The Multibeam Echo Sounder is an important device used in oceanographic research, designed to map ocean floor with a very high resolution and accuracy. It takes advantage of the physical properties of water, which is particularly efficient in transmitting sound waves, to being able to accurately measure the return time of a sound pulse emitted by a transducer, which is a function of distance. By a fairly complicated beaming technology and real time Fourier transformation done by a dedicated computer, the system is able to sweep a reasonably wide portion of the ocean floor directly underneath the ship. The trajectory of the cruise path is then designed to scan back and forth a certain portion of the sea in order to build an extensive map. In general, given the high costs of a typical expedition, ship routes are never repeated so that a increasingly complete database of the ocean floor can be constructed over the years.
Mounted underneath the ship a series of transducers (acoustic waves transmitter and receiver) are installed so that the scanning angle of the transmitter transducer is perpendicular to the ship movement. The transmitting transducers are at equidistance between each other so that when a pulsing is emitted, an interference pattern is constructed such that it is possible for the receiver transducer to identify the direction of the returning beam therefore allowing a wide angle scanning. Each unit of the transmitter transducer emits a sound pulse with a specific frequency between 11khz to 12khz. The receiver will know from which transmitter the pulse was send by measuring the frequency therefore correctly identify the return timing for each pulse.
The whole system is extremely sensitive, with the potential to measure depth with the accuracy of less than a meter. In order to achieve such accuracy, a suite of motion sensor on the ship are needed to track the ship position, tilt, pitch, yaw, roll to a high degree in order to cancel out the anomalies introduced by the ship movement.
The data acquired is then processed by a dedicated machine with special software and transformed into a 3D representation of the sea bed. Often certain degree of editing is required to clean up the bad pins resulting from inaccuracies or interferences.
Kongsberg EM122 Specification
- Depth range from 20 to 11000 m
- Swath width up to 6 times water depth/30 km
- Focused beams for transmission and reception
- High density and multiping modes for increased resolution
- Up to 864 sounings per ping
- Yaw, pitch and roll compensation and stabilization
- High accuracy
- Seabed image (sidescan) data display and recording
- Water column data display and recording
- Modular design, beamwidths 0.5 to 4 degrees
- Dual and triple frequency versions possible
- Integrated sub-bottom pro ler available
- Mammal protection