LASER HOLOGRAPHIC MEASUREMENTS OF TIDAL DEFORMATION OF A TUNNEL

A new technique based on holographic interferometry has been developed for measuring crustal deformations. the holographic recording system, consisting of an He‐Ne gas laser and associated optical elements, was first installed in a tunnel at the Amagase Crustal Movement Observatory in 1984. Tunnel deformations caused by tidal and tectonic forces have been precisely determined using the ‘real‐time’technique of holographic interferometry. In this procedure, a hologram of the tunnel wall within a section 1‐2 m in diameter is directly recorded on a photographic plate and then the plate is carefully reset in the position at which the hologram was taken. When the reconstructed image of the hologram is superimposed on the current image of the tunnel wall, many interference fringes can be seen through the hologram. the fringe displacement, formed by the deformation of the tunnel, is continuously recorded on a video‐cassette tape using a video camera and a time‐lapse video recorder. the change in the fringe patterns is analysed using the image‐processing system. Tidal deformations obtained from the holographic method are consistent with the strain changes observed with laser extensometers in the same tunnel. These observational results substantiate the tunnel deformation estimated by finite‐element calculations. Copyright © 1990, Wiley Blackwell. All rights reserved