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We investigate the penetration capabilities of microwaves, particularly L-band, for the mapping of sub-surface moisture. The experiment site is the Pyla dune, a bare sandy area allowing high signal penetration and presenting large subsurface wet structures (paleosoils) at varying depths. The SAR penetration depth is estimated by inverting a scattering model for which the sub-surface structure geometric and dielectric properties are determined by the GPR data analysis. We observed a specific phase difference between RH and W channels, due to a wet layer covered by dry sand. Our results suggest that airborne radar systems in a lower frequency range (P-band) should be able to detect sub-surface moisture down to at least ten meters.