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Hyperspectral Imaging

By [email protected] - 11th April 2011 - 23:14

Many animals and insects have the ability to see unique parts of light (the electromagnetic spectrum). In comparison, humans have much weaker sensors which are only able to see the visible portion of light. In order to see in other parts of the electromagnetic spectrum, man had to develop technology in the form of colour lenses and multispectral cameras â the latter only being available since 1962. Ironically, what some animal and insect species have mastered for millions of years, man has only just grasped.
In order to understand the concept of hyperspectral imaging, one needs to first understand the Electromagnetic Spectrum (ES). The ES is the range of all possible frequencies of electromagnetic radiation. It extends from low frequencies (radio waves and microwaves which have long wavelengths) through moderate frequencies (infrared and visible light) to high frequencies (Ultraviolet, X-ray and Gamma rays).Everything on the surface of the earth - be it a type of plant, soil, rock or mineral - absorbs and reflects light differently in many parts of the electromagnetic spectrum. Broadly speaking, when sunlight strikes a leaf, light with the longest wavelength (red) and shortest wavelength (blue) is absorbed by the green matter (chlorophyll) in the leaves. In between these two colours there is green light which is reflected by the leaf, hence it has a green appearance to the human eye.More specifically, if we measure what happens to light that strikes a leaf or a rock or mineral at discrete portions of the spectrum, we can see its spectral signature. In other words, we can see how it absorbs and reflects light at each point along the electromagnetic spectrum. For each feature, the curve that represents how it absorbs and reflects light is called its spectral signature. The spectral signature of any given object is unique and is like a signature or fingerprint. Simply put, hyperspectral imaging is the science of measuring these signatures.Scientists measure signatures of objects on the surface of the earth using a sophisticated instrument called an ASD. It is essentially a light meter that captures the reflected light from any feature on the surface of the earth. Now imagine if we had an instrument like an ASD that could take hundreds of pictures of every square meter of an area at nanometre intervals mounted on a plane. We could effectively search for signatures within the resultant imagery and detect specific minerals, rocks, soil types, plants and other earth matter automatically.Hyperspectral Imaging is one of the services provided by Southern Mapping. The hi-tech hyperspectral camera used by the company captures over 360 simultaneous images at up to 1 nanometer intervals between 450 nanometres (blue light) and 2500nm, which represents the short wave infra red portion of the spectrum (invisible to the human eye).Southern Mapping has submitted several joint research proposals to the National Research Foundation, in order to allow local skills development at universities with a strong research focus on hyperspectral technology (including Limpopo, Fort Hare, UKZN and Stellenbosch). A detailed hyperspectral course to supplement post graduate training at local universities using innovative tele-broadcast via satellite is also planned. In this way the company hopes hope to nurture the local capacity we will no doubt need as the technology continues to penetrate the main stream.Mapping with Hyperspectral ImageryWhereas hyperspectral mapping can directly detect and measure a variety of materials and chemical phenomenon (through their unique spectral properties), others can only be inferred based on correlation. Clay minerals, for example, are directly identifiable through their characteristic absorption of light at exactly 2200µm. But Copper (Cu), Gold (Au) and Silver (Ag) have no recognisable, unique, spectral features within the reflective spectral range of VNIR and SWIR imagers (0.4 â 2.5 nm). Despite this fact, hyperspectral surveys are an increasingly dominant means for guiding exploration for these precious metals. The exploration for these targets is indirect: either secondary minerals are mapped, which are known to associate with these metals or the overall spatial distribution of certain target minerals can be used to highlight probable exploration areas. In this same manner, hyperspectral data has successfully been used to address acid mine drainage and leach pad characterisations through associated precipitate markers.Mineral exploration is the dominant application for hyperspectral imagery. While detailed maps of minerals and associated clay types assist in the identification of ore bodies, the acquisition of the imagery automatically allows for the creation of a baseline environmental audit database as well. So where the data is used to identify where to mine, it doubles up and provides the means to inventory the exact environmental status of the area down to plant species level. In this way, mines are able to comply with mine closure legislation which dictates that the environment needs to be returned to its original state. Hyperspectral imaging is also able to provide extremely accurate direct measurements of chlorophyll a and b concentrations in water and is an efficient tool for quantifying water turbidity.With the technology, hyperspectral images of drill cores can also be captured with the airborne camera mounted on a modified conveyor. This allows quantifiable digital maps of mineralisations along the length of the drill core. As anyone who has been involved in the process of collecting, analysing, transporting and storing drill cores will know, the process is arduous and expensive with no room for error.Hyperspectral imaging of drill cores allows for the simplification of this process with digital storage and the ability to pull the drill core into a 3-dimensional modelling environment where the surface mineralisations can be analysed at the same time.To give an indication of the technologyâs current global status, consider the fact that by the first quarter of 2009 over 60 mines spanning 12 European countries had utilised airborne hyperspectral technology. Moreover, hyperspectral imaging has been singled out as the most efficient means for policing the EUâs mining waste directive. Apart from Europe, the USA, Canada, Greenland, Australia, Brazil and Chile have actively adopted the technology for mineral exploration and mine related environmental monitoring.The addition of hyperspectral imaging to the Southern Mapping portfolio of products, which include multispectral satellite imagery and airborne LIDAR, allows them to provide end to end solutions to the mining, infrastructure and environmental sectors. By fusing hyperspectral image reflectance values into the intensity signal of their LIDAR points, Southern Mapping will be able to continue pushing the limits of technology and provide robust and innovative products.Southern Mapping also recognises that their investments in the development of hyperspectral imaging solutions in Africa will lay a foundation for the technologyâs ultimate place in space.For enquiries please contact Southern Mapping ([email protected]).About Southern Mapping CompanySMC provides topographic surveys and mapping to assist a variety of industries and sectors. These include civil engineering and infrastructure development, mineral explorations and mine management, environmental planning and rehabilitation, and urban and agricultural planning. The company operates worldwide, but specialises throughout Africa. SMCâs staff were the first in the world to combine Lidar with digital aerial photography and now have added hyperspectral to their product offering.

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