A ship, a whale – or perhaps tremors from an earthquake? It can be a challenge to measure what is happening beneath the waves in the seas around Greenland, but GEUS and the Greenland Climate Research Centre are now tackling this challenge together.
The FATHOM research project – Fiber-optic and Acoustic Technology for Hazard and Ocean Monitoring is not only the framework for collaboration between two institutions but also a fusion of two technologies. At GEUS, researchers have experience in using Distributed Acoustic Sensing (DAS) on fibre-optic cables, whilst the Greenland Climate Research Centre has experience of using Passive Acoustic Monitoring (PAM) in the form of hydrophones. Both methods are already in use today to gain insight into what is happening beneath the sea surface.
“We are combining two existing methods in order to build on both of them and to create entirely new opportunities for monitoring activities and signals beneath the sea surface,” explains Tine Larsen, a seismologist at GEUS and project manager for FATHOM.
In addition to measuring activity in the sea, DAS can also be used to improve the monitoring of the many small earthquakes that occur around Greenland, which are normally only measured by seismographs on land.
The DAS system utilises existing fibre-optic cables as seismic measuring instruments. This is achieved by using a specialised device to send laser pulses through the cables and measure minute changes in the reflections caused by vibrations. GEUS has so far been working on applying the technology to inactive fibre cables, known as ‘dark fibres’, but in the FATHOM project the technique will be applied to cables that are simultaneously in use for communications. This opens the door to more widespread use. The technique provides monitoring data with high spatial resolution over tens of kilometres, but only within specific frequency bands.
“We can collect large amounts of high-quality data. But data are only useful if we can interpret them, and we are, to some extent, on unknown territory here. In FATHOM, we will be working on validating the data and creating a catalogue of known sources against which we can compare new measurements,” explains Tine Larsen.
And this is where PAMs, or hydrophones, come into the picture. Hydrophones are underwater microphones that are lowered to a specific location, where they record sound. When the instruments are retrieved, the data can be downloaded and decoded. Hydrophones provide extremely high-quality data across a wider frequency band than DAS, but within a much more limited geographical area and rarely with the ability to transmit data in real time.
“We use PAM to monitor wildlife and noise from ships in the waters around Greenland, but we would like to have real-time data covering a larger area. By combining our measurements with DAS, we expect to gain a better understanding of the sounds and activities in the ocean, and, not least, to develop new methods for underwater monitoring that does not affect the animals,” explains Malene Simon Hegelund, who is head of department at the Greenland Climate Research Centre at GINR and has extensive experience in bioacoustics, as the method is called.
The researchers see clear opportunities for improved monitoring at sea when the two methods are combined. First of all, they will be able to expand the frequency band from which we collect data. Furthermore, the researchers can develop tools for automatic recognition of signals, thus providing real-time classifications of detections by validating the data against each other and against known signal sources.
“We will be working on developing a dashboard where users can monitor in real time what we estimate, based on data, is happening beneath the sea surface,” summarises Tine Larsen.
The project manager emphasises that the project is just as much about identifying the limitations of the methods as it is about finding out what they are capable of.
“When it comes to monitoring, it is just as important that we are clear about what we cannot say anything about, and that is why we will push the methods to the edge so that we can identify their limitations,” says Tine Larsen.
The project is based on the waters around Greenland but could be applied elsewhere.
FATHOM has received co-funding from the Danish Ministry of Defence Acquisition and Logistics Organisation (DALO) through a funding round targeted at projects relevant to the Danish Defence’s operations in the Arctic and the North Atlantic.
The project team will regularly discuss the project’s findings with, amongst others, the Danish Defence, and expects to be able to share the research findings publicly for the benefit of other projects.
The Greenlandic telecoms company Tusass is supporting the project by providing access to its submarine cables, as the project is also expected to yield important insights for improved monitoring of the critical subsea infrastructure that these cables represent. Alcatel Submarine Systems (ASN) provides partial support to the project by enabling an extended DAS data collection period. In addition, the project team will work closely with the Marine Bioacoustic Lab at Aarhus University, with which the Greenland Climate Research Centre already collaborates closely on underwater sound analyses, as well as with Natural Resources Canada (NRCan), with whom GEUS already collaborates on, amongst other things, earthquakes in the Arctic.
