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Optical Seismometers & Tiltmeters: New Instruments for Offshore Geophysical Monitoring?

This article discusses the development and performance of optical seismometers and tiltmeters for geophysical monitoring in challenging conditions such as offshore locations, deep boreholes, volcanoes, and mountains. The optical interferometry technology used in these instruments allows for high accuracy displacement measurements and improved stability over time, making them suitable for real-time monitoring of natural hazards. The article also highlights the ongoing LINES project in France, which has developed prototypes of these instruments.

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Optical Seismometers & Tiltmeters: New Instruments for Offshore Geophysical Monitoring?

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  1. The LINES prototypes of optical seismometers and tiltmeters : New instruments for geophysical monitoring offhshore ? Pascal Bernard IPGP, France HIGH RESOLUTION GEOPHYSICAL MONITORING IN CHALLENGING CONDITIONS Far offshore, deep boreholes, volcanoes, mountains, landslides… Géosciences Montpellier: J. Chery, (F. Boudin) borehole and long base tiltmeter IPGP (Paris) – P. Bernard, A. Nercessian, C. Brunet, J. Couteau, Seismometry ESEO (Angers) – G. Plantier, (A. Sourice), M. Feuilloy, R. Feron, control/ processing ENS : F. Boudin, long base tiltmeter OSE-LAAS (Toulouse) H-C Seat, M. Cattoen, opto-electronics LSBB (Nice) - S. Gaffet - Site for underground tests

  2. Borehole tiltmeter L=1 m Seismometer Geophone 2 Hz Long Base tiltmeter L=150 m = 1308 nm mirror Mobile target Laser fiber collimateur ANR LINES: Geosciences Montpellier, LAAS-OSE Toulouse, EOST, IPGP Fabry-Pérot cavity  : Interference of the 2 reflected beams

  3. Optical interferometry for measuring displacement Reaches the best accuracies of commercial instruments (<0.1-1 nm), with the advantages + the optical fiber between mechanical sensor and optical system can be several km long + better stability with time (no electronic drift at the sensor) + long fiber insensitive to EM perturbations (lightning, telluric currents, industrial environment, high voltage power lines…) + small size mechanical sensor + sensor less sensitive to high temperature (no electronic component) + sensor simple, robust, and unexpensive Application for natural hazard real time monitoring in difficult environment: far offshore, deep borehole, mountain/volcanoes,…

  4. LINES project at LSBB, France Since 2012 3 prototypes at Laboratoire Souterrain à Bas Bruit de Rustrel (LSBB) • 1 seismometer • 1 borehole tiltmeter • 1 long base hydrostatic titlmeter

  5. Principle of the instrument: Fabry Perot interferometry + phase analysis of two reflected beams in quadrature /2 Current generation and modulation + laser diod + fiber + optomechanical sensor + fiber + photodiod + demodulation + real-time calculation of target displacement on computer

  6. OPTICAL SEISMOMETER : ORIGINAL DESIGN AND PERFORMANCE

  7. Geophone Sercel Horizontal 2 Hz + 3 km optic fiber at LSBB

  8. N displacement (m) for the Sumatra M=8.7 earthquake at LSBB 30 minutes , frequency 0.01 – 3 Hz STS2 GAS STS2 SISMO-LINES LASER SEISMOMETER 3 km long fiber Geophone 2 Hz

  9. N displacement (m) for the Sumatra M=8.7 earthquake at LSBB 50 seconds, frequency 0.1 – 10 Hz 10 s GAS – STS2 VBB seismometer 1 m SISMO-LINES – 2 Hz geophone - 3 km fiber STS2 GAS LASER SEISMOMETER 3 km long fiber Geophone 2 Hz

  10. N displacement (m) for the Sumatra M=8.7 earthquake at LSBB STS2 GAS LASER SEISMOMETER

  11. WIFI 9 m Seismometer OBS GURALP Sismo LINES optical cable 500 m Offshore test « Sea Test Base » Lanveoc, ISEN, France → spring 2018 PC internet Opto-electronic control Seismometer LINES 3 component Geophones Sercel 2 Hz +1 comp. 10 Hz +1 « zero displ. »

  12. OPTICAL LONG BASE TILTMETER PRESENT DESIGN AND PERFORMANCE

  13. Maturation industrial program with the SATTs AxLR et TTT (2014-2018) Laser Control PC • 3 prototypes • One sensor of 150 m length for the CERN • 2 sensors for the TIGF compagny (reservoir Lussagnet) on 150x150m 150 m Déport par fibre optique sur 300 m 15

  14. No-contact measurement with the new HLS longue base for the CERN laboratory Fogale voie1 CERN 150 m First TIDES Observation

  15. Noise analysis Power spectral density, Comparison of ILB with other sensor Fogale (Capacitive), GPS and Lily (bubble/resisitivity) Borehole tiltmeter GPS measurement 10-7 rad 15 months recording ILB 140 m Fogale capacitive sensor used by the CERN ILB 150 m used in LSBB site borehole tiltmeter (example LILY) GPS measurement

  16. LONG BASE OPTICAL TILTMETER: 2 POSSIBLE DESIGNS P1(t) P2(t) t In construction at ENS To be tested offshore in Lesser Antilles in spring 2019 on a 5 km optical cable

  17. Offshore applications : optical cable only, with seismometers, pressiometers, tiltmeters,... land sea • Comparison of ILB LINES and LVDT results (from Earth tide measurements) power control up to 40 km 19

  18. OPTICAL SEISMOMETER : present : - 2 Hz geophone (needs leveling) - power 100 W - fiber 10 km future : - 10 Hz oscillator, miniaturize < 4 cm, omnitilt - offshore - power < 10W - fiber 50 km ? OPTICAL LONG BASE TILTMETER Present : - measure of water level - installation in tunnels - power 100 W Future  : - measure of membrane deformation - installation in horizontal borehole - installation offshore - low power Other optical instruments : accelerometers, broad band seismometers, pressiometers, acoustic sensors... -

  19. Comparison of ILB LINES and LVDT results (from Earth tide measurements) 21

  20. Comparison between optical and LVDT measurement Hydrological loading Hydrological loading Hydrological unloading Cumulative rain in surface 40 nanorad/month

  21. M =7.1, Mexico Earthquake 2017-09-19

  22. HIPERSIS PROJECT – 2017-2018 – IPGP-ESEO – ANR, France NEW OPTICAL SEISMOMETER TARGET DESIGN AND PERFORMANCE Sensor : Miniaturization : < 4 cm : - boreholes, antennas - omnitilt - more robust - resonance 2Hz-> 10 Hz Control/recording system : integration, low power, low cost

  23. Rennes Earthquake M=3.9 2017-09-28 9 Optical pendulum CMG40 L22, optical 2 Hz GAC, optical 10 Hz TRILIUM 120

  24. P1(t) P(t) P0 P0 OFFSHORE OPTICAL PRESSIOMETER Differential pressure between two cavities separated by a deformable membrane Nominal sensitivity of the membrane: d=1 m for 0.1 mm water P i.e., d=1 nm for =10-9 and L=100 m Membrane wafer sil-tronix 0.2 mm

  25. ANR « HIPERSIS » sismo optique 10 Hz – 4 cm – omnitilt Developpement algorithmes FUI « MIRZA » Calibration pendule optique ; inclinomètre optique de forage hrizontal;acquisition intégrée faible consommation SATT-OUEST « SISMO-MARIN » : printemps 2018 Installation au Sea Test Base du sismo optique 4 comp (3x2 Hz + 1 x 10Hz) 500 m de cable, 8 m de profondeur Intégration contrôles/processing EU INTERREG  CARAÏBE  « PREST » : printemps 2019 Installation aux Saintes du sismo optique 4 comp + 2 pressiomètres optiques + 1 inclinomètre longue base optique 4 km de cable, 30 m de profondeur Cibles envisagées de capteur optique sismo et géodésie offshore, en discussion : Chili : subduction - Iquique / Valparaiso Antilles : subduction – Guadeloupe/Martinique Grèce : Golfe de Patras Naples : baie de Pozzuoli Marmara ?

  26. Measurement from ILB LINES Difference between LVD Tand ILB LINES measurements • Comparison of ILB LINES and LVDT results (from Earth tide measurements) 30

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