Imaging rainfall drainage within the Miami oolitic limestone using high‐resolution time‐lapse ground‐penetrating radar

Abstract

[1] The vadose zone of the Miami limestone is capable of draining several centimeters of rainfall within a fraction of an hour. Once the water enters the ground, little is known about the flow paths in the oolitic rock. A new rotary laser-positioned ground-penetrating radar (GPR) system enables centimeter-precise and rapid acquisition of time-lapse surveys in the field. Two-dimensional (2-D) GPR time-lapse surveying at a 3-min interval before, during, and after rainfall shows how buried sand-filled dissolution sinks efficiently drain the bulk of the rainwater. Hourly repeated 3-D imaging of a dissolution sink in response to surface infiltration shows how the wetting front propagates at a rate of 0.6-1.2 m/h traversing the 5-m-thick vadose zone within hours. At the same time, some of the water migrates laterally into the host rock guided by stratigraphic unit boundaries. Average lateral propagation measured over a 28-hour period was of the order of 0.1 m/h. On a seasonal time frame, redistribution involves the entire rock volume. Comparing 3-D surveys acquired after wet summer and dry winter conditions shows good GPR event correspondence, but also time shifts up to 20 ns caused by the change of overall water content within the vadose zone. High-precision time-lapse GPR imaging can therefore be used to noninvasively characterize natural drainage inside the vadose zone ranging from transient loading to seasonal variation.

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

“…While analysis of the groundwave has been shown to yield excellent results when the soil near the ground surface is approximately homogeneous, it is not clear whether accurate wave velocities can be obtained during an infiltration event. In this case, energy can be trapped in the low-velocity waveguide behind the wetting front, causing dispersion of the groundwave (van der Kruk, 2006). In contrast, van Overmeeren et al (1997) analyzed groundwave, reflected and refracted wave arrivals in multioffset data obtained from central midpoint (CMP) surveys to successfully determine both lateral and vertical variations in water content.…”

“…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.…”