To study something in detail you need to look at it from all directions, whether it is the Venus de Milo statue in the Louvre Museum, a car you are thinking of buying, or when using a CAT-scanner to image inside the human body. In the ERC-funded GLOBALSEIS project Professor Guust Nolet is doing this on a truly global scale, by developing a worldwide network of marine-based seismic-wave sensors that can give a much better picture of deep-earth structures and resolve a major paradox in geoscience.
Across the world’s oceans the GLOBALSEIS research team is scattering a few ‘mermaid’ prototypes – not the fishtailed variety of mythology, but rather autonomous floating devices equipped with hydrophones that listen carefully to the seismic tremors coming from deep in the earth. These mermaids are launched from ships and descend to depths of up to 2000 metres where they drift along with the deep-ocean currents – recording the seismic waves that signal earthquake events originating in the Earth’s mantle.
When enough data has been collected, they rise to the ocean surface and transmit this information through the global Iridium satellite network to the project team at the Géoazur Research Centre in the south of France.
“Simply put, through the mermaids, we are trying to understand why the Earth’s interior is so hot,” explains Guust Nolet. “When the Earth formed some 4.5 billion years ago it was very hot and with time it cooled by radiation. However, at some point in the geological past cooling slowed and the interior today is much hotter than theory tells us it should be. We do not know why."
“On the one hand, geophysicists suggest there must be high concentrations of radioactive materials in the Earth’s mantle, which decay and release heat. On the other hand, geochemists say that such high concentrations are not possible. This is a paradox. In the GLOBALSEIS project we are seeking a solution by investigating how mass, and heat are being exchanged between the Earth’s solid mantle and the surface, in particular by advection – the physical movement of material deep in the Earth.”
Rise and fall of plumes and slabs
The project is studying ‘plumes’ and ‘slabs’: plumes are massive volumes of hotter rock which rise through the mantle and eventually reach the Earth’s surface as volcanic activity. Slabs are the layers of colder rock which sink deep into the mantle from the Earth’s surface at ‘subduction zones’. Such movements involve the exchange of mass, and particularly heat, and thus could form an important element in explaining the temperatures observed in the Earth’s interior.
Prof. Nolet and his team are using seismic waves from earthquakes to search for plumes and slabs and image them by studying the waves’ properties and using advanced mathematical methods of seismic tomography. This can be done because the temperature differences between warmer plumes, colder slabs and the surrounding rock affect the properties of seismic waves (such as their velocity) in ways that can be used to produce images.
“We are particularly interested in where plumes arise and how far slabs sink,” he says, “so we are looking closely at the boundary between the upper and lower mantle at a depth of 660 kilometres. If we see that plumes and slabs cross this boundary then this would confirm a strong advective element in the Earth’s cooling, a way of getting heat out of the interior.”
However, to date, seismic tomography has not been able to produce good enough images to investigate the depth and size of these features. This is because most plumes and slabs are found under the oceans, while most seismometers are on land, which means that high-resolution imaging has proved impossible. Ocean-bottom seismic arrays do exist, but are too expensive to implement on the scale that higher-resolution seismic tomography demands.
But technical developments over the last decades now offer a solution, as Prof. Nolet explains. “I first had the idea for mermaids when I saw the oceanographers launching floats equipped with scientific instruments on to the oceans to collect data. Then, when the Iridium network of low-orbit communications satellites arrived it offered clear possibilities for locating and communicating with floating platforms over great distances. "
“The next challenge was to build a mermaid with enough artificial intelligence to recognise seismic events among all the noise of the ocean depths. It’s not quiet down there – whales, ships, oil platforms and storms create a very high noise level, especially for highly sensitive seismic sensors. We built and tested the first prototype as part of an earlier Marie Skłodowska-Curie grant, and the results were very hopeful.”
“Once dropped from a ship, mermaids are quite autonomous. They sink to a programmed depth and can stay there for months, listening for seismic events. When sufficient data has been collected, they rise to the surface and transmit the data back to us. We are hoping to mount a global, multi-disciplinary monitoring effort – a new project called MARISCOPE. By scattering mermaids around the oceans, especially in the southern hemisphere, we will be able to collect better, higher-resolution data to build 3D images of structures deep in the earth.”
The mermaids have already exceeded expectations and have enabled the research team to record earthquakes with magnitudes as low as 6 on the Richter scale, of which there are on average two every week. The next step is a fleet of 10 mermaids that will be launched around the Galapagos Islands to investigate an underlying mantle plume. However, the innovative technologies developed for mermaids are raising interest in potential commercial applications, according to Prof. Nolet. The project received an ERC Proof of Concept grant to investigate these. Together with OSEAN, a company specialising in underwater technologies, the GLOBALSEIS team is developing a more powerful mermaid.
“The new ‘Multimermaid’ will be able to carry multiple scientific instruments covering a range of research areas, such as biology and meteorology, as well as seismology. It incorporates more powerful sensors, can go deeper and can last for up to five years on a single battery charge,” explains Prof. Nolet. “In addition, it could be used for a range of non-scientific applications, such as locating groups of whales that can be a danger to shipping, or even locating black-box transponders from lost aircraft.”