A technique is described and demonstrated for generating high-resolution 3-D images of the objects on the seabed, combining synthetic aperture sonar with bathymetric (interferometric) processing. This has applications in surveying of oil wells, pipelines and shipwrecks, and in defence in mine hunting. It is shown that the technique is very sensitive to platform motion errors, and ways are discussed by which the errors may be estimated and compensated. Practical results obtained with an experimental system in a test tank are presented and discussed, and a number of areas for further work are identified. 1 Introduction Sidescan sonars are now a widely accepted and important tool of ocean technology. Broadly, they fall into one of two classes: the short-range high-resolution systems working at a relatively high frequency (e.g. about 100kHz), and the long-range low-resolution systems working at 10kHz or less. The former have a wide range of applications in surveying of oil wells, pipelines and shipwrecks, and in defence in mine-hunting. The longer range systems such as GLORIA have contributed significantly to the study of the deep ocean floor. The original method of determining seabed topography is by means of an ship-mounted echo sounder, covering a given area in a raster fashion. This is slow