Visualizing Unsaturated Flow Phenomena Using High‐Resolution Reflection Ground Penetrating Radar

Abstract

Unsaturated fl ow phenomena, such as unstable we ng fronts and preferen al fl ow, cannot be inves gated using small-scale sampling. Dye tracer experiments can help visualize the dynamics of water fl ow but are destruc ve and therefore irreproducible. We inves gated the applicability of high-resolu on ground penetra ng radar (GPR) for nondestruc ve visualiza on of unsaturated fl ow pa erns arising from a forced infi ltra on experiment. Synthe c studies using a refl ec on GPR two-dimensional fi nite diff erence me domain modeling code indicated that diff erences in water content caused by preferen al fl ow and fi ngering could be resolved. Moisture content contrasts down to approximately 2.5% within the top 2 m were detectable, but with increasing degrees of heterogeneity in the subsurface it becomes diffi cult to dis nguish these moisture content changes. We conducted a fi eld experiment in which 100 mm (900 L) of Brilliant Blue dyed water was infi ltrated across a 3-by 3-m area in rela vely homogenous and undisturbed sandy alluvial sediments. Refl ec on GPR data were collected before and a er infi ltra on. Dye-staining pa erns, revealed by excava ng a 2-m-deep trench through the infi ltra on area, were compared with changes in the GPR data. Refl ec on amplitude changes as well as refl ec on delay revealed significant diff erences within the dye-stained area. The GPR data provided informa on about the unsaturated fl ow below the extent of the dye staining, and the results of the synthe c GPR modeling, as well as the observed changes in the real GPR data set, underline the poten al of refl ec on GPR as a nondestruc ve method to map unsaturated fl ow phenomena.Abbrevia ons: BB, Brilliant Blue; GPR, ground penetra ng radar; TDR, me domain refl ectometry.The quality and quan ty of groundwater resources depend highly on the fl ow and transport properties of the unsaturated zone. Traditionally, unsaturated hydraulic parameters are estimated using retention and hydraulic conductivity experiments performed on small soil samples in the laboratory. Th ese small-scale analyses are not able to describe preferential fl ow paths or unstable wetting fronts that are observed at the fi eld scale. Prediction and characterization of fl ow in the unsaturated zone was found to be diffi cult and highly complex in several studies using dye tracers for visualization of fl ow paths (Flury et al., 1994;Schmalz et al., 2002;Weiler and Flühler, 2004). Large-scale experiments on both homogeneous and heterogeneous soil columns have shown that fi ngering and preferential fl ow exist and dominate the fl ow fi eld for soil types ranging from clayey soils to unstructured sandy sediments (Flury et al., 1994;Wildenschild et al., 1994, Schmalz et al., 2002. Šimůnek et al. (2003) and Kung (1990) furthermore showed that the infi ltration of water seemed to be dependent on the dip of geologic layers and that the dipping layers could also induce fi ngered fl ow.Dye tracing infi ltration experiments are powerful techniques for visualizing the dynamics o...

Form and function in hillslope hydrology: characterization of subsurface flow based on response observations

“…The GPR data were analyzed using an appropriate constant velocity and gridded to a 2-D transect with a regular trace-spacing of 0.02 m. There is no standard interpretation procedure for the analysis of time-lapse GPR data. Most approaches are based on calculating trace-to-trace differences (Birken and Versteeg, 2000;Trinks et al, 2001) or picking and comparing selected reflection events in the individual time-lapse transects (Allroggen et al, 2015b;Haarder et al, 2011;Truss et al, 2007). In the context of this study, however, both approaches provided only limited interpretable information.…”

“…Its short measurement times and high sensitivity towards soil moisture predestine GPR for monitoring subsurface flow processes. Nevertheless, only few field studies exist which have successfully applied surface-based GPR for the investigation of preferential flow paths or subsurface flow in general (Truss et al, 2007;Haarder et al, 2011;Guo et al, 2014;Allroggen et al, 2015b). Previous GPR monitoring studies rely on two different principles.…”

“…Previous GPR monitoring studies rely on two different principles. The first approach relies on interpreting selected reflection surfaces and comparing this interpretation result between the individual recorded GPR surveys (Truss et al, 2007;Haarder et al, 2011). The result is a shift in GPR signal travel time, which can be interpreted in terms of soil moisture changes, using a petrophysical relation (e.g., the CRIM model, Allroggen et al, 2015b).…”

Multi-offset ground-penetrating radar imaging of a lab-scale infiltration test

“…There are relatively few examples in the literature that directly illustrate the influence of soil hydrology on surfacebased GPR surveys (Freeland et al, 2006;Grasmueck et al, 2010;Grote et al, 2005;Haarder et al, 2011;Lambot et al, 2008;Moysey, 2010;Saintenoy et al, 2008;Truss et al, 2007). Truss et al (2007) performed 3-D time-lapse GPR imaging of infiltration in an oolitic limestone that revealed macroscopic funnel flow effects.…”

“…These authors also observed overall shifts in reflector traveltimes that they suggested were caused by changes in soil moisture, but they did not provide direct estimates of water content. Haarder et al (2011) used constant-offset GPR surveys to monitor an infiltration experiment where dye was applied to mark preferential flow paths that were later identified when the site was excavated following the test. These authors concluded that wetting front nonuniformity and fingering complicated the GPR images, noting impacts on both radar velocity and amplitudes, but preferential flow features themselves were not resolved.…”