Laser holographic techniques are capable of detecting three-dimensional small deformations in terms of the wavelength of a light. The technique is applied to measure the crustal deformation.
A laser holographic recording system was installed in the deep tunnel of the Amagase Crustal Movement Observatory. The hologram of the tunnel wall within the scope of 2m in diameter was recorded on a photographic plate with radiation from a helium-neon gas laser of emission power of 50 mW. Superposing the reconstructed image of hologram on the really existing tunnel wall, we can see many interference fringes through the developed photographic plate. The fringe displacement in the interference pattern, formed by the deformation of tunnel wall, has been investigated with the “real time”method. Comparing the fringe displacement with the strain change obtained from extensometers, they are consistent with each other.
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