Towards a global network of gamma-ray detector calibration facilities
Marco Tijs 1 Ronald Koomans 1 Han Limburg 1 21 Medusa Sensing BV, Verlengde Bremenweg 4, 9723JV Groningen, The Netherlands.
2 Corresponding author. Email: han@medusa-online.com
Exploration Geophysics 47(4) 302-307 https://doi.org/10.1071/EG16016
Submitted: 8 February 2016 Accepted: 11 August 2016 Published: 22 September 2016
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.
Key words: calibration, gamma ray, logging, MCNP, model pits, uranium.
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