Beam angle parameters and related calculations in Echoview

There are various acoustic parameters that are used in Echoview that relate to the shape and strength of the acoustic beam. This topic explains some instances in which the parameters are used to calculate other quantities and details adjustments to the settings that may be necessary in order to achieve valid single target analysis results. The parameters under consideration here are the 3dB beam widths and the Equivalent two-way beam angle (Equivalent TWBA).

3dB beam widths
Equivalent TWBA
Single target TWBA and across-track beam angle re-calculation

3dB beam widths

3dB beam width specifications are provided by your hardware manufacturer and in many cases are also encoded into the data files that you are using. It is recommended that the manufacturer’s values are always used for the 3dB beam width settings. A pair of (major- and minor-axis) 3dB beam widths may be read by Echoview from your data files. MajorAxis3dbBeamAngle and MinorAxis3dbBeamAngle values are displayed on the Calibration page of the Variable Properties dialog box for raw variables.

The 3dB beam widths are used to calculate the beam compensation for dual and split beam single targets, and therefore incorrect values will yield incorrect beam compensation and therefore incorrect single target TS values.

For wedge volume analysis, the major-axis 3dB beam width is used as the default value for the across-track beam width in the wedge volume_sampled calculation. An option is provided to override that default with a user-entered value for the calibration setting WedgeVolumeBeamAngle.

The major-axis 3dB beam width is valid as the across-track beam width only when:

Notes:

Equivalent TWBA

The Equivalent two-way beam angle is displayed as TwoWayBeamAngle on the Calibration page of the Variable properties dialog box for Sv and Single target variables. The Equivalent TWBA specification is provided by your hardware manufacturer for a given transducer and in many cases is also encoded into the data files that you are using. It is recommended that the manufacturer’s value is always used for the Equivalent TWBA setting in Sv variables. For data from certain echosounders the TWBA is used in Echoview to calculate the Sv values, and so an incorrect value will yield incorrectly calibrated Sv data.

For single target variables and Sv variables the TWBA is also used to calculate the value of the export variable Beam_volume_sum. It is important to note when calculating echo density (i.e. the number of single targets in an analysis domain divided by Beam_volume_sum) that when the manufacturer’s value of the TWBA is used, Beam_volume_sum is only valid if the single target maximum beam compensation is set to 6dB. See Single target TWBA and across-track beam angle re-calculation for notes on how the TWBA value in the single target variable should be adjusted when the single target maximum beam compensation is other than 6dB.

Note: Under some file formats, Echoview calculates Sv using a power to Sv equation with parameters that can include Equivalent TWBA and other transducer and environment constants. Consequently, integration export variables that use Sv in their calculations are affected by the value of the Equivalent TWBA. Check your file format for details about Sv equations.

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 widths.

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 width.

First of all, the 3dB beam widths 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 width and should not be changed on the Calibration page of the Variable Properties dialog box.

Adjustment equation for the beam width

Adjustment equation for the beam width

Notes:

Adjustment equation for the Equivalent TWBA

Adjustment equation for the equivalent TWBA

Note that the adjusted Equivalent TWBA should only be used on the Calibration page for single target variables for which Beam_volume_sum is being calculated.

The manufacturer’s Equivalent TWBA value should always be used for Sv variables.

Caveat: In both of the 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.

See also

About single targets: Analysis
Single target detection
Single target detection settings: Single beam
Single target detection settings: Split beam
Single target detection settings: Dual beam
Single target detection settings: Wideband
Beam compensation
Instrument file formats
Beam volume sum
Wedge volume sampled