Interpreting underwater symphonies

“The ocean is far less explored than we think. In fact, we know less about the depths of Earth’s oceans than we know about the surface of the Moon or Mars!” says Dr Hari Vishnu, Senior Research Fellow at the Acoustic Research Laboratory under the NUS Tropical Marine Science Institute, who listens to underwater sounds to discover the immense world beneath the waves.

“I study the use of sound to sense and understand underwater environments such as the ocean. Sound travels longer distances under water than light, and tells us a lot about what is happening in the ocean. I use sound-based sensing for diverse applications such as assessing Singapore’s marine biodiversity and the presence of marine mammals like dolphins and dugongs, improving capabilities of sonars used in defence applications, and deep-sea resource assessment,” shared Dr Vishnu.

With an academic background in computer engineering, as well as electrical and electronic engineering, Dr Vishnu was drawn to this area of research by his desire to make an impact on the under-explored world of the oceans: “Life originated in the oceans that cover more than 70 percent of the Earth, yet I feel it is underappreciated how crucial oceans are in determining its climate and supporting human survival. So, during my PhD, I decided to jump into this broad domain of ocean engineering. I specialised in studying how to process ocean sounds to interpret them and learn more about the ocean.”

Symphony of underwater life

Healthy oceanic environments such as thriving coral reefs attract fishes and crustaceans that feed in these regions, leading to a rich soundscape that can be monitored via acoustics.

“Marine mammals like dolphins and dugongs produce sound calls during foraging and communication, which can be heard over long distances. Indo-Pacific humpback dolphins, bottlenose dolphins and dugongs have been documented in Singapore’s waters, but quantifying their presence has been a challenge in the past,” elaborated Dr Vishnu.

A particularly high moment of discovery for Dr Vishnu was when a machine-learning based scanner picked up dolphin-like vocalisations in his underwater sound recordings: “I could hear these dolphin sounds clearly, it was like they were speaking to us over the recorders. It gave me the confidence that we were on the right track in monitoring these animals.”

From the tropics to the Arctic

Dr Vishnu’s expertise in Singapore’s underwater soundscape led him to a research project in a vastly different climate – the Arctic.  A unique connection between the two locations resulted in him being involved in monitoring glacier melt and how it indicated the effects of climate change.

“While Singapore and the Arctic are worlds apart in terms of climate, their underwater soundscapes are uncannily similar! The sounds due to melting ice in the Arctic remind us of the constant crackling sound in Singapore waters produced by colonies of snapping shrimp.  Our lab has many years of expertise dealing with this shrimp noise. This was how I, along with the Head of the NUS Acoustic Research Laboratory Assoc Prof Mandar Chitre, got involved in Arctic soundscape monitoring where we put our expertise to use in a different application with global impact.”

The sound of food frying in the Arctic

When warm ocean water comes into contact with glaciers, the glaciers melt and release pressurised bubbles trapped in the ice. This creates a persistent popping sound reminiscent of french fries being deep-fried. Based on this phenomenon, Dr Vishnu, in collaboration with Assoc Prof Chitre, and scientists from Scripps Institution of Oceanography and the Polish Academy of Sciences, discovered that underwater acoustic monitoring may indicate the rate at which glaciers are melting.

“For this research, we undertook a field campaign in 2019 to Svalbard, Norway. We used an array of underwater sound recorders to scan sound in a vertical direction like an acoustic camera, assessing which directions sound energy arrives from. We realised that the recorded sound tells us a story of the different events happening in the bay such as melting, calving and pieces of ice floating around,” elaborated Dr Vishnu.

The researchers found that the sound in the glacier bay was correlated with the water temperature, thus indicating a link to the glacier melt activity. The team is currently working towards building autonomous monitoring systems that can operate for long months in harsh glacial environments which can provide information on these regions.

“Research into arctic glacier acoustics will help advance our monitoring of climate-change effects and its translation to sea-level rise, which threatens the whole of humanity, and is especially relevant for planning in coastal states like Singapore,” elaborated Dr Vishnu.

Listen to the sound of melting glacier ice here: https://soundcloud.com/user-248456662/grant-deanes-melting-cie