Fig. 1. DAS technical classification based on modulated pulse frequency components. (a) Single frequency pulse φ-OTDR; (b) multifrequency pulse φ-OTDR; (c) LFM pulse φ-OTDR

Fig. 2. φ-OTDR discrete scattering element simplified model. (a) Without disturbance; (b) disturbance

Fig. 3. Discrete theoretical model of LFM pulse distributed acoustic sensing

Fig. 4. Typical structure of LFM pulse DAS system

Fig. 5. Discrete scattering model of LFM pulse in optical fiber

Fig. 6. Principle of step frequency scanning sensor. (a) Step frequency scanning; (b) data processing structure

Fig. 7. Frequency shift compensation principle of LFM pulse

Fig. 8. Sensing principle of wavelength scanning method. (a) Wavelength scanning; (b) data processing structure

Fig. 9. Basic principle of pulse compression^[50]. (a) Matched filter; (b) unmatched filter

Fig. 10. Waterfall plot over a 40 s period for a section of field fiber^[57]

Fig. 11. Waterfall plot for the full 209.4 km link produced by the real-time LFM-DAS^[57]

Fig. 12. Optical pulse compression reflectometry. (a) Experimental setup; (b) spatial resolution of Gaussian pulse^[39]

Fig. 13. LFM pulse combined with unmatched filtering technology. (a) Experimental setup; (b) strain distribution; (c) standard deviation of strain distribution^[51]

Fig. 14. Interleaved identical LFM pulses technology^[61]. (a) Experimental setup; (b) power spectral density

Fig. 15. Schematic diagram of LFM pulse principle of HDSB^[66]. (a) Sensing fiber; (b) HDSB-LFM pulse; (c) coherent time-domain signal

Fig. 16. Comparison of measurement results between DAS and geophones^[74]. (a) Measurement results of DAS; (b) measurement results of geophones; (c) comparison of detection waveforms

Fig. 17. Comparison of DAS and geophone^[83]. (a) Measurement results of DAS; (b) VSP results of the Z-direction component of the detector

Fig. 18. DAS array and detection results^[88]. (a) Layout of the DAS array; (b) location of the quarry relative to the DAS array; (c) average velocities measured in three segments before and after excavation

Fig. 19. Simulated rockfall monitoring results^[90]. (a) Rockfall along the railway; (b) rockfall monitoring waterfall

Fig. 20. Traffic changes before and after blockade based on DAS^[94]. (a) Average daily traffic volume change; (b) average traffic speed change