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

Adjustment for single target acorss-track beam angle

Beam volume angle

Equivalent TWBA for Sv analysis

Beam volume angle for single target analysis

Prior to Echvoiew 13.1

Single target beam volume angle and across-track beam angle calculations prior to Echoview 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.

Adjustment for single target across-track beam angle

The single target analysis variable Wedge_volume_sampled uses the smallest of Maximum beam compensation or Filter beam compenstation and the MajorAxis3dbBeamAngle to adjust the across-track beam angle.

Where:

MajorAxis3dbBeamAngle is the MajorAxis3dbBeamAngle in degrees on the Calibration page of the Variable Properties dialog box.

BeamCompensationdB is the smallest of Maximum beam compensation (dB) or Filter beam compensation (dB) on the Variable Properties dialog box.

Notes:

The default calculation for adjusted across-track beam angle is not used to change the general calibration values for MajorAxis3dbBeamAngle or MinorAxis3dbBeamAngle. Be aware 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.
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. Apply the value as a WedgeVolumeBeamAngle on the Calibration page of Variable Properties dialog box via 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 used for Beam volume sum and Wedge volume sampled calculations carried out on single target echoram data. See also Calculation for single target beam volume angle prior to Echoview 13.1.

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

Note, the type of single target detection operator may restrict the availability of one or more of these settings according to the nature of the detection method and the operand data types. Split-beam single target detection operators use TS and angular position data and offer both Beam compensation and Filter off-axis settings. Single beam single target detection data use only TS data and cannot offer either setting.

Beam volume angle

Where:

SingleTargetBeamAngleradians is the Single target beam angle (radians) calculated from Filter off-axis angle or Beam compensation settings (whichever results in the smallest angle).

Off-axis angle

When the Filter off-axis angle is the smallest angle:

Where:

OffAxisAngleradians is the Filter off-axis angle(degrees) on the Filter Targets page of the Variable Properties dialog box and converted to radians.

Beam compensation

Beam compensation may be specified on either the Single targets detection page or the Filter Targets page of the Variable Properties dialog box. Echoview uses the smaller of the Beam compensation settings to calculate the adjusted off-axis angle for angle comparison. When a beam compensation is the smallest angle:

Where:

BeamCompensationdB is the smallest of Maximum beam compensation (dB) or Filter beam compensation (dB) on the Variable Properties dialog box.

Mean3dbaAnglesradians is the mean of the Major3dbBeamAngle (degrees) and Minor3dbBeamAngle (degrees) on the Variable Properties dialog box converted to radians.

Single target beam volume angle and across-track beam angle prior to Echoview 13.1

Beam volume sum

Prior to Echoview 13.1, single target analysis used an adjusted Equivalent two-way beam angle to represent the single target beam volume angle. It was possible to change the angle used for the beam volume sum calculation by adjusting the equivalent TwoWayBeamAngle in an ECS file, but this requirement was easily overlooked and furthermore affected any Sv calculations in the same dataset.

A review of the calculation for Beam volume sum found that the adjustment misrepresented the single target beam volume because the volume is defined by the Maximum beam compensation, and the Filter beam compensation and Filter off-axis angles of the Variable 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.

Wedge volume sampled

Similarly, prior to Echoview 13.1, single target analysis used an adjusted across-track angle to represent the single target across-track beam angle. It was possible to change the angle used for the wedge volume sampled calculation by adjusting a 3dB beam angle or using a value for WedgeVolumeSampledAngle in an ECS file. This requirement was easily overlooked and could affect single target beam compensation calculations, or affect other calculations that used a 3dB beam angle.

A review of the calculation for Wedge volume sampled found that the adjustment misrepresented the single target beam angle because the angle is defined by the Maximum beam compensation, and the Filter beam compensation of the Variable Properties.

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.