Radargram, relative permittivity, and absolute depths of the crater B. (a) Radar profile of crater B. The colour bar represents the amplitude of the radar signals. Black dashed lines show the outlines of layered structures and crater B, and discrete scatterers. Blue arrows point to a sharp reflection. (b) Permittivity varied with depth at x = 220 m. (c) Permittivity varied with horizontal distance at three time delays (blue, red and yellow line) in a; the time delays are at t = 210 ns, t = 240 ns, and t = 270 ns, respectively. (d) Absolute depths of the outline of crater B and its error bar (blue shaded area).