Single target beam volume angle prior to Echoview 13.1

In earlier versions of Echoview (13.0 and older), the single target analysis Beam volume sum and Wedge volume sample calculations advised adjustments based on the Maximum beam compensation to the TwoWayBeamAngle and to across-track 3dB beam angle, via temporary ECS file changes to asociated calibration settings. This requirement was easily overlooked and furthermore affected any Sv calculations in the same dataset.

It came to our attention that the adjustment misrepresented the single target beam volume angle. It did not consider all the contributors to the single targets beam volume angle, namely Maximum beam compensation, Filter beam compensation and Filter off-axis angle. For further information on the current calculations refer to Beam angle parameters: Beam volume angle for single target analysis.

The historic adjustment discussion follows.

Adjusted single target Equivalent TWBA and across-track beam angle re-calculation

Often, setting a Maximum beam compensation (two-way) other than 6dB is desirable when analyzing single targets. Allowing a larger Maximum beam compensation effectively ‘opens’ the beam and allows detection of more single targets. Most manufacturers specify a limit of 12dB maximum beam compensation on the accuracy of their beam compensation formulae, which yields a sensible upper limit when accurate target strength measurements are important. However, the wedge volume and beam volume calculations assume that 6dB Maximum beam compensation is used – that is, that targets can only be detected and counted in the part of the beam described by the 3dB beam angles.

Increasing the Maximum beam compensation effectively increases the portion of the beam in which targets can be detected and counted, therefore also increasing the effective sampled volume. Since the wedge volume and beam volume calculations both represent the acoustically sampled volume, the values used to calculate those quantities need to be adjusted appropriately when the single target detection operator setting Maximum beam compensation is other than 6dB.

Adjustment equations

Using a Maximum beam compensation of X dB requires the following corrections to the Equivalent TWBA and to the across-track beam angle.

First of all, the 3dB beam angles need to be adjusted for the new beam compensation. However, these adjusted values are only for use in calculation of a modified TWBA or across-track beam angles and should not be used to change the general calibration values for MajorAxis3dbBeamAngle or MinorAxis3dbBeamAngle. It is suggested that the calculated New_beam_angle be used as WedgeVolumeBeamAngle on the Calibration page of Variable Properties dialog box via an ECS file.

Adjustment equation for the across track angle

Adjustment equation for the beam angle

Notes:

If the transducer is installed vertically with the major-axis aligned across-track, then the revised major-axis beam angle should be used as the WedgeVolumeBeamAngle on the Calibration page of Variable Properties dialog box via an ECS file.
If the transducer is elliptical and/or is installed with a different orientation, then the across-track beam angle will need to be calculated by taking the transducer’s geometry and orientation into account. Apply the value using WedgeVolumeBeamAngle on the Calibration page of Variable Properties dialog box via an ECS file.

Adjustment equation for the Equivalent TWBA

Adjustment equation for the equivalent TWBA

Where:

is the sound-speed-adjusted TwoWayBeamAngle in dB re 1 steradian, used in Sv analysis.

BeamCompensationdB is the Maximum beam compensation in dB on the Single Target Detection page of Variable Properties dialog box.

Note that this new Equivalent TWBA should only be used for single target variables for which Beam_volume_sum is being calculated. Use an ECS file to make a LocalCal change.

Caveat: In both of the beam-compensation-based adjustment calculations above, the formulae used are generic and will give approximate results. It is very likely that the manufacturer of the echosounder that you use can provide formulae that give more accurate results for their specific equipment and algorithms.

Single target beam volume angle prior to Echoview 13.1

Adjusted single target Equivalent TWBA and across-track beam angle re-calculation

Adjustment equations

Adjustment equation for the across track angle

Adjustment equation for the Equivalent TWBA

See also