oa Red Ghosts and Broadband Processing

Surface ghosts cause spectral notches. There is always a notch at 0 Hz which limits the low frequency content of the data. The objective of deghosting in data processing is to undo the physical process of surface ghosting. When scattering from irregular surfaces, such as ocean swell, is taken into account, the reflection (and transmission) coefficients become frequency dependent.

This is true for both sources and receivers. Near seismic sources, in addition to swell, particle accelerations may be comparable or even exceed the earth's gravity which is what is keeping water under air. Surface tension also plays a role.

The displacement can be large enough for stress-strain relations to become nonlinear. These nonlinear effects are frequency dependent causing the reflection coefficient to be smaller at higher frequency. Frequency independent reflection would be white but frequency dependent ghosting is red, more so near the source than near the receiver. It is useful to understand the physics of ghosting so that the relations between the up- and the down- going are understood and the up/down separation for deghosting can be made more robust.