Sea Technology

JUL 2018

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18 ST | July 2018 www.sea-technology.com Scientists have gained much knowledge and a better understand- ing of the ocean and how humans are affecting it by using hydro- phones to record marine acoustics. JRF's acoustic data may contribute to ecoacoustics by establishing a benchmark of the soundscape pat- terns along Europa's path, which could support marine conservation and resource management efforts. Building the Acoustic Payload To be able to detect whales from miles away and ensure no recorded data were lost, JRF's acoustic pay- load required significant software and hardware engineering. They acquired a "smart" digital hydro- phone called the icListen HF, made by Ocean Sonics of Nova Scotia, Canada. The icListen can record a wide range of frequencies and tar- get most cetaceans. During the design phase, and anticipating the perils of an ocean journey, they attached the hydro- phone to Europa's subsurface com- ponent, or "sub," to prevent loss due to entanglement or predation. How- ever, being directly connected, the hydrophone picked up self-noise from the platform, which interfered with acoustic sampling. Therefore, JRF embarked on a design to de- couple as much of the self-noise as possible from the hydrophone and enlisted the help of a veteran acous- tician, Michael Holt, who has de- signed devices for similar purposes. Holt helped them create a mount that would be durable but not too heavy, biofouling resistant and still enables sound transference. To con- struct this, JRF used a thick-walled 3-in. type K copper pipe, which is an excellent biofouling deterrent and provides mounting strength. They mounted it to the center of the sub to avoid drag or any change in the sub's performance. To mask mechanical noise, Holt first designed an inner casing tube out of low-density polyethylene to house the hydrophone. To keep it from hitting the bottom or the sides of the tube, they manually cut 15 circles of polyurethane open-cell foam to encase the hydrophone. JRF then sealed the tube and filled it with medical-grade castor oil, which closely matches the acoustic impedance of seawater. After testing it for leaks, they slid the inner cas- ing into the outer copper pipe mod- ified with 250 drilled holes, which reduced interference within the fre- quencies of interest. Data Analysis During the mission, which start- ed in January and ended in April 2018, the JRF team was periodically transmitting short sound files back to shore via satellite as a contin- gency for system loss or failure. The Ocean Sonics hydrophone allowed them to set five "triggers" (or ep- ochs) that detect sounds above de- sired decibel levels and within the desired frequency ranges of whales. The triggers were also used to help filter out specific frequency ranges, such as self-noise. When the epoch detected a whale, JRF would request a fast Fourier transform (FFT) text file that displayed the relative strength of fre- quencies sampled over time. Subse- quently, they would download the FFT into the Ocean Sonics' software program Lucy, where they could view it as a spectrogram (a graphi- cal representation of the audio). If JRF confirmed there was something of interest from the FFT, they could request an actual mp3 audio file via satellite. Using specialized soft- ware, they had the ability to detect humpback whales' sounds by lis- tening to and looking at the differ- ent frequencies in the mp3. For ref- erence, baleen whales' frequency range is 20 Hz to 22 kHz, and the humpback whales' song is thought to be 80 Hz to 4 kHz. The team is currently processing more than 800 surface and under- water pictures, sea surface tem- perature and salinity data, as well as analyzing more than 2,000 hr. of acoustic data. So far, the data have revealed some unusual sounds, which have yet to be substantiated. Conclusive results will be published in a scientific journal. A Second Payload: HF Voyager The HF Voyager is an entirely autonomous high-frequency (HF) UNDERWATER DRONES +33 (0)297 898 580 http://rtsys.eu/en sales @ rtsys.fr FIND YOUR APPLICATION HANDHELD SONAR FOR DIVERS UNDERWATER ACOUSTIC RECORDERS Marine Mammals Offshore Renewable Energies Defence Oil & Gas

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