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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.
.. process carried out by analogue electronics
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.
high pass input filter
.. 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:
3 wave heights
Some selection data are not logged
10 selection criteria applied 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 is a click from a distant porpoise.
The section marked by the red bar is just within the detection limit.
The click is logged as multiple replicates arriving over 30ms and spread over 100kHz
5 clicks from a dolphin click train
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.
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)
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.
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
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.
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:
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.