Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 106

to the floor of the lake. Also, glaciers (density ranging from
0.79 to 0.88 g/cm 3 ) need to be accounted for in the terrain
correction. A summary of terrain corrections, including
unusual situations, is provided by Nowell ( 1999 ).

mistaken for geological features. The process of correct-
ing data for variations in height is demonstrated using
data from the Cripple Creek mining district in Colorado,
USA (Kleinkopf et al., 1970 ). Here, world-class epither-
mal Au

Te mineralisation occurs in a Tertiary igne-
ous complex (Thompson et al., 1985 ). Mineralisation
occurs in veins and as large bodies within tectonic and
hydrothermal breccias. Potassic alteration occurs in asso-
ciation with mineralisation. Figure 3.18 shows the geol-
ogy and distribution of the mineralisation in the area plus
the gravity data before and after correction for height and
topographic effects. The pseudocolours draped onto the
topography (grey surface) in Fig. 3.19 represent the mag-
nitude of gravity and the magnitude of the various cor-
rections. The left side of the figure shows the gravity data
before height corrections are applied (a), and after the
free-air (c), Bouguer (e) and terrain (g) corrections are
applied. The magnitude of the corrections is shown on
the right side of the figure. The uncorrected gravity data
show a clear correlation between

- Ag -

3.4.6 Summary of gravity data reduction

Gravity data may be presented in a number of different
forms. Perturbations to the gravity field of non-geological
origin are compensated by applying a sequence of correc-
tions to produce observed gravity.

Observed gravity gravity meter reading

+ Eötvös correction (if required)
+ tidal correction + instrument drift correction

¼

(3.19)

Observed gravity is reduced to either the free-air anomaly
(FAA) or the Bouguer anomaly (BA).

FAA observed gravity - latitude correction

+ free-air correction

¼

(3.20)

' low ' gravity and topo-

graphic ' highs ' . This is due to the dominance of the free-

BA data is of two kinds. The Bouguer-corrected but not
terrain-corrected Bouguer anomaly is sometimes referred
to as the partial or incomplete Bouguer anomaly, and
terrain-corrected Bouguer anomaly data as the full or
complete Bouguer anomaly.

air effect (1/r 2 in Eq. (3.3) . Applying the free-air correc-

tion reveals gravity correlating with high ground,

which is mainly due to the mass of the hills. The Bouguer
correction partially removes the effect of the mass so the
correlation between topography and gravity is reduced;
see, for example, the high ground in the foreground.
Applying the terrain correction completes the correction
process. Note how the highest values of the terrain cor-
rection correlate with the largest topographic features, i.e.
where the slab approximation of the Bouguer correction
is least appropriate.
The final reduced data show a negative gravity anomaly
which correlates with the extent of the igneous complex
( Fig. 3.18d ). The mis-matches are mostly due to a lack of
gravity stations to constrain the data. The majority of the
mineralisation and associated alteration coincides with the
lowest values of gravity. Kleinkopf et al.( 1970 ) state that
density determinations show that the volcanic breccia is
about 0.20 g/cm 3 less dense than the surrounding Precam-
brian rocks, which is suf cient to explain the negative
gravity anomaly.

' high '

¼

Partial BA FAA - Bouguer correction (3.21)

Complete BA FAA - Bouguer correction

+ terrain correction

¼

(3.22)

The greatest source of error in obtaining the complete
Bouguer anomaly is uncertainties associated with the ter-
rain correction. In many mineral terrains, topography and
density are often highly correlated and highly variable. Flis
et al. ( 1998 ) examine the underlying assumptions of the
standard gravity reduction procedures for these environ-
ments and assess the errors they introduce in the context of
iron ore exploration in the rugged Hamersley iron-ore
province of Western Australia. They advocate the use of
more accurate corrections based on a

' complete Bouguer

correction incorporating a full terrain correction and a

variable density model.

'

3.5 Measurement of the Earth's
magnetic field

3.4.7 Example of the reduction of ground
gravity data

'

The correction of gravity data for height and topography
is the process most likely to create artefacts that might be

Like the gravity field, most of the Earth s magnetic eld

does not originate in the crust, and it is smaller localised