Sub-basalt converted-wave imaging : offshore west Ireland

High-velocity volcanic units that overlay lower velocity structures hinder exploration in potential hydrocarbon producing areas adjacent to the Corrib Field, offshore West Ireland. A recent 3D seismic reflection dataset acquired in the NW Corrib region is used to map major volcanic units and a heavily faulted sub-volcanic structure. The low frequency content of the seismic data is taken into account in the acquisition design. A case study is conducted to demonstrate how wave mode dependent data pre-processing, velocity analysis and 3D prestack imaging techniques should be integrated for the purpose of producing enhanced depth images of the target zone when compared to conventional PP processing. We show that the mode-converted waves of type PSPPSP provide a better image of the sub-basalt structures than the PP waves. Separation of wave modes in a pre-processing step is crucial for imaging these different wave modes separately. For carrying out the total velocity model building workflow, a priori knowledge on the area in terms of the different geologic layers and their geophysical parameters is very important.