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Unlocking mysteries beneath the surface of the sea

Hydrophones are underwater acoustic sensors used in marine geological and biological research, undersea mapping and navigation, and various commercial and military applications for carrying out interdisciplinary scientific investigations into oceanography and marine science. Within the framework of this project, a hydrophone streamer and positioning system have been developed, realised and used for collecting sea bed echoes for ocean exploration by acoustical means.
Unlocking mysteries beneath the surface of the sea
Hydrophones are submarine acoustic sensors most commonly used in either single or arrayed formation, that may contain from dozens to hundreds of individual units. Coupled with transducers that contain both transmitters and receivers they constitute a powerful tool for acquiring distance and directional data for communication, mapping and navigation.

The main objective of this project was centered on developing a special purpose device that would transmit, receive and process signals in real time. The system comprises a hydrophone streamer along with electronic equipment, including amplifiers, filters and channels, for signal processing and digitisation before on-board storage. In addition, there is also a positioning system that incorporates transmitters installed on the back of the tow-fish with both hardware and software necessary for processing the acoustical positioning signals.

There were three trial testing periods for improving the performance of the system. The system layout consists of an array of 49 identical hydrophones with wide bandwidth frequency response within the range of 10 Hz-40 kHz, receiving sensitivity at 171 dB, and maximum working depth at 300m. The system has been divided in three parts, each presenting different spatial sampling and offset characteristics. The system exploited the advantages of the Steerable Parametric Array (SPA), a transmitter unit and an integrated system for the generation of signals and recording of seabed returns. Hence, whatever the water depth and whichever the parameters of acoustical signals (grazing angle, offset constant), the steering capabilities of the SPA allowed the enhancement of the detection capabilities of the system. Based on acoustics, positioning data was collected and processed with the aid of computerised means. Data on depth measurement was also integrated into the positioning software allowing the improvement of precision for positioning.

The experience gained from this effort may lead scientists to use this technology for further underwater experimentation and ultimately, this may result in a monitoring system capable of acoustic exploration throughout much of the global ocean. Underwater acoustic monitoring allows the detection of a variety of deep-ocean processes at long distances. Some of the sound sources to be studied might include earthquakes, volcanic activities, landslides, marine mammal and fish vocalisations, weather, and manmade noises.

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