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Exploration geophysics is the applied branch of geophysics which uses surface methods to measure the physical properties of the subsurface, in order to detect or infer the presence and position of concentrations of ore minerals and hydrocarbons, e.g. Seismic Method.
i) practical application of physical methods ii) measure the physical properties of rocks or
subsurface geologyiii) to detect the measurable physical differences
between rocks that contain ore deposits or hydrocarbons.
What is Exploration Geophysics
Exploration Geophysics
• There is a need to both “identify” and “map”
subsurface sediments
• Geophysics can provide some of the
information required to “map” e.g.,
information on :subsurface architecture,
stratigraphic relationships, variation in
materials
Why use geophysics?
Using Geophysics to Characterizethe Subsurface: The Principles
Using Geophysics to Characterizethe Subsurface: The Principles
• Different geophysical surveys measure different physical properties of the subsurface
subsurface reflector
arrival of reflected wave
arrival of direct wave
source instant (t=0)
ground surface
de
pth
tim
e
• Geophysics requires subsurface targets to contrast significantly with their surroundings
choose geophysical method based on an understanding or estimate
of the physical properties of the target in relation to its environment
What geophysics?
• Where subsurface architecture/stratigraphy is complex
• Where the target produces a clear measureable signal
• Where data with a high signal-to-noise ratio can be obtained
Where should geophysics be used?
• No single survey can map all depths and/or targets
What can geophysics NOT do?
All geophysical methods have an inherent limitation on resolution depends on method, instrumentation and survey parameters, as well as site conditions (geological and cultural setting)
• In conjunction with boreholes or other information that can be used to verify the results
• With strong collaboration between geologist, hydrogeologist, and geophysicist from planning through interpretation
How should geophysics be used?
Geological SurveySurface mapping
Geochemical study
Basin study
Geophysical SurveyGravity
Magnetic
Seismic regional
Compilation & Interpretation
Data
Drilling & Testing
Coring
Logging
Testing
Formation EvaluationProspect Analysis
Risk Analysis
Reserve Calculation
Economical Study
Development
3-D Seismic Survey
InterpretationAVO
Inversion
Seismic Attributes
Drilling Development Wells
Build Platform Production Facilities
Production: Exploration Works
: Development Works
Geophysics and oil exploration
Important facts in exploration geophysics
- Two types of geophysics are available based
on the source of tool.
-We use the relative variations in the physical
properties of the earth not absolute values.
- The rocks and earth materials have overlapped
values of the physical properties.
- The application of geophysical tools depend
on the target of study and nature of the objects.
Geophysical methods are divided into two types: Active and Passive
Passive methods (Natural Sources): Incorporate measurements of natural occurring fields or properties of the earth. Ex. SP, Magnetotelluric (MT), Telluric, Gravity, Magnetic.
Active Methods (Induced Sources) : A signal is injected into the earth and then measure how the earth respond to the signal. Ex. DC. Resistivity, Seismic Refraction, IP, EM, Mise-A-LA-Masse, GPR.
0 1000 2000 3000 4000 5000 6000 7000 8000
Compression Wave Velocity, Vp (m/s)
Fresh Water
Sea Water
Clay
Sand
Till
I ce
Weathered Rocks
Intact Rocks
Steel
P - Wave Velocities
P- Waves (Compression)
Velocity of seismic waves and materials
0 1000 2000 3000 4000
Shear Wave Velocity, VS (m/s)
Fresh Water
Sea Water
Clay
Sand
Till
I ce
Weathered Rocks
Intact Rocks
Steel
S - Wave Velocities
Resistivity, (ohm- meters)
Resistivity Values ( ICE, London, 1976)
} V s = 0
S- Waves (Shear)
Mineral Densities
Potential Radar exploration depth
DEPTH (M)
References:
Physical principles of exploration methods : an introductory text for geology and
geophysics students, Beck, A. E. 1981.
Seismic tomography : with applications in global seismology and exploration
geophysics, Nolet, Guust 1987.
Handbook of exploration geophysics / by Paul A. Chapel, Paul A. 1992.
Exploration geophysics of the shallow subsurface, Burger, Henry Robert 1992.
Exploration geophysics : a review Romberg, Frederick E. 1961.
Foundation of exploration geophysics, Arnaud Gerkens, J. C. d' 1989.
Basic exploration geophysics, Robinson, Edwin S. 1988.
Exploration geophysics, Polshkov, Mikhail K. 1968.