Fig. 1. Schematic diagram of distributed vibration sensing system based on Φ-OTDR

Fig. 2. Schematic diagram of the pipeline leak positioning and early warning system based on Φ-OTDR

Fig. 3. Original time domain signals at positions (a) 426 and (b) 970

Fig. 4. Power spectra of original time domain signals at positions (a) 429 and (b) 970

Fig. 5. Approximate entropy distributions at positions (a) 429 and (b) 970

Fig. 6. Information entropy distributions at positions (a) 429 and (b) 970

Fig. 7. (a) Spatial frequency energy distribution; (b) spatial frequency energy distribution (in detail)

Fig. 8. High-pass filtered signals at (a) 429 and (b) 970

Fig. 9. Schematic diagram of the vibration location technique based on spatial-frequency energy distribution

Fig. 10. (a) Spatial frequency energy distribution of in-soil knocking; (b) spatial frequency energy distribution of in-soil knocking (in detail)

Fig. 11. (a) Spatial frequency energy distribution of in-water knocking; (b) spatial frequency energy distribution of in-water knocking (in detail)

Fig. 12. (a) Spatial frequency energy distribution of soil-buried water pipe leakage; (b) spatial frequency energy distribution of soil-buried water pipe leakage (in detail)