Using Echoview in the calibration process

This topic explains how you can use Echoview in the calibration process. It contains the following sections:

Notes:

Overview

You must calibrate your echosounder before a survey and enter calibration settings in post processing to ensure the validity of the data values (e.g. TS, Sv values) derived from power data.

If your echosounder logs raw power only (i.e. Echoview derives TS, and/or Sv values from the power data) you must enter calibration settings in post processing. If your echosounder writes TS and/or Sv values you will need to calibrate your echosounder before the survey and enter additional calibration settings during post-processing.

Note: Echoview can automatically read the calibration settings from the first ping for Simrad Ex60 echosounders.

How can I use Echoview in the calibration process?

Echoview can currently assist with the following stages of the calibration process:

Note: When working with regions, the Echoview Essentials module is necessary for calibration of both TS and Sv variables. However, it is possible to use Echoview in demonstration mode for calibration if you base your calibration on a selection.

In summary, both stages are completed as follows:

  1. Collect calibration data in accordance with techniques originally described in Foote, et. al. 1987.
  2. A calibration sphere with a known target strength is suspended on the beam axis in the transducer beam.
  3. The sphere is repeatedly insonified and the power of the returned signal measured.

Notes:

What benefits does Echoview provide?

Echoview can help you in the calibration process as follows:

Benefit

Explanation

Reduced time and cost associated with calibration

You can collect calibration data in environmental conditions where it would not normally be possible, e.g. in rougher than optimal seas. The data you collect will contain both on-axis and off-axis single target detections but you can filter out the off-axis targets and use only on-axis single targets for calibration.

If you use Echoview in conjunction with one of Echoview Software's Echolog live viewing applications, you can monitor the collection of data in real time and collect only as much data as necessary to ensure statistically valid calibration, i.e. when you have enough on-axis single targets you can stop collecting calibration data, complete the calibration process, and begin your survey.

Increased accuracy of calibration

You can filter out off axis single targets (these may be detected even in light seas) to ensure calibration is performed using only on-axis data.

Appropriateness of calibration algorithms

Echoview may use different algorithms to those used in your echosounder, e.g. for single target detection. You should calibrate using the same algorithms that you use to process your data.

Check calibration in Echoview

If you collect calibration data as part of your survey you can check the accuracy of user calibration settings during post processing and adjust them as required to more accurately calibrate your data. Different variables can then be calibrated based on the core calibration data set.

Calculating the TS calibration constant

The TS calibration constant is a value that is used in the equation that calculates TS values from power data, e.g. Transducer gain on a Simrad Ex60. The name of the TS calibration constant and its use in the equation differs between echosounders but its overall purpose is always the same (i.e. to convert from power data to TS measurements).

For TS variables derived from recorded power signals, Echoview assigns a nominal (default) TS calibration constant. This allows you to display an echogram for such variables immediately but the variable contains uncalibrated TS values as a result, which may be incorrect. Before you can draw any quantitative conclusion from your data you must enter the correct TS calibration constant. If you calibrated your echosounder before the survey you should replace the nominal value with a calibrated value. If you did not calibrate your echosounder prior to collecting survey data, or do not know this value you can calculate it using the procedure described below.

Before you begin

You can only complete this procedure if you collected calibration data in accordance with techniques describe in Foote et. al. 1987.

Note: The procedure below is generic in nature. We advise collecting additional data and following additional procedures as recommended by the echosounder manufacturer

Steps

This procedure covers how to calculate the TS calibration constant.

Note: You will need to complete this procedure separately for each variable you will analyze.

  1. Create an EV file and add the data file(s) containing the calibration data (if not already done).
  2. Verify the calibration settings for each variable in the fileset(s), i.e. on the Calibration page of the Variable Properties dialog box for each variable, verify and/or correct:
  3.    

    Simrad Ex60:

    Ek60TransducerGain

    Simrad Ex500:

    Ek5TSGain

    Precision Acoustic Systems:

    HarpChannelGain or HarpStaticGain (either setting can be used for this procedure)

    BioSonics:

    CalibrationOffset, set to 0 by default. Alternatively CalibrationOffsetTs if the transducer constants for the power to Sv and TS equations are different.

    HTI:

    CalibrationOffset, set to 0 by default

    EchoListener:

    CalibrationOffset, set to 0 by default

    Generic HAC files:

    CalibrationOffset, set to 0 by default

      
    Hint: Some of these values may be written to the data file by the echosounder and can be displayed on the Details dialog box (F9). Otherwise, check the Echoview help file for the file format support (and power to Sv and TS equations) for your data.
  1. Create a single targets variable from your TS variable,
    see Creating single target detection variables from split-beam data or Creating single target detection variables from dual-beam data
    Note: Use the same single target detection algorithm that you intend to use later for your analyses!

Tip: For clarity you may wish to apply exclusion lines to the operand (TS variable) to exclude single targets above and below the calibration sphere.

  1. On the Filter single targets page of the Variable Properties dialog box, enter settings that will filter out off-axis targets. Specify this by placing a limit on the beam compensation or the off-axis angles. You must decide how far off the beam axis is considered "off-axis". The aim is reduce the data set to echoes of consistent signal strength not attenuated by the beam pattern.
  2. Make a selection around the remaining single target detections (i.e. those representing the calibration sphere when it was on axis).
  3.  Analyze the selection and write down the TS mean value.
  4. Calculate the difference between the measured TS mean and theoretical TS of calibration sphere.
  5. Calculate the change that is required to the current TS calibration constant to make the measured TS mean equal to the theoretical TS of calibration sphere. Graphic of TS calibration calculation.

    Note:
    This is dependent on the equation that is used to calculate TS values in Echoview. This equation differs by echosounder, see the relevant page in the Reference> Algorithms > Echosounders book in this help file or contact Echoview support for more information.
  6. Do one of the following:
  1. After entering the TS calibration constant, mean TS for the data should equal the theoretical TS.  If this is not so, repeat the calibration process from step 6 above.
  2. Save the EV file.

Calculating the Sv calibration constant

The Sv calibration constant is a value that is used in the equation that calculates Sv values from power data, e.g. Sa correction on a Simrad Ex60. The name of the Sv calibration constant and its use in the equation differs between echosounders but its overall purpose is always the same (i.e. to convert from power signals to Sv measurements).

For Sv variables derived from recorded power signals, Echoview assigns a nominal (default) Sv calibration constant. This allows you to display an echogram for each variable immediately but the variable in all likelihood contains incorrect Sv values as a result. Before you can draw any quantitative conclusion from your data you must enter the correct Sv calibration constant. If you calibrated your echosounder before the survey you should replace the nominal value with a calibrated value. If you did not calibrate your echosounder prior to collecting survey data, or do not know this value you can calculate it using the procedure described below.

Before you begin

You can only complete this procedure if you collected calibration data in accordance with techniques describe in Foote et. al. 1987.

Note: The procedure below is generic in nature and Echoview Software is aware of issues with some echosounders that may require you to perform additional steps. You are advised to contact Echoview support to discuss such issues prior to starting the procedure.

Steps

This procedure covers how to calculate the Sv calibration.

Note: You will need to complete this procedure separately for each transducer from which data was logged.

  1. Complete steps 1 to 4 of the Calculating the TS calibration constant procedure.

    Note:
    If the TS calibration constant appears on the calibration page for Sv variables you will need to calculate and enter the correct value, i.e. you will need to complete the entire Calculating the TS calibration constant procedure. Specifically on variables Ex60 raw data files Transducer gain (i.e. the TS calibration constant) and Sa correction (i.e. the Sv calibration constant) are used in calculating Sv - you must first determine the Transducer gain as described above under Calculating the TS calibration constant.
  2. Using the Reduce pings operator, create a variable that only contains pings with on-axis single targets. The same criteria for "on-axis" apply as were determined under Calculating the TS calibration constant.
  3. Note: Both Step 2 and Step 3 use the Echoview Essentials module to create virtual variables.  You will require this module for calibration of Sv variables.
  1. Using the Match ping times operator, create a variable that contains Sv data for pings with an on-axis single target only:
  2. Operand 1 = Sv variable containing the calibration data
  3. On the Sv echogram containing only pings with an on-axis single target, create a region that includes all energy (not just the peak values) and only the energy returned from the calibration sphere (i.e. where fish have been attracted to the calibration sphere, their presence will add energy to the return signal and bias the resulting calibration, such data should be removed from the analysis). The height of the defined region is not critical as long as other targets are not included.

    Tip:
    Experiment with the color display minimum ((F11) to increase, (F12) to decrease) and ping graphs to help identify the data values that are returns from the sphere and data values that are background noise or are returned from another source.
  4. Analyze the region and write down values of  the Sv mean and Thickness mean (or Height mean in depth mode).
  5. Calculate the theoretical Sv value of the calibration sphere (see below).
  6. Calculate the difference between the measured Sv mean and theoretical Sv of calibration sphere.
  7. Calculate Sv calibration constant required to make the measured Sv mean equal to the theoretical Sv of calibration sphere.

    Note:
    This is dependent on the equation used to derive Sv values from the power data. This equation differs by echosounder, see the Reference> Algorithms > Echosounders book in this help file or contact Echoview support for more information.
  8. Do one of the following:
  1. Save the EV file.

Calculating Theoretical Sv of a calibration sphere

where:

TSt = TS value of the calibration sphere (theoretical TS)

R = range of the calibration sphere

T = thickness (height) mean of the region

Y = two way beam angle (dB)

References

Foote, K.G, Knudsen, H.P., Vestnes, G., MacLennan, D.N., and E.J. Simmonds 1987. Calibration of acoustic instruments for fish density estimation: a practical guide ICES Cooperative Research Report No 144 69 pp.

Honkalehto T.  and Ryan T.E. (2003) Analysis of industry acoustic observations of orange roughy (Hoplostethus atlanticus) spawning aggregations on the Cascade Plateau off southeastern Tasmania in June and July  2003. Report to the Deepwater Assessment Group. Copy available from CSIRO Marine and Atmospheric Research Library, Hobart, Tasmania, Australia.  

Simrad AS 2003a. Instruction manual. Simrad EK60 Scientific echosounder system. 91 p. ISBN 82-8066-012-7.

Simrad AS 2003b. Operator manual. Simrad ER60 Scientific echosounder system. 166p. ISBN 82-8066-011-9.