Session Keynote: Geological spectral sensing: Research driven or market pulled ? - The example of Thermal Infrared
Geological spectral sensing in the thermal infrared region (7-14 mm) has evolved over the last forty years from initially characterising the spectral behaviour of basic mineral types, to underpinning space applications for the Moon and Mars. Later breakthroughs in engineering and physics during the 80s and 90s facilitated remote TIR sensing. Technological developments more recently have seen the commencement of its routine application for the geological community, especially in resource exploration. TIR sensing technologies, involving satellite and airborne sensors, as well as for drill core logging systems, now complement mineral information provided by operational systems at shorter wavelengths.
Mapping silicates is in theory important for mineral exploration as a range of deposit styles have diagnostic non-OH bearing minerals that can be used as vectors towards potentially economic targets. These include using plagioclase composition (albite) to map porphyry Cu alteration systems; garnet Fe-Al composition to define Cu-skarn systems and pyroxene mineralogy to map Ni and PGE mineralisation in layered ultramafic complexes. However, developments in the TIR have lagged behind those in the Visible-Shortwave Infrared (SWIR), in part because of more challenging engineering and physics requirements and associated costs. This compared with the perceived simplicity of chasing hydroxyl minerals, associated with hydrothermal alteration, using the SWIR.
In the relatively slow march towards establishing TIR remote sensing systems, there have been some benchmark systems. These included NASA's Thermal Infrared Multi-spectral Scanner (TIMS) in 1983 which provided fully calibrated airborne multispectral imaging. This was followed soon after by the Geoscan multispectral VNIR-SWIR-TIR airborne imaging system, built by CSIRO and funded by private industry (Car Boyd Minerals). The first geological trials, in the early 90's, of CSIRO's active hyperspectral CO2 laser airborne profiling system, called MIRACO2LAS (Mid-infrared Airborne CO2 Laser Spectrometer), demonstrated that silicate mineral chemistry can be remotely measured with hyperspectral TIR data. MIRACO2LAS was followed by the development of the passive hyperspectral TIPS (Thermal Infrared Profiling System), designed to co-fly with airborne geophysics in line profile mode. TIPS was a collaborative effort between CSIRO and World Geoscience, designed to make spectral data at all wavelengths routinely available with airborne exploration geophysical data.
The development of the liquid Helium cooled passive hyperspectral TIR airborne imaging system, SEBASS (Spatially Enhanced Broadband Array Spectrograph System), by the Aerospace Corporation for military applications, also underwent geological trials in the late 90s. This system yielded laboratory resolution TIR spectral data that enabled measurement of even subtle mineral signatures related to plagioclase composition.
At ground (mine) level, the main tool for measuring TIR signatures has been the microFTIR designed and built by Designs & Prototypes in the US. CSIRO has also used this FTIR technology to build its TIRLogger, which measures drill core and is currently being assessed as part of a current industry and government funded research project.
The biggest recent impact of TIR sensing on the geoscience community, especially the exploration industry, is arguably the satellite-based ASTER system. ASTER was launched in 1999 as part of a joint Japanese ERSDAC and NASA project, and has been providing multispectral VNIR-SWIR-TIR imagery for the most of the Earth's surface at low cost. ASTER's five TIR bands at 90 m pixel resolution are providing broad mineralogical information, including SiO2 content.
New remote TIR sensors about to be released include the airborne TIR system, ARES, built by Australia's Integrated Spectronics, for German Aerospace. There are also plans to build satellite-based TIR systems with >20 spectral bands, although funding remains an issue.
Thus, the TIR story is one of forty years of evolution from government funded fundamental research on the spectroscopy of minerals through to engineering developments in field and airborne systems. There have been several collaborative ventures between the private and public sectors, although space initiatives are typically government funded. It is reasonable to say though, that both the value of TIR mineral sensing and operational technologies have still not been realised by the geoscience industry. To achieve this may take a "Eureka" discovery based on this technology.