Reservoir dams on Daugava river Objects The reservoir of Plavinas HPP The cascade of the Daugava hydroelectric power plants forms continuous chain of reservoirs throughout the lower stream of the Daugava river, a substantial portion of the which is located along the buried preglacial palaeovally which depth in some places is more than 150 meters. The valley was formed in the pre-Quaternary time at low standing level of the Baltic sea. For a considerable length the profile of the valley had a canyon-like shape, which contributed to the development of karst and landslide processes on its sides. Dense layers of halogen-carbonate rocks of upper Devonian sediments of Plavinas, Salaspils and Daugava units was subjected by erosion, both the clastic sandy-aleuritic sediments of Amata-Gauja unit of the upper Devonian. During the Quaternary glaciation periods, the valley and its sleeves and tributaries were completely filled with heterogeneous moraine material. The negative impact of karst processes on the safety of structures is manifested in the form of increased filtration of water from reservoirs through protective dams, subsidence of dam bodies, erosion of the foundations of drainage channels. The local nature of palaeokarst manifestations, significant differences in the physical and mechanical properties of the filling material in comparison with the surrounding host rocks, increased water saturation and filtration flows create significant anomalies in geophysical fields, which create good conditions for the use of a wide range of shallow-depth geophysical methods for their area and linear mapping. In the buffer zone of the Plavinas HPP reservoir the shallow geophysical studies were carried out during several years from 2003 to 2009. Complex of shallow geophysics methods included GPR sounding, modification of seismic reflected, refracted and surface wave techniques, electrical resistivity tomography, the methods of natural electric field, subbottom profiling. Scheme of engineering-geophysical investigation sites displacement Seismic survey on the site of drainage wells in the Plavinas HPP lower pool The pressure of the regional aquifer groundwater in the sandstones of the Gauja-Amata deposits of the upper Devonian balances the weight and maintains the body of the dam of Plavinas hydroelectric power station in a stable state. Violation of the dam drainage system operation leads to pressure increase, to “pop-up” of the dam body and its possible displacement in the horizontal direction under the action of static pressure of the reservoir mass during heavy floods. In the process of regular update of the drainage system seismic profiles in the modification of the reflected waves (CDP) were performed. In combination with the drilling data performed by Meridian Ltd (Daugavpils), the position of the sides of the buried pre-glacial valley was clarified and the areas of deluvial “plumes” – clusters of coarse-grained material with high filtration properties along the bottom of the sides, which are the target horizons for the installation of drainage filters, were identified. The profile of pre-glacial valleys on the seismic section Volumetric reconstruction of preglacial valleys and “plumes” of coarse material from the seismic data Manifestations of palaeokarst at the base of the left bank protection embankment of the reservoir The seismic profile along the top of the reservoir left-bank protective dam, crossing the buried valley, clearly shows the manifestations of buried karst at the dam foundation to a depth of 20-25 m. Signs of karst are manifested in the form of delays in the registration times of the refracted waves first breaks (upper figure), in general configuration of the first break fields (in the center) and in the form of areas of tracking missing of reflections from the horizontal boundaries in the upper Devonian sediments (bottom). Areas of paleokarst development also correspond to anomalous zones of low transverse waves velocities Vs. The reservoir of Riga HPP GPR sounding The GPR sounding as the least time-consuming method, is applied in reconnaissance studies. Performing of long profiles and significant land areas is carried out using a low frequency dipole antenna, which allows in some cases to detect and delineate the anomalous zones of the paleokarst manifestations at depths of 5-8 m. Below figure shows an example of detection of anomalous zones on the profile along protective dam of the Riga HPP reservoir. Anomalous zones are manifested at depths exceeding the depth of the clay pad in dam foundation and the roof of the gypsum and dolomites, as well as the depth of surface of the full water saturation corresponding to the water level in the reservoir. This gives reason to suggest a connection of these anomalies with the manifestation of palaeokarst in bedrock sediments and high water saturation of sandy soil of a protective dam. On seismic profile along the base of the dam karsted surface areas of bedrock dolomites occur in the form of characteristic time “delays” of first breaks and reflected waves, which is then reflected in the form of the bedrock surface correlation loss on the CDP time sections and in the low-velocity anomalies of longitudinal wave velocity sections.