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L3: Introduction
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Geophysics
The objective of geophysics is to locate or detect the presence ofsubsurface structures or bodies and determine their size, shape,depth, and physical properties (density, velocity, porosity) +fluid content
study of structure and dynamic of thesolid Earth using physical methods
Non-expensive andNon-invasive methods
less localized than borehole, but
combination of borehole and geophysicalinformation returns the optimum result
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Geology and Physics
constraining the Earths sub-surfacewith observations at/near the surface
apply the principles of physics to thestudy of the Earth
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Geophysics
Geophysical analysisis done when
1) you cant get directaccess to the rocks
2) you want to modela large area quickly
3) you know what youare looking for
but every geophysical technique onlyestimates geology (lots of interpretation)
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Active & passive
geophysical survey methods can be classified into one of two broadclasses, e.g.:
Active Passive
Electrical Resistivity, Seismic,
Electromagnetic, Radar, InducedPolarization
Depend on artificial source
Transmit a signal into the
subsurface and record what comesback
gravity, magnetic, radiometric,
certain electrical fields (SelfPotential), certain electro-magnetic fields (telluric),seismology
Depend on natural source
Measure naturally occurringphenomena
..based on the sources of the applied energy or fields
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Geophysical Techniques
wide varieties of geophysical techniques and each of these measures aspecific physical property
http://armah.netfirms.com/Geophysics.html
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Geophysical Techniques
Major geophysics and rock physical properties relationship, e.g.;
Geophysical parameter relation can be describe as
http://armah.netfirms.com/Geophysics.html
geometrical parameter
anomalygeology /engineeringinterest
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Anomalies
Ultimately what geophysics do is to measure the intensity ofwaves or force fields as they pass through geological materials
They are looking for unpredicted variations or deviations from
normal (i.e. anomalies)
Geological interpretations are made on the basis of theseanomalies
http://www.geos.ed.ac.uk/homes/jru/Silverpitseismic.jpg
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Survey scale
There are 2 scales of geophysical analysis:
small-scale
(single well; shallow penetration seismic etc.)
large-scale
(basin wide surveys etc.)
http://sepwww.stanford.edu/sep/bi
ondo/HTML/IntroRef.html
http://gsc.nrcan.gc.ca/borehole/
index_e.php
http://www.laynewater.com/bore
hole_geophysics.htmlhttps://reader009.{domain}/reader009/html5/0429/5ae5286a83ce2/5ae5287358c15.jpg
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Geophysics from a differentperspective
The techniques well talk about are used in the fields of
Environmental geophysics
Engineering geophysics
Exploration geophysics
uses surface methods to measure the physical properties ofthe subsurface Earth, in order to detect or infer the presence
and position of
ore minerals, hydrocarbons, geothermal reservoirs,groundwater reservoirs, and other geological structures
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Planning a survey & Targetidentification
http://www.geology.wisc.edu/courses/g594/
e.g.
determine whatinformation already exist
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Planning a survey & Targetidentification
http://www.geology.wisc.edu/courses/g594/
determine whatinformation already exist
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Techniques & Targets
http://www.geology.wisc.edu/courses/g594/
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Techniques & Targets Profiling vs. mapping
What is the nature of the target? 2D or 3D?
Preliminary site evaluation necessary?
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Techniques & Targets
Station spacing
Must ensure that spacing is sufficient to sample anticipated signal
http://www.geology.wisc.edu/courses/g594/
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Techniques & Targets
Limitations
Lack of sufficient contrast in physical properties
Nonuniqueness of many interpretation
Ambiguity (obvious in gravity work)
Resolution is determined by the wavelength of the signal
Noise (undesired signal)
Multiple methods
to improve uniqueness and cross-check interpretations
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Techniques & Targets
http://appliedgeophysics.lbl.gov/intro/figures/table_prop.jpg
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