Geoscience Reference
In-Depth Information
factors, but a key control is whether the right chemical conditions exist for iron in the system to form oxides (which may
be highly magnetic) or silicates (which are weakly magnetic).
There is no physical reason that the density and magnetism of a rock formation, and indeed variations in these
properties, must be related. Correlations may occur in crystalline rocks, but are less likely in sedimentary rocks. Gravity
and magnetic maps reflect fundamentally different physical characteristics of the rocks, so they may differ significantly.
Interpretation of gravity and magnetic data presented as pseudocolour images is usually based on a combination of the
'raw' data, i.e. Bouguer or free-air gravity and TMI, and some form of derivative-based enhancement of which the first
vertical derivative is most common. Care must be taken to avoid misinterpreting responses due to levelling errors, effects
of terrain and remanent magnetism.
Quantitative interpretation of gravity and magnetic data is hindered by non-uniqueness. The most reliable approach is to
create several models representing 'end-member' solutions. In its simplest form, modelling may involve automated
estimates of source depth based on anomaly amplitude and gradient relationships. Sophisticated forward and inverse
modelling methods are also available, but the results remain inherently ambiguous, although this may be reduced with
petrophysical data and other geological information.
Review questions
.....................................................................................................
1. Explain why gravity responses are monopoles and magnetic responses dipoles.
2. Explain induced magnetism, remanent magnetism, total magnetisation and the K ¨ nigsberger ratio. How does
magnetic susceptibility affect all of these parameters?
3. Explain how a change in survey height (or depth to the source) affects gravity and magnetic responses. Sketch the
various anomalies.
4. What are the benefits of gravity and magnetic gradiometer measurements over conventional field measurements?
5. Contrast the sequence of reductions applied to gravity and magnetic survey data.
6. Sketch in the principal meridian plane the magnetic field of a sphere and the induced magnetic anomaly above it for
magnetic inclinations of +90˚, +45˚ and 0˚. How do these differ in the opposite magnetic hemisphere?
7. Describe four data processing techniques you would use to assist in the analysis of low-latitude TMI data.
8. What are horizontal and vertical derivatives, the tilt derivative and the analytic signal? What benefits do they provide
over the TMI and gravity field measurements?
9. Describe the magnetic properties of magnetite. How does grain size influence magnetic properties?
10. Serpentinisation is an important control on rock magnetism and density - discuss.
11. Describe an appropriate workflow for the analysis of a set of magnetic susceptibility measurements.
12. What is Euler deconvolution?
13. How might magnetic data be used during greenfields exploration for gold deposits in Archaean greenstone belts?
FURTHER READING
Blakely, R.J., 1995. Potential Theory in Gravity and Magnetic
Applications. Cambridge University Press.
This provides a thorough description of the principles
behind the methods, and includes comprehensive math-
ematical descriptions.
Gunn, P.J., 1997. Airborne magnetic and radiometric surveys.
AGSO Journal of Australian Geology & Geophysics, 17, (2).
This a special edition of the AGSO Journal of Australian
Geology and Geophysics comprising 17 papers on virtually
every
aspect
of
the
acquisition,
processing
and
 
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