8.3.1 Single Beam Bathymetry
The first and most fundamental use of sound in water is harnessing the time
differential between the sent signal and its received echo to measure distance to the
seafloor or any other object. This is the principle behind single beam echosounding, which calculates depth as the product of half the two-way transit time of
a signal and the mean vertical sounding velocity. Measurements require a sharp
leading edge in the signal, precise timing of the bottom reflection, and an accurate
estimation of mean sound velocity in the measured water body. Frequency is also
important, since sound attenuation becomes significant at greater distances. It is,
for example, not possible to measure distances over 20 km (roughly the travel
distance to and back from the greatest depth of the ocean, which is 10,008 m at the
Mariana trench) with signal frequencies below 50 kHz (Jones 1999). Thus, different transducers are used for deep and shallow work.
After the sound pulse leaves the transducer, it travels towards the seafloor and is
subject to conical beam spreading. The further away the reflecting object, or the
smaller the object, the narrower the opening angle of the transducer should be in
order to maintain a small footprint. This is a particularly important consideration if
the seafloor is steeply sloping or highly rugose, since a large footprint does not
adequately capture the details of the surface. Further, for especially steep slopes, or
very rough surfaces, reflections may travel at an oblique, rather than perpendicular,
angle back to the ship, causing a slight upward measurement error (i.e., the seafloor is perceived deeper than it is). This need for a smaller footprint, however, is
balanced against the limitation that single beam systems only provide soundings
along the survey track. Areas between the soundings must be interpolated if a
closed surface between the soundings is desired. To avoid this, swath methods,
which allow full and complete insonification of the seafloor, were developed. In
the simplest configuration, there are several single beam transducers arranged
along outriggers that obtain a mapped swath. More modern systems use beamforming and beam-steering to generate a fan of sounding paths originating from
one centralized transducer or transducer array.
8.3.2 Side Scan SONAR
The primary purpose of side scan SONAR is to produce accurate maps of seafloor
topography. Side scan SONAR (SSS) can be ship-mounted or towed. In the latter
case, the transducer arrays are packaged in a streamlined ‘‘towfish’’ or ‘‘fish’’ that
is linked by a cable to the towing vessel (Fig. 8.8). The fish is used to decouple the
sonar from ship movements and to limit the impacts of surface noise. Ideally, the
fish is placed a few meters above the seafloor to obtain optimum coverage, where
fish height is controlled by adjusting the amount of cable and the speed of the
vessel. Many different configurations of SSS are available for different
208
B. Riegl and H. Guarin
The first and most fundamental use of sound in water is harnessing the time
differential between the sent signal and its received echo to measure distance to the
seafloor or any other object. This is the principle behind single beam echosounding, which calculates depth as the product of half the two-way transit time of
a signal and the mean vertical sounding velocity. Measurements require a sharp
leading edge in the signal, precise timing of the bottom reflection, and an accurate
estimation of mean sound velocity in the measured water body. Frequency is also
important, since sound attenuation becomes significant at greater distances. It is,
for example, not possible to measure distances over 20 km (roughly the travel
distance to and back from the greatest depth of the ocean, which is 10,008 m at the
Mariana trench) with signal frequencies below 50 kHz (Jones 1999). Thus, different transducers are used for deep and shallow work.
After the sound pulse leaves the transducer, it travels towards the seafloor and is
subject to conical beam spreading. The further away the reflecting object, or the
smaller the object, the narrower the opening angle of the transducer should be in
order to maintain a small footprint. This is a particularly important consideration if
the seafloor is steeply sloping or highly rugose, since a large footprint does not
adequately capture the details of the surface. Further, for especially steep slopes, or
very rough surfaces, reflections may travel at an oblique, rather than perpendicular,
angle back to the ship, causing a slight upward measurement error (i.e., the seafloor is perceived deeper than it is). This need for a smaller footprint, however, is
balanced against the limitation that single beam systems only provide soundings
along the survey track. Areas between the soundings must be interpolated if a
closed surface between the soundings is desired. To avoid this, swath methods,
which allow full and complete insonification of the seafloor, were developed. In
the simplest configuration, there are several single beam transducers arranged
along outriggers that obtain a mapped swath. More modern systems use beamforming and beam-steering to generate a fan of sounding paths originating from
one centralized transducer or transducer array.
8.3.2 Side Scan SONAR
The primary purpose of side scan SONAR is to produce accurate maps of seafloor
topography. Side scan SONAR (SSS) can be ship-mounted or towed. In the latter
case, the transducer arrays are packaged in a streamlined ‘‘towfish’’ or ‘‘fish’’ that
is linked by a cable to the towing vessel (Fig. 8.8). The fish is used to decouple the
sonar from ship movements and to limit the impacts of surface noise. Ideally, the
fish is placed a few meters above the seafloor to obtain optimum coverage, where
fish height is controlled by adjusting the amount of cable and the speed of the
vessel. Many different configurations of SSS are available for different
208
B. Riegl and H. Guarin