Maybe this is assumed if you know anything about dune science, but: exactly what is the ground penetrating radar measuring? Are these layers of different densit

Maybe this is assumed if you know anything about dune science, but: exactly what is the ground penetrating radar measuring? Are these layers of different density/compaction of sand? How do they form? Either way, I didn't know dunes could have that much detectable internal structure (though in retrospect I should have). Based on playing on beach dunes, I tend to think of dry dune sand as just a step above liquid.