Fig. 1. (a) Experimental setup of Φ-OTDR; (b) photo of the experimental field; (c) installation structure of the sensing cable and the road (road cross section).

Fig. 2. Original waterfall pattern.

Fig. 3. DFSI pattern at the moment of the 10th second.

Fig. 4. Short period frequency spectra at positions (a) with and (b) without cars running.

Fig. 5. DFSI processed signal, integrated over (a) 20–100, (b) 30–100, (c) 40–100, (d) 50–100, and (e) 60–100 Hz, compared with the original detected signal (f); (g) signals, ground noise, the fluctuation of noise and FR for frequency integration ranges above.

Fig. 6. DFSI processed waterfall pattern.

Fig. 7. 2D digital sliding filtering processed waterfall pattern.

Fig. 8. Waterfall detected for four cars running successively.