Otway Basin onshore seismic acquisition: less field effort = better data quality

Abstract

Within the onshore Otway Basin, the PEL 27 Joint Venture has acquired excellent quality Vibroseis data using a 240-channel system with 120-fold coverage, short (15 m) group and receiver array intervals, only 6 geophones/station, and only one sweep/VP. Not only can layers within the Crayfish Subgroup (previously difficult to image) be mapped with higher confidence, but also the high-resolution data have preserved the hitherto rarely recognised reflections from fault planes. Provided far offset requirements are met, the most important controls on data quality are high fold, short group interval and short arrays. Optimal sweep duration is important also. A reduction from 12 to 6 geophones/station had no significant effect on data quality. Source effort (time/km) was reduced by 44% compared with the previous year's seismic survey, resulting in a 24% increase in km/recording hour. Significant improvement in imaging fault-plane reflections has been achieved using finite-difference migration with a small step time of 4 ms (instead of the conventional 20 ms) and an interpolated sample rate of 2 ms (instead of 4 ms). These parameters accurately migrated reflections from fault-planes with dips to at least 50°. Conventional migration parameters, if applied to this steep dip/trace data, would migrate accurately only those events with dips less than 10°.