HR: 0800h
AN: NS11A-0163    [Abstracts]
TI: Monitoring Shallow Freeze and Thaw Processes Using High Frequency Surface Ground Penetrating Radar
AU: Steelman, C M
EM: cmsteelm@sciborg.uwaterloo.ca
AF: University of Waterloo, Dept. of Earth & Environmental Sciences, Waterloo, ON N2L 3G1, Canada
AU: * Endres, A L
EM: alendres@sciborg.uwaterloo.ca
AF: University of Waterloo, Dept. of Earth & Environmental Sciences, Waterloo, ON N2L 3G1, Canada
AB: High frequency GPR (225-900 MHz) was used for the high resolution, non-invasive monitoring of freeze and thaw processes in the shallow subsurface (i.e., within 1.5 meters of the surface) at three sites with different soil types (sand, sandy loam and silt loam). Both common midpoint (CMP) gathers and constant offset reflection profiles were repeatedly done over 2 m transects at each site. CMP gathers were used to monitor direct ground wave propagation and determine vertical velocity profiles; reflection profiles were used to image reflectivity variations and observe near vertical reflection event traveltimes. Direct ground waves were significantly attenuated during the initial freezing process due to interference with short period reflections; modelling results indicate that this phenomenon occurs until the frozen surface layer exceeds 15 cm in thickness for 450 MHz data. Differences in shallow surface freezing rates between sites were inferred from the direct wave data. Direct wave velocities indicate that surface thawing occurred rapidly in late March even though snowpack melting commenced in early March. CMP velocity profiles show the progressive development of the frozen zone and indicate the presence of liquid water in this zone one week prior to the surface thaw. Reflection profiling monitored the development of a continuous reflection event interpreted to be the interface at the base of frozen zone. Significantly diminished reflectivity of stratigraphic interfaces was observed above this interface. Measured reflection event traveltimes indicate the deeper section underwent drainage after the shallow frozen zone development terminated the seasonal autumn recharge. In addition, reflection traveltime data also found the thaw recharge started one week prior to the surface thaw.
DE: 0704 Seasonally frozen ground
DE: 0925 Magnetic and electrical methods (5109)
DE: 1823 Frozen ground
DE: 1835 Hydrogeophysics
DE: 1875 Vadose zone
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting