Set-up and development of petrophysical methods for hydrogeological applications
Begin of project: January 1, 2012
End of project: December 31, 2014
Status of project: December 31, 2014
Sample in the climatic chamber
Source: BGR
Geophysical field methods measure the distribution of physical quantities in the subsurface and provide in this way a tool for geological interpretation. However, this interpretation is difficult in many cases because the relationship of geophysical and geological quantities is naturally ambiguous. In order to improve the hydrogeological interpretation of geophysical field applications such as electrical resistivity tomography (ERT) and surface nuclear magnetic resonance (NMR), we investigate the underlying petrophysical relationships using representative samples under laboratory conditions. In this way, for instance the water content of porous material can be estimated from resistivity measurements and hydraulic conductivity estimates can be provided by NMR.
For the quantification of petrophysical models from measurements at the laboratory scale, it is necessary to avoid systematic uncertainties. Those can be associated with (i) the collection of samples, which is always an invasive action, (ii) the filling and refilling of the porous sample material when applying different laboratory methods, and (iii) the natural variability of the geological target parameters when working with different specimens of the same sample. To minimize the effects of these error sources, we developed a measurement cell for unconsolidated sediments that allows a combination of petrophysical applications: spectral induced polarization (SIP), nuclear magnetic resonance (NMR), capillary pressure, saturated and unsaturated hydraulic conductivity. The cell has a cylindrical shape (height: 100 mm, diameter: 65 mm) and is completely metal-free to avoid disturbances of the magnetic fields involved during NMR measurements (Fig. 1a). The electrical coupling for the SIP measurements is realized by electrolytes and micro-porous materials: micro-porous glass-filter plates for the current and small ceramic spikes, which are usually used for tensiometers, for measuring the voltage (Fig.1b). Beside the fixation of the sample material inside the cylinder, the glass plates have another important function: they allow the drainage of the sample by a vacuum pump with controlled pressure in the range of less than 10 up to about 900 hPa (Fig. 1c). In this way, the water retention curve (WRC) can be determined. Alternatively, the WRC can be measured applying the evaporation method, if the SIP electrodes are replaced with tensiometers. Using the evaporation method, also estimates of the unsaturated hydraulic conductivity are provided (Fig. 1d). By replacing the micro-porous glass-filters with those exhibiting larger pores, also the saturated hydraulic conductivity can be measured by flow experiments with constant or falling head (Fig. 1e). The measurement cell is successfully applied in recent research projects such as for investigating sand-pyrite mixtures in the BMBF-project ROBEHA and for investigating natural soil samples in the BGR project Benchmark soils.
Abb.1: Different configurations of the measurement cell for combined petrophysical experiments: (a) nuclear magnetic resonance, (b) spectral induced polarization, (c) capillary pressure, (d) unsaturated hydraulic conductivity using the evaporation method, and (e) saturated hydraulic conductivity
Source: BGR
Moreover, we investigated the NMR response of partially saturated sandy soils and sediments. Our results show that beside the water content also the effective pore radius can be estimated from NMR measurements of unsaturated samples. Future research is necessary to transfer these findings for the field application of NMR. If this is successful, a non-invasive tool is available with the potential to characterize water fluxes in the vadose zone.
Posters and Presentations
Ronczka, M., Costabel, S., Kirsch, R., and Yaramanci, U. (2011). Korrelation der Parameter der Spektral Induzierten Polarisation und der Kernspinresonanz auf Labor- und Feldskala. Tagungsband der 71. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 21. – 24. Februar, Köln, Deutschland.
Costabel, S., Martin, T., Hupfer, S., and Noell, U. (2014). Entwicklung einer Messzelle zur kombinierten Messung von SIP und NMR, Kapillardruck, sowie gesättigter und ungesättigter hydraulischer Leitfähigkeit. Tagungsband der 74. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 10. – 13. März, Karlsruhe, Deutschland.
Costabel, S. und Müller-Petke, M. (2014). NMR-Relaxation bei Teilsättigung unter Berücksichtigung von fast- und slow diffusion. Tagungsband der 74. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 10. – 13. März, Karlsruhe, Deutschland.