The Lines High Resolution Optical Seismometer

We present here the performance of an innovative, high resolution, robust, and low-cost optical seismometer. The instrument was developed within the LINES project ( ANR, 2009-2012), and tested at the low noise underground laboratory of LSBB (France). It is based on a Fabry-Perot interferometry between the extremity of a laser fiber and a reflecting mirror fixed on the mobile mass of a passive geophone (2 Hz ). At the other extremity of the fiber, 3 km long, an optical system includes a laser diode (1310 nm) and the recording and control systems. A double modulation (at 50 KHz and 1 KHz ) of the intensity of the diode current produces a wavelength modulation, allowing to calculate in real-time the position on the mass, thanks to fast and innovative numerical processing. It ran more than two years, recording many teleseismic earthquakes and a few regional ones. It resolves the low seismic noise of the Earth for periods as low as 5 s, and presents an acceleration floor noise lower than 1 ng/sqrt(Hz) in the 0.2-5 Hz range. The LINEs optical seismometer is thus a promising instrument for the microseismic monitoring of deep and/or hot industrial boreholes.