Remote sensing applications aid mineral exploration

Geologists have long relied on aerial photographs to supplement their surveys. Now they are able to integrate remote sensing data with geoscience information in the search for mineral deposits.

This was the message presented at a workshop on remote sensing applications in geosciences held in Toronto by the Geological Survey of Canada (GSC). Remote sensing is a method of analysing images of surface features without direct contact. The images are constructed from radar, thermal or visible light data collected by satellites in orbit around the Earth. In Canada, most remote sensing data come from the American LANDSAT and French SPOT satellites. Data are also available from airborne and satellite radar systems.

The first of five LANDSAT satellites was launched by the U.S. in 1972; a sixth is to be launched later this year. Each satellite is designed to travel in a circular near-polar orbit, passing over the daytime side of the earth several times a day. The orbits shift progressively westward so that the entire surface of the Earth can be imaged in 16 days.

LANDSAT data are collected by two sensors, the Multispectral Scanner (MSS) and the Thematic Mapper (TM). These sensors capture light reflected from the green and red, and from the near, shortwave and thermal infrared portions of the electromagnetic spectrum.

MSS images cover an 185×185-km sector of the Earth’s surface and provide useful information on regional vegetation, glacial land forms and aggregate resources. TM images are similar in size to those of MSS, but only in the former can features as small as 30 metres be delineated. TM imagery is useful in defining associations between vegetation and land forms. It can even be used to detect hydrothermally altered rocks associated with mineral deposits. SPOT data are similar to those of LANDSAT except that they have a higher resolution (up to 20 metres) and can provide stereo images.

The Canadian Centre for Remote Sensing (CCRS) collects these data at its ground stations in Gatineau, Que., and Prince Albert, Sask. Data are available for almost any area in the world, both in digital and photographic form. Digital data are available on large magnetic tapes, floppy disk and compact disk (CD-ROM). Prices for digital data start at about $1,000. Image maps are available for around $3,500.

Images can be analyzed on a desktop computer. System requirements vary, but Andrew Rencz, a research scientist with the remote sensing branch of the GSC, said images can be processed on a portable computer with a 386 DX/33 processor and a 100-megabyte hard disk. However, getting a hard copy of the data (a plotted map) can be expensive, he warned.

When it comes to analyzing satellite data, “no digital image-processing technique is perfect,”said Jeffrey Harris, a physical scientist with the GSC. He stressed that the successful application of remote sensing depends on the nature of the geological problem, as well as on the geological environment and the type of remotely sensed data.

According to Harris, the amount of structural and lithological data that can be extracted from LANDSAT is a function of the geological environment. The younger the environment, the more information can be extracted. Because of their geological complexities, Canadian Precambrian terrains are more difficult to interpret then recent terrains in places such as Thailand. During the latter part of the workshop, participants were told how remotely sensed data are being used in numerous geoscience applications. Bruce Ballantyne, who is with the mineral resources division of the GSC, described how LANDSAT data was used to help map the alteration and geology of the Sulphurets area of central British Columbia. He said satellite data can enable exploration companies to decide which targets are worth exploring in remote areas.

In another vein, Rencz discussed prospecting in glacial terrains using geobotany, biogeochemistry and remote sensing. Alteration and mineralization hidden under glacial drift can be reflected in vegetation stress, he said. Rencz added that remotely sensed data were used to study the regional structure of Ontario’s Grenville province.

Recent advances in the development of imagery software allow direct comparison and integration of geophysical, geological and geochemical data with images taken by satellites.

The course concluded with a series of hands-on displays involving computer analysis and processing of satellite images.