We have seen in the previous section, that the dominant guided wave is aliased. For this wave, aliasing is compensated by flattening data, by applying a velocity correction of 1500 m/s (approximate velocity of underwater sound).

Concerning the selected component (hydrophone or geophone X, Y, Z), the figure shows from left to right :

• Seismic section. The observation time is brought back to 0.25 s.
• The f-k plot represented with or without a normalisation in wavenumber domain.

On the components Hydro, X and Y, guided waves (underwater) are dominant. Component Z is different.

Without a normalisation in wavenumber, the guided wave has its energy concentrated on the distance 60-80 Hz. With a normalisation in wavenumber, we notice that its bandwidth is much wider (20-220 Hz). After having applied the velocity correction, we can notice that the energy associated to the guided wave is not centred on the null wave number in f-k plan. It indicates that the guided wave is dispersive. We will see in the next lesson, that a more precise group velocity correction, and a phase shift, will allow compensating the wave dispersion, and determining its dispersion law.