The acoustic wavefield generated by a vessel sailing over ocean bottom cables

During the acquisition of an ocean bottom cable (OBC) survey on the Norwegian continental shelf, a line was acquired with the source vessel sailing directly over and along one of the ocean-bottom cables without triggering the airgun sources. These OBC data have been used to assess the feasibility of estimating the acoustic wavefield generated by a vessel from data recorded by receivers located on the seabed and using this wavefield for imaging the subsurface beneath the receivers. A 2D NMO stack along the vessel path has been made and compared against a line in the same location extracted from a 3D PSDM stack volume derived based on data acquired on a dense 12.5 x 12.5m shot grid using airguns. Most reflectors observed in the data acquired with airguns can be recognized in the data acquired without active sources even though the latter data represents only ten minutes of acquisition and very limited processing. This indicates that the method may be used for monitoring purposes. Especially in areas with permanent receiver installations, it may offer a low cost and low impact monitoring solution with an opportunity for much more frequent acquisition of time lapse data.