Active Seismic Techniques

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Exploration Technique: Active Seismic Techniques

Exploration Technique Information
Exploration Group: Geophysical Techniques
Exploration Sub Group: Seismic Techniques
Parent Exploration Technique: Seismic 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.
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Active Seismic Techniques:
Active seismic techniques study the behavior of artificially-generated elastic waves in the subsurface. A seismic wave or pulse is generated at the surface by an active seismic source which can be a vibration, mechanical impact, or near-surface explosion.
Other definitions:Wikipedia Reegle



 
Use in Geothermal Exploration
 


 
Field Procedures
 
Active land seismic surveys require a source and an array of receivers, called geophones. The various sources are a sledge hammer and a steel plate on the ground; blasting caps and explosives; land air gun; accelerated weight-drop source; and vibroseis source (See Figure 1), among others. These sources have intrinsic differences related to environmental impact, depth of investigation and the extent of control the field operator has over the source signal. The source is called the shot and its location is called the shot point.

Geophones are laid out in a variety of receiver arrays and spacings, depending on the desired reflection geometry and noise attenuation design parameters of the survey. The intent of the various geophone arrays is to improve the signal-to-noise ratio obtained and this depends on survey design.

For field operations, the geophones are placed in the ground by the seismic crew (See Figure 2). Once the geophones are installed and all channels are recording properly, the source can be activated. When the desired amount of shot points have been recorded with the geophone array, then the geophones can be moved to the next survey location. [1][2]

Figure 1- Vibroseis trucks as the source for an active seismic survey.[3]

 
Environmental Mitigation Measures
 
The environmental impacts of a seismic survey vary drastically and are survey-specific. Factors to consider are: terrain, land access, land usage, survey extent, seismic crew size, source (dynamite, vibroseis, etc.), accomodation for the crew, remoteness of survey location, among others.
 
Physical Properties
 
 
Data Access and Acquisition
 
Land seismic surveys utilize an active source (such as dynamite, mechanical impact, or vibration sweep) to generate a seismic wave at the surface. This artificially-generated elastic wave propagates into the subsurface and when it encounters an interface with varying physical properties (acoustic impedance contrast), a portion of its energy is refracted or reflected back to the surface according to Snell’s Law (See Seismic Techniques). The returned wave is measured by a geophone (seismometer) at the surface and the two way travel time of each wave is recorded.[1][4]

 
Best Practices
 
Ground coupling with the geophones is crucial and should be as stable and secured as possible given soil surface conditions.

Figure 2- Installation of a geophone for a seismic survey.[5]





 
Additional References
 


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