Fundamentals of Vibration Sensing with Distributed Fibre Optic Sensors through Downscaled Experiments

Abstract : In the past few years the Distributed Vibration Sensing of fibre optic sensors (also called DistributedAcoustic Sensing) has gained great interest in geophysics, despite the fundamentals of the measurementbeing poorly understood. The technology offers immense improvement in terms of efficiency whencompared with current geophysical methods and also could be preferred for applications such aspermanent reservoir monitoring. We present a method for downscaled experimentation that enabled us todefine the fundamentals of the optical fibre measurement. We show seismic data that were recordedsimultaneously by both the fibre optical interferometer and conventional geophones, thus enabling thecomparison of sensor performance. We present results focused on wave-polarity sensitivity and thedirectionality of fibre measurements. The current interrogation method of optical fibre cables cannotdistinguish between wavefields coming from opposite azimuthal directions, which can be an issue incertain applications where the identification of up-going and down-going wavefields is essential.Observations are also presented that describe the relationship between the velocity signal that is measuredusing a geophone array and the phase difference measurement from the fibre optic sensing system.