Field study of GPR attenuation rates in natural and contaminated silt

Abstract

GPR is considered ineffectual in penetrating silty soils. Capillary tension appears to move the interstitial water dielectric relaxation frequency near the GPR range. If hydrocarbons relieve the tension, then GPR attenuation rates should decrease. We profiled these rates across a partly contaminated quartz silt diamicton at an inactive fuel terminal in Haines, Alaska. We calculated the rates from the signal amplitude decay within a series of 400-MHz moveout surveys recorded along a single transect. We sampled the hydrocarbon content along the transect and on the wall of a trench we excavated beside it, along with water content, density and resistivity. The silt was saturated and generally at 50-80 ohm-m. The ground impedance loading brought the antenna pulse center frequency down to near 200 MHz. The rates ranged from 5 to 8 dB/m in the contaminated zone and about 10 to 15 dB/m in the uncontaminated zone. The resistivities account for the contamination rates, but a dielectric relaxation centered near 1-2 GHz, using a simple mixing model, is required to account for those in the uncontaminated zone. Hydrocarbon soil analysis suggests that the threshold level for the attenuation effect is not higher than about 1000 ppm. We suggest GPR offset surveys for assessing contamination zones.