Optical-televiewer-based logging of the uppermost 630m of the NEEM deep ice borehole, Greenland

Abstract

We report on the application of optical televiewing (OPTV) to the uppermost 630m of the North Greenland Eemian Ice Drilling (NEEM) deep ice borehole, Greenland. The resulting log reveals numerous natural and drilling-related properties, including the integrity of the borehole casing and its joints, the presence of drill-tooth scoring on the ice wall of the borehole and the presence of regularly repeated layering, interpreted to be annual, to a depth approaching 200 m. A second OPTV log was acquired from a nearby shallow borehole. With the exception of the uppermost 10 m, this log shows a gradual decrease in luminosity with depth, interpreted as a decrease in light scattering with firnification. This shallow log also clearly images annual layers, allowing the construction of an age-depth scale. Comparing this with an independent core-based scale reveals that the OPTV record yields an age of 1724 at the deepest common point of both scales (80 m), 13 years older than the core-based record at 1737. However, all of this deviation accrues in the uppermost 30m of the OPTV record where highly reflective snow saturates the luminosity of the borehole image, an artefact that can be reduced by further adaptation of the OPTV system.