Distributed acoustic sensing unit and seismographs for ground motion detection

Jun 14, 2023

The aim of the research of the CRONOS project’s Component 1 is to describe seismicity in as much detail as possible, to determine the 3D structure of the Earth’s crust (seismic velocities, density and attenuation) and the anatomy of the fault system, and to understand the tectonic relationships in the area of Dalmatia, one of the most seismically active parts of Croatia. In order to achieve this, it is necessary to establish a dense network of seismographs and thus enable the collection of high-quality data – records of the movement of the Earth’s surface caused by local, regional or global earthquakes, but also records of ambient noise. And while the analysis of seismic waves caused by an earthquake is the basis of all seismological research, ambient noise – all that movement that is not the result of an earthquake – has become a new standard in the study of the Earth’s internal structure. Various methods of analyzing seismograms of earthquakes and ambient noise, combined with geological research, will help us determine a detailed 3D model of the Earth’s crust. A large number of seismological stations, together with a more suitable model of the Earth’s interior, will improve the accuracy of the location of earthquake hypocentres, will enable the calculation of the earthquake shaking scenario – the expected ground motion for probable future earthquakes, and the assessment of local seismic hazard.

A distributed acoustic sensing unit (DAS) is a device that uses an optical fiber to detect seismic waves: it sends a laser pulse and records the time it takes for the signal to return after scattering on imperfections in the optical fiber. It can be connected to an already installed network of optical cables, e.g. telecommunications.

Within the framework of the CRONOS project’s Component 1, we acquired some of the most modern instruments for the study of earthquakes and the structure of the Earth. Along with 13 new classical broadband seismographs that are the basis of every good seismological research, we must highlight a new generation of instrument that uses optical cables (optical fibers) as a sensor for ground motion detection – it is called a distributed acoustic sensing unit (DAS). The device works with the help of a laser on a principle similar to radar: it sends a series of laser pulses into the optical fiber and records the time it takes for the signal to return when it scatters on the imperfections that always exist in the optical fiber. The optical fiber is now a sensor without any additional elements, so this technology enables continuous measurements in real time along the entire length of the optical cable. At the same time, the entire optical cable behaves like a series of densely distributed sensors (e.g. every 2 m) – as if we have an extremely dense network of seismographs along several tens of kilometers of cable. Unlike conventional seismographs, the device detects the deformation (not displacement, velocity or acceleration of the ground motion) caused by a wave disturbance, e.g. a seismic wave caused by an earthquake. Because already installed telecommunication optical fibers (optical cables) can be used, this method can be used to investigate areas where classical instruments cannot be deployed or their deployment is inappropriate or difficult, for example in large cities like Zagreb or on the seabed – in fact wherever we optical cables. This dense “network of instruments” can be used to detect earthquakes or other sources of seismic waves, such as for traffic monitoring and analysis.

FEBUS A1 device with a simplified description of the functioning of a distributed acoustic sensor unit (DAS) on an optical cable. Red is the laser pulse sent from the DAS unit that travels through the fiber optic cable, and green is the signal (of small amplitude) that was scattered back on imperfections in the optical fiber an that returned to the DAS unit to be measured by the device.

In the past several years, this new type of device started to be used in seismology more often and its popularity is growing, which means that many methods are actually still under development. The Department of Geophysics of the Faculty of Science at the University of Zagreb is now in the company of not-so-many institutions in Europe that own such a instrument and use it in their research. The DAS unit collects a really huge amount of data, so its use poses some additional challenges: data storage and efficient processing, which includes today’s ubiquitous machine learning as well as data science methods.

Thanks to the CRONOS project, the Department of Geophysics of the Faculty of Science acquired a distributed acoustic sensor unit FEBUS A1 DAS from the French manufacturer FEBUS Optics, which detects ground motion as deformation based on a series of laser pulses sent into an optical fiber. The FEBUS A1 DAS device provides readings usually every 2 m along several tens of kilometers of optical cable and can be connected to an already installed optical infrastructure. This system is specifically designed to meet the requirements of a harsh environment with a single-ended connection to the sensor fiber. The value of the purchased instrument is 190,000.00 EUR (including VAT) for one sensor unit.

Collaborators on the CRONOS project – Component 1 with the FEBUS A1 DAS device from the French manufacturer FEBUS Optics.

As we mentioned earlier, we also acquired 13 high-quality seismographs from the Canadian manufacturer Nanometrics. These classic seismic instruments actually consist of two devices each: the Trillium Compact seismometer and the Centaur digitizer, which record the speed of ground motion in an extremely wide frequency range. They are characterized by simple installation in the field. The purchase value is a total of 148,750.00 EUR for 13 seismographs. Seismographs are used to record the shaking of the ground from near and distant earthquakes and to locate the earthquakes’ hypocentre, to study faults and to assess the structure of the Earth’s interior. To protect this valuable equipment and to simplify the installation of the seismic station, we also acquired custom-made protective boxes – we bought them for 5,065.55 EUR.

These instruments will make the CRONOS project’s seismograph network even denser because in October 2022 we have already installed 12 seismographs lended to us by our Norwegian partners, the Department of Earth Sciences, University of Bergen, in cooperation with NORSAR. Read more about it here and here.

The Centaur digitizer and the Trillium Compact seismometer (behind the digitizer) manufactured by Nanometrics in a custom-made protective case for quick and easy installation of the device in the field.

Prepared by Josip Stipčević and Iva Dasović