Résumé
Measurements low frequency strain associated to active faulting is mandatory for understanding the behavior of these systems. In the future, risk mitigation will depend on our capability to detect in a reliable way small precursors of large seismic events and to assess the seismic/aseismic spatial and temporal distribution and evolution of crustal strain in these unstable systems. The robustness of strain and motion detection is primary linked to measurement accuracy, but also to the number and repartition of instrument. This implies that instrument cost and maintenance are essential for the development of networks. To date, only GPS sensors are robust enough to be deployed for long period of time with limited problem of maintenance. Seismometers and strainmeters capabilities are often plagued by numerous technical problems limiting their usefulness. On the basis of existing or prototype sensors, we developed new instruments (seismometers, tiltmeters, strainmeters) using an interferometric motion measurement. Both Laser source and fringe analysis are connected to the mechanical sensor with long optic fiber (0.1-3 km) depending on applications (volcanoes, sea bottom) The fiber signal transmission appears to be a major improvement by comparison with usual electric wires (cost, data channels, lightning, weight). Also, the absence of embedded electronic on the sensor is a guarantee for reliability and toughness. The developed optical device includes a double modulation of the Laser Diode's wavelength, aiming to reconstruct the displacement of the mechanical sensor with a nanometric resolution. Differential measurements also lead removing internal sensor drift as well as the influence of atmospheric forcing. Three instruments (seismometer, hydrostatic tiltmeter, borehole tiltmeter) have been developed and tested at the Laboratoire Souterrain a Bas Bruit (LSBB), Vaucluse. We will herewith present the development of the instruments and their performance after 9 months of data recording. We discuss how these instruments could be deployed around active faults for a reliable fault motion detection at depth.