Arti cial illumination
To provided the most accurate measurements of re ectance, a eld spectrometer (Analytical Spectral Devices, Boulder, Co; ASD) was used to acquire measurements of re ectance (350-2500 nm). Spectra were acquired of homogeneous parts of the core surface, identi ed by visual inspection. The core surface was generally clean, smooth and free of debris, bio lm and dust. The cores were illuminated using a halogen lamp which was placed approximately 30 cm away from the surface. Incident light
illuminated the core surface at an angle of approximately 45◦ away from normal. The
ASD spectrometer was tted with an eight degree foreoptic placed in front of the
optical bre. The spectra were acquired from areas each of approximately 3.5 cm2.
4.2 West Angelas spectral libraries 66 Table 4.1 List of the eight principle rock types (classes) in a WA mine. The last
column indicates if the rock type is mineralised.
WA rock types Description M.
Banded Iron Formation Dark-red. Hard. High silica . No
(BIF)
Chert or Cherty BIF Hard. Very high silica content. No
(CHT) Light, creamy colour .
Martite (Hematite) Soft (very fragile) to hard texture. Yes
(MAR) Dark brown, gray colour. High iron content.
Clay Soft. White to pink colour. No
(CLY) Little to no iron, high kaolin content.
Goethite-Limonite Ochreous goethite. Chalky in appearance. Yes
(GOL) Yellow colour. Medium to high iron content.
WA Shale Soft. Pink to yellow colour. No
(SHL) Little iron content with clay content. Soft.
Shale Pink to yellow or cream colour. Kaolinitic. No
(NS3+4) Volcanic shale. Used as marker horizon
Manganiferous Shale Shale with abundant manganese. No
(SHN) Soft and light. Dark brown to black.
Kaolinitic with veins of pyrolusite
placed perpendicular above the rock surface at an constant height of 15 cm (Figure 4.1).
Care was taken not to acquire spectra of the shaded anks of the cores. A single spectrum that was recorded from an area was itself an average of 30 individual spectra, acquired within a few seconds by the spectrometer. The spectral averaging was done to increase the signal-to-noise ratio of the spectrum. A white reference was taken immediately prior to acquiring each target spectrum. The white reference was a 100 % Spectralon panel (Labsphere, North Sutton, NH, UK) with known re ectance. Data were calibrate to absolute re ectance by dividing each spectrum by the spectrum from the re ectance panel and multiplying the result by the panel calibration factor, provided by its manufacturer.
4.2 West Angelas spectral libraries 67
Rock tray
Stand
Light
source
Viewed
area
Fibre w.
foreoptic
(a)Rock tray
normal
Fibre w.
foreoptic
Viewed area
(b) (c)Figure 4.1 Setup for acquisition of spectra from cores of rock in the laboratory using arti cial illumination. Setup for the acquisition of spectra under laboratory conditions (a and b). (c) Example of a tray containing cores of rock. Red ellipsoids indicate dierent rock types; blue ellipsoids indicate dierent regions of a rock type. Green circles indicate positions from where replicate spectra were acquired. The area covered by the blue ellipsoid was removed for sample analysis using XRF and/or XRD.
4.2 West Angelas spectral libraries 68
CLY
Figure 4.2 Overview of the WAcoreLib1 core library comprised of eight rock types. Axis labels are omitted for clarity. The abscissa is in units of wavelengths (µm) and the ordinate is in re ectance. All spectra of each rock type are shown in each panel including the mean (black).
4.2 West Angelas spectral libraries 69
CLY
Figure 4.3 Overview of the WAcoreLibVal core library, comprised of eight rock types. Axis labels are omitted for clarity. The abscissa is in units of wavelengths (µm) and the ordinate is in re ectance. All spectra of each rock type are shown in each panel including the mean (black).
4.2 West Angelas spectral libraries 70 eter was tted with a re ectance probe with a high-intensity, integrated light-source. The probe measuring window was 2 cm in diameter. Acquisition was done similar to the acquisition of the WAcoreLib1, however, for measurement of the reference and target spectrum, the probe was placed into direct contact with the calibration panel and the target. The spectrometer was con gured to average forty spectra to pro- duce a single recorded spectrum. Between each measurement, the quartz window of the probe was cleaned using lens-wipes. Each spectrum was processed to absolute re ectance using the same method as above.
Natural illumination
There are several important dierences between spectra acquired under arti cial light and spectra acquired under natural light. The intensity of natural light reaching the Earth at some wavelengths around is reduced by gases and aerosols in the atmosphere, thus reducing quantities of light in these regions leading to small signal-to-noise ra- tios at aected wavelengths. In the spectral region between 0.35 and 2.5 µm, there are two major water absorption bands centred around 1.4 and 1.9 µm as well as two
minor absorption bands occurring near 0.96 and 1.1 µm (Irons et al., 1989)1. Wave-
lengths around the two major water absorptions, i.e. around 1.4 and 1.9 µm), need to be removed. The water feature around 1.4 µm, however, contains diagnostic infor- mation about clay minerals which is lost as a consequence of natural light. Further, wavelength-dependent scattering causes shifts in the wavelength-intensity distribution of the indirect light which can cause apparent changes in colour of rocks.
The objective was to obtain spectral data acquired under natural illumination as well as to modify conditions of measurement by acquiring spectra from oblique viewing angles. This was done to introduce variability in the spectra due to naturally changing illumination conditions such as sun angle and elevation and to introduce variability due to dierent viewing angles. The latter was done to simulate conditions on a mine face where a hyperspectral sensor is placed in the middle of the mine face but acquires
1Another major water absorption band occurs just outside the the used wavelength region at
4.2 West Angelas spectral libraries 71 data from a range of angles of the mine face.
To use the sun as a light source, a clear (mist free / particle free) and cloudless sky is preferable. Six spectra of each rock sample were acquired: two spectra normal to
the rock surface; two spectra oblique under an angle of around 15◦ and two spectra
oblique from the other side, i.e. −15◦ (Figure 4.4). Spectra acquired under natural
light were measured within two hours of local noon (solar azimuth of 34.69◦− 321.87◦
and solar elevation of 34.08◦− 35.79◦).
Figure 4.4 Setup for acquisition of spectra from cores of rock using natural illumi- nation.