NEW HORIZONS — Radars a lofty tool

Radar imaging is a powerful tool for mineral exploration, though, like any tool, it has its limitations.

The Landsat MSS satellite, for example, was originally promoted as a panacea for exploration companies, but users soon learned (usually after spending substantial amounts of money for its data) that this was not the case. As a result, this new science was greeted with a great deal of skepticism, some of which continues to linger.

Considering the incredible amount of ground surface information achievable with electro-optical imaging scanners (systems that require the light of the sun, such as Landsat TM and SPOT), the use of imaging radar may appear unnecessary.

Radar’s biggest selling point is its ability to penetrate cloud cover and rain such that images of earth are always available. This technology is especially useful in tropical regions that are covered by near-permanent cloud for most of the year. Acquiring cloud-free spectral imagery from such regions using electro-optical systems such as Landsat has proved to be almost impossible.

Radars are active systems, meaning they provide their own illumination of the earth. Data, therefore, can be acquired day or night. Such a system can be advantageous in polar regions, for example, where darkness prevails for several months of the year. The second biggest reason for obtaining radar data is that radars are sensitive to variations in topographic relief and are excellent at defining structure.

Radar imaging of the earth is not new. The first imaging radars were placed in aircraft during the latter years of the Second World War, and the first airborne commercial radar contract was flown in 1969.

The first satellite imaging radar, known as Seasat, was launched in 1978 by the National Aeronautics and Space Administration. Since then, similar satellites have been launched by the U.S., the former Soviet Union, Germany, the European space agency (ERS-1 and ERS-2), Japan (JERS-1) and Canada (Radarsat). Only Radarsat (which was launched in 1995 and is operated by a private Canadian enterprise), ERS-1, ERS-2 and JERS-1 remain operational.

Together, they provide near-global coverage.

Not all radars are alike, however. Each radar transmitter has its own particular system, including wavelength (or frequency), polarity, incidence angle and system noise. The most important of these are wavelength and incidence angle. The Radarsat, ERS-1 and ERS-2 systems transmit with a 5.6-cm wavelength known as “C-band,” whereas the JERS-1 system transmits a 23.5-cm wavelength known as “L-band.” The data from these wavelengths differ in that C-band systems do not penetrate vegetation canopy cover whereas L-band systems do. The topographical and structural information apparent in C-band imagery is actually vegetation that follows surface contours.

The incidence angle that is made when the radar beam intersects the earth’s surface produces distortion artifacts when information from the return beam is converted into a 2-dimensional image. These artifacts are called “shadow,” “foreshortening” and “layover.”

Shadow is actually a beneficial side-effect as it enhances topographical and structural information. However, foreshortening and, particularly layover, can produce strange geometric distortions of surface features. For example, in areas of extreme relief, such as mountainous terrain, the mountains appear to lean toward the radar on the image.

Shadow, foreshortening and layover effects can be altered if the radar transmitter has a selectable incidence beam, such as the one on the Radarsat satellite. In addition, variable incident angles mean that, usually, stereo imaging is possible and, thus, topographic information can be retrieved. If layover effects on an image are severe, geo-rectification of the data becomes a complex task requiring sophisticated software.

A disadvantage that satellite radar has over airborne radar is that the former is restricted to “looking” at the earth in the eastern and western directions, perpendicular to its orbital path. Radars enhance structural features normal to the so-called “look” direction, meaning that any lineaments oriented near parallel to that direction will not be identified.

That problem can be avoided by employing an airborne survey since flight paths can be customized to provide maximum information.

Since images can be acquired day or night and in any weather, imaging radar is a tremendous aid in the exploration of tropical environments. In addition, structural information is usually spectacular, and the radar provides a broad, synoptic view of the earth for a relatively inexpensive price.

Radars, however, are not a “mine-finding tool.” Rather, they are simply one more device used by any good explorationist to help locate an orebody.

— The author is senior geologist and head of remote sensing for BHP Minerals Canada.