R.I.318 H.Ammari Inverse boundariy value problems for helmholtz equations in an infinite periodic structure The whole paper is available as a compressed Postscript file by internet procedure FTP anonymous on host barbes.polytechnique.fr ( 129.104.4.100) in the directory pub/RI/1995 under the name ammari_318.avr.ps.gz or by Xmosaic or any other www client via the CMAP www server "http://blanche.polytechnique.fr/"