Fig. 3

Representative subsurface cavity models are used to compare the morphology of the upper hyperbolic echo pattern in the radargram. The cavities have different assumed roof shapes, which are supported by basaltic boulders. Similar models are introduced in Additional file 1: Text S2. The basic model setups are introduced in "Method" section. The first row introduces models that consider different roof morphologies of the subsurface cavity, and the bottom row shows both the observation (panel g) and corresponding simulation results. The two criteria used to determine the similarity between the observation and simulation results are introduced in "Roof shape of the low-permittivity zone" section. The red arrows denote traces of the upper hyperbolic echo pattern. Note that this comparison can only constrain the basic morphology of the roof, since various combinations of subsurface structures could also form similar morphologies of the upper hyperbolic echo pattern