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Method Name: |
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Refraction seismic tomography (surface-based) |
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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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Travel times of refracted seismic waves are interpreted in terms of subsurface seismic velocity distribution. Calculated seismic velocities can be used to characterize subsurface materials (e.g. degree of weathering or composition). |
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Keywords: |
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Seismic Techniques; Tomography (Surface-based); Tomographic inversion; Tomogram (subsurface models); Seismic velocity variation; Travel times |
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Prerequisites: |
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- The surveyed area should be larger than the area of interest.
- Accurate surveying of geometry and topography are required.
- Seismic velocities increase with depth.
- Subsurface low velocity layers cannot be detected with standard interpretation techniques; their presence could result in erroneous models
- 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.
- 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 .
- Safety is an issue when explosives are used.
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Resolution: |
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The geophone and the shot-point spacing determine the lateral resolution.The vertical resolution is a function of geophone spacing, depth to refractor signal frequency and seismic velocity contrast.The depth of investigation is usually less than 100 m in engineering applications, but depths of many kms are possible. |
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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: Processing of seismic data yields a 2-D vertical section showing depth to resolved layers; provides velocity information for each layer (usually in m / s).
Inverson of seismic data yields a velocity distribution within the subsurface .
- Interpretation:Comparison with known velocity values (VSP), or - if available - by correlation with borehole logs.
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: Reflection seismic, VSP, up-hole and gravity data
- Independent additional information: If overlapping depth of investigation: 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: high
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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 (usually with resonanz frequency between 4Hz and 15 Hz).
- Seismograph: Channel number, dynamic range (number of channels depends on equipment; 16 bit or more dynamic range).
- The field geometry (geophone spread / layout) is often a trade-off between number of available channels, smallest possible geophone spacing, and the maximum desirable offset.
- Geophone spacing (usually between one and several tens of m; for deep investigations up to several hundred m). The expected depth of the refractor(s) and the lateral resolution (which is always larger than the geophone spacing) determine the geophone spacing. The geophone spacing may be reduced at the shot end of a profile (variable spacing) to provide additional information on the shallow subsurface.
- Maximum offset (determines the depth of investigation; generally the maximum source-receiver offset should be at least three to four times the required depth of investigation, but in certain areas may it be 5 - 10 times).
- 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.
- Subsurface models (depth-distance plots; 2-D and / or 3-D subsurface models).
- Tomograms.
- 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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