Petrel Sub-basin Marine Survey (GA0335/SOL5463) : Sub Bottom Profiler Processing Report

The Petrel Sub-basin Marine Survey (GA0335/SOL5463) was undertaken in May 2012 by Geoscience Australia in collaboration with the Australian Institute of Marine Science (AIMS), as part of the Australian Government's National Low Emission Coal Initiative (NLECI). Its purpose was to acquire pre-competitive geophysical and biophysical data on shallow seabed environments within two targeted areas to support assessment of CO2 storage potential. The geophysical acquisition consisted of multibeam sonar mapping of sea floor morphology and multi-channel sub bottom profiling of the shallow sub surface geology. The aim of sub bottom profiling was to investigate regional seal breaches and potential fluid pathways by providing high resolution images connecting the sea floor map to regional seismic reflection data acquired concurrently in the area. The sub bottom profiler data were acquired aboard the AIMS research vessel (RV) Solander along 51 lines, totalling 654 line km in the Petrel Sub-basin of the Bonaparte Basin. Acquisition employed a Squid 2000 sparker as the source and a 24 channel Microeel streamer for the receivers. Group interval was 3.125 m and shot interval 6.25 m, resulting in 6 fold data. Record length was 500 ms with a sample interval of 0.25 ms. Some problems in acquisition needed to be addressed in processing. Firstly, sea conditions were far from smooth for most of the voyage. Obvious relative motion occurred between the source and the streamer, and along the streamer itself, due to the ocean swell. In some cases, acquisition commenced while the vessel was still turning onto the line and the streamer was not straight in line behind the stern. Finally, malfunction of the sparker on some half dozen lines resulted in gaps in the coverage, which could not be filled in later, due to bad weather reducing the time for the survey. Multichannel seismic reflection processing was able to compensate for some of the limitations of sparker acquisition. Mutes and filters were necessary to remove the worst of the noise, including leaked timing pulse and swell noise. Surface related multiple elimination (SRME) successfully attenuated the water bottom and later multiples. Non surface consistent trim statics were able to correct for the relative motion of the sparker and the streamer, thereby allowing alignment of reflections prior to stack, which improved the signal to noise. Minimum entropy deconvolution was a critical step in both suppressing ghosting and enhancing latent high frequencies in the data, thus improving the resolution. Migration was necessary to correctly image small channels by collapsing diffractions. Finally tidal static corrections were essential to remove mis-ties in high frequency data. The processing stream has been well documented, along with scripts employed to handle the large amount of data efficiently and consistently. This record is a manual for a much more rigorous way of processing multi-channel sparker data, and details a work flow that can be implemented within Geoscience Australia and used for future surveys. The final migrated seismic data proved to be very high resolution, allowing delineation of multiple episodes of channelling in the top 100 m of sediment. Comparison of the sub bottom profiles with older regional seismic reflection data showed just how much more detail is available in the region critical for mapping deeper faults and fluid pathways to features on the sea floor. Acquisition and processing of the sub bottom profiler data surpassed the survey expectations.