Seismic Techniques
From Open Energy Information
Exploration Technique: Seismic Techniques
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Physical Properties
The fundamental wave types for seismic surveys are the p-wave (a.k.a. longitudinal, compressional) and the s-wave (a.k.a. transverse or shear). The particle movement of a p-wave is in the direction of wave travel and involves compression and expansion. For an s-wave, the particle movement is perpendicular to the direction of wave travel.
Velocity is the physical parameter measured by seismic techniques. Seismic waves propagate through a rock body at a velocity which is governed by the elastic properties (stress and strain, Young's modulus) and density of the geological formation.[2] Additional factors which influence compressional and shear wave velocities are lithology, extent of fractures, temperature, fluid content, saturation, and fluid pressure in the subsurface.[2][4]Empirical ranges of compressional and shear wave velocities through different lithologies are shown in the figure below.[5]
Acoustic impedance contrasts are the ‘reflectors’ which are mapped using seismic techniques and may represent changes in lithology or discontinuous geological contacts. The acoustic impedance is defined as the product of the density and p-wave velocity within a rock unit. When an elastic wave encounters an acoustic impedance boundary, a portion of the wave energy is reflected off of the boundary and a portion is refracted into the second medium, according to Snell’s Law as depicted in the figure below.[4][6]
| Exploration Technique Information | |
|---|---|
| Exploration Group: | Geophysical Techniques |
| Exploration Sub Group: | Seismic Techniques |
| Parent Exploration Technique: | Geophysical Techniques |
| Information Provided by Technique | |
| Lithology: | Rock unit density influences elastic wave velocities. |
| Stratigraphic/Structural: | Structural geology- faults, folds, grabens, horst blocks, sedimentary layering, discontinuities, etc. |
| Hydrological: | Combining compressional and shear wave results can indicate the presence of fluid saturation in the formation. |
| Thermal: | High temperatures and pressure impact the compressional and shear wave velocities. |
Seismic Techniques:
Seismic methods provide information regarding the elastic properties of the subsurface through the measurement of the propagation velocity of elastic waves.
The given value was not understood.
Seismic waves are classified as elastic waves because they cause nonpermanent deformation to the material through which they propagate. The characteristics of the deformation define the wave type.[2]The fundamental wave types for seismic surveys are the p-wave (a.k.a. longitudinal, compressional) and the s-wave (a.k.a. transverse or shear). The particle movement of a p-wave is in the direction of wave travel and involves compression and expansion. For an s-wave, the particle movement is perpendicular to the direction of wave travel.
Compression and shear wave particle motion and direction of propagation.[3]
Velocity is the physical parameter measured by seismic techniques. Seismic waves propagate through a rock body at a velocity which is governed by the elastic properties (stress and strain, Young's modulus) and density of the geological formation.[2] Additional factors which influence compressional and shear wave velocities are lithology, extent of fractures, temperature, fluid content, saturation, and fluid pressure in the subsurface.[2][4]Empirical ranges of compressional and shear wave velocities through different lithologies are shown in the figure below.[5]
Typical rock velocities.[5]
Behavior of a compressional elastic wave at an acoustic impedance boundary[7]
The given value was not understood.
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