Internal Structure of Sand Dunes in the Badain Jaran Desert Revealed by GPR

In spite of the prevailing dry and windy climate conditions the mega dunes in the Badain Jaran Desert in northwestern China are relatively moist underneath a dry surface layer of less than 1 m. It is very common to find a salt lake directly at the foot of the leeward side of a mega dune. Using 50- and 100-MHz antennas we have conducted ground penetrating radar (GPR) surveys on both the windward and leeward sides of a couple of sand dunes in southeastern part of the Badain Jaran Desert. The GPR surveys clearly revealed the existence of numerous, quasi-evenly spaced bedding features on the windward side of the mega dunes, with a slope of orientation parallel to the slope of the leeward side of the dunes which closely coincides with the angle of repose of dry sands. The reason for the existing beddings is that on the leeward side sands on the dune surface can be cemented by moisture with periodic precipitation into layered beddings; and the caliches generated by these calcareous cement will be likely inducing more infiltrated water flow toward the leeward side and consequently channeling more local recharging water to the leeward side than to the windward side. The calcareous cement beddings performed as the `skeleton' of the dunes and increased the mechanic strength of the sands and consequently facilitated the build-up of the high elevation mega dunes. This trend may be one of the key factors that help the existence of the lakes in a very arid environment with high evaporation rate. The GPR profile also clearly registered the water table beneath the sand dunes that gradually elevated toward the crest, implies that the desert lakes are recharged at least partly by the groundwater from local source. Numerically simulated radar profiles precisely reconstructed the observed profiles. It is a strong support to the rationality of the proposed internal structure of the dunes.