Laser Interferometer Systems for Precise Measurements of Ground-Strains

Abstract

Four extensometers with laser interferometer systems were installed and have been successively operated in the tunnel at the Amagase Crustal Movement Observatory. Two of them (EL-1 and EL-V) are super-invar bar extensometers with laser interferometer systems consisting of simple laser sources, Michelson interferometers and photodetecting equipment with image-sensors. The remaining two components (L-1 and L-2) are laser extensometers with a frequency-stabilized laser source. Using long term continuous records obtained from EL-1 and L-1, which are orientated along the same direction, effects of instrumental and environmental disturbances have been investigated. Secular ground-strains observed with both components are inconsistent with each other. This disagreement is considered to be mainly caused by the effect of slowly increasing pressure in the enclosing pipes of L-1. Non-periodic ground-strains (residual strains obtained by subtracting the mean values of long term drifts and tidal constituents from observed data of EL-1 and L-1) are consistent with each other in the amplitude range of larger than 10^-8. But they are inconsistent in the amplitude range of smaller than 10^-8 because of different effects of instrumental and environmental disturbances. Tidal strain amplitudes obtained from EL-1 are about 27% smaller than those obtained from L-1. The former may be diminished by frictional forces between the super-invar bar and its supporting rollers.