Shallow seismic methods for monitoring a soil structure on canal erosion using Fiber Optic Distributed Acoustic Sensor (DAS).
Abstract
The setting of this case study is linked to the detection of leakage occurring through the protective clay layer lining the water bottom of canals. In order to maintain and improve the quality of water ways, understanding of the surface water - groundwater - sediment - subsurface interaction is important. Fibre Optic Distributed Acoustic Sensor (DAS) has been demonstrated to be capable of dense spatial sampling in order to determine small scale geological and geotechnical characterisation in the shallow subsurface. A monitoring setup using an acoustic source and a DAS system could be useful. As a first step this setup has been tested a laboratory setting. The DAS system is based on a novel digital optical detection of elastic Rayleigh backscatter resulting from minute built-in inhomogeneous variations of refractive index along the fibre. The acoustic energy induces strain along the fibre and the DAS interrogator measures changes in the local axial strain down to sub-nanostrain resolution.
The laboratory setup consisted of an indoor water-soil flume in Deltares' facilities in Delft. The bottom of the flume was lined with a course sand layer. A 0.1 m diameter tube with tightly wrapped fibre optical cable was positioned horizontally on the sand bed and covered by a clay layer. The wrapping increased the spatial resolution by a factor of ~100. The first experiment encountered acoustic measurements with a 3.5 kHz acoustic source. This experiment showed that the DAS recorded the acoustic wave field with sufficient spatial and temporal resolution. In the second experiment, a hole was made in the clay layer and the sand was exposed, mimicking the effects of erosion of a canal bottom. The acoustic measurements were repeated using the same source. There are marked differences between the recorded acoustic signals in the two experiments. Stacked acoustic signals from the two experiments were used to identify fibre locations with the highest changes in energy levels. Further analysis using data from different source positions relative to the hole will show whether the position of the hole can be consistently retrieved. Based on the results of the laboratory experiments, we expect that fibre-optic distributed sensing can be a future toolkit for sediment related applications, such us sediment transport and canal lining erosion detection.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H11D..06C
- Keywords:
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- 1829 Groundwater hydrology;
- HYDROLOGY;
- 1835 Hydrogeophysics;
- HYDROLOGY;
- 1865 Soils;
- HYDROLOGY;
- 1899 General or miscellaneous;
- HYDROLOGY