When there is more than one sound velocity profile, the program can use one of four options for selecting an SVP at the time of each swath:
• Previous in Time: Select the profile with a time stamp prior and nearest to the time of the swath.
• Nearest in Time: Select the profile with a time stamp nearest to the time of the swath. This can either be prior to or after the time.
• Nearest in Distance: Select the profile with a position nearest the position of the swath. This requires that positions be included in the SVP file.
The process of Sound Velocity Correction cannot be reversed.
Once it is applied, data cannot be restored to its pre-SVC state.
Sound Velocity Correction: Sound Velocity Processing
• Nearest Distance Within Time: Select the profile with a position nearest to the swath and has a timestamp nearest the time of the swath within a selected range.
The following diagram demonstrates the relationship of the profile options to the time/location of the swath.
P1, P2 and P3 all are calculated within a time period. P4 is calculated independently of the time period.
P1=Previous in Time P2=Nearest in Time
P3=Nearest Distance Within Time P4=Nearest in Distance
Sound Velocity Correction: Sound Velocity Correction
Sound Velocity Correction
The sound velocity correction process uses a ray tracing
algorithm to apply the sound velocity profiles. You can use either regular attitude data or smoothed data for applying vessel motion data to the ray-tracing algorithm (see “ATTITUDE FILTER” ON PAGE286).
The SVC process will select the profile to apply based on the method you select. You can also choose to apply the last method used.
To apply Sound Velocity Correction:
1. Select a track line or group of track lines.
2. Select the Sound Velocity Correction command.
The Sound Velocity Correction dialog box is displayed.
It is recommended that sound velocity correction be applied to bathymetry before cleaning the data. Whenever SVC is applied, the Merge command should also be applied to the data
Menu Process > Sound Velocity Correction
Tool
Sound Velocity Correction: Sound Velocity Correction
SVC will use either the sound velocity profile from your converted data, or the last SVP applied to the data. You can choose to load another profile, and you can edit a profile to use.
To use a new profile:
3. Select the Load new SVP file check box, and click Select to choose an SVP file.
The “Browse for SVP file” dialog box is displayed.
4. Select the file you want, or type the file path in the File Name text box.
The default directory is …\Hips\Svp.
5. Click Open.
The file name and path is displayed in the text box.
If you want to edit the data, click Edit to open the SVP Editor.
(See “EDIT SVP DATA” ONPAGE140.)
Profile selection method When there is more than one sound velocity profile for the data, HIPS can use one of four options for selecting an SVP.
6. Select a method from the drop-down list:
• Previous in time - Apply the SV profile that was taken just prior to the recording of the survey data.
• Nearest in time - Apply the SV profile that was taken closest in time to the recording of the survey data.
• Nearest in distance - Apply the SV profile that was taken closest to the area where the survey data was recorded.
• Nearest distance within time - Apply the SV profile with a position that is nearest to the swath and has a timestamp that is nearest to the time of the swath, within a selected range.
You also have the option of applying the last correction method last used on each line, to the currently selected data.
7. Select Last used method to apply the last selected profile method to the current SVC.
Use Surface Sound Speed
For systems which employ a flat transducer design, it is critical for the beam-forming electronics to know the exact sound speed at the location of the transducer. Therefore a sound speed probe is installed at the transducer. When this surface sound speed data is available, HIPS converts it and stores the time series data in the SSP file ( in the line directory). This data can be displayed in the Attitude Editor.
When HIPS performs sound velocity correction, the SSP data, if available, is retrieved and "inserted" into the Sound Velocity Profile (SVP) at the location of the transducer depth, replacing
If a line has not had SVC applied previously, attempting to use Last used method for correction will cause the process to fail. In this case, choose another SVC method.
Sound Velocity Correction: Sound Velocity Correction
the interpolated sound speed value at that depth. Ray-tracing then starts from that location.
To use available SSP data in sound velocity correction:
8. Select the Use Surface Sound Speed if available check box in the dialog box.
There are times, however, when the SSP data is not correct, causing the beam-forming electronics to erroneously compute beam angle data. If you determine that this has happened, you can enable the option to recompute beam angles. The recorded SSP (known to be incorrect) along with the sound speed
interpolated from SVP (assumed to be correct), will be used to compute new beam angle data. Ray-tracing will then proceed without insertion of the SSP into the SVP.
To use the available SSP data only to compute new beam angle data:
9. Select the Perform an additional recomputation check box to re-compute the steered beam angles based on a new surface sound speed that is interpolated from the sound velocity profile.
Only certain systems support the recomputation of the steered beam angles. These include: Simrad EM 12/120/2040/3000-3008/
3020/300/710/122/302 (except when using SimradSVC Licenced dll), Atlas SURF, Geoacoustics RDF, Teledyne Reson 7K (when using the 7004/7006 depth records), Benthos C3D and
SWATHplus.
Apply Delayed Heave In previous versions of HIPS, SVC used TrueHeave data in place of regular heave data when TrueHeave was available. Do not select this option if you want the original heave data to be applied.
10.Select Apply Delayed Heave to apply delayed heave values instead of regular heave.
If you choose this option, and no delayed heave data is available, there will be a warning message in the Output window, and regular heave data will be used instead.
Smoothed Sensors 11. Select which smoothed sensor data is to be applied during sound velocity correction: Heave, Pitch, Roll, Delta Draft. (Sensor data can be smoothed in the Attitude Editor. See “FILTERAND SMOOTH” ONPAGE16).
Note: Waterline values are not interpolated during SVC.
Sound Velocity Correction: Sound Velocity Correction
12..Click Process to apply the SVP file to the selected lines.
Lines which have been SV Corrected will be displayed in the Display window in the Not Merged colour (as set in the Properties for the Ship Track Lines layer).
As well, the SVP Corrected field in the Selection tab will show
“Yes” for any selected line to which SVC has been applied.
Sound Velocity Correction: Query Applied SVC Profiles