This velocity corresponds with a soil dielectric permittivity of 2.77, which is indeed very low for a porous medium. However, it is not impossible for a dry sandy soil, especially if it is in a strongly evaporative environment. Personally, I have measured velocities up to 0.15 m per ns in sandy soils (see attached paper), but this was in the Netherlands. Due to the wet climate, there always is some residual water content that can only be removed by evaporation.

I have never found velocities exceeding 0.14 m/ns, and the highest were in dry sands (dune) or dry limestone. I am not aware of v>0.15 m/ns from the literature.

The 18 cm/ns have been confirmed by comparisson with drilling results - I have been surprised as well about such high velocities - hence the question.

Very interesting question. We've measured velocities up to 0.145 m/ns in Miami Limestone under completely dry conditions in the laboratory. That formation thou is fairly special in terms of porosity, reaching values that often exceed 60%.

We have observed some areas having perfect dry sand with really high velocities, 0.18 - 0.185 m/ns.