Introduction to Acoustics

Fig. 5.57a,b Typical echogram obtained during an echointegration
campaign using a single-beam SONAR on
the Gambie river. (a) Biomass made of individual fish.
(b) Biomass made of fish school (Courtesy Jean Guillard,
INRA)
gives:
Eh = hσnφ 2 (r)
� �
exp(−2βr)
, (5.101)
r 2
where n corresponds to the density of fish per unit
volume in the layer h. Equation (5.101) is based on
the linearity principle assumption, which states that,
on the average over many pings, the intensity Eh is
equal to the sum of the intensity produced by each fish
individually [5.97].
In the case of either a single or multiple targets,
the idea is to relate directly the echo-integrated result
E1 or Eh to the fish scattering amplitudes σ or nσ. To
that goal, the time-varying gain φ 2 (r) has to compensate
appropriately for the geometric and attenuation loss. For
a volume integration, φ 2 (r) = R 2 exp(2βr), the so-called
20 log r gain is used while a 40 log r gain is applied in
the case of individual echoes.
As a matter of fact, acoustic instruments, such as
echo sounders and SONAR, are unique as underwater
sampling tools since they detect objects at ranges
of many hundreds of meters, independent of water
clarity and depth. However, until recently, these instruments
could only operate in two dimensions, providing
observational slices through the water column. New
trends in fisheries acoustics incorporate the use of multibeam
SONAR – typically used in bottom mapping, see
Sect. 5.7.2 – which provides detailed data describing
the internal and external three-dimensional (3-D) structure
of underwater objects such as fish schools [5.98]
(Fig. 5.58). Multibeam SONARs are now used in a wide
variety of fisheries research applications including:
(1) three-dimensional descriptions of school structure
and position in the water column [5.99]; knowledge of
schooling is vital for understanding many aspects of
Fig. 5.58a–d Images of a fish school of Sardinella aurita
from a multibeam SONAR. Arrows indicate vessel route.
(a) 3-D reconstruction of the school. Multibeam SONAR
receiving beams are shown at the front of the vessel. Remaining
panels show cross sections of density in fish from:
(b) the horizontal plane, (c) the vertical plane along-ships,
(d) the vertical plane athwart ships. Red cross-hairs indicate
location of the other two cross sections (Courtesy Francois
Gerlotto, IRD)
b)
0
0
20
Depth (m)
Underwater Acoustics 5.8 Acoustics and Marine Animals 197
a) Range (m)
0
0
300
20
Bottom multiple
300
Individual
fish
Bottom
fish (and fisheries) ecology [5.100]; (2) detailed internal
images of fish schools, providing insights into the
organization of fish within a school, for example, indicating
the presence of large gaps or vacuoles and areas
of higher densities or nuclei (Fig. 5.58b, d) [5.101].
In general, one tries to convert the echo-integration
result into a biomass or fish density. This requires the
knowledge of the target strength TS or the equivalent
backscattering cross section σ of the target under
a)
c)
0
10
20
Depth (m)
5
10
Depth (m)
10
–40
–30
–20
20 30 40
Distance (m)
b)
10
20
30
5
10
Fish school
40
5
Depth (m)
10 15
d)
5 10 15
Distance (m)
Part A 5.8