Geophysical surveying an active well catchment in the presence of significant topography and anthropogenic disturbances
H. Paasche, J. Tronicke, H. Maurer, A.G. Green, E. Auken and F. Stauffer
Issue: Vol 5, No 5, October 2007 pp. 309 - 319
Info: Article, PDF ( 6.48Mb )
Traditional approaches to surveying well catchments usually involve investigations at two quite different spatial scales: very high resolution borehole-based studies and low resolution tracer and pump tests. In recent years, various researchers have attempted to fill the wide gap between the two scales of acquired data by using a variety of geophysical tools that have high-to-medium resolution capabilities. Although geophysical surveying may be relatively straightforward at many locations, there are numerous regions where topography, the presence of ubiquitous metal objects and/or a lack of suitably placed boreholes cause the acquisition, processing, inversion and interpretation of certain types of geophysical data to be extremely challenging. It was necessary for us to address such problems in an investigation of an active well catchment near Zurich, Switzerland. The principal goals of our project were to determine the geometries and physical properties of the shallow sedimentary layers, with emphasis on a key water-bearing gravel unit. To achieve these goals, four different geophysical methods were employed. In a first step, frequency-domain electromagnetic measurements allowed the locations of buried metal pipes to be established. A 2D tomographic seismic refraction survey then provided information on the interface between the low-velocity surface loamy topsoil and underlying high-velocity morainal material. The high electrical resistivity of the important gravel unit allowed its upper boundary to be outlined in 2D models derived from geoelectric data and its lower boundary to be delineated in models based on 1D linked inversions of geoelectric and transient electromagnetic data.