Continuous Profiles of Electromagnetic Wave Velocity and Water Content in Glaciers: An Example from Bench Glacier, Alaska, USA

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We conducted two-dimensional continuous multi-offset georadar surveys on Bench Glacier, south-central Alaska, USA, to measure the distribution of englacial water. We acquired data with a multichannel 25MHz radar system using transmitter–receiver offsets ranging from 5 to 150 m. We towed the radar system at 5–10 kmh–1 with a snow machine with transmitter/receiver positions established by geodetic-grade kinematic differentially corrected GPS (nominal 0.5m trace spacing). For radar velocity analyses, we employed reflection tomography in the pre-stack depth-migrated domain to attain an estimated 2% velocity uncertainty when averaged over three to five wavelengths. We estimated water content from the velocity structure using the complex refractive index method equation and use a three-phase model (ice, water, air) that accounts for compression of air bubbles as a function of depth. Our analysis produced laterally continuous profiles of glacier water content over several kilometers. These profiles show a laterally variable, stratified velocity structure with a low-watercontent (~0–0.5%) shallow layer (~20–30 m) underlain by high-water-content (1–2.5%) ice.