Characterizing Englacial Structure From Analysis of Acoustic Televiewer Logs, Upper Fremont Glacier, Wyoming

摘要

Continuous ice cores were recovered from two locations on the Upper Fremont Glacier in the Wind River Range of Wyoming, USA. The study site is located at an elevation of 4000 m within the accumulation zone of this mid-latitude temperate glacier. Two boreholes were produced by a thermal drill and were logged with an acoustic televiewer to depths of 49 and 78 m, respectively. This geophysical instrument generates a digital, magnetically oriented image of the borehole wall developed from the amplitudes and transit times of acoustic waves emitted from the tool and reflected at the water-ice interface. Two categories of features intersecting the holes are identified from the televiewer images and confirmed by visual inspection of cores: (1) shallow dipping planes associated with englacial layering and/or foliation, and (2) high-angle fractures representing the mechanical failure of ice in response to the local stress field. The orientation of the shallow dipping features is found to be orthogonal to the direction of hole deviation, a situation commonly observed in rocks during rotary drilling. The strike of the steeply dipping fractures is perpendicular to the general direction of downstream ice movement, indicating that these features are a product of an extensional stress regime.