Page 150 - Introduction to Mineral Exploration

7: GEOPHYSICAL METHODS 133

interpreters become familiar with, and under- 7.4 GRAVITY METHOD
stand, the anomalies typical of the latitudes
in which they work than that they rely on The gravity field at the surface of the Earth is
processes that can significantly degrade their influenced, to a very minor extent, by density
data. variations in the underlying rocks. Rock den-
−3
Magnetic anomalies are caused by magnet- sities range from less than 2.0 Mg m for soft
ite, pyrrhotite, and maghemite (loosely describ- sediments and coals to more than 3.0 Mg m −3
able as a form of hematite with the crystal form for mafic and ultramafic rocks. Many ore min-
of magnetite). Magnetite is by far the com- erals, particularly metal sulfides and oxides,
monest. Ordinary hematite, the most abundant are very much denser than the minerals that
ore of iron, only rarely produces anomalies make up the bulk of most rocks, and orebodies
large enough to be detectable in conventional are thus often denser than their surroundings.
aeromagnetic surveys. Because all geologically However, the actual effects are tiny, generally
important magnetic minerals lose their mag- amounting even in the case of large massive
netic properties at about 600°C, a temperature sulfide deposits to less than 1 part per million
reached near the base of the continental crust, of the Earth’s total field (i.e. 1 mgal, or 10 of
local features on magnetic maps are virtually the SI gravity units or g.u. that are equal to
−2
all of crustal origin. Magnetic field variations 10 −6 ms ). Gravity meters must therefore be
over sedimentary basins are often only a few extremely sensitive, a requirement which to
nanotesla in amplitude, but changes of hun- some extent conflicts with the need for them
dreds and even thousands of nanotesla are com- also to be rugged and field-worthy. They meas-
mon in areas of exposed basement (see Fig. 4.4). ure only gravity differences and are subject to
The largest known anomalies reach to more drift, so that surveys involve repeated refer-
than 150,000 nT, which is several times the ences to base stations. Manual instruments are
strength of the Earth’s normal field. relatively difficult to read, with even experi-
Because magnetite is a very common acces- enced observers needing about a minute for
sory mineral but tends to be concentrated in each reading, while the automatic instruments
specific types of rock, magnetic maps contain a now becoming popular require a similar time
wealth of information about rock types and to stabilize. Because of the slow rate of cover-
structural trends that can be interpreted qual- age, gravity surveys are more often used to
itatively. Image processing techniques such as follow up anomalies detected by other methods
shaded relief maps are increasingly being used than to obtain systematic coverage of large
to emphasize features with pre-selected strikes areas. The potential of the systematic approach
or amplitudes. Quantitative interpretation in- was, however, illustrated by the discovery of
volves estimating source depths, shapes, and the Neves Corvo group of massive sulfide
magnetization intensities for individual anom- deposits (Fig. 7.5) following regional gravity
alies, but it would seldom be useful, or even surveys of the Portuguese pyrite belt on 100
practicable, to do this for all the anomalies in an and 200 meter grids (Leca 1990). Airborne grav-
area of basement outcrop. Magnetization inten- ity is technically difficult and, although avail-
sities are not directly related to any economic- able in a rather primitive form as early as 1980,
ally important parameter, even for magnetite is still not widely used. However, BHP-Billiton
ores, and are rarely of interest. Automatic invested several million dollars in developing
methods such as Werner deconvolution (which the Falcon airborne gradiometer system speci-
matches observed anomalies to those produced fically for use in their own mineral exploration
by simple sources) and Euler deconvolution programs and has reported several successes
(which uses field gradients) generally focus on since beginning operations in 1999. The Bell
depth determination. Direct determination of Geospace Full Tensor Gradiometer (FTG),
gradients is becoming commonplace, especially which is now available commercially, gives
in exploration for kimberlites. In airborne comparable results, equivalent roughly to
installations horizontal gradients can be meas- ground stations recorded on a 150 m grid, with
ured using nose-and-tail or wingtip sensors. 0.2 mgal noise.