Combination of electrical resistivity tomography and vertical electrical soundings for characterization of a representative hillslope as a functional unit in a catchment
Abstract:
IntroductionThe understanding and prediction of hydrological processes is a very challenging task. The topography and the subsurface structures control the complex interaction between surface and the subsurface flows (Freeze 1972, Shahedi 2008). A complete investigation of subsurface structure in larger catchment will never be economical feasible. Therefore, Zehe et al. (2014) suggest that landscape in a given geological setting can be classified into a hierarchy of lead topologies and elementary functional units (EFU's). Following Zehe et al. (2014), an EFU can be understood as a set of control volumes/landscape elements which react statistical homogeneous with respect to the functioning. After identifying these functional units in a catchment, only representative members of the different unit types must be characterized in more detail. Hillslopes can be allocated to one class of such units. For the understanding of flow processes at the hillslope scale a profound knowledge of the subsurface structures is essential. The subsurface structures should be characterize in a manner that the main features of one hillslope, which represents also other hillslope in the catchment, are captured. Thereby, it must be taken into account that the subsurface structure can vary downhill as well as over the width of the hillslope.
Concept for characterization of subsurface structures of a hillslope
To address the challenges of subsurface characterization, we propose a combination of electrical resistivity tomography (ERT) and vertical electrical soundings (VES). The application of ERT allows very detailed insights into the shallow subsurface along the investigated transect by mapping the spatial resistivity distribution in 2D. If a hillslope should be characterized, a single profile is not sufficient because it doesn't deliver information about the lateral heterogeneity perpendicular to the ERT profile. Furthermore, the results of inversion of ERT data suffer from non-uniqueness (Hoffmann and Dietrich 2004). Vertical electrical soundings are another approach to investigate geoelectrical parameter distribution of the subsurface. In difference to ERT, an assumption for the interpretation of VES data is a stratified subsurface. The correctness of this assumption can easily be proofed during field investigations by a combination of Schlumberger and half-Schlumberger arrays (Telford 1990, Yadav et al. 1997). A continuative comparison of ERT versus VES in terms of different parameters such as resolution, penetration depth, economical cost, quantification in data interpretation is given in Table 1. Considering the advantages of both geoelectrical methods, a combination of both geoelectrical methods with a measuring layout as shown in Figure 1 seems very suitable for the characterization of a typical hillslope in a catchment.
Study area
To illustrated the suitability of the suggested approach we conducted a survey in the hydrological observatory Attert Basin (Pfister et al 2002) that lies in the mid-western part of the Grand-Duchy of Luxembourg in the contact zone between the schistous Ardennes massif (Oesling) and the sedimentary Paris Basin (Gutland). The climate is characterized by high precipitation (ca. 800 mm/a), particularly in late summer. The hydrological regime is pluvial oceanic with low flows from July to September, and with high flows from December to February. For our research we consider a north facing hillslope with a slope gradient ca. 12° in the Weierbach catchment that lies in the northwest of the Attert Basin. The Weierbach catchment is covered with mixed forest (mainly beech and spruces). The general sequence of layers has a soil surficial horizon on the top underlain by periglacial deposit. The bedrock below the deposit is presented by fractured schist with thickness underlain by compact schist.
Results
In Figure 2 to 4, results of the investigations in the Weierbach catchment are shown. The ERT result in the upper part of Figure 2 shows a significant variability in the first 5 m of the subsurface in downhill direction. The results of VES (lower part of Figure 2) deliver deeper insights and information about the thickness of the different layers. As indicated in Figure 2 and illustrated by examples in Figure 3, we can see that heterogeneity of the hillslope perpendicular to the ERT profile varies along the downhill direction. For the location of VES, at which the combination of Schlumberger and Half-Schlumberger measurements indicates a lateral homogeneity perpendicular to the ERT profile, also equivalent models for the subsurface can be derived from the sounding data (example see Figure 4). This can allow a quantification of the uncertainty in the interpretation of the geophysical data.
Conclusions
Using a case study, we could illustrate that the combination of ERT and VES can be a suitable approach to characterize representative hillslopes in a catchment in an efficient manner. Thereby, we can characterize the variability of subsurface structures in downhill direction as well as along the width of the hillslope. Furthermore, we offer also an opportunity for a quantification of the uncertainty in the determination of layer thicknesses by geophysical methods.
Acknowledgements
The scientific research is conducted in the framework of CAOS-project (Catchments As Organized Systems), and is supported by the National Research Fund of Luxembourg and the National Research of Germany / Deutsche Forschungsgemeinschaft 'DFG (FOR 1598). Additional thanks to the Jörg Hausmann and Robert Nollau for excellent field work and comments, and to the colleagues from the Centre de Recherche Public' Gabriel Lippmann: Dr. Laurent Pfister and Laurent Gourdol for administrative supporting.
References
Freeze, R. (1972). Role of Subsurface Flow in Generating Surface Runoff 2. Upstream Source Areas . Water resources research, 1272-1283.
Hoffmann R., and Dietrich P. 2004. An approach to determine equivalent solutions to the geoelectrical 2D inversion problem. Journal of Applied Geophysics, 79-91.
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Zehe, E., Ehret, U., Pfister, L., Blume, T., Schröder, B., Westhoff, M., et al. (2014). HESS Opinions: Functional units: a novel framework to explore the link between spatial organization and hydrological functioning of intermediate scale catchments. Hydrology and Earth System Sciences (HESS), 3249-3313
Table 1 Comparison of ERT and VES
| Method | Advantage | Limitation |
| ERT | # automatic measurement # good lateral resolution # low personnel costs (1-2 men) |
# equipment is heavyweight # high time costs in the field for preparation and execution of measurements # bad vertical resolution # penetration depth depend on the profile lenght # quantification of equivalent models (Hoffmann & Dietrich, 2004) |
| VES | # equipment is lightweight and not expensive! # good vertical resolution (depth focusing) # deep penetration depth compared to ERT # good signal-noise-ratio # information about lateral heterogeneities (half-Schlumberger) # simple and clear interpretation using equivalent models |
# manual measurements # high personnel costs (3 men) # manual processing of data |
