Open Cut » Geology
Knowledge of the distribution of major, minor and trace elements in coals and interburden materials is increasingly important. Technological developments now suggest that such elemental distributions may be determined using a downhole logging tool. This study was undertaken to achieve two goals. The first was to survey the industry to determine those elements of interest to the coal industry and their associated detection limits. The second was to use this information as one of the inputs to select the best technique for determining elemental abundances using a downhole logging tool in both production and exploration environments.
A questionnaire was circulated widely and received a total of 15 responses. From these responses elements of interest were tabulated for reference to the Australian Coal Industry. This table of elements is complementary to the work of Les Dale and clearly shows an overlap of the elements that appear in Australian coals and the needs of industry.
Three technologies were evaluated as having potential to be incorporated into a downhole environment: X-Ray Fluorescence (XRF), Laser Induced Breakdown Spectroscopy (LIBS), and Prompt Gamma Neutron Activation Analysis (PGNAA) using a neutron generator. Various criteria were used to evaluate these techniques including operator and environmental safety, ease of implementation and operation, and performance with respect to the detection of the elements of interest to the coal industry. After due consideration, we recommend that the most practical way forward is the neutron activation (PGNAA and DGNAA) method using a neutron generator as the neutron source. The use of a switchable neutron generator allows for a significant improvement in the occupational health and environmental safety considerations, especially when compared to the previously used "always on" chemical isotope neutron sources. This nuclear approach also provides a "volume" estimate of elemental abundance and it does not have the issues of providing a representative estimate that the XRF and LIBs, "surface" techniques share. The XRF or LIBS techniques would most likely provide higher sensitivities than the nuclear neutron approach, but their implementation in a borehole environment would be much more difficult. With both of these techniques the probe is sensitive to the condition of the borehole wall and an engineering solution would be required to ensure that the material being sampled is representative of the material of interest.
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