Ultra-high-resolution FBG strain sensor for crustal deformation measurements

摘要

For measuring crustal deformation, strain sensor with high resolution on the order of tens ofrnnano-strains is required. Current sensors for this purpose such as quartz-tube extensometers andrnfree-space laser interferometers are large in size, from tens to hundreds of meters long, and hence, arerndifficult to measure spatial strain distribution practically. Optical fiber strain sensors have been widelyrnadopted for structural health monitoring of infrastructure, such as bridges and buildings. Resolution ofrnthe commercially available optical fiber strain sensors, however, is on the order of micro-strains, andrnthe sensors cannot be directly applied for crustal strain measurements. Once the strain resolutionrnbecomes good enough to meet the requirement of crustal deformation measurements, fiber strainrnsensors can be an attractive tool.rnWe have developed two types of ultra-high strain-resolution optical fiber sensors for static strainrnmeasurement and seismic waves from distant earthquakes. Each type of sensor consists of a pair ofrnfiber Bragg gratings (FBGs), one for strain sensing and the other for reference. The sensors wererninstalled into the vault at Aburatsubo, Japan, and the deformation induced by ocean tide was measuredrnwith a resolution less than 10 nano-strains. This resolution is comparable with the 38-m-longrnquartz-tube extensometer, but the FBG sensors have a much smaller baseline length (1m).rnConsidering that the resolution of FBG sensors do not rely on its baseline length, the FBG sensorsrnhave advantages to be able to discuss scale dependency of real rock masses in situ. In addition,rndynamic strain (seismic waves) up to several tens of Hertz frequency can also be measured by thernoptical fiber sensor. Thus, ultra-high-resolution FBG sensors developed by us can be a powerful toolrnfor crustal strain measurements.