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Measurement, Scaling and Calibration |
T. H. Huxley School of Environment, Earth Science and Engineering, Imperial College of Science Technology and Medicine, London SW7 2BP, UK
1 Petronas Research and Scientific Services SDN BDH, Hulu Kelang, 54200 Selangor, Malaysia
Methods for the estimation of measurement uncertainty are discussed with particular reference to concentration measurements made by a geochenmical logging tool (GLT; Mark of Schlumberger) in a borehole penetrating a cyclothem sequence at Northumberland, UK. Two components of uncertainty have been quantified for 6 elements determined by the GLT over a wide range of concentrations. The random component was estimated from duplicated determinations within this borehole over a 120 m depth interval. These uncertainty values ranged from 2.7% for Si to 71% for S, expressed at the 95% confidence limit. The systematic component of the uncertainty was estimated by determinations made on corresponding core samples by inductively coupled plasma atomic emission spectrometry (ICP-AES) over a 40 m depth interval. The ultimate basis for this estimation of bias was the certified reference materials which were analysed during the ICP-AES determinations. The bias measured by this method was typically in the range + 5% to –14% for 5 out of the 6 elements determined.
This method assumes that the samples analysed by both techniques are physically comparable. By depth averaging the ICP-AES determinations it was possible to reduce errors due to differences in sample size. However, a possible source of bias that was recognised is that samples were dried before ICP-AES determinations, whereas this was not the case for in situ GLT measurements. Such variability in the size of the systematic component of the uncertainty prevents the effective correction of this term as is recommended by the International Standards Organisation. The large values of measurement uncertainty found for some elements (e.g. S) will exert limitations on the geochemical interpretations made from the GLT measurements, in terms of ‘fitness-for-purpose’ criteria.
