AIM: A high resolution, digital marine airgun seismic reflection survey (33 lines totalling 53 line km) and a shallow resolution, analogue marine boomer seismic reflection survey (38 lines totalling 129 line km), plus secondary echosounder bathymetry and side-scan sonar topographic surveys, were carried out for BHP Petroleum Pty Ltd in May 1992, over grids centred on the proposed Troas 1 offshore drilling locations (preferred and alternate). The purpose of these site surveys was to check the sea floor for suitable drilling rig anchoring conditions, and to identify potential local drilling hazards, e.g. near-seabed shallow gas accumulations, which might occur in the uppermost continental shelf section. Analogue boomer seismic records would be used to determine shallow sub-seabed geology within the top few tens of metres of sediment, based on their acoustic characteristics, while stacked digital airgun seismic displays would be used to interpret the intermediate casing zone geology to depths of approximately 1000 metres subsea. METHODS: Survey dates: 4-19/5/92. Energy source: Airgun. CDP coverage: 4800% (digital survey only). Recording mode: 48 channel (digital survey only) - TTS 128 recorder. SP interval: 6.25 metres, group interval: 6.25 metres. Data acquisition was performed by contractor Racal Survey Australia Ltd using the M/V Tilbata Creek working out of Portland (Vic.). Processing was done by Racal in Kuala Lumpur, Malaysia, and the interpretation was also done in-house. RESULTS: The multichannel digital data has been interpreted and presented as two isochron maps illustrating the shallow structure, major faulting and local amplitude anomalies. Geology within the survey area is largely fault-controlled, with individual bed displacements ranging from 3 to 20 metres, downthrown in a SW direction. The stratigraphy can be subdivided into three units based on reflection characteristics. The shallowest unit represents weakly bedded bioclastic calcarenite containing two broad, stacked channel features. The intermediate unit represents sub-horizontally layered sandstones and siltstones/marls which are highly fractured and faulted in a NW-SE direction: at least one major fault zone crosses the centre of the survey area within this interval. The deepest mapped unit is thought to represent the even more densely fractured Early Cretaceous Eumeralla Formation, separated from the overlying Late Cretaceous Shipwreck Group by a gently northward dipping unconformity. No significant anchoring or drilling constraint is noted at the proposed Troas 1 location. Water depth over the survey area ranges from 75 to 95 metres, with localised elongate and circular stepped depression features. The seafloor is rocky and irregular and should provide good anchoring conditions. Drilling is expected to encounter high amplitude seismic anomalies at depths of approximately 340 metres through 463 metres subsea. Although no acoustic masking indicative of gas is apparent, caution is advised when drilling approaches these levels. The anomalies are believed to be associated with coarser sediments and/or differential compaction. At the alternative well location, proximity to a major fault zone may cause drilling complications at depth due to possible accumulations of upward migrating gas or a general loss of circulation fluid.