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

Direct wave arrivals are the most robust signals to determine velocity and consequently they have been used for almost a century in hydrocarbon exploration. The reason is simple as the arrival time is explicitly available. In order to acquire these direct arrivals in a seismic experimental setting it is necessary that these waves turns back to the surface after having been sent into the Earth. As is well known it is possible to turn waves back up if they encounter faster propagation velocities than have been 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 have meant that use of direct wave arrivals have been restricted to relatively shallow depths of investigation, linked to streamer length. In this paper we describe how a new and novel application of node technology has been combined with a well established physical phenomena to support the acquisition of a world first exploration-scale Ocean Bottom Node (OBN) survey.