Localization and quantification of leakages in dams using time-lapse self-potential measurements associated with salt tracer injection

Abstract : The self-potential method is the only non-intrusive method that is directly sensitive to the flow of the pore water in a porous material. We propose the use of a new protocol of self-potential measurements associated with a brine injection to locate leakages in earth dams and to quantify their permeability. Indeed, a brine solution injection upstream of an earth dam (in the assumed leakage zone) is able to change the electrical conductivity of the medium. In turn, this decreases the magnitude of the electrokinetic contribution of the self-potential signals that are related to the flow of the seepage water. The evolution of this anomalous self-potential signal (expected to be positive with respect to a reference state prior the salt injection) can be measured at the ground surface with a network of non-polarizing electrodes. The seepage flow inside the dam is localized from the evolution in space and time of the resulting transient self-potential signals associated with the transport of the brine. The mean permeability of the preferential flowpath can be determined. This method is first applied to a laboratory test to show how the passage of a salt tracer affects the self-potential response. Then, we apply this new methodology to a field test site (a dam with a proven leakage) located in the south of France. At this test site, self-potential mapping was first performed to locate the preferential flow path. Then, a network of non-polarizing electrodes was used to perform time-lapse self-potential measurements at the dam crest during a brine injection occurring upstream of the seepage zone. We used two lines of 16 non-polarizing electrodes each. From the time-lapse data, the permeability of the leaking area was estimated inside one order of magnitude.