Dr Bemis brings to the collaboration her cross-disciplinary knowledge of computer algorithms and geological phenomena occurring on the sea floor. She is interested in developing computer visualisations of changing geologic phenomena and gaining insight into fluid flow and heat transport in geologic processes. When asked what attracted her to the field, she explained: ‘I decided to study geology because I always liked rocks. As I got older, I became increasingly interested in why rocks differed, how they formed. In college, I focused on the physics of geologic processes. In graduate school, I started studying heat transfer related to hot springs at mid-oceanic ridges. This led me to underwater acoustics as a tool to study plumes of hot water from hot springs on the mid-oceanic ridges. Acoustic imaging has the advantage of both providing a 3D view of the plumes and their surroundings and of leading to quantitative measurements of fluid flow, volume flux and heat transfer.’

Dr Jackson contributes his skills and knowledge of signal processing techniques in underwater acoustics, and notably time-reversed acoustics. He tells us about how the trio came to work on oceanic hydrothermal systems. ‘Peter Rona noticed our work on using sonar to measure changes in seafloor temperature. He suggested that this technique could be applied to oceanic hydrothermal systems. Carefully designed sonar measurements can improve understanding of hydrothermal systems, much as ultrasound has improved medicine’s ability to observe the human body. Sonar provides a form of remote sensing, replacing a few point measurements with a 3D array of measured values.’ Peter Rona was an oceanographer with Rutgers University who found hot springs on the Mid-Atlantic Ridge in 1985 and is the founder of the collaboration.

Finally, Dr Xu brings his skills in scientific calculation and signal processing to the project. He is highly passionate about the underwater world and having ‘the opportunity to explore the many unknowns hidden beneath the surface of the ocean. Every day of my work is filled with the challenge of learning theories and skills and using them to better understand natural phenomena in the ocean. Deep-sea hydrothermal vents are important because they export heat and chemicals to the ocean and support unique benthic ecosystems. Our understanding of the ocean will remain incomplete without fully understanding hydrothermal vents’ impacts on and interactions with the ocean.’

COVIS is watching

The acoustic imaging of hydrothermal flow first started accidentally in 1984. Back then, a buoyant plume showed up on a routine sonar scan looking for obstacles in the path of ALVIN, a manned submarine. Later on, acoustic data processing and scientific visualisation were combined into new technologies capable of observing and quantifying what happens during hydrothermal discharges.

Drs Bemis and Jackson started making sonar observations to collect quantitative data on focused and diffuse hydrothermal flows. The project initially involved manned vehicles for deep sea exploration. However, those vehicles only allowed for an hour of exploration at a time. When they employed a Remotely Operated Vehicle (ROV), they could extend their periods of observation to a day in each session. Most recently, they, together with Dr Xu, made observations spanning years in a row by means of a sonar system connected to the NEPTUNE cabled observatory and operated by Ocean Networks Canada.

The Cabled Observatory Vent Imaging Sonar, or the COVIS, was developed with the help of the National Science Foundation. At its heart is the Reson SeaBat 7125 multi-beam sonar, capable of highresolution sonar imaging and giving the researchers a way to look into the abyss. In a paper titled ‘The path to COVIS: A review of acoustic imaging of hydrothermal flow regimes’, the team retraced the steps of scientific discovery from inception all the way to the development of leading-edge technology – the COVIS. Due to it being a cabled observatory residing almost permanently under the water, COVIS has monitored hydrothermal flows eight times a day for several years. Aided by the data the observatory sends back, the researchers were able to develop state-of-the-art diffuse flow detection techniques for estimating plume activity rhythms, bending, vertical rise, and volume and heat flux.

The importance of the research lies in two directions. ‘Heat transfer at mid-oceanic ridges is important for understanding the circulation of seawater-based fluids in the context of the plate tectonics system and from a climate perspective, for understanding the heat inputs of the ocean’, Dr Bemis explains. COVIS’ explorations started in 2010. Ever since then, its data have been used to create 3D dynamical images showing the effect of tides on the plumes, velocity measurements, heat transport estimates, and 2D images of spatial distributions of diffuse flows. But COVIS started with the question of whether acoustic imaging can really show the behaviour of the plumes. During the first five years of operations, up until 2015, COVIS performed excellent measurements and brought back a wealth of scientific knowledge. Just like bats,

COVIS sees through sound scattering and can take two-dimensional slices through the plumes, thus enabling the reconstruction of 3D images. The sonar can measure the velocity of the flows and shows the presence of diffuse hot water by detecting its shimmering effects like those of a hot roadway. COVIS can also be programmed and controlled remotely during its long missions.

The motion of plumes creates a Doppler shift in the frequency of the sonar echo, which helps determine the velocity of the water current. Together with 3D imaging, this shift helps determine the direction and strength of the current everywhere inside the plume. This is how the team made the striking discovery that the heat production can be larger than 10 megawatts, despite some plumes being thrown out of five holes, each with a size of only five centimetres. Around black smokers, the temperature reaches more than 250˚C, whereas the diffuse part of the plumes has temperatures of only a few tens of degrees. Researchers can see the diffuse flows by pointing the sonar to the ocean floor and programming the computer to listen to its ‘pings’. When diffuse flows are present, the sonar echo breaks its normal patterns and the result is transformed into maps of the flows.