Transcript Static acoustic monitoring: species discrimination by an

Cetacean click tone logging
by PODs
Chelonia Limited / Nick Tregenza
T-POD sound selection: static filters
Tonality: a/b
.. process carried out by analogue electronics
The black line represents ambient noise = background noise.
A porpoise click raises the energy in the red target band. The ratio between
the target band, and the user-defined reference band (blue) then exceeds
the user-defined threshold ratio, shown by the dotted arrow.
While that condition exists a clicks is presumed to be in progress and the
event is timed.
Any broadband sound will raise the energy in both bands and will not be
detected so the T-POD is a ‘dynamic threshold’ logger. ‘Fixed threshold’
loggers log broadband clicks if they are loud enough to exceed the threshold
in the target band.
The T-POD fails to log very loud narrowband clicks if they put enough energy
into the reference filter to raise its output so close to the ceiling that the
detection ratio cannot be achieved. However the tail end of the click, or
multipath replicates of very loud clicks, do get logged. So gaps do not appear
in the loudest section of trains logged.
C-PODs log
any tonal segments
in the stream of sound …
C-POD sound selection
high pass input filter
Tonality: a/b
b
a
background noise
b
kHz >>>
.. process carried out by digital electronics
The system is defining the target and reference bands automatically and the
user cannot control this except in one respect ….
The standard setting of the input filter is 20kHz. It can be moved down to
10kHz to increase sensitivity below 40kHz, or it can be moved up to 80kHz to
increase sensitivity above 120kHz.
Either of these changes will mean that results are not comparable with those
from C-PODs used at the normal settings.
They may be justified for some low frequency species or for porpoise
monitoring in extremely low density areas.
To be logged clicks also have to pass other tests, particularly on duration and
bandwidth. Then various measures are logged:
duration
3 wave heights
2 frequencies
bandwidth index
Some
10 selection
selection
criteria
data
are
applied
not logged
to each event
The C-POD digital processor measures amplitudes and times of waveform
maxima and minima and zero-crossings. The timing resolution is 200
nanoseconds.
The ‘average frequency’ is estimated within the first 10 cycles of the tone
(click), and the last zero-crossing interval is also logged to help identify any
sweep in frequency occurring through the click.
The last cycle may be up to 255 cycles after the start. If the tone continues a
new tone is logged. This is not uncommon when the source is a boat sonar.
This is a waveform of a dolphin click that is particularly broadband for such a
long click.
The lower graphs here are spectra from Discrete Fourier Transforms of the
waveform.
The loud part of the click is too broadband and is rejected…
… but a much quieter segment is actually much narrower-band and will be logged.
this click would be completely missed….except …. see later
Weak porpoise click
This is a click from a distant porpoise.
The section marked by the red bar is just within the detection limit.
Data captured :
time >
5 clicks from a dolphin click train
The click is logged as multiple
replicates arriving over 30ms
and spread over 100kHz
In the sea dolphin clicks are most often logged as multiple replicates, and
even clicks like the broadband one we saw earlier are not actually missed.
Two factors contribute to this:
1. Clicks become longer during propagation.
2: Multipath propagation is rife and typically produces a set of tones following
the ‘original’ click. Reflection within the animal, from the sea surface, and
from the sea bed, may be detected. Refraction by the varying sound speed in
the sea water may also be involved. These variations are driven by
temperature and salinity changes occurring at the sea surface and sometimes
elsewhere.
The pitch of the replicates tends to drift downwards as one might expect from
faster absorption of higher frequencies, but that pattern is quite irregular. The
amplitude also diminishes.
Porpoise click train: fewer replicates, frequency range < 20kHz
Porpoise buzz
frequency variation is small
Here a boat sonar is pulsing about four times per second at 50kHz and a porpoise is
clicking much faster, and at a higher frequency (130khz)
Frequency of multipath replicates from porpoise clicks and boat sonar
Narrowband sources produce multipath replicates in the same narrow band of
frequencies.
Broadband sources: the spectrum of the source is reflected in the tones of the
multipath replicates logged?
Three PODs moored within a few meters of each other detected the
same trains and show that the frequencies received for each click are not
exactly the same.
There is a small time offset between the three loggers but displaying the
inter-click intervals allows exact identification of the same click from each
logger.
So:
Over this short distance between PODs there is an effect that varies the
frequency – maybe interference within the sound beam.
The mix of frequencies received by the PODs is similar even though it is
carried in different clicks.
Tursiops click trains
Yaxis: amplitude
Colour: kHz
Click characteristics
Bottlenose dolphin
Tursiops truncatus
Harbour porpoise
Phocoena phocoena
Logging rate: > 5,000 / second
Dynamic range: > x 2000
The dynamic range is the ratio of the loudest click that the system can log to the
weakest. The very high dynamic range of the C-POD means that it logs loud
clicks that were missed by the T-POD.
This combination of fast logging and high dynamic range is suited to the task of
describing the multipath clusters that show up with tone logging.
Tone logging: summary
• Designed to produce input data for train detection at low power cost.
The power of the data reside in their coherence within trains.
• Gives some indication of click frequency spectra, derived from
clusters of tones, even though no actual spectra are logged.
advice….
• Don’t describe individual dolphin-type clicks using C~POD data.
• Good way to describe train characteristics, and the dominant
frequency of porpoise-type clicks.
To identify species is it better to look for perfect ‘specimen’ on-axis
clicks, or to try to characterise the whole ‘population’ of clicks received
by a static logger? … some relevant points:
•
•
•
•
•
•
Dolphins vary the characteristics of their clicks, so that there is a large set of ‘perfect
clicks’. For all species this set has not been fully described.
Loggers mostly receive off-axis clicks that are more diverse and less well known.
In an encounter there may be no on-axis clicks.
Sensitive detectors must work with weak received clicks. These are heavily
degraded by propagation effects and noise.
Identifying the axis of a click (= angle of origin relative to the loudest direction of the
click beam ) is difficult without multiple hydrophones spaced appropriately in relation
to the distance to the animal.
Both approaches will usually require visual identification of species.
The alternative to the ‘specimen click’ approach is the ‘click population characterisation’ approach that
seeks to identify click parameter distributions from whole encounters, and uses click rate information
from each train to inform that analysis. This may prove to be the more powerful approach.