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Sound Velocity Profiling (SVP) of the water column is essential to the acquisition of swath mapping data, and is used for ray tracing through the water column. SVP influences directly the accuracy and uncertainty of both the horizontal and vertical position of each sounding and its impact is greatest towards the outer beams of the swath (farthest sounding).
Physical processes such as fresh water influx, solar warming of the upper water column, presence of mesoscale currents, and storm mixing can affect the temperature and salinity profile, and hence the SVP. These changes can occur at various spatial and temporal scales and can sometimes be observed in the water column backscatter data.
Acquisition of SVPs must be planned to identify the relevant number and distribution of profiles and monitored carefully during the survey.
If the sound speed at transducer varies the transducer is different to the sound speed cast (by 2m/s for baseline surveys ( or 1m/s for the monitoring surveys) then another SVP should be collected. Otherwise, the outer beams of the swath shows a “smile” or a “frown” for under corrected or over corrected sound speed.
The SVP can be determined using one of the following methods:
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direct observation via deployment of an SVP measuring device
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calculation of the SVP through deployment of an expendable bathy thermograph (XBT)
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bar check
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acquired using direct SVP measuring device, XBT, bar check and CTD.
Sound speed can be affected by:
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Check for “smiles” or “frowns” in the survey outer beams. Correct the sound speed issues in your bathymetry processing software.
Pass Requirement
Sound speed check: Make sure that all the sound speed issues have been corrected in your bathymetry processing software (no “smiles” or “frown”) especially between adjacent survey swath lines.
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