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

Abstract

A lab scale infiltration experiment was conducted to evaluate the use of transient multi-offset ground-penetrating radar (GPR) data for characterizing dynamic hydrologic events in the vadose zone. A unique GPR data acquisition setup allowed sets of 21 traces at different offsets to be recorded every 30 s during a 3 h infiltration experiment. The result is a rich GPR data cube that can be viewed as multi-offset gathers at discrete moments in time or as common offset images that track changes in the GPR arrivals over the course of the experiment. These data allows us to continuously resolve the depth to soil boundaries while simultaneously tracking changes in wave velocity, which are strongly associated with soil water content variations. During the experiment the average volumetric water content estimated in the tank ranged between 10–30% with discrepancies between the GPR results, moisture probe data, and 1-D numerical modeling on the order of 3–5% (vol vol<sup>−1</sup>), though the patterns of the estimated water content over time were consistent for both wetting and drying cycles. Relative errors in the estimated depth to a soil boundary located 60 cm from the surface of the tank were typically on the order of 2% over the course of the experiment. During the period when a wetting front migrated downward through the tank, however, errors in the estimated depth of this boundary were as high as 25%, primarily as a result of wave interference between arrivals associated with the wetting front and soil boundary. Given that our analysis assumed one-dimensional, vertical infiltration, this high error could also suggest that more exhaustive GPR data and comprehensive analysis methods are needed to accurately image non-uniform flow produced during periods of intense infiltration. Regardless, we were able to track the movement of the wetting front through the tank and found a reasonably good correlation with in-situ water content measurements. We conclude that transient multi-offset GPR data are capable of quantitatively monitoring dynamic soil hydrologic processes

High‐resolution ground‐penetrating radar monitoring of soil moisture dynamics: Field results, interpretation, and comparison with unsaturated flow model

“…As a result, surface hydrogeophysical methods are increasingly recognized [e.g., Vereecken et al , 2008; Robinson et al , 2008] as an important source of soil moisture information at the field‐scale due to their larger sampling volume (i.e., dm 3 –m 3 scale), noninvasive nature, good depth of investigation (1–10 m) and high resolving power. In particular, ground‐penetrating radar (GPR) techniques have been proven to be very useful for monitoring vadose zone soil water content at the field scale [ Huisman et al , 2002; Huisman et al , 2003; Grote et al , 2003; Galagedara et al , 2005; Weihermüller et al , 2007], particularly when they are integrated with hydrological models [e.g., Saintenoy et al , 2008; Deiana et al , 2008; Looms et al , 2008; Farmani et al , 2008; Moysey , 2010; Mangel et al , 2011]. While GPR data are dependent on the same electromagnetic (EM) properties that govern TDR response, its larger sampling volume and noninvasive nature makes it less prone to the effects of macropores (e.g., root and worm holes) and air gaps on pore water content estimates compared to TDR probes [ Robinson et al , 2003].…”

“…Both of these methods are noninvasive. CMP soundings provide offset distance‐traveltime data for subsurface reflection events; the normal moveout (NMO) analysis of these data give velocity profile information that can be used to infer vertical soil moisture variations [e.g., Greaves et al , 1996; Garambois et al , 2002; Turesson , 2006; Mangel et al , 2011]. However, the quality of the results from this approach are sensitive to numerous factors [ Jacob and Hermance , 2004; Becht et al , 2006; Barrett et al , 2007; Booth et al , 2010]; this makes it difficult to obtain consistent results over the annual cycle of hydrological conditions, particularly during dynamic moisture periods [ Steelman and Endres , 2012].…”

Simultaneous multichannel multi‐offset ground‐penetrating radar measurements for soil characterization

“…The depths of the EMI and GPR layer are different by 10 cm, as they were not measured on the same day (different soil conditions). In particular, the first layer of SiMoc GPR might be influenced by the wetting front (Mangel, Moysey, Ryan, & Tarbutton, 2012) after rain events. The third layer in the EMI inversion starts at 5–10 m, 25–40 m, 45–80 m, and 80–120 m at a similar depth, then a SiMoc‐GPR layer boundary, indicating a soil changes in ε r and σ.…”