A simple pendulum borehole tiltmeter based on a triaxial optical-fibre displacement sensor

International audienceSensitive instruments like strainmeters and tiltmeters are necessary for measuring slowly varying low amplitude Earth deformations. Nonetheless, laser and fibre interferometers are particularly suitable for interrogating such instruments due to their extreme precision and accuracy. In this paper, a practical design of a simple pendulum borehole tiltmeter based on laser fibre interferometric displacement sensors is presented. A prototype instrument has been constructed using welded borosilicate with a pendulum length of 0.85 m resulting in a main resonance frequency of 0.6 Hz. By implementing three coplanar extrinsic fibre Fabry-Perot in-terferometric probes and appropriate signal filtering, our instrument provides tilt measurements that are insensitive to parasitic deformations caused by temperature and pressure variations. This prototype has been installed in an underground facility (Rustrel, France) where results show accurate measurements of Earth strains derived from Earth and ocean tides, local hydro-logic effects, as well as local and remote earthquakes. The large dynamic range and the high sensitivity of this tiltmeter render it an invaluable tool for numerous geophysical applications such as transient fault motion, volcanic strain and reservoir monitoring

Wavelength Lock-in Technique to improve Fiber Interferometer Performance for Geophysics Applications

International audienceAn automatic wavelength lock-in technique is proposed to improve fiber interferometer performance for geophysics application. The proposed approach locks the laser wavelength to the absorption lines of hydrogen fluoride gas. The lock-in scheme directly benefits from the dithering signal driving the laser to obtain the required quadrature phase shift for displacement reconstruction and thus does not affect the sensor operation

Dual-modulation fiber Fabry-Perot interferometer with double reflection for slowly-varying displacements

This Letter describes a dual-amplitude modulation technique incorporated into a double reflection extrinsic-type fiber Fabry-Perot interferometer to measure periodic, nonperiodic as well as quasi-static displacements. The modulation scheme simultaneously maintains the interference signal pair in quadrature and provides a reference signal for displacements inferior to a quarter of the source wavelength. The control and phase demodulation of the interferometer carried out via software enable quasi-real-time measurement and facilitates sensor alignment. The sensor system can be exploited in the low frequency range from 10−3 to ∼500 Hz and has a resolution better than 2.2 nm, targeting applications in geophysics

Amplitude and Phase Drift Correction of EFPI Sensor Systems using both Adaptive Kalman Filter and Temperature Compensation for Nanometric Displacement Estimation,

International audienceNanometric displacement measurements by Extrinsic Fiber Fabry-Perot interferometers (EFPI) is extremely susceptible to external environmental changes. Temperature, in particular, has a remarkable influence on the optical power and wavelength of the laser diode in use, in addition to the thermal expansion of the mechanical structure. In this paper we propose an optimization of the EFPI sensor in order to use it for very long-term (more than one year) and for high-precision displacement measurements. For this purpose, a real time and adaptive estimation procedure based on a homodyne technique and a Kalman filter is established. During a sinusoidal laser diode current modulation, the Kalman filter provides a correction of the amplitude drift caused by the resultant optical power modulation and external perturbations. Besides, stationary temperature transfer operators are estimated via experimental measurements to reduce the additive thermal noise induced in the optical phase and mechanical components

A Fiber Fabry-Perot Interferometer for Geophysics Applications

International audienceA fiber interferometer interfaced to 3 geo-mechanical elements is presented for applications in geophysics. The fiber sensor is based on an extrinsic fiber Fabry-Perot interferometer (EFFPI) which incorporates a modulation scheme to lock the interferometer at quadrature and to enable displacement measurements below a quarter of the interrogating wavelength. It operates over a relatively large frequency dynamic of ~500000 with a precision better than 2 nm. The fiber interferometer is next interfaced to a differential hydrostatic long baseline inclinometer, a 3-axis borehole tiltmeter and a single-axis seismometer, respectively. Results obtained demonstrate that the fiber interferometrically-interrogated instruments exhibit performances equivalent to or even surpassing those of the reference instruments employed for comparison during their deployment to an underground test site since March 2012. Keywords—modulation-based extrinsic fiber Fabry-Perot interferometer; fiber optique hydrostatic inclinometer; fiber optique borehole tiltmeter; fiber optique seismomete

3D tongue motion visualization based on ultrasound image sequences

Abstract The article proposes a real-time technique for visualizing tongue motion driven by ultrasound image sequences. Local feature description is used to follow characteristic speckle patterns in a set of mid-sagittal contour points in an ultrasound image sequence, which are then used as markers for describing movements of the tongue. A 3D tongue model is subsequently driven by the motion data extracted from the ultrasound image sequences. The "modal warping" technique is used for real-time tongue deformation visualization. The resulting system will be useful in a variety of domains including speech production study, articulation training, educational scenarios, etc. Some parts of the interface are still being developed; we will show preliminary results in the demonstration