Summary A detailed geological and geotechnical survey was carried out in connection with the future development of the South Essex area for a possible third London airport on Maplin Sands. Information on the position of the old buried channels of the Crouch/Roach river system was obtained by carrying out a continuous seismic profiling survey along the present channels of the two rivers. The survey provided information on the depth to the London Clay surface, the thickness and depth of the alluvial deposits, the variation of lithology within these deposits, and the position of some of the buried channels.