Seismic travel time forward modelling and inversion of fluid flow conduits in marine sediments using ocean bottom seismometers

The increasing concentration of greenhouse gases in the atmosphere and hydrosphere is causing changes to global climate. Geological carbon sequestration is a proven technology for reducing anthropogenic emissions in the atmosphere. However, leakage of CO2 along natural fluid pathways, which are imaged as seismic pipes and chimneys, may affect storage formation integrity. These fluid flow conduits are observed in marine basins globally, passing vertically through kilometres of sedimentary overburden. However, the nature and physical properties of fluid flow conduits are currently poorly understood. In this thesis, I characterise active fluid conduits in the Central North Sea and at the Svalbard Margin, as well as analogous onshore in Varna, Bulgaria. I present three-dimensional seismic P-wave travel time tomographies using ocean bottom seismometers to constrain the geometry, the geophysical properties and the material inside of fluid flow conduits.  A key aim is to compare fluid flow conduits in marine basins based on the studies of a pipe structure beneath the Scanner Pockmark, Central North Sea, and a chimney below the Lunde Pockmark, Svalbard Margin. My results imply a diverse range of the internal structures of fluid flow conduits. The analysis of the 3D P-wave velocity demonstrates that fluid flow conduits are fundamentally different in their geophysical and hydraulic characteristics depending on depth and the geological setting.
The results of my thesis highlight the complexity in evaluating fluid flow conduits, the necessity of their detailed assessment for any offshore carbon storage site selection, and the necessity to investigate their potential to function as pathways for CO2 and to ensure the integrity of the reservoir for CO2 sequestration.