In Sweden, almost 100 optical fibres are installed in dams. Today they are used for Distributed Temperature Sensing, DTS, or Distributed Temperature and Strain Sensing, DTSS. However, most of these installations can also be used for acoustic measurements such as Distributed Acoustic Sensing, DAS. In this project, researchers have carried out experiments to test the potential in recent technological developments for monitoring and analysis that can provide an additional method to monitor the structural safety of dams. The hypothesis was that it would provide additional information regarding changes of material properties and seepage flow.
By carrying out experiments at a small dam, equipped with optical fibres, the sensitivity and potential for subsurface dam monitoring using passive seismic measurements was determined. For these experiments a technique known as ambient noise interferometry was used to analyse the acoustic recordings.
The experiments show that there is great potential in using DAS and the existing fibre cables are expected to be increasingly used over time. The seismic velocities determined by this study provide a precise measurement for body, surface and coda waves and measurements proved to be stable over time.
Technological improvements and further investigation requirements
To be used to detect and monitor small deviations in the dam structure a higher precision is required than here tested. However, technological developments and evaluation techniques are developing fast with higher precision expected to be available within the coming years.
Some aspects of the results in this study were not fully understood and require further investigations. To fully assess the accuracy and precision of the results further information is required to:
- Understand the effect of temperature and rainfall on the results.
- Understand the effect of bedrock geology, the geology of the abutments and compaction in the dam.
- Create a comparison with active seismic measurements in wet and dry conditions.
Following these results the researchers laid forth several recommendations in order develop the method for further use in embankment dams.