.Note: Enter new TS values and click Apply, to assess the effects of different TS values on the echogram.
The Minimum TS threshold algorithm is applied to an Sv echogram when Apply is selected in the Minimum TS threshold section of the Data page of the Variable Properties dialog box of an Sv variable.
The intent of such a threshold is to remove samples below a specified TS value. The algorithm uses the Sv equivalent of the TS threshold in its calculations, so that samples are thresholded out in an unbiased way. The Minimum TS threshold function implements a standard step in the methodology for lake analysis1.
This page covers the topics:
It is advisable to complete the following preparations before you apply a Minimum TS threshold to an Sv variable.
.Note: Refer to TVG range correction algorithms for more information about TVG range correction methods.
Knowing the initial conditions for the Sv data and the TS value, it is possible to convert TS to Sv. To account for the fact that a single TS value (re to 1m) can correspond to multiple Sv values, where Sv varies with range, Echoview calculates a quantity called the Converted Sv for each sample in the echogram. The general form is as follows:
where:
= Sv for the ith sample in the jth ping at range Ri, j. The power to Sv equations are:
= TS Threshold (dB) setting on the Data page of the Variable Properties dialog box. The TS (dB) value is relative to 1m. The power to TS equations are:
= Absorption coefficient (dB/m) on the Calibration page of the Variable Properties dialog box for the Sv variable. = Corrected range (m) for Svi, j = 1 (m). This is the corrected range (m) for the TS threshold (dB). = Sound speed (m/s) on the Calibration page of the Variable Properties dialog box for the Sv variable. = Transmitted pulse length (s) on the Calibration page of the Variable Properties dialog box for the Sv variable. = Equivalent Two-way beam angle (Steradians).
Echoview calculatesy in the following way:
Data y Simrad Ex500 y as 10^(Y/10), where:
Y is the Two-way beam angle (dB re 1 steradian) entered on the Calibration Page of the Variable Properties dialog box.
Simrad Ex60 y as 10^(Y/10), where:
Y is the Two-way beam angle (dB re 1 steradian) entered on the Calibration Page of the Variable Properties dialog box.
Biosonics
b2 is the mean squared beam pattern factor, given by
where
qa is the major axis 3dB beam angle in radians
qb is the minor axis 3dB beam angle in radians
The above calculations depend on the Sv variable having correctly entered calibration settings.
= Sv Calibration offset.
Echoview calculates CSv in the following way:
Data CSv Simrad Ex500 -10log(Fs) Simrad Ex60 -2 x Sa correction Biosonics 0 The above calculations depend on the Sv variable having correctly entered calibration settings.
= TS Calibration offset.
You need to enter the TS Calibration offset and the TS TVG range correction method on the TS TVG correction settings dialog box that is available when Apply is selected in the Minimum TS threshold section of the Data page of the Variable Properties dialog box of an Sv variable.
Data CTS Simrad Ex500 -10log(FTS) Simrad Ex60 0 Biosonics 0
Echoview applies the following algorithm to deal with samples with Sv values smaller than a specified TS threshold.
| If Svi, j - Converted Svi, j | = 0 | The sample value is unchanged. |
| > 0 | The sample value is unchanged. | |
| < 0 | The sample value takes on a value of -999.0 |
Data page of the Variable Properties dialog box
1. Parker Stetter, S. L., Rudstam, L.G., Sullivan, P.J. and Warner, D.M. 2009. Standard Operating Procedures for Fisheries Acoustic Surveys in the Great Lakes. Great Lakes Fisheries Commission Special Publication. 09-01. Standard operating procedures for data analysis. Step 11.
