OnsetD

Ice streams are key factors in West Antarctic Ice Sheet (WAIS) stability because they are strongly out of balance and capable of rapid change. Understanding their role as buffers between the interior ice and the floating ice shelves is crucial for modeling the WAIS system and predicting the future of the ice sheet. A critically important feature of an ice stream is the transition from no-sliding to motion dominated by sliding, called the “onset region”. Hypotheses on controls of the location of the onset region are widely varied, but are all focused on the zone that includes the basal ice and its bed.

To test these hypotheses, Sridhar Anandakrishnan (Penn State University) established longitudinal and lateral transects across the onset regions of Bindschadler Ice Stream (formerly ice stream D), which flows into the Ross Ice Shelf. Along these transects he employed seismic reflection profiling to image subglacial till-layer and sub-till sedimentary strata and ice fabric boundaries and seismic refraction profiling to image the deeper sedimentary and crystalline bedrock strata. His high-density seismic coverage required a large number of shot holes in which to emplace explosive charges.

To improve source coupling and the reflection character ICDS designed and built a new rapid air movement (RAM) drill that can produce 90m deep holes. For greater speed, the great majority of holes actually drilled in the field work were 60 m deep -- just enough to penetrate below the firn-ice boundary, although one test hole reached 90 m, the deepest allowed by the air hose on the RAM drill. Previous work on ice streams has used shot holes only 15–20 m deep. Setting charges in ice rather than firn resulted in vastly higher energy coupling into the ice. In addition, deeper holes delayed the energy that reflects first off the surface to prevent it from interfering with the imaging of thin subglacial layers. During the 2002-03 field season ICDS lead driller Jay Johnson and his crew (John Robinson, Mike Jayred, Dennis Duling, and John Rhoades) drilled 226 shot holes with the RAM drill; normal drilling time for 60 m was less than half an hour.

Anandakrishnan also conducted an auxiliary seismic reflection survey on neighboring Kamb Ice Stream (formerly ice stream C), for which ICDS supplied two hot water drills that were transportable by Twin Otter. ICDS purchased these drills from contractor Dennis Duling, who trained the field operatives in their use. Anandakrishnan and his crew drilled 150 holes to 22 m on Kamb Ice Stream with no significant failures.