The ocean is a gigantic, complex ecosystem comprising life forms ranging from small enough to be invisible to the naked eye to giant enough to be compared with commercial jets. The oceans cover ~70% of the Earth’s surface, and to nobody’s surprise, have a tremendous variation in geology across the globe. This incredible variety in marine life and bathymetry, coupled with precipitation and winds, generates random underwater noise that cannot be modelled deterministically. Anthropogenic activities like commercial shipping, coastal activities, and seismic explorations add to the noise.
Why must we measure underwater noise?
Outside the realm of textbooks which have the freedom to make ideal assumptions (often dubbed ‘the real world’), no signal can be ‘noise-less.’ Communication systems are cursed with the problem of addressing noise and finding clever solutions to minimise them. To establish effective underwater communication, we must understand the noise soundscape we are working with.
Research suggests that for tropical littoral waters like that of the Indian Ocean Region (IOR), commercial shipping is the most significant contributor to the ambient noise profile of the ocean. Anthropogenic noise originating from sources like these is generally in the frequency range of < 100Hz and harms marine life. For instance, whale strandings have been reported with origins traced to noise generated by commercial ships.
Concerns for the Indian Ocean Region
Electromagnetic waves are easily absorbed by water and practically useless for long-range signal transfer underwater. Acoustic waves become the default option for underwater communications. Commercial off-the-shelf (COTS) sonar technology was designed and prototyped during the Cold War era. Naturally, it is suited for the deep, temperate waters of the Atlantic.
How do we measure noise?
Mathematical models have been developed to approximately map ambient underwater noise-based on data from commercial ships. Shipping data can be obtained from the Automated Identification System (AIS). However, these ambient maps are theoretical and require validation through experimental measurements.
Hydrophones are underwater analogues to microphones and can measure sound underwater. They can be deployed from boats or moors, but surface waves intrinsically introduce noise.
Autonomous Underwater Vehicles (AUVs)
An AUV is a self-propelled, unmanned, untethered underwater vehicle capable of being utilized as a survey platform to map the seafloor or characterize the water’s physical, chemical, or biological properties. AUVs can scan shallower waters than boats can. AUVs can also venture into areas unsafe for human divers. Due to their modular design, various sensors can be attached or removed, giving the designer the ability to use them for various purposes.
AUVs have been used to search for crashed aero planes like the Malaysian Airlines MH370. Oil-drilling companies also use them commercially to map the bathymetry before beginning operations.
Designing an AUV for noise measurement
An AUV uses a variety of sensors for dynamics (IMU, DVL, Depth Sensors), acoustics (Hydrophones), and vision (Cameras). This data is used for localization, navigation, experimentation or any specific task that an AUV might be designed for. The vessels (hulls) that house the sensors and electronics are waterproof (obviously) and extremely hydrodynamic for deep-sea missions.
Methods for designing hydrophone arrays for high-resolution ambient noise mapping are actively researched. Literature suggests the possibility of using an array of hydrophones attached to a streamer, which can be towed using an AUV. Using this apparatus allows us to take multiple measurements in the same locality, and interpolating the data can help in receiving higher resolution maps.
Sarthak has completed his internship at MRC, Pune. His research project revolved around the design and development of Autonomous Underwater Vehicles (AUVs) for underwater ambient noise validation. He is currently completing his studies at Indian Institute of Technology (IIT), Bombay.