Time lapse or 4D surveys are 3D seismic surveys repeated after a period of time, the 4D term refers to the fourth dimension which in this case is time. Time lapse surveys are acquired in order to observe reservoir changes during production and identify areas where there are barriers to flow that may not be detectable in conventional seismic. Time lapse surveys consist of a baseline survey and a monitor or repeat survey, acquired after the field has been in production. Most of these surveys have been repeated NATS surveys as they are cheaper to acquire and most fields historically already had a NATS baseline survey. Some of these surveys are collected using ocean-bottom cables because the cables can be accurately placed in their previous location after being removed. Better repetition of the exact source and receiver location leads to improved repeatability and better signal to noise ratios. A number of 4D surveys have also been set up over fields in which ocean bottom cables have been purchased and permanently deployed. This method can be known as life of field seismic (LoFS) or permanent reservoir monitoring (PRM).