Abstract
AbstractUnderstanding ecologically sensitive wetlands often requires non‐invasive methods to characterize their complex structure (e.g., deposit heterogeneity) and hydrogeological parameters (e.g., porosity and hydraulic conductivity). Here, electrical conductivities of a riparian wetland were obtained using frequency domain electromagnetic induction (EMI) methods. The wetland was previously characterized by extensive intrusive measurements and 3D electrical resistivity tomography (ERT) surveys and hence offers an ideal opportunity to objectively assess EMI methods. Firstly, approaches to obtain structural information (e.g., elevation and thickness of alluvium) from EMI data and inverted models were assessed. Regularized and sharp inversion algorithms were investigated for ERT calibrated EMI data. Moreover, the importance of EMI errors in inversion was investigated. The hydrological information content was assessed using correlations with piezometric data and petrophysical models. It was found that EMI data were dominated by the thickness of peaty alluvial soils and relatively insensitive to topography and total alluvial thickness. Furthermore, although error weighting in the inversion improved the accuracy of alluvial soil thickness predictions, the multi‐linear regression method performed the best. For instance, an iso‐conductivity method to estimate the alluvial soil thickness in the regularized models had a normalized mean absolute difference (NMAD) of 21.4%, and although this performed better than the sharp inversion algorithm (NMAD = 65.3%), the multi‐linear regression approach (using 100 intrusive observations) achieved a NMAD = 18.0%. In terms of hydrological information content, correlations between EMI results and piezometric data were poor, however robust relationships between petrophysically derived porosity and hydraulic conductivity were observed for the alluvial soils and gravels.
Highlights
The shallow subsurface structure of wetlands governs their ability to provide important hydrological and biogeochemical functions
The measured alluvial soil thicknesses are coincident with this boundary
The exact modifications of the sensitivity patterns are dependent upon the subsurface conductivity, the approach investigated by Andrade and Fischer (2018) who use McNeill’s (1980) cumulative sensitivity function, is validated by the observed similar correlations between alluvial soil thicknesses and VCP4.49 and HCP2.82 measurements, which have similar depth of investigation (4.6 and 4.5 m, respectively)
Summary
The shallow subsurface structure of wetlands governs their ability to provide important hydrological and biogeochemical functions. Prior to the 1970s, the importance of wetlands was commonly overlooked, and they were often modified for alternate land use, e.g. for agriculture or commercial and residential development (see Davidson, 2014). Since there has been significant effort in restoring, maintaining, and managing wetlands (see Wagner et al, 2008). These efforts require methods for wetland characterization. Conventional methods such as lithological sampling or piezometer installation (e.g. Grapes et al, 2005; Allen et al, 2010) may have limited spatial coverage or be prohibited due to any environmental damage they may cause
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