研究目的
To define the fundamentals of the optical fibre measurement through downscaled experimentation and compare the performance of fibre optical interferometer and conventional geophones in seismic data recording.
研究成果
The study has helped to better understand the fundamentals of the elastic measurements made using the DVS method. The straight fibre optic sensor is sensitive to the velocity of the measured wavefield and is highly directional. The optical fibre sensor orientation is a key factor for the successful use of distributed vibration measurements. The optical fibre system is not direction sensitive for seismic signals arriving from opposite directions, making the discrimination of downgoing and upgoing seismic events not possible by looking only at the phase of the arrivals.
研究不足
The current interrogation method of optical fibre cables cannot distinguish between wavefields coming from opposite azimuthal directions, which can be an issue in certain applications where the identification of up-going and down-going wavefields is essential.
1：Experimental Design and Method Selection:
The study employed a downscaled experimentation method using an optical fibre interferometer to simulate the DVS system and measure strain on the fibre optic cable. Conventional three-component seismic point receivers (geophones) were used for accurate fibre optic sensor characterisation.
2：Sample Selection and Data Sources:
A field experiment was designed using horizontally orientated, 1.5-metre-long buried fibre optic sensors and five three-component Geospace GS-11D 4.5 Hz geophones as reference sensors.
3：5-metre-long buried fibre optic sensors and five three-component Geospace GS-11D 5 Hz geophones as reference sensors.
List of Experimental Equipment and Materials:
3. List of Experimental Equipment and Materials: Optical fibre interferometer consisting of a laser, optical fibre cables, 3×3 optical coupler and two Faraday mirrors.
4：Experimental Procedures and Operational Workflow:
A walk-around acquisition geometry was used with 15-m offsets between the sledge-hammer source positions and the seismic sensors located at the centre of the semicircle.
5：Data Analysis Methods:
The phase difference between laser light from the sensing and reference arms was measured, and the fibre output was compared with the velocity measurements from the geophones.
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