Spectrometric Nuclear Logging as a Tool for Real-time, Downhole Assay – Case Studies Using SIROLOG PGNAA

The SIROLOG Prompt Gamma Neutron Activation Analysis probe is a spectrometric borehole logging tool developed by the CSIRO in Australia. The principle of the technique is the measurement of the characteristic intensity and energy of gamma (γ) rays that result from nuclear interactions when fast neutrons bombard the formation. The intensity and energy of the emitted γ rays is related to the elemental composition of the rock, thus the volumetric occurrence of the elements in a bulk rock mass intersected by a borehole, can be directly estimated. Downhole nuclear logging has certain benefits over conventional core and drill-chip assaying. A much larger volume of the material surrounding the borehole is sampled than the sample taken for laboratory analysis, thus providing better sampling statistics. Provided a suitable calibration exists, data processing can be automated for quick turnaround of results (near real-time processing). Since a continuous log is measured in situ, the data is not limited by zones of core loss or breakage or potential sample contamination. The SIROLOG PGNAA probe was demonstrated on four Anglo American mines in 2004. At Namakwa sands heavy mineral sands mine in South Africa, the technique indicated the potential to deliver quantitative estimates of whole rock geochemistry, including heavy mineral concentrations. At Sishen Iron Ore mine in South Africa, quantitative in situ determination of Fe (iron) grade was demonstrated. This is in line with published results from studies done at other iron ore mines. Encouraging results were obtained for P (phosphorous), one of the important penalty elements, although low concentrations and a limited range of grade values in the small data sample, affected the outcome. At Skorpion mine in Namibia, a good estimate of Zn (zinc) grade, the primary ore mined, was obtained in blast boreholes; however the penalty element, Ca (calcium), could not be accurately predicted due to the low abundance of this element intersected by the test boreholes. Similarly, low abundance of Cu (copper) returned poor calibrations in a study on waste dumps at Mantos Blancos mine in Chile. The results demonstrate that the PGNAA technique can quantitatively predict the abundance of certain elements in situ, although site-specific calibrations are required for best results. However, calibrations are difficult to establish where the abundance and the range of grade values of the target element are low, or the typical signature of the target element overlaps that of another element. The development of higher resolution LaBr3 γ-ray detectors can potentially alleviate these limitations. Another drawback of downhole nuclear logging is the hazard associated with the use of chemical radioactive sources. Alternative, safe sources, such as neutron activation systems, are currently being developed. These systems use non-isotopic, switchable sources such as D-T neutron generators that can be switched off when not being used.