Target strength calibration summary
In order to calibrate the ME70, a reference target (calibration sphere) with known target strength (TS) is lowered into the sound beam. The measured target strength is compared with the known target strength of the sphere, and the ME70 is adjusted accordingly. We strongly recommend that calibration surveys are done at regular intervals. As a minimum, calibration must be done prior to each survey. A summary of the target strength calibration process is provided.
Prerequisites
In order to calibrate the ME70, the following equipment is required.
•  Calibration spheres: The calibration spheres must be chosen to match the operational frequencies used by the ME70 system.
•  Winch arrangement: The winch arrangement must provide the necessary lines to lower the sphere into the sound beam.
The vessel must be anchored in a suitable position with sufficient depth, calm and sheltered water, and minimum sea currents.
Note
Each calibration sphere must be handled with care to avoid any damage to its surface. When not in use, store the sphere in a household soap solution. All suspension lines must be as thin and clean as possible. Limit knots to a minimum, and keep them small. Even knots have strong echoes!
Kongsberg Maritime can supply a variety of copper and tungsten calibration spheres dedicated for different operational frequencies.
Context
We strongly recommend that calibration surveys are done at regular intervals. As a minimum, calibration must be done prior to each survey.
Note
Calibration must be taken seriously. To achieve the best results, the calibration must be planned and done carefully.
In order to calibrate the ME70, a reference target (calibration sphere) with known target strength (TS) is lowered into the sound beam. The measured target strength is compared with the known target strength of the sphere, and the ME70 is adjusted accordingly.
Each beam mode to be used on a survey must be calibrated separately. For each beam new estimates for adjusting gain and sA correction must be calculated using the measured Target strength (TS) data and the reference sphere values.
To successfully calibrate the ME70 you need to place a number of target detections in each sector. The echoes must be distributed within the entire beam cross-section. In order to do this, the target sphere must be physically moved inside the beam during the calibration process. If the sphere location is constant, you will be rewarded with many echoes, but all will be inside the same sector. The number of sphere detections should be approximately the same for each sector, and in total not more than 100 for the whole beam cross-section.
Any adjustments to the ME70are done automatically by the calibration program. No gain adjustments are required.
Note
When you calibrate the ME70 for target strength measurements with "live data", it is very important that you choose the correct channel, and that all the other channels are switched to Passive mode. You must do this before you start the calibration process.
Before you start the collection of system parameters and target strength data, you must select the reference sphere to be used, and define the layer limits close to the expected depth of the sphere.
Procedure
1 Prepare the vessel for ME70 calibration.
2 Select the Beam Mode to calibrate on the Setup menu.
3 On the Operation menu, set Operation to Normal.
4 Lower the sphere into the beam at the specified depth. Verify that the echoes are clearly detected.
5 On the Operation menu, open the Record button, and select On.
6 On the Setup menu, select Calibration.
Image
The Calibration dialog box opens. The dialog box displays a top bar with menus and four views for displaying information regarding the echoes during the calibration process.
7 Select File menu to enter parameter settings for the calibration.
a Select Setup to define the parameters of the calibration sphere, deviation and target depth limits.
b Select Set TS Detection to define parameters for the target strength detection level.
8 Select Start to start echo data import.
Use the different views in the calibration dialog box to watch data for and position of the echoes in the individual beams as well as in the Beam mode.
9 When the appropriate number of echoes have been imported in all the beams, select Stop.
10 Stop the recording.
11 Remove the echoes you do not wish to use in the calibration.
You can suspend and select target strength data in Numerical view under the tab TS-Data.
12 Select Save or Save As to save the calibration data.
13 Stop raw data recording.
14 Close the Calibration Wizard dialog box, and resume normal operation.
Further requirements
Repeat the procedure for each beam mode you wish to calibrate.