Strainmeter to detect earth tide and seismic waves from earthquakes

Strainmeter based on laser wavelength monitoring


We have seen in a previous article that a set-up based on a laser and Optical Fiber Bragg grating (FBG) sensors can be used for strain and temperature monitoring. This technique is based on the monitoring of reflected laser wavelength. FBG sensors find one application in seismology. Strain monitoring is well known in the field of seismology for earth science observation purposes.

Strainmeters measure deformation over periods of days, months, and years. This allows them to measure signals at lower frequencies than can be detected with seismometers. Most strainmeter records show signals from the earth tides, and seismic waves from earthquakes.

Despite promising capabilities, classical FBG sensors never reached the desired sensibility (1-1000 nε) for earth science observation purposes. Recently, laboratories in France have used this technique to perform seismologic measurements of high precision. A new approach to FBG sensor using a spectrometer based on SWIFTS (Stationary Wave Integrated Fourier Transform Spectrometer) technology was presented by IPAG and IsTerre laboratories at AGU Fall meeting 2013. For the first time, they demonstrated that a FBG sensor with a field configuration (small and low power) can detect strain of the order of 4 nε, a resolution of the order perfectly suited to field deployments.

Detection of earthquakes and tidal variations via measurement of strain in the Earth’s crust requires compact and robust instrumentation with low power usage that can be deployed in the field. And strainmeters need to measure deformation over periods of days, months, and years. The set-up was therefore was requiring both a high measurement rate and also the integrity of the data for at least many days. Their measurements have been conducted in a non-lab environment during 7 days at 2kHz without any disconnection and data losses thanks to a robust Ethernet connection between the spectrometer of the Bragg interrogator and the computer.

This demonstrated the capabilities of this technical approach with SWIFTS based spectrometers, and their potential use for earth sciences investigation. This work was performed by IsTerre, IPAG, IPG Paris and LSBB laboratories, France.

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