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Method Name: |
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Seismic crosshole tomography |
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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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Determination of the seismic wave velocity distribution and attenuation between two boreholes. |
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Keywords: |
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Seismic Techniques; Crosshole tomography; Tomographic inversion; Tomogram (subsurface models); Seismic velocity variation; Travel times
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Prerequisites: |
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- Borehole deviation logs should be available (i.e., exact source and receiver coordinates are required).
- 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)
- Vertical fractures may be difficult to detect.
- Safety is an issue when explosives are used.
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Resolution: |
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Very detailed (volume) information compared to surface-based or other methods: The ""Fresnel-volume"" which is dependent on the velocity and the frequency-content of the seismic wave, determines the smallest resolvable volume and may be in the range of several dm to few m.
Maximum distance between the boreholes may vary between several tens of meters (in unconsolidated sediments) to several hundred m (in hard rock), depending on the employed source and the soil / rock quality. |
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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: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 data, Refraction seismic data, Surface-based tomographic data, VSP data, Uphole data, Sonic logs (synthetic seismograms), Geological constraints on fracture zones / fault planes
- Independent additional information:electrical and /or electromagnetic data, georadar data, georadar crosshole tomography, borehole logs
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Operating Expense: |
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- Crew size:1 key person, 1-2 assistants
- Acquisition speed: 3 - 4 tomographic vertical planes (full data sets) depends on inclination of hole; between 50 m deep boreholes may be acquired during 1 day.
- Processing: Requires 2 - 3 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 (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).
- 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.
- Surveying: source and receiver positions should be known with an accuracy of at least 1/10 of the dominant wavelength (accuracy in the range of a few cm).
- 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.
- 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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