Remote sensing involves gathering data and information about the physical “world” by detecting and measuring signals composed of radiation, particles, and fields emanating from objects. This data are collected without direct contact with the object and can be used to identify and categorize objects of interest. Remote sensing has a variety of applications. It is used in medical applications, environmental applications- and in mineral exploration. In terms of mineral exploration, remote sensing is a rapidly advancing, and extremely valuable tool. It allows mineral explorers to accurately locate a resource at a reduced cost.
The use of remote sensing in mineral exploration began some 60 years ago with hand-held cameras being pointed out of aircraft windows and has since evolved through stereoscopic aerial photography to sophisticated space age technology, with satellite and airborne multispectral and hyperspectral digital imaging systems.
When the Landsat multispectral scanner (MSS) began operations in the early 1970’s, remote sensing geology took an enormous leap forward but still functioned largely by way of photogeological interpretation of hard copy. Since then, the Landsat Thematic Mapper (TM) instruments have provided the geological user with information relating to specific groups of minerals, specifically the iron oxides and clays. However, the data remains coarse and of general purpose only with low spectral and spatial resolution, requiring sophisticated statistical processing techniques not readily understood by the average geological user.
In the 1980’s, airborne remote sensing began with the development of the Airborne Thematic Mapper by Daedalus, the Geoscan instruments (MKI and MKII) by Australian Carr Boyd Minerals Ltd., the Collins’ imaging spectrometers developed by Geophysical and Environmental Research Corporation of Millbrook, New York and the Airborne Visible InfraRed Imaging Spectrometer (AVIRIS) developed by the Jet Propulsion Laboratory, Pasadena.
All of these instruments offered increased spectral and spatial resolution over their satellite borne predecessors and, the three latter in particular, provided geological users with a means of discriminating and mapping individual mineral species and alteration assemblages. However, it has only been recently, in the 1990’s, that the means of processing this data has become available to the average geological user through the development of low cost, commercial image processing and analysis software for desktop PCs. The rapid growth in computing power and storage capacity in PCs has allowed the very large data files captured by the airborne instruments to be handled in a time frame compatible with the needs of mineral exploration. Furthermore, the data can nowadays be georeferenced, the inherent distortion in the data caused by the operation of such instruments on a relatively unstable airborne platform can be ironed out and the data integrated with other exploration datasets.
In Saudi Arabia, Bob used Landsat TM data in regional mapping and exploration before being involved in the assessment of airborne thematic mapper data to mineral exploration in the Wadi Shajiah district. Ten years ago, he became responsible for the development of mineral exploration programmes on behalf of Carr Boyd Minerals Ltd using data collected by their Geoscan MKI & MKII airborne multispectral scanners.
Fig 1 : Electromagnetic Remote Sensing of the Earth Surface
Satellite imagery is vital for early exploration and is typically one of the first data types acquired by anyone working in a new area. When combined with field mapping and geophysics it becomes a very efficient way to gain geological understanding. Today imagery obtained from a wide variety of satellite systems like Landsat, WorldView, SPOT, RapidEye and EROS, amongst others.
Satellite systems capture the red, green and blue bands of the visible spectrum, they also capture “colours” in near-infrared and shortwave infrared bands, beyond the visible spectrum. Images that include both visible and non-visible bands are known as multispectral images, and multispectral sensors may capture anywhere between four and eleven bands. Geologists get a double benefit from using multispectral images because shortwave infrared bands are sensitive to changes in soil and rock content, making it possible to differentiate some basic rock types.
A disadvantage of capturing data beyond the visible spectrum is that computer monitors only have red, green and blue pixels. They definitely don’t have infrared pixels, and even if they do we can’t see them because they’re invisible to humans. To get around this problem experts assign or map bands from a multispectral image to the red, green and blue pixels of the computer display, producing a false-colour image or false-colour composite. For example, mapping the infrared band to red pixels, the near infrared band to green pixels, and the visible red band to blue pixels.
Remote sensing images are used for Mineral exploration in two applications:
Remote sensing has rapidly advanced in the past few years. In the beginning, the primary use of remotely gathered data was comparative. If gold was found in a particular area, aerial photos of that particular area would be compared with aerial photos from other locales to find places that had similar surface features expecting to cover a valuable gold deposit. Once satellite imagery became commercially available, the same comparison and contrast was used with satellite images. To date, aerial photography is still used as an exploration tool; Aerial photographs are used to identify topographic surface features which may indicate the subsurface geology, such telling surface features as differential erosion, outcropping rock, drainage patterns, and folds/faults can be identified.
Faults, fractures and contacts often provide a conduit or depositional environment for hydrothermal or magmatic fluids in regions of known mineralization, and thus make excellent targets for further investigation. The initiation of multispectral imaging and thematic mapping has allowed surface mapping to be performed remotely, thereby enabling vast areas to be mapped in a short time at a portion of the cost of traditional geologic mapping. Known drilling results can be integrated with topographic maps, air photos, structural maps, and ore grade data, greatly increasing the accuracy and effectiveness of an exploration program.
Detailed Remote mineral data are collected from one of two ways through low-lying aircraft, or from satellites. Two main sensor types are used in remote sensing, optical sensors and synthetic aperture (SAR) sensors. Optical sensors measure the spectral data of sunlight reflected from the Earth’s surface, and SAR sensors sense electromagnetic data by transmitting microwaves and receiving the back scatter waves from the Earth’s surface.
Remote Sensing makes use of spectral signatures. For any given material, the amount of solar radiation that it reflects, absorbs, transmits, or emits varies with wavelength. When the amount coming from the material is plotted over a range of wavelengths, the connected points produce a curve called the material’s spectral signature. All objects have a unique spectral signature- and similar objects share a spectral signature.
Table below, lists characteristics of the principal remote sensing systems that are currently available for mineral exploration, some systems are installed only on satellites (Landsat, SPOT) other systems are currently installed only on aircraft (hyperspectral) systems. Radar systems are installed on both satellites and aircraft. Images acquired by satellite systems have the following advantages:
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