Fig. 15From: An impact echo method to detect cavities between railway track slabs and soil foundationConcrete slab track with cavity: a Cavity size 5 cm × 5 cm × 5 cm. b Cavity size 5 cm × 5 cm × 1.25 cm. c Cavity size 5 cm × 5 cm × 0.75 cm. d Cavity size 5 cm × 5 cm × 0.3 cm. e Non-cavityBack to article page