Download Detailed 3-D characterization of a shallow alluvial aquifer

Detailed 3-D characterization of a shallow alluvial aquifer: Integrating multiscale geophysical and hydrological data by fuzzy c-means cluster analysis
Hendrik Paasche, Jens Tronicke
(University of Potsdam, Germany)
Thomas Guenther
(Leibniz Institute for Applied Geosciences, Hannover, Germany)
Hansruedi Maurer, Alan G. Green (Swiss Federal Institute of Technology, Zurich, Switzerland)
Klaus Holliger
(University of Lausanne, Switzerland)
[email protected]
The characterization of heterogeneous aquifers in general and the detection of preferential flow
paths in particular are prerequisites for understanding groundwater flow and contaminant
transport. In this study, we present an integrated approach based on 3-D geoelectrics, 2-D
crosshole georadar and seismic tomography and direct push slug tests to identify preferential flow
paths within the alluvial aquifer at the Kappelen hydrogeological test site in northwest
Switzerland.
To consider the large differences in resolution and spatial sampling between the slug tests and the
3-D resistivity model, we performed the integration of the available data in two consecutive
steps. First, we combined the 2-D crosshole georadar and seismic tomograms using a fuzzy cmeans cluster analysis based approach to a zoned geophysical multiparameter model outlining
dominant structural units within the aquifer. A 2-D hydraulic conductivity model was then
estimated by linking the results of the sparse direct push slug tests to the integrated tomographic
model via the membership values provided by fuzzy c-means cluster analysis.
In the second step, the 2-D hydraulic conductivity model was linked to the results of a 3-D
geoelectric survey covering the entire Kappelen test site. The Kappelen aquifer is characterized
by low electrical resistivity contrasts, which mandated a non-standard inversion strategy based on
the incorporation of other available a priori information. The resulting 3-D resistivity model of
the aquifer was split into zones of high and low resistivities using again a fuzzy c-means cluster
analysis based approach. Despite significant differences in resolution between the 3-D resistivity
model and the integrated 2-D tomographic model, we find similar dominant structures in the two.
This illustrates the suitability of the 3-D geoelectric method to image hydraulic relevant
structures within the Kappelen aquifer. To this end, the individual clusters inferred from the 3-D
geoelectric model are linked to the 2-D hydraulic conductivity model, which results in a 3-D
model outlining regions of increased hydraulic conductivities acting as preferential flow paths at
the Kappelen test site. The spatial resolution of the final 3-D hydraulic conductivity model
corresponds to that of the 3-D electrical resistivity model, which is estimated to be of order of
several meters.