Fig. 1. Direct detection Φ-OTDR and intrusion detection based on time difference^[10]

Fig. 2. Representative schemes of quantitative measurement. (a) Digital coherent phase demodulation^[13]; (b) scheme using 3×3 coupler^[18]; (c) PGC scheme^[20]

Fig. 3. Interference fading and probability density distribution of scattering light amplitude. (a) Interference fading^[34]; (b) probability density distribution of scattering light amplitude^[35]

Fig. 4. Representative schemes for signal fading suppression. (a) Phase diversity^[35]; (b) mode diversity^[40]

Fig. 5. Multicolor parallel sampling and periodic non-uniform sampling. (a) Multicolor parallel sampling^[42]; (b) periodic non-uniform sampling^[48]

Fig. 6. (a) High spatial resolution technology based on pulse compression; experimental results with sensing distance of (b) 20 km and (c) 75 km^[50-51]

Fig. 7. Intrusion detection field test based on DAS^[52]

Fig. 8. Test result of deep neural network using multi-classification recognition^[60]

Fig. 9. Train track detection^[61]. (a) Field test layout; (b) waterfall pattern with DAS

Fig. 10. Railway safety monitoring based on (a) DAS and (b) confusion matrix of multi-classification recognition^[66]

Fig. 11. Detection data of DSA and conventional seismometer^[67]

Fig. 12. Seismic signals for different injected volume of CO269. (a) 27 kt; (b) 110 kt; (c) 220 kt; (d) 330 kt

Fig. 13. VSP signals based on DAS^[70]. (a) 3D distribution; (b) visualization

Fig. 14. Monitoring of composite material structure with embedded fiber^[72]

Fig. 15. Experiment of landslide detection model based on acoustic emission^[73]. (a) Side view; (b) top view

Fig. 16. Geological monitoring based on traffic noise^[76]. (a) Optical cable layout; (b) time-space distribution and (c) spectral distribution of detection signal

Fig. 17. Seismic signal detection based on communication cable. (a) Position and direction of cable^[77]; (b) seismic signals obtained by DAS^[77]; (c) seismic signals at different positions^[79]

Fig. 18. Geological detection based on existing communication cable and DAS technique^[78]. (a) Unmarked fault zone on map; (b) low-frequency harmonic noise possibly derived from waves in the earth's interior

Fig. 19. Diagram of Rayleigh scattering enhancement technique based on ultrafast laser^[84]

Fig. 20. Principle of DAS based on multimode Rayleigh scattering^[85]

Fig. 21. Model of distributed sensing array and equivalent sensing array^[87]. (a) Model of distributed sensing array; (b) equivalent sensing array

Fig. 22. Three kinds of geometry configurations of optical fibers for multi-component measurement^[89]