PS Velocity Model Building Of Johan Sverdrup OBS Data - When You Expect It To Be Trivial

Ocean Bottom Seismic (OBS) survey acquired over the Johan Sverdrup field provided a superior image to the streamer data from P-wave imaging. When processing PS data we observed that PS image ties reasonably well with PP image in depth. However, the PS image below chalk is poorer than the PP image. Despite a generally simple overburden without big shallow gas anomalies, the presence of the high velocity contrast injectite intrusions and a high contrast chalk layer, makes PS imaging difficult in the Johan Sverdrup case. The most likely reason for poor PS image below Shetland could be the mode conversion occurring at the chalk interface. Reverse Time Migration (RTM) of PP data shows improvement when compared to Kirchhoff. Similar improvement is expected from RTM migration on PS data, due better handling of multi pathing and mode converted events.