Abstract
In order to measure the ground-strain with an accuracy of 1 part in 109, the superinvar bar extensometer with a laser interferometric device was developed. The fringe displacement in the interference pattern, together with the strain change, was converted photo-electrically by an image sensor consisting of a row of 64 photodiodes. The present resolution is one-thirtieth of the fringe displacement, corresponding to the strain change -of 2.5x10-10. Strain records obtained with this laser interferometric device (L) and with the con-ventional "roller" detecting device (R) for a period of 126 days were compared and the following relationships between them were found. R/ L≤0.5 for the range of relative displacement amplitude of 1-2μm, and R/L=1 for the range of relative displacemet amplitude larger than 4μm. Tidal strain amplitudes observed with the laser interferometric device were 39x 10-9 (M2) and 3.1x10-9(O1).
