A 100Â m laser strainmeter system in the Kamioka Mine, Japan, for precise observations of tidal strains

A 100 m laser strainmeter system was installed in a deep tunnel about 1000 m below the ground surface in Kamioka, Gifu, Japan in 2003. The system consists of three types of independent interferometers: (1) an EW linear strainmeter of the Michelson type with unequal arms, (2) an NS-EW differential strainmeter of the Michelson type with equal arms and (3) an NS absolute strainmeter of the Fabry-Perot type. These are configured in L-shaped vacuum pipes, each of which has a length of 100 m. (1) and (2) are highly sensitive (order of 10−13 strain) and have wide dynamical range (10−13 to 10−6). Based on data obtained from these strainmeters (1) and (2) during the period of about 4 months in 2003, we analyzed tidal strains by employing the tidal analysis program BAYTAP-G. Observed tidal strain amplitudes of eight major constituents were compared with theoretically expected ones that were obtained from the GOTIC2 program. As a result, it was revealed that the “observed” amplitudes are about 10-20% smaller than the “expected” amplitudes. In order to explain these discrepancies, we examined the topographic effect using three-dimensional finite element model (3D FEM), and succeeded in reducing the discrepancies within several percent. This result shows that our laser strainmeter system in Kamioka has sufficient reliability to detect small strain changes of the order of 10−10 in the tidal frequency band.