Normal modes in orthorhombic media

Guided waves in a water layer overlaying an elastic half-space are known as normal modes. They are often present in seismic recordings at long offsets in shallow-water environment and generally considered coherent noise. The normal modes, however, carry important information about the near-surface and, as demonstrated by a number of authors, can be used to obtain the shallow velocity model. There is a growing evidence that the latter needs not to be isotropic due to various geological reasons. Motivated by that, we consider the normal-mode propagation in case the elastic half-space exhibits orthorhombic anisotropy. We derive the period equation that describes the normal-mode phase velocity dispersion. To simplify the complicated expression, we present acoustic and ellipsoidal orthorhombic approximations. We also outline the approach towards the group velocity and group azimuth calculation and apply it to the ellipsoidal case to obtain concise and intuitive expressions. Using numerical test, we study the relation between phase and group domains in elastic orthorhombic case. The deviation between velocities and azimuths in these domains is the strongest for low frequencies and it rapidly decreases with increasing frequency. For higher frequencies, the anisotropy effects of the underlaying half-space are barely detectable since the observed signal is composed mainly of the direct acoustic wave, resulting in the two domains being nearly indistinguishable.