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Multibeam Bathymetry - Scientific Instrumentation

Introduction

The research vessel James Clark Ross was fitted with a multibeam echosounder in the summer of 2000. Since then it has been used to provide high resolution maps of the sea bed to support many different types of science. The main areas surveyed are within the Southern Ocean but the ship has also mapped areas in the Arctic and on passage between the UK and the Antarctic.

One of the transducer units being fitted in the hull of the ship

How does it work?

An echosounder works by sending a sound signal into the water from a transducer at the bottom of the ship. The sound travels through the water, reflects off the sea floor and returns to the transducer where the time taken for the round trip is measured. Water depth can be calculated using the simple formula -

Depth = speed x time/2

where the speed of sound in water is approximately 1500m per second. The shorter the time taken for the sound pulse to return to the ship then the shallower the water. The sound pulses are sent out at regular intervals as the ship moves over the water and the depths are continuously recorded.

Why is it multibeam?

Many ships have singlebeam echosounders for navigational purposes. These systems send out one sound pulse at a time and find the depth directly under the ship. A multibeam system sends out an array of sound pulses in a fan shape and returns depths from underneath the ship and from either side as well. This is sometimes referred to as a swath bathymetry as it produces a swath of depth information. The multibeam system on the James Clark Ross has 191 beams and can map a swath width of about 4 times the water depth.

Multibeam bathymetry fan - Shows how the multibeam system works

Is it really that simple?

In theory yes, but it is much more complicated in practice. The ship is constantly moving (rolling, pitching and yawing) and the transmitting beams have to take this into account. The system corrects for the ship's motion by steering the beams so they reflect off the correct part of the sea floor. More information about the technical aspects of the multibeam system is available on the Engineering pages.

Who uses it and why?

The bathymetric maps produced by the multibeam instrument  are used for a wide variety  of science. Marine geologists use the maps to study how the ocean crust formed, how glaciers once moved over coastal areas and as a tool to find sites for sediment coring. Oceanographers can better model how ocean currents will flow with detailed bathymetry maps. Marine biologists can use the data to map wildlife habitats and it can be used as an important tool to define fishing areas in places like South Georgia. Finally, much of the Southern Ocean is very poorly surveyed so there is an element of discovery when the ship travels to a new area.

Multibeam bathymetry example 1 - Bathymetry around the South Sandwich Islands