AIM: To add detail to several leads identified by the 1990 Saint Clair Seismic Survey, and to provide further regional coverage, particularly in the north-western part of the licence area. The proposed programme of dip and strike lines will address Pretty Hill Sandstone reservoir objectives, both within fault block traps on the northern flank of the Saint Clair Trough, and also within hanging wall anticlines on its southern flank. METHODS: Survey dates: 20/2 - 2/4/91. Energy source: Vibroseis. CDP coverage: 11,000%. Recording mode: 440 channel digital - GEOCOR IV recorder. VP interval: 30.0 metres; group interval: 15.0 metres. RESULTS: 23 lines totalling 228 line km were acquired by Party 205 of Geosystems, and processed in Brisbane, Qld, by Digicon. The detailed seismic data confirmed the previously mapped structural style of the Saint Clair Trough half-graben, which has mainly an east-west orientated trend. Faults seen on the seismic records are down-to-the-north normal faults, with varying throws within the Pretty Hill interval ranging from 10 ms near the upper unconformity to 300ms at depth. Within this formation, also, the various stratigraphic sub-units exhibit changes in thickness and lithology consistent with local well data and their position relative to the trough axis. Five reflector horizons were interpreted and contoured at top and intra-Pretty Hill levels. All five resulting maps show similar structural styles within the licence area. In the north, faulting is intense and follows a NW-SE trend which extends from the Lake Eliza High in the NW to the Diamond Swamp High in the SE. This fault trend is interrupted only by a small saddle lying between the Saint Clair Trough and a part of the Penola Trough that extends into PEL 39. The major axis of the Saint Clair Trough has a crescent-shaped outline, with the trough's deepest part found just east of Lake George. From there the axis appears to swing to the NW towards the offshore extension of the Lake Eliza High, and as it does the trough shallows. In the south of the licence area a terrace is mapped between the Saint Clair Trough and the Beachport High. Here the structure comprises fault blocks trending WNW-ESE with a regional dip to the SE. Further to the SE the terrace develops into a trough which can be identified on seismic data recorded in PEL 40. The western end of the terrace runs under Lake George where no seismic has been acquired to date. The Pretty Hill maps at all levels show a regional dip to the south and south-east. The depth at the top Pretty Hill unconformity varies from about 1300 metres in the north to 1900 metres at the deepest part of the trough, while at near basal Pretty Hill level the depth range is from 1700 to over 4000 metres. The Reedy Creek trapdoor-style structural play on the northern flank of the trough has been matured for drilling by the Saint Clair Detail Seismic Survey. It contains a predicted complete stratigraphic section of Pretty Hill Sandstone lithologies, which is estimated to be in excess of 2000 metres thick. Two other strong structural leads, Saint Clair and Woakwine, which occur in the southern rollover zone and terraced area respectively, require further seismic detailing to confirm closure and size uncertainties before they can be considered mature.