Distribution of preferred ice crystal orientation determined from seismic anisotropy: Evidence from Jakobshavn Isbrae and the North Greenland Eemian Ice Drilling facility, Greenland

摘要

Preferred crystal orientation fabrics (COFs) within an ice sheet or glacier are typically found from ice cores. We conducted experiments at the North Greenland Eemian Ice Drilling (NEEM) facility ice core location, where COF data were available at Jakobshavn Isbrae west Greenland, to test if COF can be determined seismically. We used observations of anisotropic seismic wave propagation on multioffset gathers and englacial imaging from a 2D reflection profile. Anisotropy analysis of the NEEM data yielded mean c-axes distributed over a conical region of 30 degrees to 32 degrees from vertical. No internal ice seismic reflectors were imaged. Direct COF measurements collected in the ice core agreed with the seismic observations. At Jakobshavn Isbrae, we used a multioffset gather and a 2D reflection profile, but we lacked ice core data. Englacial reflectors allowed the determination of ice column interval properties. Anisotropy analysis found that the upper 1640 m of the ice column consisted of cold (approximate to - 10 degrees C) and mostly isotropic ice with c-axes distributed over a conical region of 80 degrees from vertical. The lower 300 m of the ice column was characterized by warm (> -10 degrees C) ice with COF. These observations were consistent with complex ice fabric development and temperature estimations over the same region of Jakobshavn Isbrae. This study demonstrated that the ice sheet and glacier ice anisotropy information can be gained from seismic field observations.