propagated inside a wide across-track and narrow along-track sector, while the
receiver array is usually directed perpendicularly to the transmit array. The
receiver array is steered simultaneously across the different narrow across-track
directions by a beam-forming process, and the system then performs spatial filtering of the acoustic signals backscattered from the different directions.
The resolution within the swath is referred to as across-track resolution (the
greater number of individual SONAR beams formed, the finer the resolution the
survey). Many multi-beam SONAR systems can counteract the degradation in
survey resolution caused by conical beam-spreading inherent in single-beam
systems (i.e., counteract the issue that when the insonified footprint gets larger,
many more surface irregularities are included within a single sonar footprint and
hence not resolved). The along-track resolution of multi-beam SONAR is the ratio
of acoustic wavelength to the length of the array, which defines the resolution at a
given depth. For typical multi-beam systems this ratio is on the order 1:60–1:400
(i.e., resolutions from 1 m at 60 m to 1 m at 400 m ranges, respectively). A longer
array will increase this ratio, but fitting such a long array is not always possible or
practical. Higher frequencies will also increase the ratio, but limit the achievable
range due to higher absorption of the signal. Synthetic Aperture SONARs can be
used to overcome these limitations by utilizing data from several consecutive
pings to synthesize a longer sonar array.
Different frequencies are used to map different water depths. Higher frequencies ([100 kHz) are generally used in shallow water, while the lower frequencies
(\30 kHz), are preferred for deep water. With different frequencies there is a
trade-off in resolution, with higher frequency systems providing greater spatial
resolution than lower frequency systems. Importantly, due to conical beamspreading, swath width varies with survey depth (i.e., the deeper the water, the
wider the swath) and the coverage area of these systems is thus a direct function of
water depth. Most systems provide coverage ranging from approximately 2 to 7
times the water depth.
An early SONAR system, the Seabeam, is used here to illustrate the functioning
of a multi-beam system. Several transducers are mounted in an array along the keel
of a vessel and transmit pulses that insonify an area of seafloor normal to the ship’s
track. A hydrophone (receiving transducer) array on the ship’s keel has its axis set
in the fore and aft direction. Thus, two separate sonar arrays are oriented
orthogonal to one another, one for transmitting and one for receiving. This
arrangement is referred to as a Mills Cross Array. The arrays and the associated
analogue electronics provide 90 9 1°-wide unstabilized beams. Roll and pitch
compensation reduces this to 60 9 1°-wide stabilized beams, which permits
mapping a 60° swath of the sea floor with each ping. This system allows survey
vessels to produce high-resolution coverage of wide swaths of the ocean bottom in
far less ship time than would have been required for a single-beam echo sounder,
greatly reducing the costs of mapping.
In more modern systems, most of the signal processing, including beamforming, has been moved from analogue signal processing into the digital (discrete) signal processing domain using digital signal microprocessor (DSPmP)
8 Acoustic Methods Overview
213
receiver array is usually directed perpendicularly to the transmit array. The
receiver array is steered simultaneously across the different narrow across-track
directions by a beam-forming process, and the system then performs spatial filtering of the acoustic signals backscattered from the different directions.
The resolution within the swath is referred to as across-track resolution (the
greater number of individual SONAR beams formed, the finer the resolution the
survey). Many multi-beam SONAR systems can counteract the degradation in
survey resolution caused by conical beam-spreading inherent in single-beam
systems (i.e., counteract the issue that when the insonified footprint gets larger,
many more surface irregularities are included within a single sonar footprint and
hence not resolved). The along-track resolution of multi-beam SONAR is the ratio
of acoustic wavelength to the length of the array, which defines the resolution at a
given depth. For typical multi-beam systems this ratio is on the order 1:60–1:400
(i.e., resolutions from 1 m at 60 m to 1 m at 400 m ranges, respectively). A longer
array will increase this ratio, but fitting such a long array is not always possible or
practical. Higher frequencies will also increase the ratio, but limit the achievable
range due to higher absorption of the signal. Synthetic Aperture SONARs can be
used to overcome these limitations by utilizing data from several consecutive
pings to synthesize a longer sonar array.
Different frequencies are used to map different water depths. Higher frequencies ([100 kHz) are generally used in shallow water, while the lower frequencies
(\30 kHz), are preferred for deep water. With different frequencies there is a
trade-off in resolution, with higher frequency systems providing greater spatial
resolution than lower frequency systems. Importantly, due to conical beamspreading, swath width varies with survey depth (i.e., the deeper the water, the
wider the swath) and the coverage area of these systems is thus a direct function of
water depth. Most systems provide coverage ranging from approximately 2 to 7
times the water depth.
An early SONAR system, the Seabeam, is used here to illustrate the functioning
of a multi-beam system. Several transducers are mounted in an array along the keel
of a vessel and transmit pulses that insonify an area of seafloor normal to the ship’s
track. A hydrophone (receiving transducer) array on the ship’s keel has its axis set
in the fore and aft direction. Thus, two separate sonar arrays are oriented
orthogonal to one another, one for transmitting and one for receiving. This
arrangement is referred to as a Mills Cross Array. The arrays and the associated
analogue electronics provide 90 9 1°-wide unstabilized beams. Roll and pitch
compensation reduces this to 60 9 1°-wide stabilized beams, which permits
mapping a 60° swath of the sea floor with each ping. This system allows survey
vessels to produce high-resolution coverage of wide swaths of the ocean bottom in
far less ship time than would have been required for a single-beam echo sounder,
greatly reducing the costs of mapping.
In more modern systems, most of the signal processing, including beamforming, has been moved from analogue signal processing into the digital (discrete) signal processing domain using digital signal microprocessor (DSPmP)
8 Acoustic Methods Overview
213