Publication Date:
2003-01-01
Description:
In August 2000, we used cross-borehole ground penetrating radar (GPR) measurements to investigate sites on Fort Wainwright, Alaska, and east of the Cold Regions Research and Engineering Laboratory (CRREL) permafrost tunnel near Fox, Alaska. The sites are characterized by perennially frozen soils characteristic of much of interior Alaska. The purpose of this investigation was to define variations in GPR signal velocity and attenuation that may be indicative of hydrocarbon contamination and to determine if downhole GPR methods could detect petroleum contaminants. To acquire background information for comparison, we conducted detailed investigations in non-contaminated areas to define general conditions before we profiled at the Fort Wainwright Tank Farm where extensive contamination has been documented. Results showed that cross-borehole GPR is a useful tool for detecting changes in electrical characteristics in permafrost and contaminated environments. Bulk electrical properties (velocity and attenuation) were observed to vary systematically between frozen and unfrozen materials, and produced distinct signal responses to frozen/unfrozen sediments, bedrock and massive ground ice. There is considerable overlap in both velocity and attenuation for soils and rock. Fine-grained silts exhibit higher attenuation (9-10 dB m-1) than coarser sand and gravel (1.5-2 dB m-1). Bedrock values are intermediate (2-7 dB m-1). Unfrozen sediment and rock exhibit velocities between 0.05 m ns-1 and 0.1 m ns-1 and attenuation ranges from 4 dB m-1 to 11 dB m-1 at 100 Mhz while frozen materials are between the velocities of 0.1 m ns-1 and 0.18 m ns-1 and 1.5 dB m-1 and 10 dB m-1 at -0.3 {degrees}C. The general trend is for frozen materials to have higher velocities than their unsaturated or saturated counterparts. Preliminary results where petroleum contamination was pervasive show that the attenuation decreases from 6 dB m-1 to 7 dB m-1 in uncontaminated schist to 4 dB m-1 in the hydrocarbon-contaminated schist. Additional measurements are required to determine that the observed decrease in attenuation is primarily related to petroleum contamination.
