What is sonar and what exactly does a sonar system do?
Swath bathymetric data (partially processed) of seafloor sink hole.
The word “sonar” is an abbreviation for “SOund, NAvigation, and Ranging.” A sonar system consists of a transmitter, transducer, receiver, and display.
In the simplest terms, an electrical impulse from a transmitter is converted into a sound wave by the transducer and sent into the water. When this wave strikes an object, it rebounds. This echo strikes the transducer, which converts it back into an electric signal. This signal is then amplified by the receiver and sent to the display.
Since the speed of sound in water is constant in a given locale at a given time (approximately 4800 feet per second), the time lapse between the transmitted signal and the received echo can be measured and the distance to the object determined. This process repeats itself many times per second.
Traditional Sonar: Single Beam Sounding
Traditional sonar is referred to as single beam sounding. While such systems have existed for over 50 years, confidence in the technology is often misplaced. The beamwidth of most single beam sounders varies between 10 and 30 degrees.
The transducer is customarily mounted in a vessel, and although the vessel may be equipped with a heave sensor, there is normally no way to measure the orientation of the transducer during operation.
The method utilized in single broad-beam bottom detection relies on estimating the shortest slant range to the seafloor that is ensonified within the main lobe of the beam. As there is already an existing plus or minus 5 to 15 degree angular uncertainty (based upon the 10 to 30 degree beamwidth), there is very little point in analyzing the vessel’s pitch and roll characteristics, as these are usually below this existing uncertainty.
Thus, the method for determining the minimum slant range provides an answer that is merely an estimate. While this method is reasonably adequate when the sea state is low and where the seafloor is reasonably flat, it has obvious limitations.
Further, even in these “ideal” conditions, no information is gathered about the topography of the seafloor outside the sonar beam footprint. This, in turn, leads to guesswork about what lies between survey lines.
Sidescan Sonar
Sidescan sonar was developed to address the limitations of single beam sounding. Sidescan sonar, like a single beam sounder, transmits sound energy and thereafter analyzes the strength of the return signal that has bounced off the seafloor or other objects.
The sound energy, however, is transmitted from the sides of a towfish, creating a fanlike beam on either side that sweeps the seafloor. The return signal is continuously recorded, creating a “picture” of the seafloor and any other objects.
However, while the contours of the seafloor and objects on it can be imaged quite well, most sidescan sonar systems cannot provide accurate bathymetric data.
Multibeam Sonar
A significant advance over sidescan sonar systems is multibeam sonar. As its name implies, multibeam sonar employs a multitude of individual sonar beams to ensonify the seafloor. Such sonar can be transmitted from a towfish or from a hull mounted transducer.
Multibeam, or swath, sonar systems provide fan-shaped coverage of the seafloor (similar to sidescan sonar), but the output data is different. Instead of continuously recording the strength of the return echo, the multibeam system measures and records the time for the acoustic signal to travel from the transducer to the seafloor and back.
Some of the advantages of multibeam systems over conventional systems include: increased detail of the seafloor (100% coverage), confidence that all features and hazards are mapped without voids, the ability to map inaccessible areas (e.g. - under jetties, structures, and vessels; near breakwaters, shoal areas, and retaining walls), fewer survey lines (which translates into less required survey time), optimum seafloor detail for route and dredge programs, and the ability to comply with the highest order International Hydrographic Organization (IHO) and U.S. Army Corps of Engineers (USACE) hydrographic standards.
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