Keynote Speakers

Progresses of underwater acoustic technologies
Tomonari Akamatsu, Research Organization for Nano & Life Innovation, Waseda University

Introduction
In underwater environments where electro-magnetic waves and light cannot travel far, sound waves have long been used for exploration and communication. This lecture will introduce the recent developments of active technologies, which emit sound waves to detect reflections from objects, and passive technologies, which detect sounds from phonating objects. It will also explore future technological prospects.

Development of Active Technologies

In active technologies, a major turning point has been the application of multi-channel and wide-band methods. For example, multibeam sonar equips multiple receiving elements for the beam forming to separate reflected sounds from various directions as well as the echo delay time, which is enabling the visualization of 3D object distributions underwater. This has been applied in mapping seabed topography, fish school shapes, subsurface exploration for mining using air guns, and even underwater visual-like imaging using acoustic lenses. Additionally, wide-band technologies provide various type of target information in addition to the location and time. For example, power spectrum shape of wide-band echo from a fish can be used for the species identification. Impulse response of a target could reflect the internal structure of a fish that is the bio-mimetic echosounder of dolphin sonar. Wide-band frequency modulated signals also contribute the fine time and spatial resolutions to obtain fine imaging of seabed and fish schools.

Ocean sound propagation experiments by Munk and others in 1991 demonstrated that low-frequency underwater sounds can travel thousands of kilometers in the sound channels. This technique has been applied to observe average water temperature and current speed based on the sound travel time between two acoustic stations having a transmitter and a receiver. One pinnacle of the application of this technology is ocean acoustic tomography, which calculates the 3D distribution of water temperature and current speeds by solving simultaneous equations of multiple sound paths.

Underwater communication is challenging theme, which is even difficult comparing with space communication. The speed of sound is significantly slower than light and is more susceptible due to multiple reflections and attenuation. Techniques like phase modulation and convolution processing have been employed to mitigate these effects. The improvement of the distance and the speed of communication is an essential technology for future underwater engineering for controlling autonomous underwater vehicles (AUVs) and other independent underwater robots.

Development Passive Technologies
Passive technology development has been largely driven by the detection of submarines. During the Cold War, a network known as SOSUS (Sound Surveillance System) was deployed across the Atlantic. Interestingly, what was once considered noise turned out to be the vocalizations of whales. After the Cold War, underwater acoustic technologies were made available for civilian use, and what was once regarded as noise became the target of observation. Not only marine mammals like whales, dolphins, and dugongs, but also sound-producing species such as fish and crustaceans, are now being monitored using passive technologies. These techniques are widely used globally for environmental assessments of the oceans. For examples, long-term continuous monitoring of presence of odontocetes using high frequency pulse event recorders have been used for the impact assessment of offshore windfarm development.

A more recent development that has attracted attention is Distributed Acoustic Sensing (DAS), which uses weak reflections from lattice defects in fiber optic cables to measure underwater sound pressure. Though the sensitivity is still insufficient, existing fiber-optic communication cables can be used as hydrophone arrays to detect surrounding sound sources.

Future Issues and Anticipated Technologies
Ocean is no longer out of reach domain. Underwater Domain Awareness is strongly demanded for the sustainable use of ocean. For active technologies, precise mapping and wide-band communication in long range is needed. Localization in the water like an underwater GPS will be required for the operation of underwater robots. For passive technologies, identifying the sources is a big issue. Annotated reference data of phonating targets is needed. The rising underwater noise levels from various human activities have become an international concern due to their impact on marine life. The International Maritime Organization has already set non-mandatory guidelines for ship noise. In coming future, two major ocean developments are the floating type offshore windfarms and deep-sea mining. The issue is that we do not know the baseline data of ocean noise and status of intact marine ecosystem in far offshore and deep waters. This means no control data is available to assess the impact of future ocean developments. Ocean engineering should care the ocean ecosystems to meet sustainable development goad 14^th. Active and passive acoustic technologies will provide compromised solutions.