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Method Name: |
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Seismic surface waves methods |
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Method Type: |
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Seismic Techniques
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Assigned Problems: |
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Principle: |
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Seismic surface waves are investigated to estimate subsurface (shear-wave) velocity-depth models. |
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Keywords: |
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Seismic Techniques; Seismic surface waves methods; Spectral analysis of surface waves(SASW); Rayleigh wave methods; Seismic depth sections; Subsurface (shear-wave) velocity-depth models |
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Prerequisites: |
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- The surveyed area should be larger than the area of interest.
- Moderate topography
- Accurate surveying of geometry and topography are required.
- Complex subsurface geology may lead to misinterpretation in 2-D profiles.
- Area with rough surface topograhpy should be avoided because of difficult static corrections.
- Subsurface consists of several layers, each with approximately constant seismic velocity.
- Layers must have sufficient velocity contrast and thickness.
- Target must be characterized by a seismic impedance contrast.
- Significant absorption of seismic energy in shallowest subsurface layers (e.g., unconsolidated moraines) may limit utility of survey.
- Subsurface must consist of quasi-horizontal layers with different seismic velocities.
- Source and receiver coupling is critical, such that data quality is site-dependent and should be checked in tests prior to main survey.
- Ambient seismic noise (e.g., traffic, rain, wind) may reduce data quality significantly.
- Vertical fractures may be difficult to detect.
- Safety is an issue when explosives are used.
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Resolution: |
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Lateral resolution is approximately equal to the surface-wave wavelength. As the surface wave wavelength decreases with increasing depth, lateral resolution decreases with increasing depth. Penetration in depth of surface waves is wavelength dependent and equals approximately 0.4 times the wavelength |
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Expected Results: |
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- Measured parameter: Velocity of ground motion (as determined by the voltage generated by the calibrated geophone recording system).
- Data analysis: Inversion of dispersion curves yields a velocity-depth function.
- Interpretation: Seismic interpretation assumes that the resolved reflectors represent true lithological interfaces. Additional geological or geophysical surface data may be required for reliable interpretation.
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Combination with other Methods: |
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- Required additional information: Geological information is necessary for the interpretation.
- Related add-on information: Refraction seismic data, Surface-based tomographic data
VSP data, S-wave reflection / refraction, Geological constraints on fracture zones / fault planes
- Independent additional information: electrical and /or electromagnetic data
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Operating Expense: |
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- Crew size: 1 key person, 1-2 assistants
- Acquisition speed: Is given by the number of geophones / channels (usually 24 - 48), geophone and shot-point spacing, type of empoyed source and topographic conidtions (terrain, access): 50 to 200 shot-points per day.
- Processing: Requires 1 - 2 days per acquisition day.
- Equipment rental costs: intermediate
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Parameters to specify: |
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- Source type / Source parameters (e.g., amount of explosive, hammer, weight-drop, vibrators)
Geophone type (natural frequency should be maximum 10 Hz).
- Seismograph: Channel number, dynamic range (number of channels depends on equipment; 16 bit or more dynamic range).
- Profile length (usually 50 - 200 m).
- Geophone spacing (usually between several decimeters to few meters) should be less than half the dominant surface-wave wavelength.
- Source-point interval (usually between one and three times the geophone spacing). The geophone shot-point separation is smaller for shallow refractors and larger for deep refractors.
- Sampling rate: Depending on required resolution and field condition (usually around 0.25 ms for high resolution).
- Record length (depending on maximum expected travel times, e.g. target depth).
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QC Documents: |
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- Coordinates and map of shot and receiver locations.
- Geodetic survey.
- Accuracy of travel time picks.
- Daily checks: noise level; impedance of geophones and cables; dynamic range and gain adjustment of seismograph.
- Trigger accuracy.
- Field notes (e.g., all activities, effective time schedule, present personnel).
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Products: |
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- Raw data and geometry files.
- Measurement of noise level.
- First-arrival times and / or amplitudes of seismic signals.
- Shear wave velocity vs. depth functions.
- Interpretation
- Optional: Test measurements (i.e., ""walk-away"" tests, source tests, geometry test of array).
- Optional: Modelling of the detectability of an anomaly with the employed source-receiver geometry.
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