REMOTE SENSING
Airborne CO2 laser system maps mineral deposits
Paul Mortensen
A team of Australian scientists working at the Commonwealth Scientific and Industrial Research Organization (CSIRO, Sydney) has developed an airborne, laser-based remote-sensing system for mineral mapping. The CO2 based device tunes every 2 ms to more than 100 different frequencies in the wavelength band between 9.1 and 11.2 µm. The system can discriminate between silicate and carbonate minerals, the surface distribution of which can indicate hidden bodies of mineral ore.
According to Lew Whitbourn, senior principal research scientist at CSIRO`s division of exploration and mining, geological remote sensing in the thermal-IR region has typically relied on passive systems such as NASA`s thermal-IR multispectral scanner (TIMS) and Geoscan, which measure the radiation emitted from the surface of the earth. Variations in the intensity of the emitted radiation, however, are dominated by surface-temperature effects, caused by topographical changes and the bulk thermal properties of surface materials, which complicate separation of the desired spectral emissions.
"TIMS gives full images but not enough spectral information to discriminate the minerals," explains Whitbourn. Active remote sensing overcomes this problem because it directly measures the surface reflectance. In the 9- to 11.2-µm wavelength window for high-spectral-resolution mineral mapping--especially important in the deeply weathered Australian environment--the laser system measures 100 wavelengths, while TIMS measures just six.
System size a challenge
The CO2 laser was seen as a suitable mid-IR source because it can span a 20% bandwidth between 9 and 11 µm at high-power levels. To make a practical airborne system, however, CSIRO scientists were faced with the challenge of reducing the size of a traditional CO2 laser and its auxiliary equipment and finding a way to tune it through a broad range of wavelengths.
Their solution was a laser with a 3-m glow discharge in a Z-folded resonator. The difficulty caused by wall reflections of the beam was solved by aligning four mirrors. A rotating octagonal mirror sweeps the intracavity beam of the laser across a diffraction grating, producing one burst of laser pulses every 24 ms. Each burst lasts about 2 ms and contains u¥to 100 pulses. Each pulse has a typical duration of 10 µs and a peak power of 100 W and is at one of 100 discrete wavelengths between 9.1 and 11.2 µm.
The system, currently operated from a F-27 Fokker Friendshi¥aircraft, is designed to produce contiguous line profiles of ground reflectance spectra. The 700-kg laser/telescope assembly, measuring 1 ¥ 1 ¥ 2.4 m, sits in the aisle of the aircraft and looks downward through a 300-mm-aperture hole in the floor. The divergence of the 10-mm-diameter laser beam is adjusted slightly by a 50-mm-aperture ZnSe sending telescope to illuminate a 2-m-diameter target at an altitude of 500 m. The weak mid-IR signal reflected back to the aircraft is collected by a 0.3-m-aperture Cassegrain telescope.
Two data streams record the time and intensity of laser pulses from a reference detector and the corresponding reflected ground signal. Raw ground reflectance includes atmospheric, altitude, and instrument effects that are subsequently removed by computer processing. Comparison of airborne and laboratory spectra shows good agreement for a number of minerals, including quartz, clay/feldspars, garnet, talc, dolomite, amphibole and mixtures of these (see figure).
The research team has tested the system over a wide variety of known geological areas. Joint tests with TIMS also have been completed at Broken Hill where the combined system showed promise--TIMS maps mineralogical and structural boundaries while the laser system verifies the mineralogy.
The next ste¥in this research will be development of a compact, passive thermal infrared profiling spectro meter, which will give comparable spectral resolution to that of the laser spectrometer. As a forerunner to this, the CSIRO team is collaborating with an Australian-based geophysical company to build a visible to short-wavelength IR profiling spectrometer. By flying gridded flightlines and recording data from a global-positioning system, the spectrometer will produce remotely sensed mineral maps based on the spectral signatures of rocks and soils.