Highly-sensitive And Broadband Optical Fiber Distributed Acoustic Sensing Technology And Its Applications

Optic fiber distributed acoustic sensing(DAS)has the significant advantages of anti-harsh environment,long-distance,high-resolution,fully-distributed and real-time response,which owns broad development prospects in the field of large-scale acoustic detection such as seismic imaging,resource exploration and structural health monitoring.In this dissertation,oriented to the requirements of practical applications,the theoretical and experimental research on DAS key technologies from the aspects of optimizing system noise floor,improving acoustic pressure sensitivity and broadening frequency response band are performed,and then the application explorations in the fields of seismic imaging,oil well detection and hydroacoustic detection are carried out.The core contents and innovative work of this dissertation are summarized as follows:(1)For the intensity fading in the heterodyne coherent DAS system,a fading noise suppression technology based on multi-carrier frequency modulation and temporal differential-vector-sum algorithm is proposed.The influence of the intensity fading noise on the signal-to-noise ratio(SNR)of acoustic sensing is analyzed theoretically.Based on the heterodyne coherent system with phase acousto-optic two-stage modulation,the temporal differential-vector-sum algorithm is proposed for phase synthesis of multi-carrier.Compared with the single-carrier frequency system,the experimental results demonstrated that the fading sensing channel’s(root mean square greater than 0.2 as the reference)probability of this scheme can be reduced from 70.07% to 4.76%,and the static noise floor of the demodulated signal can be reduced by 11.7 d B at most.(2)For the intensity fading noise and out-of-band acquisition noise of the system,a denoising scheme of spatio-temporal joint oversampling-downsampling is proposed.On the one hand,the oversampling-downsampling fading noise suppression technology along fiber spatial dimension is developed to eliminate intensity fading noise.On the other hand,the oversampling-downsampling processing algorithm along optical fiber temporal dimension is proposed to further suppress the out-of-band noise.Through spatio-temporal joint oversampling-downsampling method,under the 0～500 Hz target bandwidth,the influence of coherent fading can be eliminated from the experimental results.The SNR of dynamic acoustic signal is improved by 20.8 d B,and the strain resolution based on the denoising algorithm can reach 2.58 pε/√Hz@100～500Hz.(3)For the low acoustic pressure sensitivity of the linear structure single mode fiber,a double sensitization method of discrete scattering enhanced optical fiber(DSEOF)combined with highly-sensitive acoustic transducer is proposed.The SNR improvement and fading noise suppression mechanism of DSEOF is analyzed theoretically,and the acoustic pressure sensitivity is optimized from the perspective of SNR.Further,the highly-sensitive cylindrical transducer is designed,whose sensitization mechanism and dynamic response are studied by the mechanical analyze.The experimental results show that the system can achieve high-fidelity recovery of signal waveforms,and the maximum error is less than 3.07%.The airborne acoustic pressure sensitivity is as high as-112.5 d B re rad/μPa@500Hz～5k Hz.Compared with ordinary optical fiber,the SNR is enhanced by 18 d B.(4)For the trade-off between sensing distance and frequency response bandwidth of distributed sensing system,a time-slot multiplexing bandwidth enhancement technology based on DSEOF is proposed.Utilizing the redundant time-slot resource between two adjacent discrete peaks,multiple probe pulses can be multiplexed into one measurement cycle to increase optical sampling frequency.The cross-correlation algorithm is developed to realize the demultiplexing of the composite detected trace,and the phase is recombined according to the time sequence to demodulate the acoustic signal.Utilizing the DSEOF with the backscattering enhacnced point interval of 20 m and the length of 1020 m,the maximum detected bandwidth up to 300 k Hz can be realized from the experimental results,and the frequency response bandwidth is increased by 6 times.(5)Based on the above key technology research,the applications of highly-sensitive broadband DAS in the fields of seismic imaging,oil logging and hydroacoustic detection are carried out.The active seismic sound source and urban background noise are recorded and analyzed respectively.The high-quality data of oil well VSP measurement are successfully obtained.Besides,the detection and tracking of underwater acoustic target are realized through Lake trial application,which shows the application potential of the DAS system.