Geoelectrical monitoring of embankment dams for detection of anomalous seepage and internal erosion — experiences and work in progress in Sweden

The demand for better methods for condition control and monitoring of embankment dams will increase in a near future as a result of expected more extreme weather situations. Furthermore the availability demand on hydropower will increase as a result of switching from fossil fuels to wind and solar power, in some countries in combination with decommissioning of nuclear power. Temperature monitoring is a powerful method for detecting anomalous leakage indicative of internal erosion, but the possibilities to install sensors in existing dams is often limited to the dam toe. Electrical resistivity tomography (ERT) on the other hand can be used to monitor the interior of the dam, and detect flow induced variation in the resistivity caused by the temperature changes, which has been demonstrated in long-term monitoring of several dams in Sweden. Furthermore short-term monitoring of embankment dams in connection with a substantial change in the reservoir water level detect anomalous leakage paths via differential wetting of zones with different hydraulic properties. In Sweden the electrodes are generally installed along the dam crest, in the top of the core, using a 2D ERT approach. This has the advantage of focusing the sensitivity to the core itself, which is the most critical part for internal erosion, but there are however a number limitations. The orientation of the electrode layout in combination with the 2D approximation leads to severe 3D effects, which distorts the inverted model resistivities and geometry. Furthermore the resolution decreases with depth, which is a major limitation for high dams. A way ahead would be if electrodes could be installed on deeper levels inside the dam close to the core, which might be possible using modern drilling technology, and using 3D inversion to take into account the geometry. This is being tested in two embankment dam installation that are initiated during the autumn, with electrodes and various sensors.