The health of marine ecosystems is linked to that of all living beings on Earth. Around the world, people are waking up to this truth and making efforts to find out what goes on in the murky depths of the oceans. One of the foremost indicators of the health of oceans is determining the population size of Cetacean species, which includes porpoises, dolphins and whales.
However, that is easily said than done. Over the centuries, man has recklessly hunted these gentle giants to the point of almost near extinction. Those left over, are in the endangered list. Conservation status and population size of these mostly endangered species is only possible by collecting extensive amounts of data about them.
Researchers are carrying out studies on the ecological impact of industrial activities such as human-generated noise on the coasts. A team from the University of Sao Paulo in Brazil is carrying out underwater acoustic monitoring. They quantify the human-generated noise while studying sound generated by cetaceans. Water being an efficient medium for transmission of sound, such research is useful because water allows easy detection of the tones and clicks cetaceans produce, even when they are hundreds of kilometers away.
The team first went about their research using commercial underwater recorders. However, they found these to be expensive and inflexible. Almost all the underwater recorders had proprietary hardware and software, which they found impossible or difficult to modify. Therefore, the team built a flexible, low-cost autonomous hydrophone recorder based on the Raspberry Pi or RBPi, an affordable single board computer. They published a paper about their effort in the open-access journal PLOS ONE.
The system is an underwater sound recorder. The entire unit consists of a 50 cm PVC case with 9.5 mm thick walls, closed on both ends. One end has a protective cage made of stainless steel, and this houses the actual hydrophone or an underwater microphone. All the other electronics is located within the PVC case, which is sturdy enough to withstand pressures up to 10 bar. Pressure-chamber testing establishes this as equivalent to pressures experienced almost 100 m underwater.
After signal conditioning, the analog output from the hydrophone passes into a USB linked analog to digital converter, which generates the digital data corresponding to the analog signal. The RBPi stores the digital information along with an appropriate time stamp. Five ordinary D-sized Duracell batteries power the RBPi and there is adequate room within the PVC enclosure to add four more such packs in parallel. Power from the batteries reaches the RBPi via a power management module. The RBPi also includes a real-time clock for generating the time stamp.
The team monitored marine traffic including dolphin and whale population on the eastern and southeastern coast of Brazil. They used their own RBPi based hydrophone recorders and these worked satisfactorily. According to the results of their tests, the devices recorded for two weeks continuously when powered with five battery packs. If they recorded for one hour each day, the researchers estimate the five batteries would power the recorder for over four months of recording.
