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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 usingelastic 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 frequencydecreases with depth, seismic resolution decreases with depth. Typical values are: Investigationdepth: ~30 m: vertical resolution ~0.5 m. The depth of investigation is typically from lake bottom toseveral 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 ofreflection seismic data yields an image of reflectors (either in travel-time or depth: seismic timeor depth section). Migration and / or depth-conversion is required for one-to-one correlation withother geophysical or geological data.

- Interpretation: seismic interpretation assumes that the resolved reflectors represent truelithological interfaces. Additional geological or geophysical surface data may be required forreliable interpretation. Features dipping greater than 45° on stacked (un-migrated) seismicsections are unlikely to be real reflections.

Combination with other Methods:- Required additional information: geological information is necessary for the interpretation.

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- Related add-on information: refraction seismic data, dual-frequency digital echo-sounder; sidescan sonar; sub-bottom profiler (SBP), geological constraints on fracture zones / fault planes.

- Independent additional information: borehole logs, drilling core.

Operation 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 isalways larger than the hydrophone spacing) determine the hydrophone spacing. Thehydrophone spacing may be reduced at the shot end of a profile (variable spacing) to provideadditional information on the shallow subsurface.

- Maximum offset (determines the depth of investigation; generally the maximum source-receiveroffset should be at least three to four times the required depth of investigation, but in certainareas 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.