Hiroya Matsui

Ultrahigh Resolution Multiplexed Fiber Bragg Grating Sensor for Crustal Strain Monitoring

We demonstrated a multiplexed fiber Bragg grating (FBG) sensor with static strain resolution of 10 nanostrain (nε) for crustal strain monitoring. Each sensor unit consists of a pair of identical FBGs for strain sensing and reference, respectively. A narrow linewidth tunable laser is used to interrogate the FBGs, and a cross-correlation algorithm is incorporated to demodulate the wavelength difference induced by strain. When no strain is applied, an ultrahigh wavelength precision corresponding to strain resolution of 3.3 nε was obtained, indicating the ultimate resolution of the sensor system. With a variable strain applied by a piezo-stage, strain resolution of 17.6 nε was demonstrated. When the sensor is adopted for the in situ monitoring of crustal deformation, the strain induced by oceanic tide is clearly recorded with a resolution of 10 nε, providing a potential tool for the geophysical measurements.

Field demonstration of 10-nano static strain resolution multiplexed FBG sensor for geophysical applications

We have developed an FBG sensor with a strain resolution better than 10 nano-strain (10-8) for geophysics applications. The sensor consists of a pair of identical FBGs, one for strain sensing and the other for reference. A narrow linewidth tunable laser is used to interrogate the two FBGs simultaneously. Cross-correlation algorithm is utilized to extract the Bragg wavelength difference between the FBGs with high precision. Multiplexed sensing is achieved using WDM technique. With this sensor, the crustal deformation induced by oceanic tide at Aburatsubo Bay in Japan is clearly observed with a strain resolution better than 10 nε. This is the first that 10 nε order static strain resolution is demonstrated with FBG sensors, providing a potential tool for the geophysics applications.

Development of borehole multiple deformation sensor system

The multiple deformation sensor system for small diameter borehole is required for the long term monitoring of deformation of rock mass of high-level radioactive waste disposal site. The conventional electric monitoring systems are difficult to apply for a long term monitoring in many cases because of the sensor failure caused by lowering of insulation or other problems and only available for the large borehole and for 6 or less measurement sections. The Borehole Multiple Deformation Sensor System was developed based on the FBG (Fiber Bragg Grating) sensor technology that is expected to have longer life time than electric systems. The developed system can be set in a 66 mm diameter borehole and available for 9 or more measurement sections that can be hardly achieved by electric systems. The sensor system is applying for the monitoring of ground deformation in the Horonobe Underground Research Laboratory in Japan.

Current Status of Phase II Investigations: Mizunami Underground Research Laboratory (MIU) Project

The Mizunami Underground Research Laboratory (MIU) Project, a comprehensive research project investigating the deep underground environment in crystalline rock, is being conducted by the Japan Atomic Energy Agency at Mizunami City, Central Japan. The MIU Project is being carried out in three overlapping phases: Surface-based Investigation (Phase I), Construction (Phase II), and Operation (Phase III), with a total duration of 20 years. The overall project goals of the MIU Project from Phase I through to Phase III are: 1) to establish techniques for investigation, analysis and assessment of the deep geological environment, and 2) to develop a range of engineering techniques for deep underground application. Phase I was completed in March 2004, and Phase II investigations associated with the construction of the underground facilities are currently underway. Phase II investigation goals are to evaluate the geological, hydrogeological, hydrogeochemical and rock mechanical models developed in Phase I and to assess changes in the deep geological environment caused by the construction of underground facilities. Geological mapping, borehole investigations for geological, hydrogeological, hydrochemical and rock mechanical studies are being carried out in shafts and research galleries in order to evaluate the models. Long-term monitoring of changes in groundwater chemistry and pressure associated with the construction of the underground facilities continue in and around the MIU site, using existing boreholes and monitoring systems. This report summarizes the current status of the MIU Project on results of the Phase II investigations to date.Copyright