About time varied gain

Time varied gain (TVG) is signal compensation that removes transmission loss effects from echosounder data. Transmission loss is two-way and is due to acoustic beam spreading and absorption in the acoustic medium. Signal compensation is applied by the echosounder or by receiver electronics through analog or digital signal processing. The desired result is that targets of the same size produce echoes of the same size, regardless of target range.

• Nominal TVG
• TVG in Echoview
• TVG range correction

The discussion on this page draws on assumed knowledge of hydroacoustics, echosounding techniques and echosounding equipment.

Nominal TVG

The nominal form of the TVG function in the dB domain is as follows:

For TS:	TVG(R) = 40 log R + 2αR
For Sv :	TVG(R) = 20 log R + 2αR

where:

R	=	c t /2 where: c = the speed of sound (m/s) in the medium t = the receive time(s) of the target echo (Simmonds and MacLennan, p180, p189.)
α	=	the absorption coefficient ( dB/m)

The nominal TVG is programmed into echosounder systems however Simmonds and MacLennan1 state that it only provides adequate compensation at large distances from the transducer. Using nominal TVG can introduce significant errors at the short ranges used for calibration1 and at the short ranges of measurements around the transducer's near field to far field transition 2Rb*. See also derived from the transducer active radius.

Rb	=	the boundary* between the near field and far field (refer to Near field depth). *described in Simmonds and MacLennan, p39-40.
a	=	the linear distance (m) across the transducer face
λ	=	the wavelength of sound in seawater (m)

Note: The coefficient of the log R term in Echoview is called the TVG range coefficient.

TVG in Echoview

Data formats and calibration

Echoview applies (nominal) TVG according to the requirements of the data format and the settings on the Calibration page of the Variable Properties dialog box. From Echoview version 6.1 onwards, TVG is applied to samples with ranges greater than 1 meter.

Echoview help file topics on the application of TVG in Sv and TS calculations include:

• EK60 power to TS and Sv
• EK500 power to TS and Sv
• FSV magnitudes and Sv
• Multibeam magnitude to Sv

See also:

• Additional uses of Calibration settings
• About stop ranges and start ranges

Other applications of TVG in Echoview

• Background noise analysis variables
• Background noise in Echoview
• Data generator algorithms
• Time varied threshold

TVG range correction

The purpose of TVG range correction (as offered as a solution in Echoview) is to correct for the introduced errors from nominal TVG and apply TVG at the correct range, particularly for data collected at short ranges. If your data is located in the transition from near field to far field conditions (short ranges), then the application of TVG at the correct range becomes important. Using TVG range correction for Sv and TS data helps to minimize error for further calculations such as the estimation of biomass.

TVG range correction is also important in the choice of Echoview single target detection method for your data.

The Echoview support team has developed a linked set of topics called "TVG range correction: Its impact on Single Target Detection algorithms and the calculation of Sv.". The discussions include:

• Time varied gain range correction overview
• Are my Echoview Single Target Detection results affected?
• What is the error if my Single Target Detection results are affected?
• I am interested in the theory and would like to understand the background to this issue - Single targets.
• Is my Sv data affected?
• What is the error if my Sv data is affected?
• I am interested in the theory and would like to understand the background to this issue - Sv.

TVG range correction setting

Echoview offers TVG range correction as:

• the TvgRangeCorrection setting and where required the TvgRangeCorrectionOffset setting on the (raw variables) Calibration page of the Variable Properties dialog box or (virtual variables) Calibration page of the Variable Properties dialog box. For information about available algorithms, defaults and usage refer to Common calibration names: TvgRangeCorrection.
• the TVG Range correction method setting on TS TVG Correction settings for the Minimum TS threshold section on the Data page of the Variable Properties dialog box

Note: Changes in calibration settings used for range calculations may shift ping data in the depth dimension but will not automatically adjust the position of regions and lines. You should review any existing regions and lines to ensure they are still valid. For more information refer to Range calculation effects on regions and lines.

1. Simmonds and MacLennan, p118.