Changing OBC Acquisition Geometry Using FWI Modelling

The ACG field is located 100 km east of Baku in the Caspian Sea. Imaging challenges include shallow gas and collapsed mud volcanoes. To date, standard reflection tomography on conventional towed streamer data has failed to build a detailed velocity model. OBC surveys were acquired in 2007 and 2008. Using Full Waveform Inversion (FWI) to update the velocity model has provided a significant uplift in the seismic image with improvements in continuity and resolution especially over the crestal areas, though the flank areas were degraded. Previous work has shown that refracted waves play a big part in the inversion and large offsets are needed for imaging steep dips and at depth. The suggestion here was that the refracted energy has adequately sampled the subsurface over the crest only and that the flanks were poorly sampled. In 2010 another OBC survey was planned over one of the fields and a data study was embarked upon to understand if acquiring a larger acquisition footprint would improve the effectiveness of the FWI application. This paper presents the results of the modelling study, describes the recommended acquisition programme and gives the results of FWI on the 2010 OBC survey.