Abstract
Abstract. Ground-penetrating radar (GPR) reflection tomography algorithms allow non-invasive monitoring of water content changes resulting from flow in the vadose zone. The approach requires multi-offset GPR data that are traditionally slow to collect. We automate GPR data collection to reduce the survey time significantly, thereby making this approach to hydrologic monitoring feasible. The method was evaluated using numerical simulations and laboratory experiments that suggest reflection tomography can provide water content estimates to within 5 % vol vol−1–10 % vol vol−1 for the synthetic studies, whereas the empirical estimates were typically within 5 %–15 % of measurements from in situ probes. Both studies show larger observed errors in water content near the periphery of the wetting front, beyond which additional reflectors were not present to provide data coverage. Overall, coupling automated GPR data collection with reflection tomography provides a new method for informing models of subsurface hydrologic processes and a new method for determining transient 2-D soil moisture distributions.
Highlights
Preferential flow is ubiquitous in the vadose zone, occurring under a wide variety of conditions and over a broad range of scales (Nimmo, 2012)
We suggest that ground-penetrating radar (GPR) reflection tomography could fill this need by quantitatively mapping changes in water content through space and time at the sub-meter scale
Reflection tomography in the post-migrated domain is a viable method for resolving transient soil moisture content in 2-D associated with an infiltration and recovery event in a homogeneous soil
Summary
Preferential flow is ubiquitous in the vadose zone, occurring under a wide variety of conditions and over a broad range of scales (Nimmo, 2012). Reviews such as those by Hendrickx and Flury (2001) and Jarvis (2007) illustrate that a basic mechanistic understanding of preferential flow exists. Reflection GPR is commonly used to image subsurface structures, but is well suited to understanding hydrologic variability due to the strong dependence of EM wave velocities on soil volumetric water content (Topp et al, 1980). Note that GPR methods are not applicable in media with relatively high electrical conductivity
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