Geology Reference
In-Depth Information
before interpretation can begin. Interpreters tend to concentrate on anoma-
lies - that is, on differences from a constant or smoothly varying back-
ground. Anomalies take many forms. A massive sulphide deposit containing
pyrrhotite would be dense, magnetic and electrically conductive (Table 1.2).
Typical anomaly profiles recorded over such a body by various types of
geophysical survey are shown in Figure 1.10. A wide variety of possible
contour patterns correspond to these differently shaped profiles.
Background fields also vary and may, at different scales, be regarded as
anomalous. A 'mineralisation' gravity anomaly, for example, might lie on
a broader high due to a mass of basic rock. Separation of regionals from
residuals is an important part of geophysical data processing, and even in
the field it may be necessary to estimate background so that the significance
of local anomalies can be assessed. On profiles, background fields estimated
by eye may be more reliable than those obtained using a computer, be-
cause of the virtual impossibility of writing a computer program that will
produce a background field that is not influenced by the anomalous val-
ues (Figure 1.11). Computer methods are, however, essential when deriving
backgrounds from data gathered over areas rather than along single lines.
The existence of an anomaly indicates a difference between the real world
and some simple model, and in gravity work the terms free-air anomaly ,
Bouguer anomaly and isostatic anomaly are commonly used to denote de-
rived quantities that represent differences from gross Earth models. These
so-called 'anomalies' are sometimes almost constant within a small survey
area - that is, the area is not anomalous! Use of terms such as Bouguer
gravity (rather than Bouguer anomaly) avoids this confusion.
1.5.9 Wavelengths and half-widths
Geophysical anomalies in profile often resemble transient waves but vary
in space rather than time. In describing them the terms frequency and fre-
quency content are often loosely used, although wavenumber (the number
of complete waves in unit distance) is pedantically correct. Wavelength may
be quite properly used of a spatially varying quantity, but where geophysical
anomalies are concerned the use is imprecise, since an anomaly described
as having a single 'wavelength' would be resolved by Fourier analysis into
a number of components with different wavelengths.
A more easily estimated quantity is the half-width , which is equal to half
the distance between the points at which the amplitude has fallen to half the
anomaly maximum (cf. Figure 1.10a). This is roughly equal to a quarter of
the wavelength of the dominant sinusoidal component, but has the advantage
of being directly measurable on field data. Wavelengths and half-widths are
important because they are related to the depths of sources. Other things
being equal, the deeper the source, the broader the anomaly.
Search WWH ::




Custom Search