GPR Surveys across a Prototype Surface Barrier to Determine Temporal and Spatial Variations in Soil Moisture Content

Abstract

Engineered surface barriers are expected to play a critical role in the closure of waste sites within the DOE complex and currently require monitoring to verify performance. The most comprehensive approach to assess performance is by water balance evaluation, which requires knowledge of the soil water storage. However, water storage measurements are still made mostly by point sensors and as result field-scale estimates are subject to much uncertainty. The objective of this study was to assess the viability of using ground penetrating radar (GPR) to monitor changes in soil moisture distribution, and therefore water storage, at multiple scales. Profiles were collected at four times during the year March, May, September and the following January to track the decrease in storage from the spring to the summer followed by the increase in the winter. A series of 40-m long profiles were collected using 100-Mhz antennas with a sample interval of 0.8 ns and 500 samples per trace in a 400-ns window. A common midpoint (CMP) survey was first used to determine an optimal antenna separation (3.5 m) after which data were collected using a wide-offset reflection geometry. Travel times were used to calculate the electromagnetic velocities which were then used tomore » calculate water content using the Topp’s equation. Changes in the GPR response were easily observed over the course of the study and indicated spatial differences in moisture, which owing to the relatively uniform soil, can be attributed to differences in water removal by evapotranspiration. Water content also showed a strong seasonal strong seasonal dependence that correlate well with seasonal changes in precipitation and plant water uptake. An investigation of the effects of soil water content on the amplitude of the ground wave showed weaker amplitudes in the drier spring and summer months than in the winter suggesting a correlation between amplitude and water content. Results show that GPR can provide accurate non-invasive estimates of spatial and temporal changes in water content and therefore soil water storage.« less