Geology Reference
In-Depth Information
Figure 1.12 Image-processed magnetic data over an archaeological site.
(Reproduced by permission of Professor Irwin Scollar.)
field quality control. Contoured cross-sections ( pseudo-sections )areusedto
display the results of some types of electrical survey.
In engineering site surveys, pollution monitoring and archaeology, the
objects of interest are generally close to the surface and their positions in
plan are usually much more important than their depths. They are, moreover,
likely to be small and to produce anomalies detectable only over very small
areas. Data have therefore to be collected on very closely spaced grids and
can often be presented most effectively if background-adjusted values are
used to determine the colour or grey-scale shades of picture elements ( pixels )
that can be manipulated by image-processing techniques. Interpretation then
relies on pattern recognition and a single pixel value is seldom critically
important. Noise is filtered by eye, patterns such as those in Figure 1.12
being easily recognised as due to human activity.
It can also be revealing to overlay contoured results on a Google Earth
or other image. Many tools are available for doing this, ranging from full
Geographic Information Systems (GIS) to simpler packages such as Global
Mapper. Some also allow the transparency of overlaid pixel-based images
to be adjusted so that features on the ground can be correlated with pat-
terns in the geophysical data. This can be a powerful interpretation tool,
provided, of course, that the ground features imaged were actually there
at the time of the survey. It is also a valuable way of showing results to
clients.
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