Towards a global network of gamma-ray detector calibration facilities

Abstract

Gamma-ray logging tools are applied worldwide. At various locations, calibration facilities are used to calibrate these gamma-ray logging systems. Several attempts have been made to cross-correlate well known calibration pits, but this cross-correlation does not include calibration facilities in Europe or private company calibration facilities.

Our aim is to set-up a framework that gives the possibility to interlink all calibration facilities worldwide by using ‘tools of opportunity’ – tools that have been calibrated in different calibration facilities, whether this usage was on a coordinated basis or by coincidence.

To compare the measurement of different tools, it is important to understand the behaviour of the tools in the different calibration pits. Borehole properties, such as diameter, fluid, casing and probe diameter strongly influence the outcome of gamma-ray borehole logging. Logs need to be properly calibrated and compensated for these borehole properties in order to obtain in-situ grades or to do cross-hole correlation. Some tool providers provide tool-specific correction curves for this purpose. Others rely on reference measurements against sources of known radionuclide concentration and geometry.

In this article, we present an attempt to set-up a framework for transferring ‘local’ calibrations to be applied ‘globally’. This framework includes corrections for any geometry and detector size to give absolute concentrations of radionuclides from borehole measurements. This model is used to compare measurements in the calibration pits of Grand Junction, located in the USA; Adelaide (previously known as AMDEL), located in Adelaide Australia; and Stonehenge, located at Medusa Explorations BV in the Netherlands.

Gamma-ray tools are used heavily in borehole logging, both in oil and gas, as well as in mining applications. For proper functioning, these tools need to be calibrated against a source of known activity and geometry. In this paper, we present a holistic approach to tool calibration, aiming to interlink several calibration facilities around the world, allowing quantitative interpretation of data.

Key words::

• calibration, gamma ray, logging, MCNP, model pits, uranium.

Acknowledgements

The authors wish to thank Annick Henriette, Patrick van Eyll (ALT), John Stowell (Mt Sopris), Duncan Cogswell (Borehole Wireline) and Paul Worthington (Robertson Geologging) for sharing data and commenting on our research.