Geophysical data acquisition, processing and interpretation - Geophysics for the Mineral Exploration Geoscientist - page 58

Where one colour is at low levels, the remaining two
combine to create a shade close to the relevant secondary
colour ( Fig. 2.31a ). Each component dataset can be
stretched independently to improve resolution and colour
balance. An example of a ternary display of radiometric
data is shown in Figs. 4.26 and 4.27 .
The HSI colour model allows the component datasets to
control the colour parameters in a different way. One
dataset might be assigned to hue (colour), another to
saturation, and the third to intensity. A common HSI
display uses intensity variation to display shaded relief,
hue to display amplitude, and the colours are usually fully
saturated. This is sometimes called a colour drape, and
several example displays combining different data process-
ing and display methods are shown in Fig. 2.36 . The
simultaneous display of amplitude and textural variations
(see Section 2.10.1 ) is an extremely powerful combination,
particularly for mapping oriented interpretations. Note
that variations in any parameter of the HSI model produce
a new colour in RGB colour space ( Fig. 2.31b ) , so when
working with multichannel data, changes in image colour
cannot be easily related to variations in a particular data
channel. This is unlike the RGB colour model where vari-
ations in the intensity of a primary colour are directly
related to variations in its associated data channel, making
interpretation easier.

a)

b)

c)

2.9 Data interpretation - general

Interpretation begins with a qualitative analysis of the
geophysical data. If the aim is simply to identify and target
individual anomalies, i.e. regions with anomalous
responses, then this is, in principle, a straightforward task
provided the data are appropriately processed and pre-
sented (see Sections 2.7 and 2.8 ) . If the aim is to create a
pseudo-geological map, then this is usually a more
demanding task, not least because it requires both geo-
logical and geophysical expertise. For this reason, our
description of interpretation is focused more towards the
requirements of geological mapping. We leave the issues of
interpreting depth and pseudo-depth sections to our
descriptions of the individual geophysical methods. Also,
we focus on the interpretation of data displayed in the near
ubiquitous form of pixel images.
When a target is identified or when difficulties are
encountered in making a pseudo-geological map,

0 2

Kilometres

Figure 2.36 Various composite displays of the same aeromagnetic
dataset. The pseudocolour emphasises amplitudes whilst the textural
information is contained in the grey-scale shaded relief component
as follows: (a) grey-scale shaded relief and (b) wet-look grey-scale
shaded relief. (c) High-pass

filtered data in pseudocolour with
grey-scale shaded relief. All images are illuminated from the north.
Data are aeromagnetic data from the vicinity of the Mount Polley
alkalic Cu

-

Au deposit, located in British Columbia, Canada. Pit
outlines shown by the solid lines. Data are reproduced with the
permission of the Minister of Public Works and Government
Services Canada, 2006, and courtesy of Natural Resources Canada,
Geological Survey of Canada.

full spectrum of 16.7 million colours. Areas where all three
datasets have coincident high values appear white in the
ternary image, whilst low levels of all colours appear dark.

the

observed data can be analysed with the aid of the