Reducing bottom and surface reverberation with the RCG (Reverberation Controlled Gain) function

When you transmit an acoustic pulse into the water to search for a target, echoes will also be returned from any other objects in the beam. The energy reflected from these "unwanted" objects is referred to as reverberation. Reverberation is often a limiting factor, especially if you are searching for smaller targets.

Reverberation is often regarded as noise.

There are several types of reverberation.

Volume reverberation

The volume reverberation is caused by minute plankton and air-bubbles in the water. This reverberation has often daily cycles in intensity.

Surface reverberation

The surface reverberation depends on the weather condition and the waves.

Bottom reverberation

The bottom reverberation depends on the roughness and physical properties of the sea bottom.

The RCG (Reverberation Controlled Gain) controls the receiver processing. The algorithm handles each receiver beam individually. The filter reduces the unwanted echoes from the bottom and from the sea surface. This enhances the visibility of schools.

Note:

The numerical settings do not express an increasing filter strength. They represent specific settings using different algorithms.

The algorithms in the filter may interpret scattered fish as reverberation. Use reverberation controlled gain carefully if you wish to detect scattered fish.

Tip:

We recommend that the reverberation controlled gain is active all the time, but be careful in the vertical presentation.The filter works best on small schools.

This rough guide to the settings can be used as a starting point for own experiments.

  • Off: The filter is disabled.
  • Weak: Use for scattered fish.
  • Medium: Use for schools.
  • Strong: Use for schools with higher density. This setting can also be used in horizontal views to detect schools close to the bottom.
  • 4: Use for scattered fish located close to the bottom. This setting uses a slightly different algorithm than the other scattered fish settings.
  • 5: Use for scattered fish located close to the bottom. This setting uses a slightly different algorithm than the other scattered fish settings.
  • Scattered: Use for scattered fish. This setting uses a slightly different algorithm than the other scattered fish settings.
  • 7: Use for scattered fish. This setting uses an algorithm dedicated for higher bandwidth.
  • 8: Use for schools. This setting uses an algorithm dedicated for higher bandwidth.
  • 9: Use for schools. This setting uses an algorithm dedicated for higher bandwidth.
  • Experimental: These options are provided for software development purposes.

The filters in the ST90 system are implemented in “series�. The reverberation controlled gain adjustment is made late in the sequence, so any changes to the preceding filters may have an effect on how it works.

Procedure

  1. Click in any view to make it active.

    The active view is identified with a thicker border. Unless you use the Apply to all function, all changes you make will only be applied to this view.

  2. Open the Active menu.
  3. Select RCG (Reverberation Controlled Gain).
  4. Select a value from the options provided.

    The choice you make is by default only applied to the currently selected (active) view.