Q. Is it possible to just log the clicks from the animals instead of trains?A. Many cetacean clicks are not very distinctive, especially those from dolphins and from very distant animals. The T-POD logger doesn’t know whether it is picking up a very loud source a long way off, like a dolphin, or a very weak source very nearby. A logger that relies on recognising the presence of the animal from the characteristics of individual clicks can work reasonably well for porpoises, but not for dolphins, and in either role it can be misled by boat sonars operating at the frequency of the animal. Picking out those clicks that fall into trains, and then the trains that resemble those produced by the animals, hugely increases the accuracy of the system and gives some evidence of the behaviour of the animal. Q. What is the probability model used in TPOD.exe?A. A train, by definition, has some regularity. Cetacean clicks trains are not very regular, but a click cannot come anywhere in time and if the prevailing rate of arrival of clicks is so high that a click is likely by chance to fall in the possible time zone for a particular click in the train, then that click cannot be considered to have contributed to the improbability of that train being a chance event. An identified train can be evaluated in this way to give it an overall improbability estimate. In practice there are major limitations to this approach (e.g. the strongly non-random nature of tonal noise in the sea which can carry temporal signatures of surface waves, conduction pathways, propeller blade rates and other things) but it does still have practical value. Q. How are boat sonars identified?A. The 'seeking boat sonars' stage of processing a file looks for the constant duty cycle of boat sonars. Within that cycle there may be a sequence of several inter-pulse intervals and a constant interpulse interval is becoming unusual. Long clicks are also sought. If you have data that you know is boat free, you can switch off the detection of boat sonars. Q. Why is 'seeking cetaceans' quicker than finding boat sonars?A. Unlike the search for boat sonar, the software does not have to use any extended auto-correlation. To find boat sonars, an auto-correlation is carried out on the whole of each scan, using a fast algorithm that skips all 'mutually negative' time slots. The potentially huge number of calculations per day's data are further speeded up by prior reduction of the time resolution, but it is still a long process. All the mathematics in TPOD.exe is done using integers which makes it very much faster (this does include those calculations that give results, shown as decimals). The cetacean search is also able to skip very noisy patches as no train can ever be found in them. The image below shows the duration of each click as a vertical line. A boat sonar is pulsing regularly at 50 kHz with surface waves affecting the received duration of successive reflected pulses. Meanwhile a bottlenose dolphin injects a much faster and accelerating train of shorter clicks that are also at 50 kHz. Both fisher and dolphin are seeking the same prey.
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