Analysis of convections in laboratory by geoelectrical 3D-tomography

Many natural bodies such as the Earth’s mantle, as well as materials inside industrial installations like molten glass inside furnaces, exchange matter through convection resulting from differences in temperature, density and chemical concentration. In Earth Sciences the analysis of plumes and convection inside the Earth’s mantle is a classical area for numerical modelling. However, experimental methods in the laboratory help as analogue models to verify the validity of computer models. The inherent problem of such analogue experiments is the visualisation of the flows and the determination of the parameters of interest as for example temperature. In nature and in industrial installations it is difficult to measure the temperature inside the object of interest directly. Therefore, the goal of this research study was the development of a new method for temperature measurements that allows temperature data acquisition without influencing the flow pattern in the laboratory simulation of Earth’s convection. Contrary to the three-dimensional optical techniques used by some of the scientist, DC-geoelectrical 3-D tomography works without tracers inside fluids. This is the precondition for the determination of temperatures inside molten glass, that has to remain uncontaminated.

The basis for the use of DC-geoelectrical 3-D tomography is the adjusted Arrhenius temperature dependence of the electrical conductivity. In order to verify the applicability of the method, thermally driven flows were generated in a viscous material. Therefor, dissolved polymer polyethylene glycol was heated in a tank with a base of 48 x 30 cm and a height of 30 cm. During each geoelectrical measurement, two electrodes supplied a current into the tank to generate an electrical field. The further 26 electrodes, placed on the sides of the tank, contemporaneously recorded the voltages between themselves and the reference electrode. Finally, the voltages were inverted using a 3-D inversion program. The model of the three-dimensional distribution of electrical conductivities obtained was converted into temperatures. In order to verify the validity of the method these temperatures were compared with the temperatures recorded by the 29 thermocouples on the sides of the tank. There were high positive correlations between the temperatures measured directly and the temperatures determined by geoelectrical 3-D tomography. This, as well as the low root mean square deviations normalised to the mean of the temperatures of the respective geoelectrical model demonstrates the success of this new method.

It should be noted that small-sized structures such as hot plumes with a diameter of 3 cm possess lower temperatures in the reconstructed models than in reality because of the relatively low spatial resolution. This means that geoelectrical 3-D tomography can be used for the measurement of the general distribution of temperature, but not for analysis of small structures.

Bock, M., Regenauer-Lieb, K., Lotze, M., Wilke, T. and Rücker, C. (2010): Analysis of thermally induced flows in the laboratory by geoelectrical 3-D tomography, J. Geophys. Res., 115, B12410, doi:10.1029/2010JB007462.

Bock, M., Regenauer-Lieb, K., Lotze, M., Wilke, T., Lentes, F.-T. & Siedow, N. (2006): Untersuchung von Konvektionsströmungen im Laborversuch mit Hilfe der geoelektrischen 3D-Tomopgraphie. 66. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Bremen, März 2006 (Poster).

Bock, M., Regenauer-Lieb, K., Lotze, M., Wilke, T., Lentes, F.-T., Siedow, N. & Rücker, C. (2007): Untersuchung von Konvektionen im Laborversuch mit Hilfe der geoelektrischen 3D-Tomographie. 67. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Aachen (Poster).

Bock, M., Regenauer-Lieb, K., Lotze, M., Wilke, T., Lentes, F.-T., Siedow, N. & Rücker, C. (2007): Analysis of convections in laboratory by geoelectrical 3D-tomography. Subduction Zone Geodynamics Conference, Montpellier / France (Poster).

Bock, M., Regenauer-Lieb, K., Lotze, M., Wilke, T. & Rücker, C. (2008): Anwendung der geoelektrischen 3D-Tomographie für die Analyse thermisch induzierter Strömungen im Labor. 68. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Freiberg (Vortrag).

Bock, M., Regenauer-Lieb, K., Lotze, M., Wilke, T. & Rücker, C. (2008): Analysis of thermally induced flows in the laboratory by geoelectrical 3-D tomography. European Geosciences Union General Assembly 2008, Vienna / Austria (Poster).

Melanie Bock (Institut für Geowissenschaften, University of Mainz, Becherweg 21, 55099 Mainz, Germany)

Klaus Regenauer-Lieb (Western Australian Geothermal Centre of Excellence, CSIRO Earth Science and Resource Engineering & School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia)

Martin Lotze, Thorsten Wilke, Frank-Thomas Lentes (Schott AG, Hattenbergstrasse 10, 55122 Mainz, Germany)

Carsten Rücker (Institut für Geophysik und Geologie, University of Leipzig, Talstraße 35, 04103 Leipzig, Germany)