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6: REMOTE SENSING 115

probability of representing the same kind of Structural maps
surface material, e.g. alteration. This deci-
sion is best made by computers. Multispectral The synoptic view afforded by MSS and TM
classification is particularly useful for a large imagery is ideal for regional structural ana-
area or a region covered by dense vegetation, lyses, especially if the scene chosen is illu-
as differences in the vegetation reflect the minated with a low sun angle (e.g. autumn or
underlying geology. spring) which emphasizes topographical fea-
3 Principal component analysis is used to tures. Manual lineament analysis on overlays
enhance or distinguish lithological differences, of photographic prints is then carried out (usu-
because spectral differences between rock types ally after a spatial filtering technique has
may be more apparent in principal component been employed) and lineaments are digitized.
images than in single bands. The reflectances The original apparently random pattern can
in different bands in MSS, TM, or ASTER be quantified by computer processing, thus
images have a high degree of correlation. Prin- providing an objective method for evaluating
cipal component analysis is a commonly used lineaments. A rose diagram is prepared to
method to improve the spread of data by redis- depict the strike–frequency distribution. At the
tributing them about another set of axes. This Helvetia porphyry copper test site in Arizona,
effectively exaggerates differences in the data. the three trends identified by these methods
correspond to deformation events in the Pre-
cambrian, the Palaeozoic, and the Laramide
6.2.7 Interpretation Orogeny (Abrams et al. 1984).
Two approaches are used to extract geological The most common structures seen on
information from satellite imagery. Landsat images are faults, fractures, linea-
1 Spectral approach. Spectral properties are ments of uncertain nature (see Fig. 9.10), and
used to separate units in image data based on circular features. In conjunction with the stat-
spectral reflectance. This is done interactively istical approach above, deductions regarding
on computers using multispectral data in areas stress patterns in an area can be made. These
with or without dense vegetation. structural studies may yield clues to the loca-
2 Photogeological approach. Known weather- tion of concealed mineral deposits. Linear
ing and erosional characteristics (topographical zones, such as the 38th Parallel Lineament of
expression) are used, on black and white or the USA (Heyl 1972), consist of weak crustal
color prints, to imply the presence of geological areas that have provided favorable sites for
structure or lithology. Photogeological ele- upwelling intrusives and mineralizing fluids.
ments include topography, erosion, tone, tex- These areas contain faults or fractures asso-
ture, drainage pattern, vegetation, and land use. ciated with deep-seated basement structures
These elements are discussed in section 6.4. and the intersections of these features with
other faults can provide favorable loci for
mineralisation.
6.2.8 Applications of Landsat satellite
imagery
Lithology and alteration maps
The use of satellite imagery is now a standard
technique in mineral exploration (Nash et al. Landsat imagery has made significant con-
1980, Goetz & Rowan 1981, Peters 1983, Drury tributions to the advancement of geological
2001). It has also been used in structural inves- mapping both in known and unmapped areas
tigations (Drury 1986) and in hydrogeology of the world. Many third world countries now
(Deutsch et al. 1981). In mineral exploration have geological maps which were previously
Landsat imagery has been used to provide basic too expensive to produce by conventional field
geological maps, to detect hydrothermal altera- mapping. Today regional geological mapping
tion associated with mineral deposits, and to often starts with Landsat imagery, followed
produce maps of regional and local fracture by rapid field reconnaissance for verifica-
patterns, which may have controlled miner- tion where possible. The maps thus produced
alisation or hydrocarbon accumulations. are used to select exploration target zones on