Deep-water Seismic Bright Spot Reflections in the Southern Canary Basin

A combined vertical-incidence and wide-angle seismic study provide strong evidence for the presence of large deep-water fluid reservoirs in Middle Miocene-age sediments with areal extents of more than 50 km2 in the Southern Canary Basin. Pronounced seismic bright spots at 300-800 m below seafloor are well imaged on multichannel reflection seismic sections (MCS) and Ocean-Bottom-Hydrophone (OBH) records, acquired on R/V POSEIDON in 1997 and 1999. Normal incidence reflection coefficients of about -0.4, calculated from MCS shot records, indicate strong negative acoustic impedance contrasts. Mud diapirs, observed in the vicinity of the bright spots, outline vertical sediment migration paths. MCS surveys approaching the deep-water commonly lack sufficient wide-angle reflections. In this study, wide-angle reflections have been imaged on OBH records deployed at the seafloor (3800 m water depth) to constrain the seismic origin of the bright spots. The strong direct water wave, generally masking shallow primary reflections, has successfully be suppressed, enabling the application of semblance based velocity analysis. Results from amplitude-versus-offset analyses and full waveform inversion confirm the presence of a low acoustic impedance zone at the bright spots. However, natural origin of fluids associated with the prominent bright spots remain unresolved. Circumstantial evidences such as occurrence of bright spots within the hydrate-stabilty-zone and existence of large magma volumes in the lower crust and upper mantle, point towards methane or carbon dioxide-rich fluids trapped in the sedimentary sequence.