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Method Name: |
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Seismic noise measurements |
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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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Technique to estimate the fundamental frequency of the local site that yields information about the local structure. |
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Keywords: |
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Seismic Techniques; Seismic noise measurements; Ambient vibrations; H/V-ratio; Fundamental frequency; Local subsurface structure |
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Prerequisites: |
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- Complex subsurface geology may lead to misinterpretation in 2-D profiles.
- Surface- and subsurface-topography small relative to the thickness of soft sediments.
- Subsurface consists of several layers, each with approximately constant seismic velocity.
- Layers must have sufficient velocity contrast and thickness.
- Subsurface must consist of quasi-horizontal layers with different seismic velocities.
- S-velocity contrast between softer sediments and bedrocks of factor 2 or larger.
- Instrument positioning: Good coupling to soil required, remove surficial layer (grass, clay), asphalt or compacted ground ok for lower frequencies.
- Instrument positioning: For sediment thickness of less than 30 m a similar distance to buildings and constructions is recommended.
- Undisturbed recording for 15-30 min required (no person moving within 10 m of the sensor; at least 100 m distance away from strong sources like motorways, railways, industry etc.
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Resolution: |
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Down to the first strong S-wave velocity contrast, resolution of soft sediments from 10 m down to several hundreds of meters, 1-D layer resolution decreasing with depth, resolution low (25 m or larger) due to long wavelength involved. Precision of peak identification is +/-15%. |
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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). Fundamental frequency of resonance of the ground, and the S-wave velocity average or profile if the thickness of the soft sediments is measured by a single 3-component sensor in the frequency range 0.2-15 Hz.
- Data analysis: Inverson of seismic data yields a velocity distribution within the subsurface. Determining the H/V-ratio, from the resonance frequency, the amplitude and the shape of polarization determine the structure. The estimation of the S-wave velocity is possible. Analysis of maximum amplitudes vs. frequency.
- Interpretation: 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. Core logging and / or petro-physical logs for interpretation, synthetic seismogram from sonic and density log for better calibration, caliper log to detect good geophone positions in open hole case. Sensitivity may vary within a structure; thus, foregoing measurements at several locations may be necessary.
- Related add-on information: VSP data, Geological constraints on fracture zones / fault planes, Downhole seismic data, Crosshole seismic data, Drilling measurements, Laboratory measurements.
- Independent additional information: borehole logs, Numerical modeling.
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Operating Expense: |
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- Crew size: 1 person
- Acquisition speed: 45min per point
- Processing: half a day for 5 points
- Equipment rental costs: intermediate
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Parameters to specify: |
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- Instrument: measurements useful down to eigenfrequency of seismometer, 5 s or 20 s sensors recommended.
- Number of points: several at a site to insure 1-D approximation, distance between points of the order of the sediment depth.
- Duration of recording per point: Minimum 15 min, 30 min and more recommended for 1 Hz fundamental frequency and lower.
- Time window length for Fourier transform needs to contain at least 10 periods of the lowest frequency of interest.
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QC Documents: |
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- Coordinates and map of shot and receiver locations.
- 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.
- Subsurface models (depth-distance plots; 2-D and / or 3-D subsurface models).
- Shear wave velocity vs. depth functions.
- H/V curves (horizontal to vertical ratio).
- Fundamental frequencies (table of location and f0).
- Interpolated map of frequencies (image, grid).
- Interpretation.
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