AIM: Shell's R 5 Marine Geophysical (seismic plus magnetic) Survey, the most extensive yet conducted in the SA offshore, was spread over its entire concession area. The object of the seismic work was to define and confirm prospects for drilling in EPPs 6 and 7 that had been partly outlined by earlier Shell surveys, besides gathering semi-detailed infill of the previously established reconnaisance grid in EPPs 5, 10 and 11. A shipborne magnetometer survey of all permits was performed along with the seismic coverage, to obtain estimates of basement depth within the deep basins, and to help identify areas of igneous activity. Limited seismic refraction data was also recorded in EPPs 6 and 7 to investigate the nature of problematic, localized shallow bad data zones noted on previous surveys. The survey acquisition and processing was contracted partly to Geophysical Service International (GSI) and partly to Western Geophysical Corporation (WGC), with each company independently operating a survey vessel acquiring separate parts of the programme, so as to achieve the required production within the summer weather window. METHODS: - [GSI portion of R 5 programme] Survey dates: 22/10 - 9/12/70. Energy source: Airgun. CDP coverage: 2400%. Recording mode: 48 channel digital - Texas Instruments DFS III recorder. SP interval: 50.0 metres, group interval: 50.0 metres. 103 lines totalling 7674 line km (7599 line km of reflection, 75 line km of refraction) were shot across EPPs 6, 7 and 10 by GSI using the M/V R.C. Dunlap. Of this total 4411 line km were shot across EPPs 6 and 7 to detail drilling targets. The seismic data was also processed by GSI in Sydney, while the magnetic data was processed by Geosurveys of Australia Pty Ltd in Adelaide. Survey control was achieved using extended range Shoran, set up and monitored by Offshore Navigation Inc. - [WGC portion of R 5 programme] Survey dates: 20/11/70 - 24/1/71. Energy source: Aquapulse. CDP coverage: 2400%. Recording mode: 24 channel digital - Western SDS 1010 recorder. SP interval: 50.0 metres, group interval: 100.0 metres. 50 lines totalling 3188 line km (all reflection) were shot across EPPs 5 (1159 line km), 10 (1143 line km) and 11 (886 line km) by WGC using the M/V Marchant 3. The seismic data was also processed by WGC in Sydney, while the magnetic data was processed by Geosurveys of Australia Pty Ltd in Adelaide. Survey control was achieved using extended range Shoran, set up and monitored by Offshore Navigation Inc. RESULTS: The R 5 survey seismic data quality was only rated as poor to fair, with a low signal to noise ratio evident on most records. Despite this, the survey was successful in delineating a mappable shallow regional unconformity (the 'A' Horizon), a number of closed structures beneath this unconformity, and a prospective deep water basin beyond the edge of the continental shelf in the Great Australian Bight. Within EPPs 6 and 7, two structural prospects, Echidna and Platypus, were mapped sufficiently accurately to allow selection of drill sites. The prospects lie within the newly defined Duntroon Basin, a tectonically distinct area of the continental shelf immediately south of Eyre Peninsula, where up to 12,000 feet of sediments have been deposited in a highly faulted continental margin setting. Elsewhere on the Bight continental shelf, only a thin veneer of sediments over shallow basement is indicated. One minor exception is a narrow basement graben, the Elliston Trough, cutting across the eastern part of EPP 5, where local depocentres may approach 10,000 feet in thickness. Further to the south, several large but still poorly recognized structural leads occupy the continental slope zone of EPPs 10 and 11, where water depths of 2000-4000 feet are beyond the scope of present offshore drilling techniques.