PEERING INTO THE SUBSURFACE: USING GROUND PENETRATING RADAR TO STUDY ICE WEDGE GEOMETRY

In continuous permafrost environments, polygonal terrain is often evidence of subsurface ice wedges. It has been estimated that these environments can contain wedge ice in over 30 percent of the top meter of ground. Being the surface expression of ice wedges, polygons have a variety of appearances which can depend on the surface materials and the subsurface ice wedge geometry. Because polygonal terrain is widespread in permafrost environments and has been observed on Mars, this project uses ground penetrating radar (GPR) to help characterize the shapes and volumes of ice wedges. Although part of a larger study of using geophysical methods to detect ground ice, these results focus on data collected at two locations on Devon Island – a site that has also been used as a Mars analog.

Polygon surface troughs on Devon Island vary from narrow depressions to those more than a meter wide. Although subsurface ice was not found beneath every trough, polygons with more notable troughs contain fairly large ice wedges. The two sites studied here have different surface materials, allowing for comparisons of polygon appearances in fine sediments to those in gravel deposits.

A GSSI SIR-3000 GPR system was used to collect data at 200 and 400 MHz. These data show the depth to the active layer, the widths of the ice wedges, and other subsurface stratigraphic features very well. Locations and widths of wedge ice were confirmed by augering and trenching to the tops of a sample of the ice wedges.

GPR data reveal the ice wedges and clearly delineate their edges. This allows a fairly accurate estimate of the ice wedge widths. Interestingly, surface tensional cracks at one location correlate with wedge edges detected in the GPR data. The tensional cracks are likely due to subsidence that may result from downward melting of the ice wedge in response to increasing temperatures over several years or more. As a surface indicator of active layer thickening, these tensional cracks can be useful for studying past and current climate change in Arctic regions. Additionally, by better understanding the relationship between surface expression and subsurface ice volume, the surface characteristics of polygonal terrain may be used to predict locations and volumes of subsurface ice on Earth and on Mars.