Geophysical Mapping: Method Details
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Method Name: Shear-wave reflection seismics
Method Type:   Seismic Techniques
Assigned Problems:
+ Depth of Overburden-bedrock interface Civil Engineering
+ Fractures Groundwater
+ Gravel, clay, limestone, salt exploration Natural Resources
+ Heat mining Natural Resources
+ Host sediments, hydogeological settings Hazardous Waste
+ Landslides Natural Hazards
+ Quality / Thickness of aquifer/aquitard Groundwater
+ Soil / rock quality Civil Engineering
+ Young's / shear modulus, Poisson's ratio Civil Engineering
0 Quality and thickness (Natural resources) Natural Resources
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   Measurement of travel times of elastic transversel waves (S-waves) reflected at interfaces in the underground (depth range few m to several tens of m).
Keywords:   Seismic Techniques; Shear-wave reflection seismics; 2-D / 3-D reflection seismics; Transverse / secondary waves; Seismic depth sections; Subsurface models
Prerequisites:  
  • 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.
  • 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.
  • 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.
Resolution:   Vertical and horizontal resolutions depend 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 resolutions and different depth of investigation are:
  • Depth of investigation (min / max): few m / several tens of m
  • Vertical resolution (min / max): several dm / few m
  • Horizontal resolution (min / max): few m / few tens of m
  • S-waves provide higher resolution but have minor depth of investigation because of higher attenuation than P-waves
Expected Results:  
  • 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). Processing of reflection seismic data yields an image of reflectors (either in travel-time or depth: seismic time or depth section). Migration and / or depth-conversion is required for one-to-one correlation with other geophysical or geological data.
  • 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. Features dipping greater than 45 on stacked (un-migrated) seismic sections are unlikely to be real reflections.
Combination with other Methods:  
  • Required additional information:Geological information is necessary for the interpretation.
  • Related add-on information:Refraction seismic data, Surface-based tomographic data, VSP data, Uphole data, Geological constraints on fracture zones / fault planes
  • Independent additional information:electrical and /or electromagnetic data, georadar data gravity data, borehole logs
Operating Expense:  
  • 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 3 -4 days per acquisition day.
  • Equipment rental costs: high
Parameters to specify:  
  • Source type / Source parameters (e.g., amount of explosive, hammer, weight-drop, vibrators) Geophone type (usually 3-component geophone; geophones with only one component in horizontal direction are limited use).
  • 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)
QC Documents:  
  • 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).
Products:  
  • Raw data and geometry files.
  • Measurement of noise level.
  • Recordings (particle velocity in three orthogonal directions) with frequency analysis.
  • First-arrival times and / or amplitudes of seismic signals.
  • Subsurface models (depth-distance plots; 2-D and / or 3-D subsurface models).
  • Data after pre-stack processing.
  • Seismic time section (stacked data).
  • Time-migrated seismic section.
  • Seismic depth section.
  • Velocity-depth functions; (P-wave / S-wave velocity ratio).
  • 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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