NUCLEAR BOREHOLE LOGGING TECHNIQUES DEVELOPED BY CSIRO-EXPLORATION AND MINING FOR IN SITU EVALUATION OF COAL AND MINERAL DEPOSITS

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1 AU NUCLEAR BOREHOLE LOGGING TECHNIQUES DEVELOPED BY CSIRO-EXPLORATION AND MINING FOR IN SITU EVALUATION OF COAL AND MINERAL DEPOSITS M. BORSARU and J. CHARBUCINSKI CSIRO, Division of Exploration and Mining, P.O.Box 883, Kenmore, Queensland 4069, Australia SUMMARY. Commonwealth Scientific and Industrial Research Organisation, Exploration and Mining has developed nuclear spectrometric techniques for borehole logging and surface quantitative analysis for the coal and metalliferous mining industries. The paper reviews the latest developments of this technology. 1. INTRODUCTION Geophysical techniques are well established in the resource industries lie oil, gas, uranium, coal and minerals. Nuclear borehole logging which represents a subset of this group has been widely used in the oil, gas and uranium industries for a long time. Due to the deep penetration of neutrons and gamma rays, nuclear techniques are suitable for borehole logging applications and they are maing inroads in other industries lie coal and minerals. Many holes drilled in the coal and metalliferous mining industries are cored and the core is sent to the laboratory for Chemical analysis. The chemical analysis of the core provides all the information which is usually extracted from a borehole. However, this does not mae nuclear borehole logging redundant. The benefits which can be derived from nuclear logging are: i. It samples a much larger volume of the material surrounding the borehole than the core sample and therefore provides better sampling statistics especially in heterogeneous deposits. ii. The results are instantaneous. iii. The cost of drilling open holes is cheaper than the cost of cored holes. Considering that the full information provided by the laboratory analysis is not always needed and that in some mineral deposits the core cannot be fully recovered, nuclear logging and the laboratory analysis of the core are complementary. The Commonwealth Scientific and Industrial Research Organisation, Division of Exploration and Mining has developed the spectrometric nuclear logging system, SIROLOG, based on natural-gamma, gammagamma and neutron-gamma techniques. In this system the whole gamma ray spectrum is recorded for each preset logging interval. By recording and analysing the whole gamma-ray energy spectrum from a spectrometric measurement one can extract more information from the logging data. A dedicated software pacage for spectrometric data analysis and interpretation has also been developed. The SIROLOG logging system was developed for both the coal and metalliferous mining industries. However, the data interpretation and the source/detector configuration is usually specific to the type of application, ie, it is tuned for each specific application. The SIROLOG logging system and its application to the coal and metalliferous mining industries was described in a previous paper (1). This paper deals mostly with new developments and applications of SIROLOG. A Coal Face Analyser was also developed recently and is described in this paper.

2 2. INSTRUMENTATION Being spectrometric, gain stabilisation is an essential part of the system. The upgraded SIROLOG is fully digitised. Pulses produced by the gamma-ray detector are processed in the probe and transmitted to the uphole PC computer. The transmission of the signal is noise proof and is not susceptible to attenuation in the cable as was the case with the previous analogue system. A CPU is incorporated in the probe and all the software is written in C. The whole system consists of the probe, winch and a laptop computer. When low activity sources are used and a source transporter is not required, the system is portable and does not require a dedicated logging vehicle. This maes SIROLOG suitable for logging in areas where the only way of access is by helicopter. The standard logging probe has a diameter of 60 mm and can accommodate a scintillation detector of 37 mm diameter. The length of the probe is about 2 m. When logging large diameter boreholes it is advantageous to use larger volume detectors which are more efficient for gamma ray detection. Larger diameter probes are constructed for these applications. Scintillation detectors are used in the SIROLOG system. The most common scintillator used in the gamma-gamma tool is Nal(Tl), while BGO (bismuth germanate) is the preferred scintillator for natural-gamma and neutron-gamma logging. 3. APPLICATIONS 3.1 Coal The spectrometric gamma-gamma technique was developed for the determination of ash content of coal (2). The ash determination is based on the correlation which exists between Zeq (atomic equivalent number) of coal and ash. The technique wors well in both dry and watterfilled boreholes. A combined gamma-gamma/natural-gamma probe was developed for borehole lithology logging for brown coal (3). The identification of the three basic components -sand, clay and coal- is possible because they have different densities and intrinsic natural radioactivities. The combination probe employs one BGO detector and a 137 Cs gamma-ray source. The gamma-gamma and natural-gamma responses are recorded simultaneously due to the spectrometric feature of the probe. A technique based on the prompt neutrongamma method was also developed for the determination of ash in coal seams intersected by boreholes (4). The technique wors both in waterfilled (4) and dry (5) holes, and uses a 252 Cf neutron source and a BGO detector. The capture gamma-rays produced in the neutron capture process by the main constituents of coal ash ( Al, Si, Fe, Ca and S) have energies above 3 MeV and therefore have large penetration ranges. Assuming, for most rocs and coal, a cm range of penetration for the neutrons produced by the Cf neutron source, the deeply-penetrating gamma-rays emanate from a large volume of coal. This maes the neutron capture technique less sensitive to the rugosity and condition of the borehole than the gamma-gamma technique. Apart from sampling larger volumes of coal, neutron-gamma can also measure some of the major constituents of coal ash. Figure 1 shows a plot of the chemical assays and the neutroncapture predictions for iron content in the coal seams given by regression analysis (6). 6 5 I 4 2» 3 S! * %Fe (laboratory assays) Figure 1. Comparison of iron content of coal by chemical assays and nuclear logging. Iron content of the ash can be easily determined from the ash values obtained from the same measurement. The determination of iron in coal is important in some coal deposits with high variation in Fe content because the slagging index of coal is largely affected by the iron content of the ash.

3 3.2 Metalliferous Mining The three nuclear borehole logging techniques, natural gamma, gamma-gamma and neutron gamma have found applications in the iron ore mining industry. Natural-gamma can be used for delineating the iron ore body based on the big difference in natural gamma radiation between the iron ore (low in natural gamma radiation) and the shaly roc. It can also provide a means for determining alumina contamination of iron ore based on the correlation between alumina and the aolinitic material of the ore matrix (7). Neutron-gamma logging can provide both the iron ore grade and the silica content (8,9). The spectrometric gamma-gamma technique enables the simultaneous measurement of iron grade, density and borehole diameter on a stratigraphic basis in wide (310 and 380 mm) blast holes in iron ore deposits. The primary gamma-ray source used is Co. Both neutron-gamma and neutron activation techniques have been used for the determination of the manganese content of manganese ore (10). The neutron-gamma technique has also been used for the estimation of the nicel content of sulphide ore deposits. 4 IN-SITU ANALYSIS USING ULTRA- LOW RADIATION INTENSITY GAMMA- RAY SOURCES Although nuclear techniques are widely used in the mining industry, some mines are still reluctant to use them due to the extra care required when woring with radioactive sources. Wor has been carried out over the last years to develop environmentally friendly techniques for in-situ analysis using ultra-low radiation intensity gamma-ray sources. Equipment for in-situ analysis using low activity sources significantly simplifies safety procedures and reduces to a minimum the source radiation ris. Logging systems using very low activity sources are much more liely to be accepted by the mining industry. Two instruments have been developed: i. a borehole logging probe and ii.a coal face analyser. i. Logging probe Two source - shield - detector configurations were tested for the logging probe. The one source configuration (11) comprises a 1.8 MBq 137 Cs gamma-ray source placed along the axial centreline of the detector with a conical 30 mm thic lead shield between the source and the detector. The lead is not thic enough to stop all gamma-rays reaching the detector and the 662 ev Cs pea, produced by gammaradiation penetrating the lead shielding, is used for gain stabilisation. The distance between the source and the bottom end of the detector varies between 30 and 57 mm depending on the application. The three source configuration comprises three 0.36 MBq 137Cs gamma-ray sources placed circumferentially (at a distance of 22.5 mm from the bottom end of the detector) around a cylindrical iron/lead shield. This configuration is a modified version of the Zero Probe (12). Due to the very short source to detector distance the probes provide the best possible delineation of coal seams. Both probes operate in the pre-inversion zone of the calibration curve. In this zone the count rate recorded by the detector is proportional to the density of the matrix logged. The logging tool has an external diameter of 60 mm. ii. Coal Ash Face Analyser Coal ash determination on the coal face falls into the category of in-situ measurement, which is mainly applicable to the production phase in open-cut pits and underground. Two coal face analysers have been developed. One coal face ash analyser was based on natural gamma radiation and utilises the existence of a correlation between the natural gamma radiation of coal and its ash content (13). In the second coal face ash analyser, the determination of the ash content of coal on the coal face was based on the bacscattered gamma-gamma technique in a 2% geometry (14). It is practically the same configuration described in (11) adapted to a 2n surface measurement. The primary source of radiation is a 1.8 MBq 133 Ba gamma-ray source separated by 40 mm of lead from the 37dia x 25 mm NaI(Tl) scintillation detector. An extra 0.35 MBq 137 Cs gamma-ray source is used for

4 gain stabilisation. I33 Ba was chosen as the primary source of radiation because of the lower energy gamma-rays produced by this source. The low energy region of the bacscattered gamma-ray spectrum is affected mostly by changes in Zeq of the matrix, and the analyser is more sensitive to changes of ash content of coal when a low energy gamma-ray source is used. The instrument is portable, hand held, weighs 2 g and does not expose the user to unacceptable levels of radiation. 4.1 Applications Coal Both the logging probe and the face analyser were developed primarily for the coal mining industry. The laboratory and field tests demonstrated that both the single source and three source configurations were suitable for delineation of coal seams and ash determination in wet and dry boreholes. Figure 2 shows a comparison between the ash content determined by the 3 source probe and chemical analysis in a dry 100 mm borehole. The face analyser using the 133 Ba gamma-ray source was developed for quantitative measurements of ash on the coal face. Figure 3 shows three spectra for shale and coal of 3.7 and 21 %ash. The spectrum for shale shows lower count rate in the low energy region due to the high Zeq of shale. Higher count rate is recorded in the high energy region due to the high density of shale. It is evident from this figure that the instrument is sensitive to changes of ash content in coal. Figure 4 shows a cross-plot between the ash content of a coal face predicted by the regression equation versus laboratory assays. The r.m.s. deviation given by the regression equation was 2.6 %ash with a correlation coefficient of 80%. The standard deviation of the population was 4.3 %ash i 2000 O F A -^ shale 21 /<as» Energy (ev) h TJSO o t "" 30 i A" Figure 3. Face Analyser recorded spectra. 25 " 10 < so < * Figure 2. Comparison of coal ash content by chemical assays and nuclear logging. The coal face analyser based on the natural gamma radiation could only provide a semiquantitative value for the ash content of coal on the coal face. Due to the heavy shielding from the natural radiation which does not originate from the coal face, the instrument is also heavy (15 g) % Ash (cherried) Figure 4. Comparison of ash content by chemical assays vs Face Analyser predictions Iron ore The single source probe was tested in an iron ore deposit (15). The probe proved suitable for delineation of the ore body and also for predicting its grade.

5 -.... J Pb-Zn ores Both single source and three source configurations were tested for orebody delineation and grade control of Pb-Zn ore (16,17). Lead grade is determined from the 80 ev K X-Ray pea excited by the multiscattered gamma-rays. Figure 5 shows the bacscattered spectra collected with both configurations in two bul (200 1) samples with low (0.5 %Pb) and high (7.9 %Pb) lead content. The 80 ev Pb X-Ray pea shows up very strongly in both configurations. The probes were field tested in two cored holes, reamed later to a diameter of 142 mm. The holes were water-filled. Figure 6 shows a cross plot of the predicted %Pb versus the laboratory assays. The r.m.s. deviation given by the regression equation was 0.3 %Pb and the standard deviation of the population was 1.7 %Pb g O f J -Ji Energy (ev) -»-0.5%Pb3sou rce 7.9%Pb3sot rce -*- 0.5%Pb 1 so urce -*-7.9%Pb1sou rce * * 400 Figure 5. Spectra recorded in two bul samples of lead ore with one and three source configuration. The gamma-gamma probe is not able to measure the concentration of zinc directly. The probe's response is related to the overall contributions given by the major components with high atomic number which are present in the Pb-Zn ore, eg Pb, Zn, Fe and Mn. Because Pb concentration can be measured directly, the determination of Zn is possible if the Fe and Mn concentrations in the ore are constant, or can be estimated in a different way /^(labotafcry assays) Figure 6. Comparison of %Pb determined by laboratory analysis and nuclear logging *** ^ ] /<Zn (laboratory assays) Figure 7. %Zn estimated by nuclear logging vs laboratory assays. Figure 7 shows the cross-plot between Zn concentration given by the laboratory analysis and nuclear determination. The concentrations of Fe and Mn were estimated from statistical information obtained from a geological data base. The r.m.s. deviation for the determination of %Zn was 2.4 %Zn and the correlation coefficient was The standard deviation of the population of 72 samples used was 4.55 %Zn. 5. CONCLUSIONS The spectrometric SIROLOG system for insitu analysis developed by CSIRO has proved itself in the Australian mining industry. The new fully digitised, portable systems using ultra-low radiation intensity gamma-ray sources will mae the system even more competitive. The system has great potential in both the coal and metalliferous mining industries.

6 6. ACKNOWLEDGMENTS The authors acnowledge the financial assistance from the Australian Coal Association Research Program in developing this technology. The authors also wish to than Mr Robert Dixon and Michael Barry of CSIRO, Exploration and Mining who developed the newly digitised system and Mr Za Jecny for his assistance during the laboratory and field trials. REFERENCES 1 Borsaru M., Charbucinsi J. and Eisler P. L Nuclear in-situ analysis techniques for the mineral and energy resources mining industries, Proc. 9th Pacific Basin Nuclear Conference, Sydney, Australia 1-6 May 1994, vol.1, pp Borsaru M., Charbucinsi J., Eisler P. L. and Youl S. F Determination of ash content in coal by borehole logging in dry boreholes using gamma-gamma methods. Geoexploration 23, pp Huppert P., Borsaru M., Charbucinsi J., Ceravolo C. and Eisler P. L Combined natural-gamma/gamma-gamma borehole lithology logging. Nucl.Geophys.3, p Charbucinsi J., Youl S. F., Eisler P. L. and Borsaru M Prompt neutron-gamma logging for coal ash in water-filled boreholes. Geophysics 51, pp Borsaru M., Charbucinsi J., Eisler P. and Ceravolo C Coal ash determination in dry boreholes by the neutron capture technique. Nucl. Geophys. 2, pp Borsaru M., Biggs M. S. and Nichols J.F Neutron-gamma logging for iron in coal and implications for estimating the ash fussion characteristics at Callide mine. Nucl. Geophys. 7, pp Charbucinsi J., Millitz P. and Ceravolo C In situ assaying of iron ore in blast holes for alumina content, CSIRO, Div. of Geomechanics Int. Report (Newseries) No Eisler P.L., Huppert P., Mathew P. J., Wylie A. W. and Youl S. F Use of neutron capture gamma radiation for determining grade of iron ore in blast holes and exploration holes, Proc. of IAEA Symp. on Nuclear Techniques and Mineral Resources, Vienna, pp Charbucinsi J., Comparison of spectrometric neutron-gamma and gammagamma techniques for in situ assaying for iron grade in large diameter production holes, Nucl. Geophys. 7, pp Aylmer J. A., Charbucinsi J., Eisler P. L. and Youl S. F Quantitative borehole logging of manganese ore by prompt neutrongamma and neutron activation methods. SPWLA 25th An.Log.Symp. New Orleans,p D 11 Borsaru M. and Ceravolo C A low activity spectrometric gamma-gamma borehole logging tool for the coal industry. Nucl. Geophys. 8, pp Charbucinsi J., 1993b, The "ZERO PROBE"- low radioactivity borehole logging tool, Trans. IEEE, Nucl. Sci. Symp. San Francisco, California. 13 Borsaru M., Ceravolo C, Waddington P. and Wenhao Gu A coal face ash analyser based on natural gamma-ray activity. Nucl. Geophys. 6, pp Borsaru M., Ceravolo C, Carson G. and Tchen T. Low radioactivity coal face ash analyser. To be published, Int.J.Appl Rad. Isot. 15 Borsaru M., Ceravolo C. and Tchen T The application of the low activity borehole logging tool to the iron ore mining industry. Nucl. Geophys, 9, pp Almasoumi A., Borsaru M. and Charbucinsi J. Determination of the lead concentration of Pb-Zn ores in laboratory boreholes using gamma-gamma techniques with very low activity sources. To be published, Int. J. Appl. Rad. and Isot. 17 Charbucinsi J., Borsaru M. and Gladwin M. Ultra-low radiation intensity spectrometric probe for orebody delineation and grade control of Pb-Zn ore. To be published