- Global geophysics - structure of the Earth
- Exploration geophysics
- petroleum geophysics - structures that trap oil
- mining geophysics - detection of ore bodies
- Geotechnical & environmental geophysics
- site investigations - mechanical properties and depth
- groundwater studies - depths, thicknesses
- contaminant studies - detection, flow direction
What is the objective of geophysics?
to locate or detect the presence of subsurface structures or bodies and determine their size, shape, depth, and physical properties (i.e. physical parameters).
Physical properties
Measurements made on the surface of the Earth are controlled by lateral and vertical variations of the physical properties of the subsurface, i.e. the intensity or propagation path of a force field or wave field is determined by the physical properties of the material which directly underlies the area where measurements are made.
Physical properties of the subsurface that we exploit:
- elastic parameters - bulk modulus, rigidity modulus, Poisson's ratio
- density
- electrical conductivity
- electrical capacitance
- electrical inductance
- magnetic susceptibility and remanence
- dielectric constant
These lateral and vertical variations in physical properties give rise to geophysical anomalies which we use to map out subsurface lithology contrasts.
An anomaly is a deviation from the uniform.
Geophysical methods
Geophysical measurements are categorize into two groups:
- Passive methods - these involve measuring the spatial variation of static or natural fields of force, e.g. gravity and magnetic fields. Lateral variations in physical properties give rise to spatial variations in the field. It is difficult to separate size from density or susceptibility, and therefore, there is an inherent ambiguity in interpretation.
- Active methods - these involve measuring the characteristics of a wave field, i.e. travel times of elastic waves, and amplitude and phase of electromagnetic waves. Energy is introducted into the ground and you control the source and detector, therefore there is less ambiguity
Gravity
- Measures the spatial variations in the strength of the gravitational field of Earth
- Lateral variations in density give rise to spatial variations in the field
Figure 1
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Magnetics
- Measures the spatial variations in the strength or intensity of Earth's magnetic field
- Lateral variations in magnetic susceptibility and remanence give rise to spatial variations in the field
Figure 1
Figure 2
Seismic Refraction
- Measures the travel times of refracted waves
- The propagation velocity and path are controlled by the density and elastic moduli of the material through which the waves pass
Figure 1
Figure 2
Seismic Reflection
- Measures the travel times of reflected waves
- The propagation velocity and path are controlled by the density and elastic moduli of the material through which the waves pass
Figure 1
Figure 2
Electrical Methods
- Measures the resistance or potential drop of the ground between two electrodes
- Resistance measured at the ground surface is controlled by laterial and vertical variations of electrical conductivity
Figure 1
Figure 2
Figure 3
Electromagnetics
- Measures the components of a secondary electromagnetic field resulting from the induction of a primary electromagnetic field
- Secondary elctromagnetic fields are a function of the subsurface electrical conductivity and inductance
Figure 1
Figure 2
Ground Penetrating Radar
- Measures the travel time of reflected radar waves
- Velocity of the radar wave is controlled by the dielectric constant of the subsurface materials
- Dielectric constant - a measure of the capacity of a material to store a charge when an electric field is applied
Figure 1
Figure 2
Petroleum geophysics
- gravity first used to fine oil bearing structures along flanks of salt domes
- refraction seismology was next - fan shooting
- reflection methods are almost exclusively used today, looking for structures, direct detection of gas -- bright spots
Mining geophysics
Most successful in locating:
- Massive sulfides ore deposits
- principle metals of copper, nickel, lead, and zinc; found in minerals such as chalcopyrite, pyrrhotite, galena, and sphalerite
- properties include: high conductivity, high density and high magnetic susceptibility (from the mineral pyrrhotite)
- electromagnetic, Induced Polarization (IP), magnetic and gravity are best
- Disseminated sulfide ore deposits
- sources of copper and molybdenum; found in the minerals chalcopyrite, chalcocite, bornite, molybdenite and pyrite
- IP best
- iron ores
- contain magnetite and hematite
- magnetics and gravity are best
- chromite and gold mixed results.
