Geoscience Reference
In-Depth Information
alteration zone that is more magnetic than its host rocks.
The prospective host stratigraphy is the cause of the grav-
ity anomalies in the coal examples from the Bonnet Plume
Basin (
Figs. 3.69
) and Jharia (
Fig. 3.77d
)
, and the Port
Wine placer gold example (
Fig. 3.70
). Other examples of
magnetic responses associated with mineralisation are
provided by Gunn and Dentith (
1997
).
Summary
.....................................................................................................
•
Gravity and magnetic surveys are relatively inexpensive and are widely used for the direct detection of several different
types of mineral deposits and some types of mineralising environments, and for pseudo-geological mapping.
•
Variations in rock density cause variations in the Earth's gravity, and variations in rock magnetism, which depends on a
property called magnetic susceptibility, cause variations in the Earth's magnetic field. The magnetism is induced by the
Earth's field, and the rock may also carry a permanent or remanent magnetism.
•
Several remanent magnetisms may coexist in a rock and their combined magnetism is called the natural remanent
magnetism. The ratio of the strength of the induced and remanent magnetisms is called the K ¨ nigsberger ratio.
•
Density is a scalar quality, and magnetism is a vector quantity. This causes gravity anomalies to be monopolar and
magnetic anomalies to be dipolar.
•
Commonly occurring minerals may exhibit diamagnetism, paramagnetism or ferromagnetism, the latter being of two
types, antiferromagnetism and ferrimagnetism. Ferromagnetism is much stronger than diamagnetism and paramagnetism.
The most important ferromagnetic mineral is magnetite, which can carry strong induced and remanent magnetisms.
•
Relative measurements of the strength of the vertical component of the Earth's gravity field are made using a gravity
meter. Gravity gradiometers are used to measure spatial gradients. Survey height above the geoid is an essential
ancillary measurement required for the reduction of gravity survey data.
•
The reduction of gravity data involves a series of corrections to remove temporal, latitude, height and terrain effects that
all have significantly higher amplitude than the signal. The effects of terrain variations surrounding the survey station are
particularly significant and are difficult to remove.
•
Absolute measurements of the total strength of the Earth's magnetic field (the geomagnetic field), i.e. the total magnetic
intensity (TMI), are made with a magnetometer.
•
The direction and strength of the geomagnetic field varies within and over the Earth, in particular with latitude. This
produces variations in the form of the induced magnetic anomalies associated with magnetic geological features.
•
The reduction of magnetic data principally accounts for temporal variations in the geomagnetic field. This is achieved by
continuously recording the field at a base station and through levelling the data based on repeat measurements at tie-
line/survey line intersections.
•
Magnetic responses can be simplified using the pseudogravity and reduction-to-pole transforms, but remanent
magnetism is a source of error. Wavelength filtering can be applied to both gravity and magnetic data to separate the
responses of large, deep and shallow sources. Enhancements to increase resolution of detail are most commonly based
on spatial gradients (derivatives).
•
Density variations in crystalline rocks are controlled by the densities and proportions of the constituent minerals. When
the rocks have significant porosity, as is the case for most sedimentary rocks, then the amount of porosity and the pore
fluid become the dominant control on density.
•
The magnetism of rocks is roughly proportional to their magnetite content. Other, sometimes significantly, magnetic
minerals are haematite and pyrrhotite. Whether an igneous or metamorphic rock contains magnetite depends on various
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