A Coil Shooting Survey in the Angola Deep Offshore

Abstract

The Calulu PDA (Pre-Development Area) is located in the Angolan deep offshore. The water depth ranges from 1500m to 2500m. The area is characterized by a very complex geological settings caused by the presence of extended salt canopies. The main reservoir levels are turbiditic sands located in highly structured sub-salt areas. The area was covered in 1999 by a conventional Narrow Azimuth Towed Streamers (NATS) 3D seismic survey. As most acquisitions of that period, the data suffer of an intrinsic limitation: the maximum recorded offset is about 3.5 km. Despite recent reprocessing efforts with the latest state of the art techniques (3D SRME and RTM) the final data quality was still not sufficient to correctly image the complex salt tectonics structures and the steeply dipping anticlinal flanks. For this reasons, in 2011, TEPA and Partners decided to acquire a long offset (more than 7 km) NATS seismic survey over the area (~1300 Sq. km) complemented by a coil shooting survey over a sub-area of interest (~700 Sq. km). The required turnaround deadlines for the whole project (Fast Track and Final Processing) was a real challenge for all the parties involved (TOTAL and WesternGeco): a raw TTI (tilted transverse isotropic) RTM PSDM was available within 4 and 5 months respectively after the last shot point for the NATS and Coil. As a seamless workflow, the full processing took then the pace to deliver the final results within 12 months after the last SP. Scope of the final processing was to refine the existing TTI velocity model and produce improved images, with respect to the Fast Track, of the Coil data set using various migration algorithms (Beam, Kirchhoff and RTM). The most representative results among the different techniques from both surveys are presented and compared in this paper. Acquisition overview The coil shooting technique allows the acquisition full azimuth (FAZ) seismic data. It consists of a single vessel navigating in circular path and recording data over the full range of offsets and azimuths. This technique has the potential of improving seismic imaging in subsalt provinces. In our specific case the coil shooting survey was designed and implemented in order to deliver high fold and a full azimuth distribution over an area of interest (Khaled & al, EAGE 2012).