Geophysical Mapping: Method Details
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Method Name: Shallow Water Seismics - Lake Seismics
Method Type:   Seismic Techniques
Assigned Problems:
+ Depth of Overburden-bedrock interface Civil Engineering
+ Earthquakes / paleoseismology Natural Hazards
+ Fractures Groundwater
+ Gravel, clay, limestone, salt exploration Natural Resources
0 Foundations of ancient structures Buildings and Structures
0 Host sediments, hydogeological settings Hazardous Waste
0 Quality and thickness (Natural resources) Natural Resources
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   Mapping surface and subsurface features of rivers, lakes, estuaries and coastal zones using elastic waves reflected at interfaces in the underground.
Keywords:   Seismic Techniques; lake seismics; 2-D / 3-D reflection seismics; seismic velocity contrast; seismic depth sections; subsurface models
Prerequisites:  
  • 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.
  • 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 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 values are: Investigation depth: ~30 m: vertical resolution ~0.5 m. The depth of investigation is typically from lake bottom to several tens, exceptionally a few hundreds of m.
Expected Results:  
  • Measured parameter: change of water pressure (as determined by the voltage generated by the calibrated hydrophone 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: 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, dual-frequency digital echo-sounder; side scan sonar; sub-bottom profiler (SBP), geological constraints on fracture zones / fault planes.
  • Independent additional information: borehole logs, drilling core.
Operating Expense:  
  • Crew size: 1 key person, 2-3 assistants
  • Acquisition speed: geophone chains or reverse VSP (source in borehole) can significantly reduce acquisition time.
  • Processing: Requires 2 - 3 days per acquisition day.
  • Equipment rental costs: high
Parameters to specify:  
  • Source type / Source parameters at surface: airguns, water guns, sparkers, boomers and pingers.
  • Seismograph: Channel number, dynamic range (number of channels depends on equipment; 16 bit or more dynamic range).
  • Hydrophone 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 hydrophone spacing) determine the hydrophone spacing. The hydrophone 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 hydrophone spacing).
  • 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.
  • 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.
  • First-arrival times and / or amplitudes of seismic signals.
  • Subsurface models (depth-distance plots; 2-D and / or 3-D subsurface models).
  • Seismic time section (stacked data).
  • Time-migrated seismic section.
  • Seismic depth section.
  • 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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