We released water and brilliant blue dye (4 g/L) into a 1-m long trench cut into the hillside at the Shale Hills CZO and monitored downslope migration of the solution through the unsaturated zone using time-lapse ground penetrating radar (GPR). The site was then excavated to photograph the dye pathways. The goals were to improve our understanding of how GPR can be used to interpret infiltration by comparing radargrams and photographed dye pathways and to evaluate the importance of the soil-bedrock interface and saprolite fabric on infiltration pathways. Time-lapse GPR data can enhance the signal, but processing is challenging. Directly differencing radargrams produces spurious anomalies because the addition of water not only changes the reflection coefficients but also alters the subsurface radar velocity, misaligning reflectors of the before and after time sections. Constant-offset GPR provides insufficient velocity information for migration to realign the peaks. We investigated an alternate approach called Dynamic Time Warping (DTW), originally developed for speech recognition. DTW nonlinearly stretches and contracts the time scale to find an optimal match between waveforms. Once aligned, the radargrams can be differenced to find changes in reflection strength. GPR radargrams showed a fingering pattern similar to the dye trace photographs, but with less obvious resolution of the structural fabric. The depth of penetration of the dye (approximately 0.5 m) and lateral migration (about 0.2 m) were also captured by the radargrams. Although the depth of penetration was controlled by bedrock, the fingering observed in the dye did not seem to be influenced by undulations in the bedrock but was set by the fabric encountered in the saprolite. GPR provided a rapid method for mapping bedrock, which was an important control on depth of infiltration. GPR also suggested fingering occurs, but detailed pathways were difficult to resolve without dye tracing and excavation.
GPR2 Advances and Recent Applications for GPR
GPR, unsaturated, time-lapse, hydrogeophysics, infiltration
NYQUIST, Jonathan, TORAN, Laura, LIN, Henry, and PITMAN, Lacey (2014): Dynamic time warping of time-lapse GPR data to monitor infiltration at the Shale Hills Critical Zone Observatory. SAGEEP 2014 March 16-20, 2014 Boston, MA USA.
This Paper/Book acknowledges NSF CZO grant support.