Application of laser holographic techniques to investigate crustal deformations

Laser holographic techniques can be used to detect small deformations of three-dimensional objects, of the order of the wavelength of the laser light. We report the use of laser holography to measure the crustal deformation in a deep tunnel at Amagase, Kyoto, Japan. A recording system consisting of a helium–neon gas laser and associated optical elements was installed in the observation tunnel in 1984. Using this sytem, holograms of a section of the tunnel wall ∼2m in diameter are directly recorded on photographic plates. When the reconstructed image of a hologram is superposed on the current (‘real-time’) image of the tunnel wall, many interference fringes can be seen through the developed photographic plate. The fringe displacement, formed by the deformation of the tunnel, is continuously monitored with a video camera and a video-cassette recorder. The measured fringe displacement in the interference pattern is consistent with strain changes obtained from extensometers which have been installed in the same tunnel.