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.

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)

Rb

= the boundary* between the near field and far field.

a

= the linear distance (m) across the transducer face

λ

= the wavelength of sound in seawater (m)
 
*described in Simmonds and Maclennan, p39-40.

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*.

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:

See also:

Other applications of TVG in Echoview

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:

TVG range correction setting

Echoview offers TVG range correction as:

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.