Multi-beam echo-sounder may be considered as a series of single-beam echo sounders mounted on an array. For example, in SeaBat 8101, there are altogether 101 transducers (beams), each with 1.5 degree alongtrack and acrosstrack respectively. Every ping of signal emitted will be equivalent to a fan-shape transmission which results in the receiving of 101 soundings across the track of the vessel.

A Typical Multi-beam Echo Sounding Plan

Geometric Consideration of Multi-beam Echo-sounder

Errors in using multi-beam echo sounder

It is easily conceived that the accuracy of sounding using multi-beam echo sounder will deteriorate from the beam at nadir to the outer side beams because of the dynamic movement of the vessel. The most significant effect is due to the movement in roll, pitch and heave.

Effect of Roll

The roll rotation will cause lateral displacement of the whole swath. This will cause a tilting of seabed as well as smaller guaranteed surveyed coverage. The vertical error it produces is also considered to be the most significant one in multi-beam hydrographic survey.

Effect of Pitch

This will displace the sounding forward or aft by the tangent of the pitch angle and thus causes horizontal error.

Effect of Heave

Heave alters the instantaneous elevation of the transducer with respect to the seabed and directly produces a vertical error of the same magnitude onto the sounding.

As a result, motion sensors are required to measure the above movements in order that the accuracy of soundings in the outer beams may be comparable to the soundings obtained at nadir or by using single beam echo sounder.