Geoscience Reference
In-Depth Information
seismic velocity in the subsurface, allowing construction of a velocity cross-section. In-seam and tomographic surveys are
used at a local scale to map mineralization.
Complex data acquisition and processing are required in order to detect and resolve the weak signals in seismic
reflection data. Important data processing techniques include stacking and migration. Stacking involves the use of repeat
measurements to enhance the signal and migration corrects for distortions caused by reflections from points located
away from the midpoint beneath source and detector.
Seismic reflection surveys produce the most detailed images of the subsurface of any geophysical method, allowing
structure and stratigraphy to be mapped at the scale of metres to tens of metres at depths of less than a few hundred
metres.
The resolution of subsurface features depends on the size of the feature relative to the wavelengths of the seismic
waves. Features with dimensions less than ¼ of the dominant wavelength cannot be resolved.
Review questions
.....................................................................................................
1. Describe what is meant by the terms wavefront, ray and Fresnel zone, and explain their relationship to each other.
2. Explain the principle of geometric spreading. By how much do the energy and amplitude of body waves and surface
waves change when the wavefront expands to double, and four times, its radius?
3. Describe the main controls on variations of velocity and acoustic impedance across the three main rock classes.
4. Describe the effects of metamorphism, serpentinisation and weathering on seismic velocity and acoustic impedance.
5. Describe the process of stacking seismic reflection data. What assumptions are made about the seismic properties of
the subsurface, and how might they affect the results in an area of complex geology?
6. How is a synthetic seismogram produced and how can it be used in the interpretation of seismic data?
7. Describe the fundamental phenomenon that controls the resolution of seismic reflection data.
8. Compare and contrast the seismic refraction and reflection methods describing the strengths and weaknesses of
both methods.
9. What type of seismic survey would you use (a) to map the thickness of the sediments in a palaeochannel, (b) to detect
massive sulphide mineralization between drillholes and (c) to map a coal seam over a distance of several kilometres?
Explain your choices.
FURTHER READING
Ashcroft, W., 2011. A Petroleum Geologist
s Guide to Seismic
Included are datasets from various deposits and mineralised
terrains.
Gochioco, L.M., 1991. Tuning effect and interference re
'
ection. Wiley-Blackwell.
This topic is an excellent summary of the seismic re ection
method and, although written for the petroleum geologist, is
highly recommended for anyone wishing to work with
re ection data.
Eaton, D.W., Milkereit, B. and Salisbury, M.H., 2003. Hardrock
Seismic Exploration. Geophysical Development Series 10,
Society of Exploration Geophysicists.
A comprehensive set of papers describing the application of
the
Re
ec-
tions from thin beds and coal seams. Geophysics,56,
1288
1295.
Gochioco, L.M., 1992. Modelling studies of interference
re ections in thin-layered media bounded by coal seams.
Geophysics, 57, 1209
-
1216.
These two papers describe seismic responses from coal
seams and are excellent illustrations of how synthetic
seismograms can be used to understand the relationship
-
seismic
reflection method in hard-rock terrains.
 
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