High‐resolution radar mapping of internal layers at the North Greenland Ice Core Project

Abstract

Abstract. Existing accumulation maps with reported errors of about 20% are determined from sparsely distributed ice cores and pits. A more accurate accumulation rate might be obtained by generating continuous profiles of dated layers from high-resolution radar mapping of near-surface internal layers in the ice sheet (isochrones). To generate such profiles we designed and developed an ultrawideband radar for high-resolution mapping of internal layers in the top 200 m of ice and tested it at the North Greenland Ice Core Project drill site. Reflection profiles of 2-and 10-km length reveal horizons that we correlate with electrical conductivity measurement (ECM) recordings. Our results show that the radar-determined depth of internal layers is within _+2 m of that in an ice core collected at a nearby location. Preliminary frequency analyses of layer reflections reveal that the reflections are strongest at the 500-1000 MHz frequency range. Long-term accumulation rate computed from radar data is within 5% of that obtained from snow pits.

Annual Greenland accumulation rates (2009–2012) from airborne snow radar

“…Hence, the two primary methods used to generate large-scale (hundreds of kilometers) accumulation-rate patterns are model outputs and radar-derived accumulation rates . High-resolution, near-surface radar data have shown good fidelity at mapping spatial patterns of accumulation over ice sheets at decadal and annual resolutions from both airborne and ground-based radars (Kanagaratnam et al, 2001(Kanagaratnam et al, , 2004Spikes et al, 2004;Arcone et al, 2005;Anshütz et al, 2008;Müller et al, 2010;Medley et al, 2013;Hawley et al, 2006;2014;de la Peña et al, 2010;Miège et al, 2013). Radars detect the lateral persistence of isochronal layers within the firn.…”

“…Early studies by Spikes et al (2004) in Antarctica and Kanagaratnam et al, (2001 and in Greenland used high/very-high-frequency (100 to 1000 MHz) ground-based and airborne radars, with vertical resolutions of ∼ 30 cm, to measure decadal-scale accumulation rates between dated ice cores. These high/very-high-frequency radars can penetrate hundreds of meters in the dry-snow zone and tens of meters in the ablation zone (Kanagaratnam et al, 2004).…”

A Wideband Radar for High-Resolution Mapping of Near-Surface Internal Layers in Glacial Ice

“…The primary sources of error in the computation of accumulation rate are the uncertainties in the range measurements and the density profile assumptions (or measurements, if available). We estimate that the error in our assumed density profile is %, with the range uncertainty partly dependent on this error [13]. In addition, there is also an absolute uncertainty of m due to the vertical range resolution of the radar.…”

“…These results agree very well with our radar-measured accumulation rate. The computation of the uncertainty in our accumulation rate estimate has been detailed in [13]. The primary sources of error in the computation of accumulation rate are the uncertainties in the range measurements and the density profile assumptions (or measurements, if available).…”

“…Annual accumulation and melt events register a change in the density, whereas volcanic events produce a change in conductivity [12]. By mapping shallow internal layers and combining these data with published information on density and thickness, we can estimate the accumulation rate [13]. The spatial coverage possible with an airborne system can be used to reduce the errors due to local variability and study the effect of such variability by collecting data from the near-surface layers over distances greater than several ice thicknesses.…”