Remote Sensing – Eyes of mining technology
In simple terms, Remote Sensing is a technology that allows us to study the earth and its surface using data from satellites or airplanes, with sensors and cameras on-board. Remote sensing uses reflected solar radiation and reflected thermal radiation from the earth’s surface to map changes in surface materials through variation, seen as different colour patterns on an image through their spectral absorbance. Some minerals, for example, reflect as distinctly strong colours, showing characteristic absorbance features that alter with changes in crystalline and molecular shape and constituent materials like water. These patterns of spectral variations can enable an understanding of geological processes and may provide mineralogical evidences to sites of mineralisation or deposits.
Today one can find wide ranging applications of remote sensing, especially as satellite imagery particularly in environments where geological information is limited, image processing techniques, such as mineral maps, false colour images, band ratios and principal components help identify geochemical signatures.
Popular Remote sensing technologies
While remote sensing in mineral exploration is a rapidly advancing field, currently most imagery is performed through the following technologies:
Landsat
The Landsat program is known to offer the longest continuous global record of the Earth's surface. Till this day, it continues to deliver beautiful yet scientifically valid images of the Earth’s surface. Ladsat1 was first commissioned into orbit in 1972. Many satellites have been launched after that. Presently most in use for exploration is Landsat 7 which was successfully launched on April 15, 1999 from the Western Test Range of Vandenberg Air Force Base, California. The Earth observing instrument on Landsat 7, the Enhanced Thematic Mapper Plus (ETM+),is as capable as its predecessors. The ETM+ also includes additional features that make it a more versatile and efficient instrument for global change studies, land cover monitoring and assessment, and large area mapping than its design forebears. Landsat 9 is Earth observation satellite, tentatively scheduled for launch in December 2020. NASA is in charge of building, launching, and testing the system, while the United States Geological Survey will process, archive, and distribute its data. Scenes from all Landsats are available from the Landsat Archive at the United States Geological Survey (USGS) EarthExplorer website for free. Iron oxides, Phyllosilicates and Carbonates are most easily interpreted with false-colour images.
ASTER
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a Japanese sensor on board the Terra satellite launched into Earth orbit by NASA in December 1999. ASTER data has 14 spectral bands, ranging between 15 and 90 meters, designed specifically to measure diagnostic absorption bands related to mineral groups like clays, carbonates, silica and iron oxides. From these bands the CSIRO’s Centre for 3D Mineral Mapping (C3DMM) has developed a processing methodology to transform the raw data into 17 geoscience products that reflect relative mineral abundance derived from the visible and near infrared (VNIR)-shortwave infrared (SWIR) bands. These are available as the ASTER regional mineral maps series that further provide detailed information on surface temperature of land, emissivity, reflectance, and elevation. In 2009 as a joint initiative of NASA and Japan’s METI, Global Digital Elevation Model (GDEM) was released. GDEM is the most comprehensive mapping of the earth ever made, covering 99% of its surface which also includes a detailed mapping of the polar regions. In 2011, version 2 of GDEM was also released, improvising on accuracy, resolution and importantly the realistic values over water bodies.