Seismic Imaging of Sub-Glacial Sediments at Jakobshavn Isbræ and NEEM Greenland

Abstract:

Sub-glacial sediment conditions can have a major control on glacier flow yet these are difficult to measure directly. We present active source seismic reflection experiments that imaged sub-glacial sections at Jakobshavn Isbræ, West Greenland and at the North Greenland Eemian Ice Drilling (NEEM) location. At Jakobshavn Isbræ we re-processed an existing 9.8 km-long high-resolution seismic line using an iterative approach to determine seismic velocities for enhancing sub-glacial imaging. The seismic profile imaged sediments ranging in thickness between 35 and 200 meters, and the underlying bedrock. Based on the geometry of the reflections we interpret three distinct seismic facies: a basal till layer, accreted sediments and re-worked till. The basal till and accreted sediments vary in thickness from less than 5 m to nearly 100 m thick and are interpreted as the zone of most recent deposition. A reflection polarity reversal observed at a low topographic region along the ice-sediment interface suggests the presence of liquid water spanning approximately 200 m along the profile. At NEEM we acquired a 5.8 km long-offset shot gather. Seismic imaging revealed two prominent reflections at the base of the ice. The upper reflection is interpreted at the base of ice – top of till interface whereas the lower reflection is interpreted as the base of till - top of bedrock. The thickness of the subglacial sediment section at NEEM is estimated to approximately 50 m using seismic imaging. The NEEM ice core drilled through the upper part of this section and ceased drilling before reaching bedrock.