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In the present paper we combined two different non-destructive techniques to detect the water content spatial variation in a porous material. The main goal of this work was to verify the ability of radar “early time” signals, using the proton density values measured with the unilateral NMR (Nuclear Magnetic Resonance) technique, in order to detect the spatial distribution of water in the subsurface of a concrete slab. This site was chosen because it was the best compromise between an uniform, porous and slow drainage material and the antenna-coupling, affected only by the surface permittivity variations due to the change in water content. The GPR (Ground Penetrating Radar) survey has been conducted using a bistatic radar unit (Sensors & Software, Inc) operating at 1000 MHz; on the other hand, NMR data were collected using portable unilateral NMR operating at 16.3 MHz. The results obtained in this paper confirm that the “early time” radar signal and the NMR response can be applied to estimate some physical properties of both natural and man-made materials. In fact, the data show a matching pattern of the water distribution in the shallow part of the slab measured by GPR and NMR, and a very high correlation coefficient between the radar signal amplitude and the NMR signal integral. Finally, the results highlight the importance of this new combined approach to evaluate different physical parameters at the same time.