Designing an exploration-scale OBN: Acquisition design for subsalt imaging and velocity determination

Direct wave arrivals are the most robust signals to determine velocity. They have been used for almost a century in hydrocarbon exploration. This is because the arrival time is explicitly available and provides a direct measurement of the average velocity of the subsurface raypath. To acquire these direct arrivals in a seismic experimental setting, it is necessary that the waves turn back to the surface after they start traveling into the earth. As is well known, it is possible to turn waves back up if they encounter faster propagation velocities than previously experienced. Using these simple concepts, we show how it is possible to design a seismic acquisition to measure subsalt velocities when the salt cover is very thick and potentially not homogeneous. Until now in marine seismic surveying, the physical limitations of the earth meant that the use of direct wave arrivals was restricted to relatively shallow depths of investigation. By combining the application of node technology with a well-established physical phenomena (i.e., refraction in the basement), it is possible to retrieve subsalt velocities from seismic surveys.