An interpretation of all seismic data in SA part of the Otway Basin is part of a joint geological and geophysical study with the Victorian and Tasmanian Department of Mines and the BMR to produce a basin - wide interpretation. The geometry and structure of the pre-Cretaceous basement surface has been elucidated from the seismic data over part of the basin, supplemented by interpretation of depth to magnetic basement where acoustic basement is not discernible. Maps of interpreted depth to top of the Late Cretaceous Sherbrook Group and top of the Early Cretaceous Otway Group have been prepared. Basement interpreted onshore and near - offshore shows characteristic irregular relief and commonly displays normal faulting. Several distinct E to SE trending troughs in this basement may represent continental rocks. A second type of acoustic basement in the most distant offshore seismic sections occurs as prominent ridges above the abyssal plain, and may form the southern margin of the basin. The Cretaceous units show structures typical of passive continental margins with intensive en-echelon basin dipping listric faulting, landward dipping of beds within faulted blocks and minor antithetic landward - dipping faulting.