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. Demer et al (2015) discuss aspects of acoustic instrument calibration with respect to quantitative analysis of data for aquatic resource abundance. This page explains instances in which the parameters are used to calculate acoustic metrics and details adjustments to the settings that may be necessary in order to achieve accurate acoustic measurements. The parameters under consideration here are the 3dB beam angles and the equivalent two-way beam angle (Equivalent TWBA).

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

3dB beam angles

For single beam echosounders, Demer et al (2015) recommend the use of 3dB beam angles estimated from sphere calibration results.

3dB beam angle specifications are provided by your hardware manufacturer and in many cases are also encoded into the data files that you are using. A pair of (major- and minor-axis) 3dB beam angles 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. Use an ECS file to change the values for MajorAxis3dbBeamAngle and MinorAxis3dbBeamAngle.

The 3dB beam angles 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 angle is used as the default value for the across-track beam angle in the wedge volume_sampled calculation. An option is provided to override that default with a user-entered value for the calibration setting WedgeVolumeBeamAngle in an ECS file.

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

Notes:

Equivalent TWBA

For single beam echosounders, Demer et al (2015) recommend the use of an adjusted Equivalent TWBA value for Sv variables. See also Calibration Assistant: Two way beam angle correction.

Adusted equivalent two way beam angle equation

Where:

is the TwoWayBeamAngle in steradian, adjusted for local sound speed.

is the TwoWayBeamAngle in dB re 1 steradian, adjusted for local sound speed.

is TwoWayBeamAngle in steradian measured by the manufacturer under sound speed cw_Manufacturer (m/s).

The relationship between the linear and dB TwoWayBeamAngle is: relationship equation between linear and dB

cw_Manufacturer is the sound speed (m/s) recorded by the manufacturer when was measured.

cw_Local = The sound speed of water (m/s) at the location the Sv data was recorded.

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, measured under controlled conditions at a known sound speed (cw_Manufacturer), is provided by your hardware manufacturer for a given transducer. Often the cw_Manufacturer is reported in dB re 1 steradian but the linear form is required for calculation. In many cases cw_Manufacturer is encoded into data files and read by Echoview. Use an ECS file to change the value of TwoWayBeamAngle.

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 or the sound-speed-adjusted 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 used by the single target variable should be re-calculated 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 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.

Adjustment equation for the beam angle

Adjustment equation for the beam angle

Notes:

Adjustment equation for the Equivalent TWBA

Adjustment equation for the equivalent TWBA

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.

Sv variables require the manufacturer's TWBA or a sound-speed-adjusted TWBA.

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.

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