Geophysics

Question 1

Gravity basics

Answer 1

• Variability of gravity
o Earth is oblate spheroid – distance from centre at pole is greater than at outside of equator – less gravity at poles
o Acceleration due to gravity is less at altitude
o Sensitive to density
• Could be an array of sediment types from a single density reading – see chart
• Acceleration due to gravity can be used to determine the density structure of the subsurface
o Rocks have different densities

Question 2

Use of magnetics?

Answer 2

Magnetic minerals – magnetite
Interacts with the magnetic filed
Produced by the dynamo effect caused by the circulation of fluid in the core
Potential field such that the force between two magnetic poles is:

F_m=(m_1 m_2)/(4πμr^2 )
m1 and m2 are pole strength, μ is the magnetic permeability and r the seperation
Don’t know how earths magnetic field flips (in a relative short geological time)
Magnetics allowed for plate tectonic discovery – basalt sheet between S. America and Africa cooled past curie temperature– preserved the magnetic field of the time – magnetic reversals show older rock further away from spreading ridge in Atlantic

Question 3

Electrical methods?

Answer 3

• Areas which are more insulator than conductors are more likely to be hydrocarbon reserves
• Ohm’s Law: V=IR (Resistivity: ability of rocks to conduct electricity)
o Sensitive to metallic ores, fluid and porosity/permeability
• We actually measure the electrical properties of the pore fluid, not the rock, rocks can therefore have different values depending on what’s in the pores

Electromagnetic (Magnetotelluric)
• Changes in the solar wind perturb the Earth’s magnetic field which generates currents in the ionosphere which in turn induces currents in the Earth subsurface
• Can sense resistivity structure to >200km

Electromagnetic (Ground penetrating radar)
• High-frequency capable of imaging to a few metres (or the water table)

Question 4

Seismic methods?

Answer 4

• Earthquakes
o Creates both compressional and shear waves
• Measure travel times at a remote station
• Invert these times to show the seismic velocity structure of the earth
• It’s a function of rock type and state
• How we know the core is liquid – shear waves can’t penetrate
o Surface waves = damage to buildings
o If resonant frequency of buildings correlates with earthquake that building will shake itself apart
• Controlled source:
o Technique with the highest resolution (10’s of metres) over a depth range of surface to sub-moho
o Seismic velocity is dependent on:
• Porosity
• Lithification
• Pressure
• Fluid saturation
o Back-scatter
• Reveals structure as it detects echoes of a source pulse from changes in the impedance of the rock layers
• Preferred method for hydrocarbon exploration and exploitation
o Forward-scatter
• Sensitive to rock velocity only and can be used to detect the gross shape of basins
Seismic stratigraphy:
• Map faults, unconformities, layering within the rock column
• Using the shape of the reflected seismic pulse to determine the pore fluid content