Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 119

b)

a) c)

e)

d) f)

g) h)

Source

Depth

Figure 3.25 The effects of various filter operations on synthetic and real gravity data. (a) Calculated gravity field due to a vertically sided square
prism and (b) after upward continuation. Note that the anomaly now has lower amplitude and longer wavelength. (c) Residual anomaly
obtained by subtracting the upward continued response (b) from (a). This produces a sharper short-wavelength response. (d) First vertical
derivative of the anomaly in (a). Note the more localised response compared with the original anomaly. (e) Total horizontal gradient of the
anomaly in (a). Note how the maxima are coincident with the edges of the source, revealing its geometric form. (f) 3D analytic signal of
the anomaly in (a). The crests of the ridges are also coincident with the edges of the source, revealing its geometric form. (g) Tilt derivative
of the anomaly in (a). Note the positive response above the source. (h) Second vertical derivative of the anomaly in (a). The response is
localised over the source edges.

of about 25 gu. This dataset is chosen to demonstrate the
various enhancements in a situation where anomaly detec-
tion is the primary requirement. Stations are on loose grid
with a spacing of about 250 m, with more detailed data
over the orebody.
The airborne TMI survey data ( Fig. 3.28 ) are from the
vicinity of

amphibolite and gneiss which have experienced multiple
phases of deformation and metamorphism. The Broken
Hill mineralisation does not give rise to a magnetic
response but the local structure and stratigraphy is clearly
seen from the pattern of anomalies. This dataset was
chosen to demonstrate the use of data enhancements to
assist geological mapping. Haren et al.( 1997 ) and Maid-
ment et al.( 2000 ) describe the interpretation of these
data and argue for structural as well as stratigraphic

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the world-class Broken Hill Zn Pb Ag

deposit, in New South Wales, Australia. The regional

geology comprises high-grade metasediments,