Geoscience Reference
In-Depth Information
mapping season, especially to remote and poorly mapped areas
(e.g. Worked Example 10.1). The image quality in some areas
allows features such as roads, paths, streams and exposures to
be resolved, providing useful logistical information that can
guide mapping strategy in the fi eld.
Additional data
Geophysical or geochemical data may be available for some
areas, and maps of gravity or magnetic anomalies can provide
valuable insights into large-scale subsurface structures that may
infl uence the surface geology.
10
Worked Example 10.1 ASTER satellite mapping of rock units in southern Tibet
Accurate maps are not readily available for this
region, so before starting a research project
there, satellite imagery was viewed online,
along with a false-colour, composite Landsat
image (Figure 10.2a). ASTER (Advanced
Spaceborne Thermal Emission and Refl ection
Radiometer) satellite images for specifi c areas
were also acquired. Different rock types
showed contrasting emissions, which were
enhanced with further processing (Figure 10.2b).
Comparing the images with existing, small-scale
geological maps of Tibet suggested that the
ASTER images could be used to defi ne the
outcrop extent of certain units, because
vegetation is sparse.
During two subsequent fi eld seasons, rock units
were recorded in the fi eld, and their distribution
compared with different colours on the ASTER
images. Making observations in the fi eld that
assess the validity of inferences from remote
sensing data is known as 'ground-truthing'. The
satellite images discriminated well between
granite, Palaeozoic metasediments and younger
rock types, and highlighted other granite domes
in the region (Figure 10.3, p. 212).
29° N
28° 30' N
(a)
88° E
89° E
(b)
Figure 10.2
(a) Landsat image of part of southern Tibet, using bands 7, 5 and 3 (2.08-2.35
µ
m,
1.55-2.75
µ
m, 0.60-0.69
µ
m wavelength respectively) to make a composite RGB image. Granites and
gneisses in domes show up pale pink or beige, while metamorphic schists mantling the domes appear dark
purple. Boxes show areas of images in part (b). (b) ASTER satellite images of the two boxed areas shown
in (a). Granite appears red; gneisses appear pale blue; Palaeozoic schists appear dark blue to purple.
(Landsat data in (a) courtesy of the U.S. Geological Survey/NASA; ASTER images in (b) from Watts
et al.
2005.)
