Converted-wave Imaging in the Caspian Sea - A View of Mud Volcanoes

BP's field developments in the Caspian Sea are being enabled by ocean-bottom seismic (OBS), almost exclusively through wide-azimuth P-wave imaging. Mud volcano-related features, once thought to act like diffuse gas clouds to P-waves, complicate the propagation of P-to-S (PS) converted waves and are not invisible to them. Following initial data processing of two OBS surveys in 2003 and 2004, an internal study into data quality and propagation issues associated with PS waves focused on understanding how much of the poor imaging was fundamental to the data, and how much could be improved through data processing and more-refined model building. Processing trials supplemented by forward modelling of plausible scenarios for the internal geometries of the mud volcanoes begin to explain the range of PS image quality within each OBS survey, and between surveys, as well as the conundrum of poor PS image quality where P-wave imaging is not overly compromised. This suggests discrete zones of poorly-consolidated, possibly fluidised, material which have a significant impact on PS propagation. So far this has proved as much of a challenge in many areas as the effects of distributed gas on PP propagation.