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GPR Imaging of Icy Debris Fans

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esearchers from Bucknell University in Lewisburg, Pennsylvania used ground penetrating radar (GPR) to non-invasively investigate the subsurface characteristics of icy debris fans. We would like to share some of their work. Although fans in Alaska and New Zealand were studied during their research, this article focusses on the work done on the McCarthy Glacier, Alaska, USA.

Icy debris fans are found at the head or along the side of valley glaciers where high-level ice caps are detached from valley glaciers (Figures 1 and 2). These unstable, rapidly changing landforms have recently been described as deglaciation features on Earth, however, the subsurface characteristics remain unknown and the processes that lead to their formation are poorly understood.

Figure 1
Icy debris fans were imaged using a pulseEKKO® GPR system with antennas with a center frequency of 100 MHz.

Figure 2
Ice debris fans associated with the McCarthy Glacier in Alaska. The numbered red lines indicate the approximate locations of the GPR lines shown in Figures 3, 4 and 5.

The surface processes building these features have been observed to include ice avalanches, rockfalls, icy debris flows, and slushflows, which results in composition including snow, ice, and lithic (rock) deposits for icy debris fans. Recent deposits on icy debris fans are hundreds of meters long, tens of meters wide, and meters thick.

Periods of intense ice melting or significant rockfall produce concentrations of significant rock deposits.

To better understand the structure of icy fans and the processes that create them, we used a pulseEKKO® system with 100 and 200 MHz center frequency bi-static, unshielded antennas to collect the GPR data. The time sampling rate depended on the frequency of the antenna; 100 MHz data were sampled every 0.8 ns, while 200 MHz data were sampled every 0.4 ns. All GPR data were collected with 16 stacks per trace (see article TIPS: Noise, Stacking and DynaQ® for more information about stacking) and a time window of 3000 ns.

GPR profiles (Figure 2) were used to determine the subsurface geometry of the fans and common mid-point (CMP) and wide-angle reflection/refraction (WARR) soundings were used to measure the GPR signal velocity in the subsurface. Using the CMP Analysis routine in the EKKO_ProjectTM software, these measurements provided an average velocity of 0.16 m/ns, which is a typical velocity for ice. The CMP/WARR soundings indicated little velocity variation from surface material to depths up to 53 m within the icy debris fans. For more information about CMPs, see Common Mid-Point survey using the DVL-500P

The GPR profiles (Figures 3 to 5) indicate that there is a significant difference in observed GPR signal characteristics above and below the prominent reflector (green boundary); the primary difference is the amount of diffraction patterns in each layer. The shallowest material is layered, with few diffractions, while the material below the green reflector shows significantly more diffractions. These are interpreted as brittle failure planes associated with cracks and crevasses in ice so, we interpret this boundary as the separation between icy debris fan material with high porosity above and ice below.

Figure 3
Line 3 plotted with a topographic correction applied. It shows two strong GPR reflectors, the shallower one (green line) interpreted as the interface from the icy debris flow material and the ice and the deeper one (blue line) as the top of the talus (loose rock) under the ice.

 

Figure 4
Line 1 is a downslope line parallel to Line 3 in Figure 3 but a couple of hundred meters to the right. It shows two strong GPR reflectors, the shallow green one interpreted as the interface from the icy debris flow materials and the ice and the deeper blue one, the top of bedrock under the ice.

 

Figure 5
Line 4 generally runs along an elevation contour line and crosses Lines 1 and 3. It shows a significant thickening of the ice where the bedrock/talus reflector gets deeper.

 
There is a deeper, strong interface (blue boundary) that is interpreted to be the bedrock or possibly older glacial ice in Line 1 (Figure 4). However, for Line 3 (Figure 3) we interpret this to be talus (loose, piled rock), as can be seen in the photo in Figure 2.

Some of the conclusions from this research are:

  • GPR reflections from within the icy debris fan appear to be associated with rock-rich interfaces. These interfaces may be produced by melting ice concentrating the rocky materials or rockfall events.
  • GPR was useful to image the base of large icy debris deposits.
  • Internal GPR reflections become less coherent with depth, may be offset by faulting, and may indicate a rotation.
  • The range of GPR signal velocities measured is consistent with ice-rich material with varying amounts of liquid water.

While many reflectors and reflector characteristics still need to be ground truthed in ongoing research, GPR has provided the first images into the structure of icy debris fans.

Find more information about this research here:

Data and story courtesy of Dr. Robert W. Jacob, Bucknell University

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