wall. These properties allowed us to simulate BOS measurements using a ray-tracing model with the borehole wall represented as a diffusely reflecting surface. Ray-tracing simulations of different layering configurations show that both the interaction of the multiple reflections of the light in the borehole and the reflectance of the firn directly within the camera's field of view are important in determining the BOS signal; for a 5 cm layer, the former effect is about half of the latter. The presence of bumps or scars on the walls can also produce both local and long-range variations in brightness. The current BOS system has two primary limitations: 1) the single light source cannot distinguish between changes in grain size and density as the cause of brightness variations and 2) the system cannot be used on cores because the multiple reflections in the borehole are necessary to produce noticeable variations. To address these limitations, we have developed an instrument which uses two light sources, yellow (590nm) and near-IR (950nm), to estimate the degree to which scattering in the borehole wall transports photons laterally. An estimate of the scattering density is determined from the spreading distance. This allows us to correct the infrared-visible spectral differences giving an estimate of grain size or bubble density at high spatial resolution. We present measurements on ice cores from 50 meters depth at Summit, Greenland, where our measurements show strong annual-scale layering.