Extended Range Optical Televiewer Imaging of the NEEM Deep Ice Borehole

The North Greenland Eemian (NEEM) Ice Drilling Project borehole reached the base of the ice sheet in August 2010, hitting the bed at a depth of 2438 m beneath the surface. The NEEM core is therefore one of the longest ever drilled, and it is hoped that it will provide a high-resolution record of past climate extending through the last (Eemian) interglacial period. Additional complementary information relating to the core and the physical makeup of the local ice sheet can be provided by viewing the borehole that has been left following the removal of the core. An advanced technique for such logging (optical televiewing; OPTV) has recently been developed by Robertson Geologging Ltd (RGL) for use in the mining industry. OPTV has many advantages over previous (directional) borehole video technology in that it provides a geometrically accurate image of the entire borehole wall (i.e., 360 degrees around the borehole and along its full length) at a very high resolution (less than 1 mm per pixel). The applicant has, over the past three years, worked with RGL to adapt this equipment for use in ice and applied it to several test sites on glaciers and ice sheets. The results of these pilot studies have completely transformed the information that can be recovered from ice boreholes - including identifying different ice types, identifying different layers and structural generations, identifying and characterizing bubble and debris inclusions held within the borehole wall and, not least, providing orientated virtual images of the core removed from the borehole (by 'rolling' and inverting the image of the borehole wall). Despite the potential of OPTV to revolutionize ice drilling research, the technique is currently restricted to use at depths of less than 1000 m by issues relating to the pressure rating of the sonde itself and signal communications along an extended length of relatively lightweight (and therefore transportable) cable. The former issue has now been resolved, and RGL is currently in the late stages of solving the signal-processing challenges and will have an extended-range OPTV system (EOPTV) developed by the end of 2011 (see Letter of Support). This small-grant application is to apply this new EOPTV system to the full (~2.5 km) length of the NEEM deep borehole, both to evaluate the performance of the new system and to address several specific science questions (listed under Objectives).