Research On Elastic Wave Field Response Characteristics Of Distributed Optical Fiber Acoustic Sensor (DAS)

Distributed acoustical optical fiber sensing system is a new type of sensor which can receive and record elastic wave signals reflecting underground strata.Optical fiber has the trend of replacing traditional seismic sensors for detection because of its unique advantages.The receiving record of optical fiber is consistent with that of seismic geophone,but there are some complex responses.The complexity of these responses is not only related to the demodulation mode of the optical fiber demodulator and the different types of optical fibers,but also mainly due to the particularity of the elastic wave field response characteristics of optical fibers.At present,there is little research on the forward experiment of optical fiber,most of which are based on the actual experiment of optical fiber to study its response parameters,which are greatly influenced by the instrument itself.Therefore,from the point of view of numerical simulation to study the characteristic response of elastic wave received by the optical fiber itself,it is of great practical significance to the design of the early observation system and the data processing in the later stage.By referring to the numerical response of elastic wave connected to seismic geophone,two numerical simulation methods,namely,numerical simulation of optical fiber signal based on plane wave particle displacement and velocity formula and numerical simulation of optical fiber signal based on finite difference of three-dimensional wave equation,are used in this paper.The mutual conversion of received signals by optical fiber and seismic detection are successfully realized by numerical simulation of optical fiber signal.The results show that the two methods have the same effect,but the first simulation method can not realize the transformation of complex wave field because of its particularity,so the second method is often used in the later trial calculation and analysis.The amplitude and angle sensitivity of P-wave,S-wave and Rayleigh surface wave are discussed by different propagation directions and azimuth angles.The particularity of distributed optical fiber receiving body wave and surface wave is verified.The reception response characteristics of the former are further confirmed by setting a three-dimensional isotropic viscoelastic model in the later stage.Firstly,the single-shot recording and single-channel waveform of forward data are analyzed,and the response characteristics of elastic wave field received by optical fiber in two-dimensional detection are summarized.Secondly,combined with the data received by three-dimensional line in different azimuths in the same direction of propagation,the receiving response characteristics of different types of waves are analyzed more deeply through the processing.Finally,the polarity of the orthogonal line and the signal-to-noise ratio of the optical fiber are analyzed to confirm the similarities and differences between the response of the optical fiber and the seismic geophone,and then different optical fiber layouts of DAS are analyzed,which provides guidance for seismic exploration of shallow artificial sources.