In marine seismic, a wavefield recorded at long offsets is dominates by so-called normal modes — dispersive horizontally propagating waves resulted from multiple reflections and refractions in water layer. There is a potential to use the normal modes for shallow sediments characterization. Seismic anisotropy has a strong effect on properties of the normal modes and in case of azimuthal anisotropy (due to uneven horizontal stresses or fractured sediments), the normal modes characteristics are azimuthally dependent. We derive and numerically analyze an exact period equation (main tool for study of the normal modes) in elastic orthorhombic media and consider acoustic and ellipsoidal orthorhombic approximations.


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