2:45 PM - 3:00 PM
[HSC06-05] Ground Surface Deformation Monitoring by Distributed Fiber Optic Strain Sensing during Pilot Field Tests
Keywords:distributed fiber optic strain sensing, ground surface deformation, uplift and subsidence, static load, airbag inflation and deflation test
Introduction: The Earth's surface deformation (uplift, subsidence) induced by the CO2 injection may cause immense problems when they occur. Geodetic measurements can identify these deformations; however, they do not provide a complete solution to the issues involved in deformation measurement. One of the limitations of these methods is an insufficient number of coherent points due to decorrelation; they are also limited to onshore sites. To address the risks associated with these deformations, we suggest using distributed fiber optic strain sensing (DFOSS) for surface deformation monitoring by installing a single-mode optical fiber cable in a shallow trench to establish. We conducted three experimental field tests (iron plate load and airbag inflation) to confirm the strain sensitivity of DFOSS for static loads, excavation, subsidence, and uplift. This study also presents two installation methods to couple the fiber cable with the ground under various environmental conditions; the successful installation method must be chosen to avoid environmental effects, protect the cable to enhance the fiber lifetime, prevent heterogeneity soil effects, and achieve cable coupling with the ground while maintaining a high strain sensitivity. Hence in this study, the fiber cable was installed in a shallow trench with one part buried in the soil and another part covered with cement.
Method: A fiber cable with a total length of approximately 2.9 km was installed in a shallow trench less than 50 cm deep. One part of the cable was covered with soil, while the other part was covered with cement. The first test was performed with the same load weight conditions for the two sections where Iron plates were loaded above the fiber cable. In each stage, three plates (2.4 tons) were loaded, with a time interval of approximately 40 min between each step; a total of five stages were applied for fifteen plates with a total weight of 12 tons. The strains were measured separately to confirm the best surface installation method; then, We conducted the airbag inflation and deflation tests to simulate uplift and subsidence caused by the fluid injection and production in the subsurface. The surface deformations resulting from both tests were observed from Rayleigh frequency shifts recorded by a tunable-wavelength coherent optical time-domain reflectometer utilizing Rayleigh backscattered signals. Rayleigh frequency shifts can be converted to strain or deformation.
Result and Discussion: Our results suggest that covering the cable with cement is a practical approach for installing a fiber cable for ground surface deformation monitoring because the baseline is stable, with limited environmental effects; hence, for long-term monitoring, this method will be suitable for obtaining stable baseline conditions and for accurately detecting deformations. In addition, our results confirmed that DFOSS is an effective monitoring technology with high accuracy and reliability for locating anomalies along a cable and has a high sensitivity for monitoring subsidence and uplift. Furthermore, ideal surface monitoring can be achieved with DFOSS by incorporating well-based strain sensing, allowing ground deformation (horizontal in the surface and vertical in the subsurface) to be monitored in three dimensions.
Method: A fiber cable with a total length of approximately 2.9 km was installed in a shallow trench less than 50 cm deep. One part of the cable was covered with soil, while the other part was covered with cement. The first test was performed with the same load weight conditions for the two sections where Iron plates were loaded above the fiber cable. In each stage, three plates (2.4 tons) were loaded, with a time interval of approximately 40 min between each step; a total of five stages were applied for fifteen plates with a total weight of 12 tons. The strains were measured separately to confirm the best surface installation method; then, We conducted the airbag inflation and deflation tests to simulate uplift and subsidence caused by the fluid injection and production in the subsurface. The surface deformations resulting from both tests were observed from Rayleigh frequency shifts recorded by a tunable-wavelength coherent optical time-domain reflectometer utilizing Rayleigh backscattered signals. Rayleigh frequency shifts can be converted to strain or deformation.
Result and Discussion: Our results suggest that covering the cable with cement is a practical approach for installing a fiber cable for ground surface deformation monitoring because the baseline is stable, with limited environmental effects; hence, for long-term monitoring, this method will be suitable for obtaining stable baseline conditions and for accurately detecting deformations. In addition, our results confirmed that DFOSS is an effective monitoring technology with high accuracy and reliability for locating anomalies along a cable and has a high sensitivity for monitoring subsidence and uplift. Furthermore, ideal surface monitoring can be achieved with DFOSS by incorporating well-based strain sensing, allowing ground deformation (horizontal in the surface and vertical in the subsurface) to be monitored in three dimensions.