Seismic expression of the phreatic surface in the Neuquén Basin. A special case of "direct gas indicator". The passage from liquid to gas within the poral space of rocks can produces significative changes in the acoustic impedance, which in turn generate a visible response in the seismic data. Sometimes, sharp amplitude changes can be observed, with or without a related flat spot, and are usually used as direct indicator of hydrocarbon occurrence. The seismic interpretation (both 2-D and 3-D) in a sector within the eastern flank of the Chihuidos High ("Dorso de los Chihuidos"), in the Neuquén Basin, allows to define a particular fluid contact case, involving the phreatic surface. It is related to a strong change in the seismic amplitudes of a reflector package within the Rayoso and Neuquén groups. The change happens when the dipping reflectors cut across an almost constant TWT. Strong amplitude reflections are observed above that TWT, and weak ones below. In some cases, a sub horizontal reflector (flat spot) runs along the amplitude change, crossing the dipping reflectors. We interpret that the high amplitudes above that surface are generated by strong impedance contrast between high impedance shale packages and high porosity, gas saturated (air from the vadose zone in this case), low impedance, sandstones. Crossing the phreatic surface, sandstones became water saturated, and the impedance contrast with the shales reduces, thus the reflection amplitudes diminish. Then, both the observed amplitude change and flat spot are generated by the phreatic surface. The elevation estimated for this seismic event is around 200 and 300 m.a.s.l., values that coincide approximately with the known for the phreatic level in the zone, reinforcing the anomaly assignation. Several coinciding factors, in this basin sector, allowed the phreatic surface being "visible" to the conventional reflection seismic. Among these, clearly come up the outstanding air-filled layer (vadose zone) thickness, which combined with the dipping attitude of the bedding, allowed the impedance contrast remained visible in the seismic data trough the registration and processing treatment.