Fróðskaparrit - 01.01.1992, Side 36

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TESTING ACOUSTICAL METHODS .. .
energy available. However, since overall
signal energy is very high, detection may
still be feasible (Møhl, 1990).
Implementation
To explore this possibility we have devel-
oped a tow-able acoustic sensor. The sensor
- or acoustic fish - is made of a 2.5 m poly-
ethylene tube, 11 cm in diameter, tapering
in both ends. A shock absorber (2 m of thin-
walled rubber tubing) connects the fish
with the towing cable (100 m of 5 mm steel
wire, 4 leads). Two hydrophones (sono-
buoy surplus type), spaced 1 m apart, are
suspended inside the water filled tube in
rubber bands. The fish also has a pressure
gauge for depth indication. 10 m in front of
the fish a number of lead weights can be
attacheđ to the wire in order to adjust tow-
ing depth to towing speed. The fish is
flooded with water at launch. This opera-
tion requires the ship to be stopped.
On board, the signals from the hydro-
phones are fed to a two channel heterodyne
converter with pass-band centre frequen-
cies adjustable from 10 to 100 kHz. The
bandwidth is 3 kHz, and the dynamic range
is 40 dB.
The Simrad SONAR
r/v “Magnus Heinarsson” is equipped with
a SimRad SU Survey P661E SONAR,
operating at 18kHz. During the cruise we
realized that this instrument could be
utilised in its passive mode to listen for
odontocetes when the ship was making 8
knt. Its working principle is that of a het-
erodyne converter, but selectivity is intro-
duced at the transducer level, as is direc-
tionality.
Findings
Sperm whale (Physeter macrocephalus)
A single specimen was spotted at an esti-
mated distance of 5 NM. The ship proceed-
ed at 2 knt towards the point of diving. The
acoustic fish was deployed and signals
recorded with the heterodyne converter set
at 10 kHz. Repetition rate was steady at
about 1 pulse every two seconds. A time
series of a single pulse is given in Fig. Al,
showing double path transmission (proba-
bly due to reflection at the surface). The
time difference between the two sets can be
used to estimate the depth of the source,
provided a range estimate is available. The
somewhat higher signal amplitude in the
delayed path can be interpreted as an indi-
cation of the animal’s acoustical axis being
more aligned with the reflecting point than
with the acoustic fish.
Also, the multi-pulsed structure so char-
acteristic of this species is shown. From the
interpulse interval of 4 ms, an estimate of
15 m for total body length of the specimen
can be made (Adler-Fenchel, 1980). This
information reveals the sex, since only
males grow to this length. Although this
information is no big surprise (only males
are observed at these latitudes), it serves to
show the kind of information obtainable
with the acoustic fish.
In Fig. A2 is shown the simultaneous
output from the two transducers in the fish
for the first pulse of a click. The time dif-