Collaborative Research: Sonic Logging of GISP2, GRIP and NGRIP Boreholes

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Abstract OPP-00-82469LamoreyOPP-00-82469WaddingtonInterpretation of paleoclimate records from ice cores depends on understanding the ice sheet flow to determine depth-age relationships and whether the ice has been affected by folding. The alignment of crystals in ice, called fabric, is an important factor in understanding ice sheet flow because preferentially aligned crystals cause the ice to flow more easily in certain directions. Fabrics have traditionally been determined by using thin sections cut at intervals in the ice core. A new method of determining fabric is sonic logging, where a probe is lowered into a borehole to measure the velocity of compressional waves through the ice. Sonic logging is valuable because it provides a continuous profile of the ice fabric and averages the alignment of ice crystals over a larger volume than do thin sections. The Principal Investigators will measure sonic velocity profiles at the Greenland Ice Sheet Project 2 (GISP2), Greenland Ice Core Program (GRIP), and Northern Greenland Ice Core Program (NGRIP) and use them to: 1) improve the understanding of the relationship between thin section and sonic velocity data; 2) determine if sonic velocity data can be used to identify the depth at which paleoclimate record continuity is lost and if so, formulate criteria to identify this depth; and 3) provide verification and input data for anisotropic flow law models.Sonic velocity profiles, combined with thin section data, will be used to investigate how sonic velocities and thin section data can be combined to better determine ice fabric. A fabric estimation model will be developed that uses thin section data and velocity profiles to predict fabric parameters and their uncertainty and spatial variability. The sonic velocity profiles will be used to determine if the depth at which the paleoclimate record continuity is lost can be identified from sonic logging. This determination was facilitated at GISP2 and GRIP by comparing two paleoclimate records because they correlate highly above this depth and diverge below it. NGRIP will provide another important set of data because this location was chosen specifically to recover ice that was found to have disturbed stratigraphy at GISP2 and GRIP. Finally, the GISP2, GRIP, and NGRIP sonic logs will be used to verify and constrain the latest generation of ice sheet flow models that use an anisotropic flow law to include the effect of fabric.