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Method Name: |
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Vertical seismic profiling (VSP) |
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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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Vertical seismic profiling (VSP) is a technique that measures seismic waves between a borehole and the surface. |
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Keywords: |
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Seismic Techniques; Vertical seismci profiling (VSP); high resolution 2-D seismic section; Seismic velocity contrast; Seismic depth sections; Subsurface models |
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Prerequisites: |
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- Moderate topography.
- Accurate surveying of geometry and topography are required.
- Borehole deviation logs should be available (i.e., exact source and receiver coordinates are required).
- Surface- and subsurface-topography small relative to the thickness of soft sediments.
- Target must be characterized by a seismic impedance contrast.
- Ambient seismic noise (e.g., traffic, rain, wind) may reduce data quality significantly.
- Good coupling between casing and formation (poor coupling yields delayed arrival times and attenuated amplitudes).
- In open hole case, coupling may be difficult or impossible (large breakouts).
- Safety is an issue when explosives are used.
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Resolution: |
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Horizontal resolution: information limited to the area close to the borehole. Vertical resolution depends on seismic velocity and the dominant signal frequency. Because seismic velocities generally increase with depth whereas the dominant frequency decreases with depth, seismic resolution decreases with depth. Typical depth of investigation varies between borehole depth and several tens to several hundreds of m ahead of the borehole (engineering-scale applications). |
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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 reflection seismic data yields an image of reflectors (either in travel-time or depth: seismic time or depth section). Processing of VSP data yields a so-called corridor stack: a plot of seismic reflectors vs. travel-time of the seismic waves (or depth).
- Interpretation: Seismic interpretation assumes that the resolved reflectors represent true lithological interfaces. Additional geological or geophysical surface data may be required for reliable interpretation. Walk away VSP allow high resolution 2-D seismic section to be produced.
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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.
- Related add-on information: Reflection seismic data, Refraction seismic data, Surface-based tomographic data, Sonic logs (synthetic seismograms), Surface reflection seismic data: lateral extension of information, Geological constraints on fracture zones / fault planes
- Independent additional information: electrical and /or electromagnetic data, georadar data, gravity data, borehole logs
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Operating Expense: |
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- Crew size: 1 key person, 1 - 3 assistants (Simple equipment operated by 3 - 4 persons may be used for shallow holes (< 100 m). Deeper holes may require more operating expense).
- Acquisition speed: Geophone chains or reverse VSP (source in borehole) can significantly reduce acquisition time.
- 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 in borehole: explosives, piezo-electric sources, vibrators, borehole hammer.
- Geophone type: single-, or multicomponent geophones, or hydrophones.
- Seismograph: Channel number, dynamic range (number of channels depends on equipment; 16 bit or more dynamic range).
- Source-point/ geophone spacing (usually several decimeters to few m).
- 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.
- Borehole information (i.e., coordinates, casing (type and length), diameter, deviation, fluid properties).
- 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).
- Seismic section for walk away VSP.
- Corridor stack.
- Velocity-depth functions; (P-wave / S-wave velocity ratio).
- 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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