oa Seismic on the edge – a 3D transition zone seismic survey from concept to final volume

The Speculant 3D Transition Zone (3DTZ) Seismic Survey was acquired by Origin Energy in the Otway Basin, approximately 30km east of Warrnambool, Victoria during November and December 2010. The objective of the survey was to fill a data gap between an existing Marine 3D Seismic Survey (3DMSS) and Land 3DSS, with the target located offshore in the shallow waters of the coastal zone. Although the survey covered a small surface area, the coastal location combined with the economic and environmental significance of the region dictated numerous exclusion zones. To achieve coverage of the target under these exclusion zones, the survey was designed to simultaneously use two independent recording systems; a cable free system on land and an Ocean Bottom Cable (OBC) system offshore, two different sources (GI air gun offshore and Vibroseis on land) and three different receiver types. The final survey design was an orthogonal geometry with a high density of marine shots and a high density of land receivers, with large holes due to access restrictions. The field data were a collection of six different source - receiver combinations, each with unique noise characteristics and statics, covering separate but overlapping CMP’s. Combining these disparate data sets was the key challenge of the processing. The lessons learnt during planning, acquisition and processing are shared in this paper along with examples of the effect that survey design, field conditions and equipment use and limitations can have on field records and processed data.