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

Abstract

Abstract.A lab scale infiltration experiment was conducted in a sand tank to evaluate the use of time-lapse multi-offset ground-penetrating radar (GPR) data for monitoring dynamic hydrologic events in the vadose zone. Sets of 21 GPR traces at offsets between 0.44-0.9 m were recorded every 30 s during a 3 h infiltration experiment to produce a data cube that can be viewed as multi-offset gathers at unique times or common offset images, tracking changes in arrivals through time. Specifically, we investigated whether this data can be used to estimate changes in average soil water content during wetting and drying and to track the migration of the wetting front during an infiltration event. For the first problem we found that normal-moveout (NMO) analysis of the GPR reflection from the bottom of the sand layer provided water content estimates ranging between 0.10-0.30 volumetric water content, which underestimated the value determined by depth averaging a vertical array of six moisture probes by 0.03-0.05 volumetric water content. Relative errors in the estimated depth to the bottom of the 0.6 m thick sand layer were typically on the order of 2 %, though increased as high as 25 % as the wetting front approached the bottom of the tank. NMO analysis of the wetting front reflection during the infiltration event generally underestimated the depth of the front with discrepancies between GPR and moisture probe estimates approaching 0.15 m. The analysis also resulted in underestimates of water content in the wetted zone on the order of 0.06 volumetric water content and a wetting front velocity equal to about half the rate inferred from the probe measurements. In a parallel modeling effort we found that HYDRUS-1D also underestimates the observed average tank water content determined from the probes by approximately 0.01-0.03 volumetric water content, despite the fact that the model was calibrated to the probe data. This error suggests that the assumed conceptual model of laterally uniform, onedimensional vertical flow in a homogenous material may not be fully appropriate for the experiment. Full-waveform modeling and subsequent NMO analysis of the simulated GPR response resulted in water content errors on the order of 0.01-0.03 volumetric water content, which are roughly 30-50 % of the discrepancy between GPR and probe results observed in the experiment. The model shows that interference between wave arrivals affects data interpretation and the estimation of traveltimes. This is an important source of error in the NMO analysis, but it does not fully account for the discrepancies between GPR and the moisture probes observed in the experiment. The remaining discrepancy may be related to conceptual errors underlying the GPR analysis, such as the assumption of uniform one-dimensional flow, a lack of a sharply defined wetting front in the experiment, and errors in the petrophysical model used to convert dielectric constant to water content.

Resolving precipitation induced water content profiles by inversion of dispersive GPR data: A numerical study

“…Significant advances have recently been made using groundpenetrating radar (GPR) to study near-surface water content in both static (Greaves et al, 1996;Grote et al, 2010;Huisman et al, 2001;Vellidis et al, 1990) and dynamic conditions (Haarder et al, 2011;Mangel et al, 2012;Moysey 2010;Truss et al, 2007;van Overmeeren et al, 1997). In time-lapse studies, it is common to see large increases in water content at the surface associated with infiltration events which create a high permittivity region of the soil column.…”

Estimating field-scale soil water dynamics at a heterogeneous site using multi-channel GPR

“…For instance, monitoring wetting/drying fronts at natural and artificial conditions has been demonstrated by, for example, Binley et al (2001), Stoffregen et al (2002), Deiana et al (2008), Haarder et al (2011), andMangel et al (2012). Following this direction, the geophysical data are used to constrain unsaturated flow models.…”

Preferential flow systems amended with biogeochemical components: imaging of a two-dimensional study

“…The process of infiltration in soils and the instability of wetting fronts has been studied primarily in two-dimensional tanks where it is possible to image the generation of a two-dimensional fingered flow (e.g., Bauters et al, 2000a;DiCarlo, 2004;Glass et al, 1989a, c, d, Selker et al, 1992aJ. S. Selker et al, 1992), and more recently using geo-electrical imaging (Ganz et al, 2014;Liu and Moysey, 2012) and ground-penetrating radar (Mangel et al, 2012(Mangel et al, , 2015. Laboratory experiments designed to examine the infiltration process in homogeneous porous media have confirmed the ubiquitous formation of fingered flow (Bauters et al, 2000b;Glass et al, 1989d;Selker et al, 1992a;Wang et al, 2003bWang et al, , 2004.…”