6 Adjusting the TVG in the Echogram dialog box

The Time Varied Gain (TVG) can be defined in the Echogram dialog box, or by means of the TVG function on the Active menu.

Context

When an acoustic pulse is sent through the water, it will gradually lose its energy. The greater the distance between the transducer and the target(s), the greater the loss of energy.

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Geometric spread: Once transmitted, the acoustic energy will spread out to form a circular beam. The width of this beam increases with the physical distance to the target(s).

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Absorption loss: Depending on the salinity and temperature, the water will absorb some of the energy from the transmission. The absorption loss increases as the physical distance to the target(s) increases.

Both the geometric spread and the absorption will also have an effect on the returned echo signal. That is why we normally refer to these factors as the two-way transmission loss.

The TVG (Time Variable Gain) compensation is designed to counteract the natural phenomena of geometric spread and absorption loss. In the EK80 system, the TVG compensation is made using digital signal processing software.

The TVG compensation is expressed as a logarithmic curve. You can choose from a selection of curves. Each curve has a different slope creating a different gain compensation. Each curve is identified with the equation X log TVG. The coefficient "X" is an integer. Typical values for "X" are 10 to 40.

Several TVG compensation settings are available.

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No TVG: TVG compensation is not implemented. This option is hardly ever used.

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Sv (20 Log): Volume backscattering strength

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Sp (40 Log): Point backscattering strength

Note

The setting is only applied to the currently active echogram.

Procedure

Click in the echogram view you want to change.

The active view is identified with a thicker border.

Open the Active menu.

Select Echogram.

Observe that the Echogram dialog box opens.

In the Echogram dialog box, select the Echogram tab to open the page.

Choose the required setting.

Apply the change you have made.

Select Apply if you wish to apply the chosen setting only to the currently active echogram view.

Select Apply to All if you wish to use the chosen setting on all the echograms of the same type.

Select OK to save the selected settings and close the dialog box.