Correcting for water-column variations

Marine surveys are acquired as a collection of independent sail lines that can be shot days, months, and (in the case of 4D) years apart. Even when the acquisition is well managed (for example, source and receiver positions can be accurately defined for coverage in 3D or for repeatability in 4D), one key element cannot be controlled: the properties of the water column that can change significantly during the course of the acquisition.