The world experience shows that the construction of large reservoirs provokes, as the results of its filling and exploitation, the development of hydrogeological phenomena undesirable or risky for adjacent lands. Filtration processes increase a loss of the reservoir water and, in addition can cause an ascent of the underground water level, a growth of the water mineralization and the salinity of the cultivated lands. That is why the geological and geophysical study is very important on the exploration stage of the hydrotechnical works to predict and monitor the hydrological and hydrogeological changes. El Cuchillo reservoir on San Juan river supplies with the drinking water more than 3.5 million inhabitants of the metropolitan area of Monterrey in Mexican state Nuevo Leon. The capacity of the reservoir restricted by the dam and dike is 1784 Mm3, its maximum depth reaches 35 m. The territory of the reservoir is composed by rocks (intercalating sandstones and shales) of the Eocene age with east dip at the angle 5-100. The sediments were affected by tectonic disjunctions with two principal directions: northeast (NE) and southeast (SE). One fault of the NW direction is located in zone of the dike with horizontal displacement 50-75 m. Two orthogonal fracture systems (N-S and E-W directions) are observed in the sandstone formations. Two types of the underground water exist in the study area: superficial with the salinity until 0.4 g/l related to the flow of San Juan river and brackish one (from 2-3 until 6-7 g/l) obtained from the deeper parts of the Eocene rocks. The rocks were affected by weathering, which reaches the depth of 15 m according to drilling data obtained near the dam and dike. The dam of 4.8 km long has a clay core and a concrete screen. Furthermore bentonite cement was injected to depths of 15 to 25 in its base. The dike of the total length 6 km delimits the reservoir from the North. It is constructed from loose material with the clay core. It has no screen and had only injection of bentonite cement. The geological and geophysical studies were carried out in two stages: during the construction (1992) and filling (1995-1997) El Cuchillo reservoir. During the first stage (1992), the zones of probable filtration were predicted geophysical, geological and geomorphologic data. On the second stage (1995 and 1997), after filling the reservoir, the forecasts were verified and the filtration mechanisms were revealed on the base of repetitive vertical electrical sounding (VES), made with Schlumberger array with AB/2 distances from 1.4 until 100 m with the step 300 - 500 m along profile. Three factors complicate the hydrogeological situation in this zone: the existence of fractures and faults which are perpendicular to the dike; the absence of the waterproof screen in its construction; as well as the decrease of the relief about on 15 m outside of the dike with respect to the ordinary reservoir level. To verify the proposed forecast and to reveal changes of the formation resistivity, which have been provoked by water filtration, VES observations were carried out along the dike in 1995 (2.5 years after the first geoelectrical study and 1.5 years after the beginning of the reservoir filling). Apparent resistivity cross-sections are displayed on fig.1. Changes in the period 1992- 1995 are greater than that in 1995-1997.


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