Multibeam Magnitudes to Sv
Echoview provides Sv variables for data generated by multibeam echosounders. These are derived from the signal magnitudes (or intensities) stored in the respective data files (see Echosounder file formats for more information). The method of calculating Sv is as follows:
If a time varied gain was applied to the data at the time of recoding, then:
Sv = Pr - x log r + 20log r + C + Cb
Otherwise,
Sv = Pr + 20log r + 2a r + C + Cb
where:
| Pr |
= |
a number proportional to the received power (dB) - determined from the data file |
| x |
= |
the TVG range coefficient, included in the recorded signal strength - determined from the data file (typically 20, 30 or 40) |
| r |
= |
range (m) - determined from the data file |
| a |
= |
absorption coefficient (dB/m) - specified on the Calibration page of the Variable Properties dialog box |
| C |
= |
A calibration offset common to all beams (dB) - specified as CalibrationOffset on the Calibration page of the Variable Properties dialog box |
| Cb |
= |
A calibration offset specific to beam b (dB) - specified as BeamOffsets on the Calibration page of the Variable Properties dialog box |
Note:
- This manner of specifying calibration settings is very general and many physical factors are included in the terms C and Cb, including the transmitted power, the transmitted pulse length, the equivalent two-way beam angle, the signal frequency, the transducer gain and a factor for converting the stored signal strength to voltage.
- Most multibeam data formats store the signal strength proportional to voltage (let us call it v) with some value for the TVG range coefficient included. The term Pr then, is determined as 20log(v) and the time varied gain is removed (in the equation above) before adding the 20log(r) TVG required for Sv. See your echosounder file format for further information.
- The DIDSON echosounder records signal strength as an intensity proportional to power - that is, Pr is read from the DIDSON data file and does not include a time varied gain in the recorded signal. However a TVG range coefficient setting is available for DIDSON Sv variables on the Calibration page of the Variable Properties dialog box. This setting allows you to tune the TVG range coefficient to reflect recording conditions or for other purposes.