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Abstract: Electromagnetic induction (EMI) has been used to characterize the spatial variability of soil properties since the late 1970s. Initially used to assess soil salinity, the use of EMI in soil studies has expanded to include: mapping soil types; characterizing soil water content and flow patterns; assessing variations in soil texture, compaction, organic matter content, and pH; and determining the depth to subsurface horizons, stratigraphic layers or bedrock, among other uses. In all cases the soil property being investigated must influence soil apparent electrical conductivity (ECa) either directly or indirectly for EMI techniques to be effective. An increasing number and diversity of EMI sensors have been developed in response to users' needs and the availability of allied technologies, which have greatly improved the functionality of these tools. EMI investigations provide several benefits for soil studies. The large amount of georeferenced data that can be rapidly and inexpensively collected with EMI provides more complete characterization of the spatial variations in soil properties than traditional sampling techniques. In addition, compared to traditional soil survey methods, EMI can more effectively characterize diffuse soil boundaries and identify areas of dissimilar soils within mapped soil units, giving soil scientists greater confidence when collecting spatial soil information. EMI techniques do have limitations; results are site-specific and can vary depending on the complex interactions among multiple and variable soil properties. Despite this, EMI techniques are increasingly being used to investigate the spatial variability of soil properties at field and landscape scales. [Copyright &y& Elsevier]

Assessing and managing the spatial variability of hydropedological properties are important in environmental, agricultural, and geological sciences. The spatial variability of soil apparent electrical conductivity (ECa) measured by electromagnetic induction (EMI) techniques has been widely used to infer the spatial variability of hydrological and pedological properties. In this study, temporal stability analysis was conducted for measuring repeatedly soil ECa in an agricultural landscape in 2008. Such temporal stability was statistically compared with the soil moisture, terrain indices (slope, topographic wetness index (TWI), and profile curvature), and soil properties (particle size distribution, depth to bedrock, Mn mottle content, and soil type). Locations with great and temporally unstable soil ECa were also associated with great and unstable soil moisture, respectively. Soil ECa were greater and more unstable in the areas with great TWI (TWI > 8), gentle and concave slope (slope < 3%; profile curvature > 0.2). Soil ECa exponentially increased with depth to bedrock, and soil profile silt and Mn mottle contents (R = 0.57), quadratically (R = 0.47), and linearly (R = 0.47), respectively. Soil ECa was greater and more unstable in Gleysol and Nitosol soils, which were distributed in areas with low elevation (< 380 m), thick soil solum (> 3 m), and fluctuated water table (shallow in winter and spring but deep in summer and fall). In contrast, Acrisol, Luvisol, and Cambisol soils, which are distributed in the upper slope areas, had lower and more stable soil ECa. Through these observations, we concluded that the temporal stability of soil ECa can be used to interpret the spatial and temporal variability of these hydropedological properties. [ABSTRACT FROM AUTHOR]

Repeated electromagnetic induction (EMI) surveys have merits of revealing temporal changes in heterogeneous soilscapes such as subsurface hydrologic dynamics. We conducted eight repeated EMI surveys from 1997 to 2009 over a 19.5-ha agricultural field that revealed soil and water patterns. The first two surveys were done in 1997 and 2006 and compared different EMI meters (EM38, EM31, and Dualem-2), dipole orientations, and geometries. Another six surveys were conducted in different seasons in 2008 to 2009 using the EM38 operated in vertical dipole orientation. Soil apparent electrical conductivity (ECa) collected during the wetter periods (>10-mm antecedent precipitation during the previous 7 d) showed greater spatial variability (greater sills and shorter spatial correlation lengths), indicating the influence of soil water distribution on soil ECa. During a relatively short time period, most soil properties controlling ECa (e.g., texture, organic matter, and depth to bedrock) remain unchanged. Thus, repeated EMI surveys can capture the dynamics of soil moisture change and related subsurface flow paths in the landscape. Significantly (p < 0.05) higher ECa was detected in areas close to simulated subsurface flow paths, especially during the wetter periods; however, such flow paths could not be pinpointed directly from the ECa maps because of the limitation in ECa spatial resolution. In wetter areas, significant correlation between the relative difference in ECa and that in measured soil moisture was observed (r2 = 0.59-0.77), but not in drier areas. During drier periods or at drier locations, the influences of soil moisture and flow path on ECa were masked by terrain and other soil properties. Thus, the optimal use of EM! for detecting subsurface hydrologic dynamics would be during wet periods or in wet areas across the landscape in this study. [ABSTRACT FROM AUTHOR]

Locating existing septic systems and determining the extent of soil contamination after septic system failure can be destructive, time consuming, and a nuisance to homeowners. The objective of this study was to determine the effectiveness of noninvasive electromagnetic induction (EMI) for locating a failed septic system in fine-textured glacial-till-derived soils. Components of a failed septic system were located with a push probe, georeferenced with a theodolite, and surveyed with a dual receiver EMI sensor (DUALEM-2) in December 2001 (wet soil moisture condition) and July 2002 (dry soil moisture condition). Three transects located perpendicular to the soil absorption field trenches were sampled to a depth of 1.2 m and used to ground reference the EMI survey. Near-surface (1-m depth) apparent conductivity (ECa) was significantly correlated to unweighted average electrical conductivity from soil saturated paste extracts (ECsat; r = 0.79). The ECa below the soil absorption field was higher than the surrounding native soil under both dry and wet soil moisture conditions. Individual soil absorption trenches had a higher ECa than background ECa under both soil moisture conditions. A higher ECa pattern that was apparent in December 2001 associated with discharge of wastewater at shallow depths was not evident in July 2002 after the system had been abandoned for 6 mo. While more research is warranted, results from this study suggest that electromagnetic induction is a promising technique to identify the location of septic system components, failed septic systems, and their associated effluent plumes. [ABSTRACT FROM AUTHOR]

Abstract: Experimental evidence has shown that soil layering can significantly alter water movement through soil profiles, especially in sloping landscapes. Detailed knowledge of soil layering and its position in the landscape is therefore necessary for determining dynamic subsurface flow. In this study, we used Ground Penetrating Radar (GPR) in combination with high resolution real-time soil water monitoring to investigate seasonal changes in GPR signals to identify soil horizon interfaces and their impacts on water flow in two contrasting soils. The results showed that 1) in the deep Rushtown soil located in a concave hillslope, GPR reflections in the Bw–BC and BC–C horizon interfaces became clearer as soil became wetter. Such increased contrast along interfaces was due to the presence of water associated with water-restricting BC horizon and lateral flow from upslope area; and 2) in the shallow Weikert soil located in a planar hillslope, GPR reflections in the soil–bedrock interface and the weathered–unweathered rock interface become intermittent as soil became wetter. This scattering of GPR reflection was due to the non-uniform distribution of water in fractured shale bedrock and possible tree root effects. While wet condition permitted the enhanced detection of water distribution in both soil profiles, optimal time for detecting soil layering was different for these two soils: wet condition was more suited for detecting water-restricting dense layer in the deep Rushtown soil, while dry season was optimal for identifying the soil–bedrock interface in the shallow Weikert soil. Overall, this study demonstrates the value of repeated GPR surveys in different seasons to capture subsurface hydrologic processes and the impacts of soil layering and preferential water distribution on seasonal changes of GPR signals. It also provides a useful reference for selecting optimal time for GPR survey in the field. [Copyright &y& Elsevier]