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.

3dB beam angles

Beam volume angle

Equivalent TWBA, Sv analysis

Beam volume solid angle, single target analysis

Prior to Echvoiew 13.1

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

the transducer is installed with the major-axis aligned across-track
the single target detection operator setting Maximum beam compensation (dB) is set to 6dB

Notes:

Review the Echoview support for 3dB beam angle calibration settings for your file format. Calibration settings and values can be edited through an ECS file.

Beam volume angle

Equivalent TWBA for Sv analysis

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

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:

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 where the Sv data was recorded.

The Demer (2015) 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.

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.

Beam volume angle for single target analysis

The single target beam volume is defined by the single target detection variable's specified values for Maximum beam compensation, Filter off-axis angle and Filter beam compensation. The single target Beam volume angle is calculated using the smallest adjusted off-axis angle of the following settings:

the Maximum beam compensation on the Single Target Detection page
the Filter off-axis angle on the Filter Targets page
the Filter beam compensation on the Filter Targets page

Off-axis angle

When the Filter off-axis angle is the smallest adjusted off-axis angle, the BeamVolumeAnglesteradians value is calculated based on the Filter off-axis angle.

Filter beam compensation

When the Filter beam compensation is the smallest adjusted off-axis angle, the BeamVolumeAnglesteradians value is calculated based on the Filter beam compensation.

Maximum beam compensation

When the Maximum beam compensation is the smallest adjusted off-axis angle the BeamVolumeAnglesteradians is calculated based on the Maximum beam compensation.

Echoview uses a beam volume angle derived from the 3dB beam angles for Beam volume sum and Wedge volume sampled calculations. Prior to Echoview 13.1, single target analysis used the TwoWayBeamAngle to represent the beam volume angle for Beam_volume_sum.

Notes:

For elliptical transducers the same formula applies, but must be applied separately to major- and minor-axis beam angle via an ECS file.
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 taking the transducer’s geometry and orientation into account.

Prior to Echoview 13.1

Prior to Echoview 13.1, single target analysis used the Equivalent TWBA for Sv analysis to represent the beam volume angle for Beam_volume_sum. Equivalent TWBAOld is in dB .

Adjustment equation for the equivalent TWBA

This misrepresented the volume for single target analysis because single target beam volume is defined by the variable's Maximum beam compensation and Filter off-axis angles properties.

The correction to single target beam volume affects previous fish density estimates obtained using echo counting, including echo density calculated by the Cell statistic [Single targets] operator. The magnitude of this effect will be proportional to the difference between the equivalent two-way beam angle and the single target based solid angle. Note that the beam volume sum reported with echo integration exports (from Sv echograms) has not been changed: this volume calculation still uses the equivalent two-way beam angle.

Similarly, the calculation of Wedge_volume_sampled now incorporates any reductions to the single target beam volume angle due to the identified single target variable properties. The correction to wedge volume sampled affects point target density estimates obtained via an echo counting method in which the number of fish tracks is divided by the wedge volume sampled. The magnitude of the effect will depend on the difference between the 3 dB major-axis beam angle (or WedgeVolumeBeamAngle) previously used, and the angle defined by the settings in single target detection (dependent on Maximum beam compensation and Filter off-axis angle). For example, if the maximum beam compensation is set to less than 6 dB, the angle used by wedge volume sampled will be smaller than the 3 dB major-axis beam angle.

For further assistance relating to this calculation change please contact support@echoview.com.