Kruonis pumped storage hydroelectric power plant (PSPP) Objects Study of the dam concrete walls erosion and of water leakage from the reservoir The studies were performed in the borehole with depth of 32 m, which were drilled in the concrete wall of the upper reservoir dam of Kruonis PSPP – pumped storage power plant (Lithuania) with the aim of identifying and establishing the extent of concrete decompaction zones in the place of water leakage from the upper reservoir along the temperature seam. The study of the properties of concrete by the method of non longitudinal VSP was carried out at excitation of elastic oscillation by blows of a 1 kg weight hammer on the surface of dam concrete block across the network of parallel profiles with the distance between them 2 m. Points of excitation step along profiles was 2 m. The piezo-hydrophonic 16-channel probe was placed in a well filled with water and moved up with step 25 cm as all the excitation points on the surface were worked out. Thus, the step between the receiving channels in the compilation seismograms was equal to 25 cm. Non longitudinal VSP method work scheme from the dam surface Acoustic scanning method work scheme from the observation shaft The study of the concrete properties by method of acoustic scanning was performed at excitation of elastic oscillations by blows of a 1 kg weight hammer in the observation shaft located parallel to the observation well at a distance of 2.6 m. Piezo-hydrophone 16-channel probe was placed in the hole filled with water. Examples of non longitudinal VSP records and seismic acoustic raying On the resulting records of non longitudinal VSP (left) and acoustic transmission from inspection shafts to the well (right) the first breaks are well traced of the waves in the steel reinforcement of walls with velocity of about 5300 m/s, and in concrete with a velocity of 3100-3200 m/c. The central frequency of the oscillations spectrum in both cases was in the range of about 1600 Hz. Dam block concrete wall decompaction map in the area of leak According to the time-distance curve of the wave first break in concrete at a depth of 28-32 m in the wall of the cassette block of the dam adjacent to the deformation seam, a decompaction zone was revealed in which velocity of the longitudinal wave propagation was below 2,700 m/s. Thickness of this zone near the wall was more than 4.5 m, gradually decreasing towards the inner surface of the cassette block wall. This zone corresponds to the depth of interval in which the water flows from upper reservoir fixed along the deformation seam, causing leaching and erosion of concrete wall of the block. Volumetric reconstruction concrete decompaction zone and a cavity in the area of the leak Obtained by a sufficiently dense network velocity cross-sections in vertical planes were used to construct a volume of wave velocity distribution in the block of dam and for reconstruction of a decompaction zone spatial position in concrete and a cavity in the wall of the block resulting from the leaching action of water flow.