Unravelling Acoustic Challenges in India Ocean Region

  • The Indian Ocean has vast economic potential owing to its mineral resources.
  • The acoustic sensors aid in understanding the oceanic processes using underwater wave propagation systems.
  • The topography, marine ecosystem and chemical characteristics of the Indian Ocean lead to distinct underwater wave propagation
  • Most acoustic systems are based on Western conditions and do not cater to local conditions, thus reducing their efficiency in tropical waters.
  • R&D investments are needed to develop regional-specific systems to understand the unique conditions better.

The Indian Ocean Region (IOR) is a key global focal point due to its strategic, economic, and environmental significance. With immense potential in both economic and ecological aspects, a comprehensive understanding of its oceanic resources is vital. Ongoing oceanographic research aims to provide valuable insights into seafloor topography and landmass distribution, essential for geological mapping and seafloor exploration. Monitoring systems are crucial in environmental observation, enabling scientists to track sediment composition and distribution changes over time. This information is pivotal for understanding environmental impacts and facilitating informed management decisions. Additionally, the central Indian Ocean Basin holds polymetallic nodules, concentrations of metals like manganese, cobalt, and nickel, crucial for high-tech industries.

Advancements in acoustic technology have transformed our comprehension of ocean basins. Sonar systems, initially developed during World War I for submarine detection, now play a pivotal role in oceanic studies. These systems reveal crucial information about the ocean floor’s topography by transmitting sound waves into the water and capturing returning echoes. The disappearance of Malaysia Airlines Flight MH370 highlighted the insufficient bathymetric data in vast sea areas globally. The distinctive tropical conditions in the IOR pose challenges for effective acoustic sensor operation. These systems are indispensable for diverse applications, including habitat mapping, resource exploration, marine engineering, and coastal zone management. They offer insights into benthic ecology, potential mineral and fuel resources, and the impact of sediment processes on coastal ecosystems.

The Indian Ocean’s depth can be understood in two ways: the literal depth, i.e. hypsometrically, and the acoustic depth, influenced by the chemical composition of water. With an average depth of 3,890 metres, the Indian Ocean is in the middle of the Pacific and Atlantic Oceans. The acoustic depth is affected by water characteristics, creating instances where areas that seem shallow in literal depth may appear deep acoustically and vice versa.

“The Indian Ocean boasts diverse seafloor features like seamounts, troughs, ridges, and active and dormant underwater volcanoes. These landforms interact with sound waves, causing reflections, refractions, and scattering that can complicate distinguishing between seabed echoes, signals, and actual objects. ”

This interference makes it challenging for tools like sonars and profilers to operate efficiently in the ocean’s depths.

In the Indian Ocean, the speed of sound is influenced by temperature, salinity, and pressure. The sound speed in warmer northern regions is faster than in colder and less salty oceans. In the deep Indian Ocean at mid-latitudes, where temperatures decrease with depth and salinity varies, the sound speed reaches its maximum due to high sea surface temperature. Generally, as you go deeper, temperature drops, salinity can change, and pressure always increases. Research indicates that temperature changes more near the surface in mid-latitudes than at the ocean bottom, while salinity changes only slightly. The speed of sound in water increases with rising water temperature, salinity, and pressure.

“Just beneath the ocean surface is the sonic layer, where changes like heating, cooling, and wind impact the speed of sound. The depth linked to the near-surface maximum in sound speed is called the sonic layer depth (SLD). The SLD is lower in the Arabian Sea than in the Bay of Bengal due to the intrusion of Persian Gulf and Red Sea waters. ”

The sound channel axis, corresponding to the sound-speed minimum, forms a channel in the ocean known as the Sound Fixing and Ranging or SOFAR channel.

“This channel allows low-frequency sounds to travel long distances. In tropical waters, the SOFAR channel is deeper. In the Arabian Sea, the depth of the channel axis increases towards the northern latitudes, while in the Bay of Bengal, it decreases. This is due to low-saline cold water in the northern Bay of Bengal, reducing sound velocity. ”

The SOFAR channel is shallower in the Bay of Bengal. Consequently, the effective sound channel lies much deeper beneath the sea surface.

In the Arabian Sea and the Bay of Bengal, the sound speed profile is depth-limited, meaning the top part of the water is faster than the bottom. This has a significant impact on how sound travels in these regions. Sound waves in a depth-limited ocean will be surface-refracted and bottom-reflected. Typically, the Arabian Sea has a faster sound speed than the Bay of Bengal under normal conditions. Strong winds, running almost parallel to the coasts of Somalia and Arabia, cause the water to rise, decreasing temperature and sound speed. Following this temperature trend, the sound speed increases from west to east in this area.

The Indian Ocean is a bustling hub for various human activities like fishing, shipping, offshore drilling, and international trade. These activities generate noise from engines, propellers, and seismic surveys. This human-made noise can disrupt important sonar signals and mix with other underwater sounds. Additionally, there are underwater data cables in the Indian Ocean, facilitating large data transfers between countries. The collective noise from these cables might pose challenges for sonar devices in the region. Therefore, it’s crucial to assess the impact on acoustic habitats along the warm coastal areas of the Indian Ocean.

The unique features of the Indian Ocean’s underwater landscape, such as deep-sea trenches and ridges, provide diverse habitats for marine life. Various species like whales, dolphins, and other marine animals inhabit the open ocean areas. These living creatures impact how sounds travel underwater by producing noise, which can distort and scatter signals. Coastal animals, especially those near continental shelves, contribute background noise. Active sonars may mistake whales or groups of fish for actual targets. Additionally, barnacles and other organisms can indirectly affect sonar performance by accumulating on sonar domes and sensor faces, contributing to hull noise known as self-noise on ships.

Only a few companies worldwide specialise in designing and developing underwater acoustic sensors, and these technologies are considered restricted. Importing them requires an end-user certificate. The United States and Scandinavian nations primarily produce well-known acoustic sensors. However, in the warm coastal waters of the IOR, using these systems efficiently poses significant challenges. The sonars utilised for underwater surveys in this region could perform better, limiting their effectiveness. The tropical littoral waters in the IOR, influenced by surface and bottom interactions, exhibit characteristics like shallow waters, impacting acoustics.

“The constant changes in surface and bottom conditions, typical of tropical weather, contribute to acoustic signal distortions through multipath propagation. As a result, sonars used for underwater monitoring struggle to perform optimally in the warm coastal waters of the IOR due to their short acoustic depth, with their efficiency reduced by 60% to 70%. ”

India must enhance its capability to manufacture underwater acoustic gadgets, requiring strong engineering and material science skills. To achieve this, there’s a need to develop expertise in acoustics, including training initiatives for scientists, researchers, and marine professionals.

“Access to advanced acoustic tools like sonars, robots, and AUVs, along with comprehensive training on their maintenance and usage, is essential. Practical training should cover data collection, processing, and sediment classification methods to interpret acoustic data effectively and draw conclusions about sediment composition and distribution. ”

By fostering collaboration, sharing best practices, and supporting research initiatives, India can establish a solid foundation for integrating acoustic sediment analysis into environmental management, policymaking, and scientific research. Collaboration with international partners will further enhance capabilities and knowledge sharing, contributing to a comprehensive understanding of aquatic ecosystems and promoting sustainable practices.


Romit Rajendra Kaware

About Author

Romit Rajendra Kaware is a recent graduate from the Indian Institute of Technology (IIT) Delhi, holding a Bachelor’s degree in Civil Engineering. Currently, he is serving as a research fellow at MRC. His primary focus during his fellowship is on scientific research in structures and materials, as it enhances comprehension of the methodologies employed in constructing resilient infrastructure. Romit  is deeply enthusiastic about applying his expertise in civil engineering to the maritime sector to foster sustainable development for our ecological resources.

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