Using 3D seismic reflection data to find fluid seeps from the Costa Rica accretionary prism

Submersible dives planned using 2D and 3D seismic reflection data off Costa Rica successfully found numerous sites of fluid expulsion predominantly along surface fault scarps. These data were used because more typical data, such as deep-towed side-scan sonar, bottom photography, and heat flow, were unavailable. Because fluids and fluid pressure distribution profoundly affect the way an accretionary prism deforms, detecting fluid pathways and recovering fluids is a key to its deformation processes. Detailed seismic interpretation of the shallow subsurface identified apparently active faults which are most likely to act as fluid pathways. Due to a quasi inverse relationship between surface dip and reflection amplitude, the amplitude of the seafloor reflection, displayed in map-view, shows the orientation and extent of surface scarps, many of which appear related to subsurface faults. Because the seafloor amplitude response depends on seafloor morphology as well as near-surface physical properties, removing the effect of the surface dip, calculated directly from picked surface structure, leaves a residual that may be related to variation in physical properties. Although the search for fluid seeps by submersible dives over the Costa Rica prism was sparse and irregular, the seeps that have been discovered are all on structural highs of the apron/prism boundary.