Bench Glacier Project

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Borehole video is being used to investigate: 1) debris content with the ice and entrainment processes; 2) characteristics and distribution of englacial water conduits; 3) characteristics of the glacier bed. A small water tight video camera is lowered down each borehole; footage is viewed in real time and recorded at the surface. The holes are filmed right after drilling and periodically thereafter.

	Debris within the ice, about 10 m above the bed. Englacial debris is not common in Bench Glacier… it was only encountered in one location in the 18 holes that span the length of the glacier; this hole is at the site nearest the terminus. Another hole located just 20 m across glacier, had very little debris.
	The bed of the glacier in the accumulation zone. Particles range in size, up to about 5 cm in diameter. The bed is mixed with areas of small particles and areas with bedrock or very large boulders.
	Rare imaging of an englacial conduit dumping water into a borehole. The water level in the borehole is just below the level of the conduit. Depth below the ice surface is 28 m. Quicktime movie of englacial water conduit.

OPENING OF A DEEP CREVASSE

During the May/June 2002 field season a particularly unusual englacial feature was recorded on video. We apparently witnessed the opening of a large crevasse at a depth of approximately 138 m.

The borehole did not drain during drilling or when the bed was intersected (172 m depth). No crevasse was present the first few times we filmed the borehole. Approximately 2 weeks after the borehole was drilled, we noticed turbidity erupting into the borehole at a depth of about 138 m (approximately 35 m above the bed). The turbidity was restricted to a 2-3 m section of the borehole. By this time the water level in the borehole was sitting about 30 m below the glacier surface.

In the same region as this turbidity, a large crevasse opened across the borehole over a period of about 1 day. The crevasse did not extend to the surface. The borehole was eventually offset by the displacement between the crevasse walls, making it impossible to lower the camera past the past the crevasse and into the old hole to the bed. Instead, the camera went into the crevasse where the water was slowly moving down. The opening of the crevasse was about 5 cm and it extended with a steep dip for a minimum distance of 5 m.

Two additional boreholes were drilled a few meters away from the initial borehole, both of which intersected the erupting turbidity. The turbidity continued for at least four days, although it appeared to diminish somewhat in intensity. Unfortunately, on our field season ended on the fifth day.

	Image of the crevasse at 138 m depth. The cable in the image is the video camera’s cable and is about 0.75 cm in diameter. Quicktime movie of crevasse
	Erupting turbidity in a borehole drilled 1 m away from the initial borehole. White cloud in center of image is turbid water. Quicktime movie of erupting turbidity.