Can Seismic Measurements Detect Flaws in the Core of an Embankment Dam? A Real-Scale Experiment in Älvkarleby, Sweden

Embankment dams are the most common dams worldwide. The long-term integrity of these structures may be affected by internal erosion of their body and foundation. In November 2019, an experimental embankment dam was built in Älvkarleby (Sweden) by the hydropower operator Vattenfall, with built-in flaws of unknown position and size within its core. In order to assess the capability of new methods to detect with precision the position of the defects, seismic, resistivity, temperature and acoustic sensing measurements were and will continue to be collected for some months more. Additionally, geotechnical instrumentation monitors the dam's behavior. This contribution focuses on testing seismic methods to detect these built-in damages. Five hydrophone lines were installed within the 20 m long and 4 m deep dam, three at the top and two at the bottom of the dam's core, totaling 120 pressure sensors. Every two months, surveys using high-frequency seismic P- and S-wave sources are performed, shooting in 25 shallow and 4 deep boreholes as well as in the water reservoir upstream. Preliminary results of synthetic P-wave reflection seismic data and 3D traveltime tomography suggested potential for detecting the defects with real seismic data. Based on the synthetic data, the defects' position can, in general, be well recovered by tomography, not, however, their velocity contrast and size. The detection of defects highly depends on the seismic ray coverage. P-wave reflection seismic results from both before and just after filling in the reservoir do not have high enough frequency content for locating the damages. However, results from shooting upstream in the water reservoir show higher frequency content in the range of 400 to 2000 Hz, which previous 2D/3D seismic modelling studies performed at Uppsala University indicated as a requirement for detecting reasonably sized defects within this type of structure. Future work will continue with P-wave traveltime tomography using real data, time-lapse comparison of the reflection seismic data, seismic interferometry, and estimation of the shear-wave modulus within the core using S-wave velocity data. Once the dam is completely saturated, more information about the full potential of the seismic methods for the detection of the built-in flaws can be obtained.