1.4 GHz radar penetration and evidence of drainage structures in temperate ice: Black Rapids Glacier, Alaska, USA

We have tested the ability of a 1.12-1.76 GHz bandwidth airborne Frequency Modulation-Continuous Wave (FM-CW) radar with an effective pulse duration of 3 ns to penetrate temperate ice of the ablation zone of Black Rapids Glacier, central Alaska, U.S.A. We used high-gain horn antennas to suppress clutter, and tested over cold and nearly ideal surface conditions. Englacial horizons dipping to at least 60 m depth were found along three sections of one axial profile. More narrow-band (1.21-1.29 GHz), low-resolution (24 ns pulse duration) profiles from a fourth section detected events at about 100-150 m depth. Comparative profiles recorded with a 100 MHz short-pulse-type radar reproduce the horizons of two of the sections, and verify the penetration in all cases. All horizons are composed of diffractions. We interpret voids from the phase of the 100 MHz diffractions within one of the horizons. The diffraction nature of the horizons, the void interpretation and the proximity to a nearby and up-glacier pothole field lead us to conclude that the horizons within two of the sections are meandering drainage channels. A more complex, branching structure with near-surface horizons profiled within the third section much farther down-glacier may also be a complex drainage system fed by near-surface melting. The FM-CW signal-to-clutter-noise ratios of some of the targets predict that they could be detected at 200 m depth in the 1-2 GHz range. Significant performance improvements at maximum vertical resolution could be achieved with higher-gain antennas.