Geophysics

What is Geophysics and Geology – Definition & Techniques

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What is Geophysics and Geology – Definition & Techniques

Geophysics

Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth’s shape; its gravitational and magnetic fields; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation.[1] However, modern geophysics organizations use a broader definition that includes the hydrological cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial relations; and analogous problems associated with the Moon and other planets.

Geophysics
Geophysics

 

Mechanical Wave Measurements

  • Crosshole Tests (CHT)
  • Downhole Tests (DHT)
  • Spectral Analysis of Surface Waves
  • Seismic Refraction
  • Suspension Logging

Electromagnetic Wave Techniques

  • Ground Penetrating Radar (GPR)
  • Electromagnetic Conductivity (EM)
  • Surface Resistivity (SR)
  • Magnetometer Surveys (MT)

Mechanical Wave Geophysics

  1. Nondestructive measurements (gs < 10-4%)
  2. Both borehole geophysics and non-invasive types (conducted across surface).
  3. Measurements of wave dispersion:  velocity, frequency, amplitude, attenuation.
  4. Determine layering, elastic properties, stiffness, damping, and inclusions
  5. Four basic wave types: Compression (P), Shear (S), Rayleigh (R), and Love (L).
  6. Compression (P-) wave is fastest wave; easy to generate.
  7. Shear (S-) wave is second fastest wave.  Is directional and polarized.  Most fundamental wave to geotechnique.
  8. Rayleigh (R-) or surface wave is very close to S-wave velocity (90 to 94%).  Hybrid P-S wave at ground surface boundary.
  9. Love (L-) wave: interface boundary effect

Geophysical Equipment

Shear Wave Velocity, Vs

  1. Fundamental measurement in all solids (steel, concrete, wood, soils, rocks)
  2. Initial small-strain stiffness represented by shear modulus:  G0 = rT Vs2   (alias Gdyn = Gmax = G0)
  3. Applies to all static & dynamic problems at small strains (gs < 10-6)
  4. Applicable to both undrained & drained loading cases in geotechnical engineering.

Seismic Piezocone Test (SCPTu)

Automated Seismic Source

  • Electronically-actuated
  • Self-contained
  • Left and right polarization
  • Modified beam uses fin to enhance shear wave generation
  • Successfully tested to depths of 20m
  • Capable of being used with traditional impulse hammer

Downhole Shear Wave Velocity

Electromagnetic Wave Geophysics

  • Nondestructive methods
  • Non-invasive; conducted across surface.
  • Measurements of electrical & magnetic properties of the ground:  resistivity (conductivity), permittivity, dielectric, and magnetic fields.
  • Cover wide spectrum in frequencies (10 Hz < f < 1022 Hz).

Surface Mapping Techniques:

  • Ground Penetrating Radar (GPR)
  • Electrical Resistivity (ER) Surveys
  • Electromagnetic Conductivity (EM)
  • Magnetometer Surveys (MS)

Downhole Techniques

  • Resistivity probes, MIPs, RCPTu
  • 2-d and 3-d Tomography

Ground Penetrating Radar (GPR)

  • GPR surveys conducted on gridded areas
  • Pair of transmitting and receiver antennae
  • Short impulses of high-freq EM wave
  • Relative changes in dielectric properties reflect differences in subsurface.
  • Depth of exploration is soil dependent (up to 30 m in dry sands; only 3 m in wet saturated clay)

Electrical Resistivity (ER) Surveys

  • Resisitivity rR (ohm-m) is an electrical property.  It is the reciprocal of conductivity
  • Arrays of electrodes used to measure changes in potential.
  • Evaluate changes in soil types and variations in pore fluids
  • Used to map faults, karst features (caves, sinkholes), stratigraphy, contaminant plumes.

Electromagnetic Conductivity (EM)

Magnetometer Surveys (MS)