DE BEERS AUSTRALIA EXPLORATION HYPERSPECTRAL BUSINESS UNIT

HyMap MK1 AND THE HYPERSPECTRAL SURVEY PIPE LINE

De Beers has utilised reflectance spectroscopy in its mineral exploration programs since the mid 1980's. This resulted in De Beers commissioning the first of the HyMap series of hyperspectral scanners in 1994 and this instrument has been used operationally since 1996. Since 1996 over 300,000 km2 has been surveyed world-wide with some individual surveys exceeding 60,000 km2. This has resulted in the De Beers HBU building an unrivalled system for the acquisition, processing, interpretation and delivery of map-quality image products. The experience and software available to De Beers ensures that delivery of map products, customised to client requirements, is achieved shortly after data acquisition has been completed. Where specialist interpretation knowledge is required, the De Beers team works with specialists from a variety of application areas. Surveys have been conducted using the De Beers system for applications in mineral exploration, geological mapping, vegetation assessment, environmental base-line and contamination studies.

TARGET AND AREA DEFINITION

DATA ACQUISITION DATA PROCESSING SYSTEM CORRECTION

DATA PROCESSINGDATA REDUCTION

DATA PROCESSINGTHEMATIC MAP PRODUCTION

The data is reduced to endmember un-mixed images that map the distribution of various minerals. In the example shown, nine endmembers were recognised from the SWIR2 data (above centre). Un-mixed index images have been produced showing the distribution of these minerals as red pixels on a background image. Shown above are the index images for:

Fe Chlorite Mg Chlorite Pyrophyllite

Kaolinite Endmember Spectra Sericite

Endmember images can be combined to facilitate geological mapping. In the example below, the un-mixed endmembers images for Fe oxide, phengite and nontronite (above) have been overlain onto a background image in red, green and blue respectively. Where mineral occurrences overlap, complementary colours occur, eg. Fe oxide and phengite overlap areas are yellow.

The HyMap MKI is an airborne hyperspectral scanner designed and optimised for the detection and mapping of absorption features that identify differences in minerals, rocks, soils, vegetation and other materials on the earth's surface. The system is flown in a light aircraft (above) at 2,500m to 5,000m A.G.L. and can collect data from up to 2,200 km2 per survey day.

The system (cross-section below) consists of three spectrometers mounted on an optical bench beneath which is a two facet axe head mirror assembly. As the mirrors rotate, light from each pixel is focused onto the detectors via mirrors, a collimator, lenses, and diffraction gratings that distribute the light across the detector arrays. Each detector has 32 elements resulting in the generation of 32 images per spectrometer at 15nm intervals.

TECHNICAL SPECIFICATIONS HyMap MK1 AND FAQ

IFOV: 2.5 mr along track, 2.0 mr across track

FOV: 61.3o (512 pixels)

GIFOV: 5m-10m (at flying heights 2,500m to 5,000 AGL)

SPECTRAL BANDS: 96 - BAND WIDTH: 15nm

VNIR: ~530-1,010nm SWIR1:~1400-1995nm SWIR2:~2,014-2,488nm

SPECTRAL RESOLUTION - ~15nm

SNR: > 500:1 ( 50% reflector at noon)

DATA COLLECTION RATE: 3 GIGABYTE PER HOUR

DAILY COVER: 1,200km2- 2,500km2 (5m - 10m pixels)

GEOMETRIC CORRECTION: - +/- 20m (C-MIGITS IMU/GPS)

HyMap MK1 SPECTRAL COVER VERSUS HyMapTM (PROBE 1 & 2)

SURVEY PLANNING

If the target sought has a distinct spectral signature, then a hyperspectral survey is the most efficient method to locate and map the target within an area. If the target’s spectra cannot be located in existing spectral libraries, then field and laboratory spectrometer studies may be required to determine that the target has a diagnostic spectral signature.

Hyperspectral surveys require cloud-free conditions (preferably when the sun’s azimuth is above 40o). This limits surveys to a period up to 2 hours either side of solar noon, depending on latitude and time of year.

Surface moisture severely reduces the reflectance of SWIR light, so the surface should be dry (soil moisture < 20%).

Geological and other mapping applications are restricted if vegetation or other materials obscure, on average, more than 80% of the surface.

If the above conditions are met, the survey is planned. Flight lines are planned (above) with a 15% overlap between strips. To reduce hot spot effects and cross track shading, they are usually orientated north-south, i.e. into and away from the sun.

Surveys are conducted using chartered unpressurised aircraft (such as a Cessna 404) that have been equipped with a standard aerial camera port and a GPS flight line navigation system.

DATA PROCESSING - SYSTEM CORRECTION

The system is calibrated before and after each survey. In this process the position of the band centres wavelength are determined to an accuracy of +/-2nm. Gain factors are also determined by measuring the spectra of a standard laboratory lamp and calculating the difference between system and standard lamp spectra.

Once the data has been collected, it has to be corrected and converted into reflectance data. This is a two-stage process and its effects on the data are illustrated in the spectra above. The top spectrum above is from raw uncorrected data (target vegetation) as collected by the system.

In the first processing stage, this raw data is converted into radiance data, centre spectrum above. Five corrections can be applied to the data at this stage. Firstly, the dark current values, calculated from readings recorded for alternate scan lines, are subtracted from the data. Secondly, each pixel’s band value is multiplied by the gain factor determined from the calibration of the system. The values are then in units of milliwatts per cm2 per nanometer per steradian (uW/cm2/nm/sr)*1000. At this stage in the processing, the data may also have a tan theta correction applied to compensate for scan line offset and be rotated so every image strip has an origin (pixel 1, line 1) in the NW corner. A cross track illumination correction can also be applied during this processing stage.

The final processing stage is to convert the data to reflectance (bottom spectra - green vegetation). This is achieved by subtracting modeled atmospheric spectra from the radiance spectra for each pixel. The model spectra are calculated to account for: the latitude, longitude, UTC time, aircraft altitude and terrain height as well as a water vapour value obtained from the data using the intensity of absorption at 940nm. This is achieved using a program called ACORN, which is part of the ENVI processing software.

Once the data has been converted to reflectance, further processing to map the distribution of the target spectra can be carried out. This is achieved using a combination of commercially available software (ENVI) and De Beers’ proprietary software, developed by Quantum Mineral Exploration in South Africa.

DATA PROCESSING - ENDMEMBER UN-MIXING

Endmember un-mixing is the procedure whereby the reflectance image is processed to determine the component spectra that account for the spectral variation in the image. Ten to fifteen endmember spectra may be distinguished from a 32 band image (each spectrometer’s data is processed separately). These spectra are stored in a library and then the spectrum from each pixel is compared to these library spectra by the process of un-mixing. This then creates an image with bands corresponding to the library spectra. Each pixel’s gray level value in the un-mixed image is a measure of its match to its endmember spectra. Subsequently the un-mixed images are converted to index images or colour composites that highlight the targets of interest. There are several algorithms for endmember selection and un-mixing and De Beers uses proprietary software for these processes and index image generation.

GEOMETRIC CORRECTION AND MAP PRODUCTION

When imagery is acquired, it is distorted by the movement of the aircraft and changes in velocity (top left). These distortions have to be removed and this is done by one of two processes:

1) Control Point Warping. This requires locating features in the image and finding their coordinates from a map. Using these coordinates the image is warped to the map projection, resulting in an image map with an accuracy of +/- 100m. This is the process required if no GPS and/or C-MIGITS IMU/GPS data is available. It can take several hours to correct one flight line image using this technique.

2) Automated Ray Tracing. Using data from the C-MIGITS IMU/GPS motion detection unit to give the exact location of the nadir pixel, ray tracing can be used to reconstruct the image with the correct geometry as a rectified map. If this process is combined with data from a Digital Elevation Model (DEM), the accuracy will be +/- 20m, whereas without a DEM, accuracy may be between 30m and 50m. This correction can be carried out in under 5 minutes for a standard flight image.

Normally the system operates when mounted in a Zeiss aerial camera stabilised platform which minimises the amount of distortion in the imagery by compensating for aircraft roll, pitch and yaw. The top left image was collected without the platform operating so that the distortions are exaggerated. This image was automatically corrected using data from C-MIGITS IMU/GPS (top right). This demonstrates the precision of correction achievable with this system.

Once the index or colour composite images have been produced and geometrically corrected, they can be combined into mosaics, have location grids added and be produced as image maps (bottom - index image mosaic). This process is carried out using ENVI or ER Mapper software.

For information and quotes for surveys using the HyMap MKI contact:

Dr Michael C Hussey

Tel: +61 8 9378 0024 or +61 414 648 661

Fax: +61 8 9378 0020

Email: Michael.Hussey@debeersaustralia.com

The USGS has produced a mineral spectral library in which there are over 200 spectra, not only of rocks and minerals, but also of differing green and dry vegetation, including leaves and woody material. These spectra are high- resolution laboratory spectra with a spectral resolution of ~3nm. A selection of some common minerals is shown (above left). It is possible to re-sample the spectra to the 16nm band width of the HyMap MKI. This has been done to the spectra above right. Only the wavelength region from 2.0um (2,000nm) to 2.5um (2,500nm) is shown, but these re-sampled spectra preserve all of the character of the higher resolution spectra over the same wavelength range to the right of the red line. This is equally true for shorter wavelength regions. This indicates that HyMap MKI imagery can be used to identify materials that have diagnostic spectra in the 530nm to 2,500nm wavelength range.

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