Fróðskaparrit - 01.01.1992, Side 37

TESTING ACOUSTICAL METHODS .. .
41
ference shows that the whale was at a bear-
ing 17° off the towing direction. To deter-
mine the side, the course of the ship will
have to be changed during recording.
When the speed of the ship was
increased, propeller noise was driving the
hydrophone amplifiers into saturation.
Pilot whale (Globicephala melas)
A pod of 6 animals were observed in calm
sea. They were heading away from the
ship, and before the acoustic fish was
deployed and operated, distance was esti-
mated to be 1 NM. Both whistles (Fig. Pl)
and clicks (Fig. P2) were detected. The best
signal to noise ratios were obtained at 20
kHz.
In pursuing the pod, it was again made
abundantly clear that the acoustic fish in its
present configuration is useless when oper-
ated from a ship of this size at speeds more
than 2 knt.
White-sided dolphin (Lagenorhynchus acu-
tus)
A school of about 10 animals was encoun-
tered while the Simrad SONAR was used
in its passive mode. The school was initial-
ly detected acoustically while the ship was
steaming at 8 knt. The course was changed
to the direction indicated by the SONAR,
and after 6 min, the school was seen sur-
facing at an estimated distance of 0.5 NM.
Thus, the acoustic detection distance was
roughly 3 times the visual one.
The SONAR recordings show both
whistles (Fig. Ll) and clicks (Fig. L2). The
former contain rather little usable informa-
tion, since the selectivity of the SONAR
slices out the energy only in its passband of
less than 1 kHz (centred at 18 kHz). Since
delphinids are known to whistle over a
range of about an octave, only the parts that
happen to sweep through the “window” of
the SONAR are detectable. Consequently,
nothing can be said about duration, fre-
quency span, or general sonic activity from
such records. But since the whistles have
high detectability and low directionality
they obviously can be used for detection
purposes.
Click trains obtained with the SONAR
accurately portrait pulse repetition, and rel-
ative intensity at the transducer (Fig. L2).
However, the received sequences are
fairly short. This is consistent with an
assumption about high directionality of the
source. The inverse of the repetition rate is
believed to indicate roughly the two-way
transmission time to the target of interest.
The rate in Fig. L2 accordingly suggests a
target about 50 m from the animal, possibly
the ship.
When the acoustic fish was deployed,
signals of the same type as obtained with
the SONAR were recorded. However, the
greater bandwidth of the heterodyne sys-
tem resulted in more diversity in whistle
structures.
Conclusions
Although exposure to odontocete species
during the cruise was conspicuously low, it
sufficed to demonstrate the viability of the
basic idea of utilising the ultrasonic part of
the acoustic spectrum for detection. Clicks
were detected by the acoustic fish from all