Geophysical data acquisition, processing and interpretation - Geophysics for the Mineral Exploration Geoscientist - page 49

presentation of 2D datasets, when both the spatial coordin-
ates are ground locations, is a map. The various types of
map displays are described in Section 2.8.1.1 .
Sometimes one of the spatial coordinates in a 2D dataset
is the estimated depth at which the response originates, so
the presentation comprises a cross-section. Often, a param-
eter related to the signal

2.8.1.1 Types of 2D display
Regardless of whether the plotting axes are geographic
coordinates, or depth/pseudo-depth versus location on
the survey line,

the same forms of display are used

( Fig. 2.30 ).
The simplest presentation for data comprising parallel
survey lines is a series of pro les ( Fig. 2.30a ), one for each
survey line and plotted so that each pro le has the same
scale and direction for the independent variable (magnetic
field strength in the example). The dependent variable of
the pro le, i.e. location, coincides with the spatial position
of the measurement. These are known as stacked pro le,or
a stack plot. A distinct advantage of these over other types
of display is that every data point is presented, unlike
gridded data where an averaged value at a larger data
interval is presented. Furthermore, short-wavelength vari-
ations in the traverse direction are preserved and can
potentially be correlated between adjacent traverses. The
disadvantage of this form of display is that it can be
confusing when the geology is highly variable, making
correlations between the traverses difficult. Also, a large
range in amplitude can cause adjacent profiles to overlap,
obscuring low-amplitude features. Locating the actual pos-
ition of features along the survey traverses can sometimes
be dif

is depth penetration is used as a
proxy for depth, e.g. time for time-series data such as
seismic data (see Chapter 6 ); or frequency for frequency-
series data (see online Appendix 4 ) , or transmitter-to-
receiver spacing (distance) for some types of electromag-
netic and electrical data (see Chapter 5 ) . Data presented in
this way are called a pseudosection: see for example the
resistivity data in Fig. 2.29c . In this case, an integer multiple
(n) of the spacing between transmitter and receiver is used
as a proxy depth parameter (see Section 5.6.4.1 ). A variable
other than depth is used because of the uncertain relation-
ship with true depth. It is emphasised that these are pseudo-
depths and not true depths and that the discrepancy
between the two will vary with depth and across the dataset
as the physical properties of the subsurface vary, resulting
in a distorted representation of the subsurface.
A more sophisticated presentation displaying depth
involves modelling (see Section 2.11 ) the survey data to
infer the subsurface distribution of the physical properties.
This invokes a model of the subsurface with associated
simplifying assumptions. The simplest approach is based
on modelling the data from each survey station/point
individually with the assumption that the subsurface phys-
ical property distribution below each station comprises a
1D model (see One-dimensional model in Section 2.11.1.3 ).
Neighbouring 1D models along the survey line are merged
to display a cross-section of the computed true value of the
parameter as a function of depth. This is known as a
parasection, and computed depth is used as the vertical
axis, but the simpli

'

cult, especially where gradients are large or anom-
alies are superimposed on longer-wavelength features, or if
the survey lines are not straight. The data may bene t from
amplitude scaling (see Amplitude scaling in Section 2.7.4.4 )
and the application of some form of high-pass filter to
remove unwanted longer wavelengths (see Section 2.9.2 ).
Stack plots are commonly used to display aeromagnetic
data to reveal the responses of potentially diamondiferous
kimberlite or lamproite pipes. These exploration targets are
comparatively small and may occur on one traverse only,
depending upon survey line spacing, resulting in a subdued
expression in the gridded data (Jenke and Cowan, 1994 ) .
Stacked profiles are also essential for displaying the weak,
short-wavelength, magnetic responses sometimes associ-
ated with ilmenite-rich heavy-mineral sand strandline
deposits (Mudge and Teakle, 2003 ) .
A classic form of 2D display is the contour plot. Here the
contour lines

cation of the subsurface in the model-
ling means that the depths may not be accurate. Parasec-
tions are a common form of display for electrical and
electromagnetic data.
When the constraints of the 1D model are relaxed so
that the physical property is allowed to vary in one lateral
direction (depth and along-line direction

but constant in
the across-line direction), it is known as a 2D model, or a
3D model when the property is free to vary in all directions
(depth, along-line and across-line directions) (see Section
2.11.1.3 ) . Although more difficult to calculate than the 1D
model, they are more realistic representations of the sub-
surface so the results are referred to as cross-sections.

- levels of constant amplitude across the

display area. The contour lines are usually black
( Fig. 2.30b ), but contours colour-coded according to amp-
litude (or some other parameter) add information to the
display that allows the eye to quickly identify and correlate
areas with similar characteristics. Contour plots are espe-
cially effective for de ning gradients and are the only form

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