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

Tonality: a/b

.. process carried out by digital electronics

kHz >>>

ab b

high pass input filter

background noise

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

2 frequencies

10 selection criteria applied to each event

Some selection data are not logged

bandwidth index

duration

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 :

5 clicks from a dolphin click train

The click is logged as multiple replicates arriving over 30ms and spread over 100kHz

time >

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)

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?

Frequency of multipath replicates from porpoise clicks and boat sonar

Tursiops click trains Yaxis: amplitude Colour: kHz

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.

Bottlenose dolphin Tursiops truncatus

Harbour porpoise Phocoena phocoena

Click characteristics

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.