Page 151 - Introduction to Mineral Exploration

134 J. MILSOM

0 500 530
1000 m

540

535
NevesNeves 535
Corvo

Graca

Zambujal
530
535

FIG. 7.5 Gravity anomalies over the Neves Corvo group of orebodies in the Portuguese pyrite belt. The main
bodies, indicated by crosses, coincide with local culminations in a regional gravity high associated with high
density strata. Under these circumstances it is difficult to separate the anomalies from their background for
total mass or other analysis. (Data from Leca 1990.)

Gravity tends to increase towards the poles. anomaly or Bouguer gravity. Provided that the
The difference between the polar and equato- density of the topography has been accurately
rial fields at sea level amounts to about 0.5% of estimated, a Bouguer gravity map is usually a
the total field, and corrections for this effect are good guide to the effect of subsurface geology
made using the International Gravity Formula on the gravity field, although additional cor-
(IGF). Gravity also varies with elevation. The rections are needed in rugged terrain. Although
0.1 g.u. (0.01 mgal) effect of a height change much can be achieved with careful processing
of only 5 cm is detectable by modern gravity (e.g. Leaman 1991), terrain corrections can be
meters and corrections for height must be made so large, and subject to such large errors, as to
on all surveys. Adding the free-air correction, leave little chance of detecting the small anom-
which allows for height differences, and sub- alies produced by local exploration targets.
tracting the Bouguer correction (which appro- Geophysical interpretations are notoriously
ximates the effect of the rock mass between ambiguous but the gravity method does pro-
the station and the reference surface by that vide, at least in theory, a unique and unambigu-
of a uniform flat plate extending to infinity in ous answer to one exploration question. If an
all directions) to the latitude-corrected gravity anomaly is fully defined over the ground sur-
produces a quantity known as the Bouguer face, the total gravitational flux it represents