Onland and Offshore Extrinsic Fabry–Pérot Optical Seismometer at the End of a Long Fiber

Abstract

We report here the design, performance, and in situ demonstration, on‐land and offshore, of an innovative high‐resolution low‐cost optical (laser) seismometer. The instrument was developed within the Laser Interferometry for Earth Strain project (French Agence Nationale de la Recherche [ANR] program), and first tested at the low‐noise underground laboratory Laboratoire Souterrain à Bas Bruit (LSBB, France). It is based on Fabry–Pérot optical interferometry between the extremity of a probing optical fiber and a reflecting mirror secured to the mobile mass of a passive 2 Hz geophone. The detection technique is based on the wavelength modulation of the laser diode (1310 nm), which allows the separation of the optical power into two signals in quadrature, thanks to an heterodyne technique. The relative displacement of the mobile mass is retrieved in real time by the phase unwrapping of these two signals. At LSBB, the fiber was 3 km long. It recorded many teleseismic earthquakes and a few regional ones, and resolves the low‐seismic noise of the Earth for periods up to 6 s, presenting an acceleration noise floor lower than 1 ng/Hz in the 0.3–5 Hz range. A three‐component version of this fiber‐based interferometric 2 Hz geophone has been recently constructed, shielded in a hyperbaric container, and installed offshore for test in Brittany (France) in April 2018, with an improved control system. Its record of the marine ambient noise matches those of a collocated commercial broadband seismometer for periods up to 50 s. This opens promising perspectives for large‐scale ocean bottom instrumentation with up to 50‐kilometer‐long optical lines; an installation is planned for 2020, off Guadeloupe, with a 5‐kilometer‐long fiber cable. It may also prove useful for installations in other challenging and exposed environments, such as deep hot boreholes, active volcanoes, unstable landslides, for real‐time monitoring in regions with high natural hazard, but also for seismic monitoring of geoindustries.

First Optical Seismometer at the Top of La Soufrière Volcano, Guadeloupe

“…To decipher it, a real-time adaptive estimation algorithm (Kalman filter) followed by a phase unwrapping yields the distance D(t) of the mobile mass from the other side of the optical cavity (Chawah et al (2012)). This seismometer and signal analysis presents very high-resolution characteristics, similar to those of the best commercial short period or medium band seismometers (Bernard et al (2019b)) and to the optical seismometers of Zumberge et al (2010)…”

“…and Institut de Physique du Globe de Paris (IPGP) have been working together for about 10 yr to develop optical seismic sensors and systems able to withstand hostile environmental conditions (Bernard et al (2017); Bernard et al (2019b); Bernard et al (2019a)). We developed our latest prototype and the topic of this report as part of the HIgh PERformance SeISmometer (HIPERSIS) project (French ANR).…”

“…The project chose this collaboration's bid to put a permanent installation at La Soufrière. Although our optical seismometers have already been tested in rather protected environments (Chery et al (2012)), with 3kilometer-long fibers, and in natural environments during an underwater measuring campaign (Brest, France) (Bernard et al (2019b)), this is the first time one is in a critical area and exposed to immediate natural hazards.…”

An Atypical Shallow Mw 5.3, 2021 Earthquake in the Western Corinth Rift (Greece)

“…For instance, while DAS systems interrogating submarine optic cables would considerably help with detecting and locating shallow events (<5 km) beneath the gulf, the Brillouin Optical Time‐Domain Reflectometer (BOTDR) system for long‐term, repeatable strain measurements on dedicated submarine cables would allow detecting a continuous or transient slip in outcropping faults zones crossed by the fiber. Cabled OBS located in the gulf could also greatly improve real‐time monitoring of the rift, in particular with innovative optical seismometers (Bernard et al., 2019; Feron et al., 2020), which may considerably reduce the cost of their installation and maintenance. Such high‐end equipment and techniques would meet societal needs for seismic and tsunami early warning.…”

Fiber optic sensors for seismic wave detection

“…Fiber optic sensors (FOSs), with the advantages of all optical sensing, anti-electromagnetic interference, and long-distance transmission, have been used in seismic wave detection. According to the different requirements in seismic wave detection, FOSs for detecting the different physical parameters have been developed, such as fiber optic strain sensors for crustal deformation observation [1,2], fiber optic seismometers for earthquake monitoring [3,4], and fiber optic jerk sensor for earthquake engineering [5].…”