Details

TVG (Time Variable Gain)

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.

The TVG (Time Variable Gain) compensation is designed to counteract the natural phenomena of geometric spread and absorption loss. 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.

Select the TVG (Time Variable Gain) setting you want to use. Several TVG compensation settings are available.

No TVG

TVG compensation is not implemented. This option is hardly ever used.

School (20 log R)

This setting offers weaker amplification close to the bottom. It has been designed to provide the best echoes for schools. The physical size of a school allows you to detect it even with reduced gain.

Fish (40 log R)

This setting offers larger amplification closer to the bottom. It has been designed to provide the best echoes from single fish.

Targets (40 log R)

This setting uses the same TVG compensation as Fish (40 log R), but additional filtering and compensation is implemented.

Only echoes detected and interpreted as single fish are shown. The echoes are compensated for their physical position in the beam (beam compensation). This means that fish in the outer region of the beam are adjusted to correct size even though their echoes are weaker.

Target mix (40 log R)

This setting uses the same TVG compensation as Fish (40 log R), but additional beam compensation is implemented. Echoes detected and interpreted as single fish are adjusted to their correct size according to their physical position in the beam.

User

This setting allows you to define your own TVG curve. To choose your own TVG curve, use the dedicated function in the Echogram dialog box.

Echogram Type

Use this function to select what kind of echogram you wish to see in the current (active) view.

Surface

A Surface echogram is mainly used when you wish to look at the entire water column starting from the sea surface and down to the sea bottom. Since this echogram is referenced to the sea surface, the sea bottom contour will vary with the actual depth. If you set up the Start Range and Range depths to place the sea bottom contour at the lower end of the echogram, you will have good opportunity to study the echoes from the water column.

In the surface echogram, all calculations are made from the sea surface and down to the detected sea bottom. Use this echogram type to obtain correct calculation of the biomass. It will also provide valid data for the Size Distribution information pane.

Bottom

A Bottom echogram is mainly used when you want to examine the echoes from fish close to the sea bottom. Since this echogram is referenced to the sea bottom, the sea surface will vary with the actual depth, while the bottom is drawn flat. This makes it easy to study the echoes from the sea bottom. You can investigate the sea bottom conditions and hardness, and detect fish.

The echogram is only drawn for pings that have a successful bottom detection.

Pelagic

A Pelagic echogram is mainly used when you wish to look at the water column starting from any distance below the sea surface down towards the bottom, but without seeing the bottom contour. Pelagic echograms are useful when you work in deeper waters. The reduced range and the fact that you do not need to wait for the bottom echo means that the ping rate is increased.

In a Pelagic echogram the calculations disregard any bottom detection. All calculations are based on the entire echogram shown in the view. If the bottom echo is present in the echogram, the biomass calculation will be wrong.

Bottom TVG

Use the Bottom TVG function to specify a separate Time Variable Gain (TVG) compensation for the bottom detection.

The ST90 system allows you to use different TVG compensation for the echoes from the water column than from the bottom. By making the bottom echoes individually adjustable, you can fine tune the presentation from the bottom to investigate the bottom type and conditions. The Bottom TVG adjustment has no effect on fish close to the bottom.

If you set Bottom TVG to None there will be no gain change in the TVG at the detected bottom depth. The TVG selected for the echogram is then used for the entire depth range.

User TVG

If you are very familiar with the TVG principles, you can choose to use your own curve. In order to use the chosen curve, the TVG mode must be set to User. You can also select TVG setting with the TVG function. The TVG (Time Variable Gain) function is located on the Active menu.