[S24-1] Methods and instruments of experimental geothermics - application and recent evolution I

[S24-1-02] Laboratory measurements of rock thermal conductivity and diffusivity by transient divided bar and pulsed needle probe methods

Thue S. Bording, Soeren B. Nielsen, ^○Niels Balling (Department of Geoscience, Aarhus University, Aarhus Denmark)

For the determination of terrestrial heat flow and regarding any aspect of subsurface thermal modelling, accurate information on rock thermal properties is of vital importance. Several methods are available for laboratory measurements of thermal conductivity, while fewer are available for thermal diffusivity, and accurate determinations are not straightforward. We have revisited the classical divided bar and needle probe methods with methodological improvements to measure thermal conductivity and volumetric heat capacity simultaneously, and thereby also thermal diffusivity. The divided bar technique is generalised to the transient case (Bording et al. 2016) and for the needle probe, we apply a pulsed mode. For both methods, measured temperature history is fitted by numerical finite element forward modelling, and sample properties are determined using inverse Monte Carlo analysis. This methodology provides a robust framework for deriving sample thermal properties with associated uncertainty estimates.
A series of synthetic tests were performed, and both techniques were applied for laboratory measurements of various materials including different rock samples. The transient divided bar provides excellent reproducibility and high accuracy. For conductivity the obtained uncertainty may be reduced to 1–3 per cent, and for diffusivity to about 3–5 per cent. The main uncertainty originates from the presence of thermal contact resistance across the internal interfaces in the bar, and they need to be minimized. The pulsed needle probe yields equally accurate values for thermal conductivity. The determination of high quality values of thermal diffusivity requires a dense sampling of the temperature rise function at early times and a very good contact between probe and sample material.

Bording, T. S., Nielsen, S. B. and Balling, N., 2016. The transient divided bar method for laboratory measurements of thermal properties. Geophys. J. Int., 207, 1446–1455.