We have seen in the previous section, that the dominant guided wave is dispersive. We see in this section, that a phase shift after a precise group velocity correction allows compensating the wave dispersion, and determining its dispersion law. Concerning the selected component (hydrophone or geophone X, Y, Z), the bottom 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 which allows to better appraise low and high frequencies

In this study, we keep the recordings, which have been obtained with source-sensor distances from 250 to 900 m. In f-k domain, right figure, energy is localized in f-k domain, according to a curve which can be drawn near to its tangent, at a given frequency. We will successively correct the noise shot from the group velocity, then from the dispersion (phase shift), in order to render, in the dominant frequency band of the guided wave, a curve (f, k) tangent to the vertical line k=0. The group velocity in indicated in m/s. The phase shift is expressed in degrees. The dispersion curve, phase velocity variation depending on the frequency, is shown on the top figure. The values of the group velocity and of the phase shift can be modified with cursors.