Single target detection dual beam method

Method 1

This operator detects single targets using a modified version of the single target detection single beam (method 1) algorithm that applies compensation estimates (based on difference between a narrow beam and wide beam signal) to the peak selection criteria.

Echoview accepts operands of the following data types as input:

Operand 1	Operand 2
TS (narrow beam)	TS (wide beam)

Each operand must have been associated with a different transducer and the 3dB beam width settings must be defined for each transducer. You associate a transducer with a variable and corresponding 3dB beam angles on the Calibration page of the Variable properties dialog box.

The average of the 3dB beam angles from the transducer of the first operand must not be greater than the average of the 3dB beam angles from the transducer of the second operand, otherwise no output data will be calculated.

For algorithm information see Dual beam (Method 1).

Method 2

This operator detects single targets using a modified version of the single target detection single beam (method 2) algorithm that applies compensation estimates (based on the difference between a narrow beam and wide beam signal) to the peak selection criteria.

Echoview accepts operands of the following data types as input:

Operand 1	Operand 2
TS (narrow beam)	TS (wide beam)

Each operand must have been associated with a different transducer and the 3dB beam angle settings must be defined for each transducer. You associate a transducer with a variable and corresponding 3dB beam angles on the Calibration page of the Variables properties dialog box.

For algorithm information see Dual beam (Method 2).

Settings

The Single Target Detection Dual Beam (Method 1 or Method 2) Variable Properties dialog box pages include (common) Variable Properties pages and these operator pages:

Operands page

Filter Targets page

Single Target Detection page

Single Target Detection parameters

Setting

Description

TS threshold (dB re 1m2) (compensated TS)

Method 1 operators apply a compensated TS threshold before the detection of initial narrow beam and wide beam targets.

Compensated TS threshold represents the TS of the smallest single target of interest to you. Determine what TS you are interested in (the TS for fish rather than the TS for plankton) and set TS threshold to a value that is a bit smaller.

The minimum value for this setting is -120 dB re 1m2. This minimum value will detect spike noise, especially if you also use a short minimum pulse length.

Method 2 operators apply a compensated TS threshold after the detection of initial narrow beam and wide beam targets.

The minimum value for this setting is -120 dB re 1m2. This minimum value will detect spike noise, especially if you also use a short minimum pulse length.

Pulse length determination level (dB re 1m2 for method 1)

Pulse length determination level (dB re 1W for method 2)

This is theoretically and in-practice 6 dB (and rarely changed). Method 2 operators remove TVG before searching for peaks.

PLDL is calculated relative to the pulse's TS peak. Samples above the PLDL are included in the detection. Normalized pulse length and Standard deviation of angles are based on included samples.

Minimum normalized pulse length

Minimum normalized pulse length together with Maximum normalized pulse length are used to reject pulses that fail the estimate for expected echo length.

Too short a pulse length allows too many false detections. Too long a pulse length allows the loss of too many real detections. (Pers. comm. I. Higginbottom).

The minimum value is 0.01.

EK60

Samples are separated by 1/4 pulse length. Apparent Echoview normalized lengths will be 0.25, 0.5, 0.75, 1, 1.25, 1.5. These values are due to Simrad's sampling and the fact that Echoview does not interpolate.

Typically a 0.7 minimum normalized pulse length results in three samples in a pulse.
1.3 results in 5 samples in a pulse.
Over 1 and less than 1.25 results in only 4 samples, which is one normalized pulse.

BioSonics

BioSonics data has many more samples in a pulse. The estimate for echo length is not as constrained as for Simrad data.

Maximum normalized pulse length

Minimum and Maximum normalized pulse length are used to reject pulses that fail the estimate for expected echo length. See Minimum normalized pulse length.

Too short a pulse length allows the loss of too many real detections. Too long a pulse length allows too many false detections. (Pers. comm. I. Higginbottom).

The maximum value is 10.

Maximum beam compensation (dB re 1m2)

Beam patterns can be modeled and can be used to estimate what the TS value would be if the target was central in the beam.

Higher values of beam compensation result in more single targets. Many beam compensation models cite limits of validity.

"... In shallow lakes (western Lake Erie) or in lakes with few targets (e.g. Lakes Superior and Huron), we recommend using a larger beam compensation to increase the number of detected single-fish echoes. Our analysis shows less than a 1 dB difference when beam compensation is increased from 3 to 12 dB. This potential bias has to be weighed against the increase in the precision of in situ TS measures due to more identified
targets..." (Parker-Stetter et al: Section 9.2 Single-echo detection).

"In effect the higher the value the further from the beam center targets will be accepted. The default value of 4 dB is optimized for accurate TS estimation, because there is more uncertainty in the compensation the further off axis. For just detecting targets, especially strong targets that might be in the edge of the beam, it is fine to use a larger setting... More noise and false single targets are detected as Maximum beam compensation is increased." (Pers. comm. I. Higginbottom).

The maximum value is 35 dB re 1m2 (split beam and dual beam only).

Note: The value for Maximum beam compensation is important when using the export variables Beam_volume_sum and Wedge_volume_sampled. For further information refer to Beam angle parameters.

Over axis angle threshold (dB re 1m2)

The BioSonics dual-beam beam compensation algorithm uses the Over axis angle threshold to deal with single targets that may produce unrealistic beam compensation estimates. When beam compensation is less than the Over axis angle threshold then the target is discarded. When beam compensation is less than zero and is greater than the Over axis angle threshold, beam compensation is set to zero.

The minimum value is -99 dB re 1m2 (dual beam only).

Exclusion

The Exclusion section is used to select lines to limit targets and minimize processing.

Setting

Description

Exclude targets above line

These are analogous to the Exclude-above and Exclude-below lines used on the Analysis page.

The exclusions on the Single target detection page are applied before the echogram is displayed (affecting the number of targets displayed) and the exclusions on the Analysis page are applied after the echogram is displayed (affecting the on-screen and exported analyses). See About setting variable properties for an illustration of which pages apply when.

Exclude targets below line

The correct configuration of calibration settings (environment and echosounder system) and transducer geometry for the operands is important. You may need to configure raw variables prior to using the operator. For more information refer to Creating single target detection variables from dual beam data.

If you are familiar with Single target detection (or TS-Detection) on Simrad echosounders, see Simrad and Echoview single target detection terminology for a comparison of the terms used.