Cuxhaven field campaign
Introduction
The work area Cuxhaven is dominated by a glacial buried valley containing an important groundwater aquifer. In order to complement the existing seismic (Rumpel et al., 2006) and electromagnetic (Siemon et al, 2006) measurements, we carried out a large-scale geoelectrical experiment in 2006. At 10 positions we injected currents up to 40A in 3 directions and registered the potential difference at 20 stations, as well in three directions.
Abb. 1: Measuring area and field layout - injection points (black) and measuring stations (red)
Data processing
The registered signals are cut and re-sampled. Figure 2 shows a typical time series and its Fourier spectrum. By using a specially developed least squares method we determine the resistance as a signed correlation coefficient between current and voltage signal. Additionally we obtain an error measure that is used for the inversion.
Abb. 2: Typical signal (top) and its Fourier spectrum
Inversion
The data a processed by different methods. Mainly we apply a smoothness-constrained inversion using the Finite-Differences based software DC3dInvRes (Günther, 2004). The determined error measures are used for weighting the individual data, such that more reliable measurements are fitted more accurately than noisy registrations. Figure 3 shows such an inversion result.
Abb. 3: Inversion result as depth slices. We see the conductive Lauenburg clay inside the vallay, the resistive Geest in the West and the salt-water intrusion at depth from Northeast.
Due to the chosen star-shaped injection and registration we can derive an apparent resistivity tensor (Bibby, 1986) from each 4 measurements. Its invariants are much more independent of local inhomogeneities and can show preferred directions of current flow. By modelling and sensitivity studies we are able to obtain an understanding of the fields. The work is currently being summarized in the PhD thesis of Jörn Schünemann and a recent paper (Schünemann et al., 2009).
References
BIBBY, H. M. (1986). Analysis of multiple-source bipole-quadripole resistivity surveys using the apparent resistivity tensor. Geophysics, 51 (4), 973-983.
GÜNTHER, T. (2004). Inversion Methods and Resolution Analysis for the 2D/3D Reconstruction of Resistivity structures from DC Measurements. Dissertation, University of Mining and Technology, Freiberg.
SCHÜNEMANN, J.; GÜNTHER, T.; JUNGE, A. (2007): 3-dimensional subsurface investigation by means of large-scale tensor-type dc resistivity measurements. - 4th International Symposium on Three-Dimensional Electromagnetics, Freiberg.
SCHÜNEMANN, J.; GÜNTHER, T.; JUNGE, A. (2009): 3-Dimensional Geoelectrical Investigation of Coastal Aquifer Structures by a large-scale Dipole-Dipole Experiment. accepted for Near Surface Geophysics Journal.