Reverberation

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 generated when the echoes from your transmissions are reflected not only by fish and valid targets in the water, but also from the sea bottom and the sea surface. 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.

Reverberation is often a limiting factor, especially if you are searching for smaller targets. The echoes from the bottom will always depend on the sea bottom properties. How uneven the seabed is will affect the echoes. What kind of bottom you have under your vessel is also significant, as rock, mud or sand will provide different reverberation. The bottom reverberation usually makes it difficult to investigate targets located close to the seabed.

Your current tilt setting will also have an effect on the reverberation. The echoes from the sea surface will vary with the current sea state and your current tilt setting.

The illustration demonstrates how the sea bottom and the sea surface return echoes back to your sonar system.

  1. In this situation you will be able to identify the upper school of fish, but the lower school will be hidden in the shadow of the seabed rock formation.
  2. In this situation you will see several weak echoes from the waves and from the rock on the bottom, but you will still be able to identify the school of fish.

The reverberation is caused by the transmission pulse itself. It can only be reduced by shortening the pulse duration or using a smaller beamwidth.