Time-lapse monitoring of DNAPL in a controlled cell

摘要

A multi-component GPR antenna is used to perform time-lapse measurements in a controlled experiment simulating DNAPL release. The DNAPL monitoring is based on the delay time of a reflection from the back side of a cell and on spectral analysis of traces. The study shows that co-polar and cross-polar antennas detect a similar pull-up of the reflection from the back of the cell induced by the DNAPL. The amplitude of the reflection from the DNAPL is weaker in the cross-polar data with respect to the co-polar data and in actual cases it could not be detected by both antenna configurations. In addition we observe a change in the amplitude spectra of the traces collected at the DNAPL release with respect to those collected in an uncontaminated area. The amplitude spectrum variations occurred mainly in the co-polar antennas. The spectra of cross-polar antennas show variations over time that are not easily linked to the DNAPL position. We observed that the data collected 141 hours after the first DNAPL injection and two hours of water flow, show a pull-up of the reflection from the back of the cell and variations in the amplitude spectra of the traces located at the same position as the DNAPL injection. This suggests that the DNAPL probably remained trapped by sediments and was not totally removed by the water flow. Forward models, simulating the experiment, confirmed that the DNAPL induces a pull-up of the reflection from the back of cell and showed that in a controlled experiment the DNAPL produces a reflection whose amplitude depends on the DNAPL saturation. In a real case the presence of small amounts of DNAPL can be at best barely visible because of induced small amplitude reflections. This is truer if no GPR data are available from before the DNAPL spill, so there is little chance that GPR responses can be positively attributed to DNAPL presence.