Underground » Geology
A mineralogical core logging system based on spectral reflectance (HyLogger™) has been used to detect and quantify mineralogies in coal and coal measure sediments. The HyLogger™ system, as tested, operates in the visible-to-shortwave infrared spectral region, where iron oxides, sulphates, hydroxyl-bearing and carbonate minerals have characteristic spectral responses. Specialized software assists with mineral identification and data display. Three Phases of activity were undertaken.
In Phase I, carbonates (siderite, ankerite, calcite) and clays (halloysite, dickite) were successfully detected and mapped in coal. Repeat measurements taken from one of the cores after three months demonstrated the reproducibility of the spectral approach, with some spectral differences being attributed to variations in moisture content and oxidation. Also, investigated was HyLogger™’s ability to create a “brightness-profile” on coal materials, and these results were encouraging.
In Phase II, geotechnically significant smectitic clays (montmorillonite) were detected and mapped in cores of clastic roof and floor materials. Such knowledge would be useful for mine planning and design purposes.
In Phase III, our attempts at determining whether phosphorus-bearing minerals such as apatite could be spectrally detected were less than conclusive. A spectral index could only be created for apatite, and the relationships between the spectrally-derived apatite-index, the XRD results and the analytically-derived phosphorus measurements were ambiguous.
A data mining technique based on self organizing maps was used to assign the spectral results into meaningful “packages” and relate them to lithology. Despite the coal ash responses being marginally above the level of the noise, the spatial coherence of the spectra suggests meaningful results. The higher ash, dull coals tended to show more consistent spectral mineralogical assignments than bright coal. Other encouraging results include the recognition of clay alteration due either to weathering or thermal maturation; and the detection of variations in moisture content (though all estimates were qualitative). Differences between the XRD and spectral reflectance mineralogical assignments can be explained in terms of the degree of mineral ordering in the materials being measured.
The HyLogger™ is a means to characterize the mineralogy of a sedimentary package of rocks providing the required minerals are detectable by spectral methods. Minor modifications to the illumination configuration would optimize the system’s ability to collect a coal brightness profile. And particular minerals of interest (e.g., apatite and goyazite) should be added if possible to the automated spectral interpretation software for coal applications. The detection of mineral matter in coal using spectral reflectance techniques will continue to be a challenge; however, when the thermal infrared HyLogger™ module becomes available, quartz, feldspar and other silicates should be detectable.