Page 133 - Introduction to Mineral Exploration

116 M.K.G. WHATELEY

the basis of favorable geology and structure. more sophisticated than Landsat, having a high
Detailed follow-up photogeology and field resolution visible (HRV) imaging system, with
work then takes place. a 20 m (Spot 5, 10 m) ground resolution in MS
Ferruginous residual deposits (gossans) mode and a 10 m (Spot 5, 5 m) resolution in
which overlie mineralized ground can often be panchromatic mode (Table 6.1). SPOT also has
identified from color anomalies on enhanced an off-nadir viewing capability which means
false color ratio composites, whereas these that an area at 45°N can be imaged 11 times in
may not be detected on standard imagery, e.g. the 26-day orbital cycle. Repeated off-nadir
at the Silver Bell porphyry copper site (Abrams viewing introduces parallax from which stereo-
et al. 1984) and the Cuprite Mining District in scopic image pairs can be produced. The com-
Nevada (Abrams & Hook 2002). mercial nature of SPOT means that images
are slightly more expensive, but the increased
resolution and stereoscopic capability mean
6.2.9 Advantages of Landsat imagery
that the additional cost may be warranted in
1 This imagery provides a synoptic view of an exploration program, although SPOT does
large areas of the Earth’s crust (185 km × not have as good a spectral range as TM.
185 km), revealing structures previously un-
recognized because of their great extent.
2 The evolution of rapidly developing dyn- 6.3.2 Aster
amic geological phenomena can be examined NASA’s satellite, Terra, was launched in
through the use of successive images of the December 1999. Terra is the first of a series
same area produced at 16- to 26-day intervals, of multi-instrument spacecraft that are part of
e.g. delta growth or glaciation. NASA’s Earth Observing System (EOS). The
3 Some geological features are only inter- program comprises a science component and a
mittently visible, e.g. under certain conditions data system supporting a coordinated series
of climate or vegetation cover. Landsat offers of polar-orbiting and low inclination satellites
“revisit capability” and multitemporal cover- for long-term global observations of the land
age (Viljoen et al. 1975). surface, biosphere, solid Earth, atmosphere,
4 Digital images can be displayed in color. and oceans. It houses the Advanced Spaceborne
This is useful in mapping rock types and altera- Thermal Emission and Reflection Radiometer
tion products, either direct from rocks or from (ASTER) sensors. ASTER covers a wide spectral
vegetation changes (Abrams & Hook 2002). region with 14 bands from the visible to the
5 Landsat is valuable in providing a tool for thermal infrared parts of the spectrum with
rapid mapping of regional and local fracture high spatial, spectral, and radiometric resolu-
systems. These systems may have controlled tion (Abrams & Hook 2002). An additional
ore deposit location. backward-looking near-infrared band provides
6 It is very cost effective, with an outlay of stereo coverage. The spatial resolution varies
−2
only a few pence km for map production. The with wavelength (Table 6.1).
Global Land Cover Facility of University of
Maryland (GLCF 2004) offers the largest free
source of Landsat data. 6.3.3 High resolution satellite systems
7 Computer processing enables discrimination The Ikonos-2 satellite was launched in Septem-
and detection of specified rocks or areas. ber 1999 and has been delivering commercial
data since early 2000 (Infoterra 2004). Ikonos
is the first of the next generation of high spatial
6.3 OTHER IMAGING SYSTEMS resolution satellites. Ikonos data records four
channels of multispectral data (0.45–0.53,
0.52–0.61, 0.64–0.72, and 0.77–0.88 µm) with
6.3.1 SPOT
a resolution of 4 m and one panchromatic
The first French satellite, SPOT, was launched channel with 1 m resolution (0.45–0.90 µm).
in February 1986, followed by others with Ikonos radiometric resolution is far greater
improved technologies (Table 6.1). They are than the Landsat scenes, with data collected as