Self-potential Imaging of Seepage in an Embankment Dam

Abstract

We have investigated seepage in Les Cedres embankment dam (Valleyfield, Canada) by combining self-potential tomography (SPT), electrical resistance tomography (ERT), thermometry, electromagnetic (EM) conductivity and magnetic measurements. SPT consists of inverting self-potential data to retrieve the source current density distribution associated with water flow pathways (streaming current density) in embankment dams. The SPT inverse problem relies on the resistivity model of the dam that is obtained by 3-D ERT. Our 3-D SPT code is based on Occam’s inversion. The forward problem is solved using the finite-volume scheme. The investigated embankment dam is used to channel water from the Saint-Lawrence River to a hydroelectric plant. It separates in its upstream and downstream sides Les Cedres and St-Timothee reservoirs respectively. St-Timothee reservoir is emptied during the winter and filled during the summer. Temperature monitoring was done in a borehole installed in the middle of the survey zone. To build a better understanding of water flow through the dam, it is important to separate the part of the source current density caused by the electrokinetic effect from the other sources (principally electro-chemical). In order to achieve that, ERT, EM31, magnetic and thermometric measurements have been used in the interpretation. EM conductivity maps allowed identifying two linear anomalies caused by metal-shielded electrical cables. The magnetic survey shows an important anomaly zone that is probably related to a metallic object. Therefore, all measurements near these zones were discarded from inversion. SPT shows a few seepage sources on the upstream dam side at a depth between 4 and 5 m. Two of them are confirmed by geotechnical testing. The water flow through the dam appears complex. It is partly controlled by a permeable zone that is well identified in the resistivity model. In the vicinity of the vertical temperature profile the SPT shows that the water flows parallel to the dam orientation and not through the borehole used for thermometry. This is why there is no clear indication of water seepage in temperature measurements. Finally, all observable seepage outlets on the downstream side can be related to the SPT anomalies and are observed as conductive zones in the resistivity model.