Slowly-attenuating P-SV leaky waves in a layered elastic halfspace. Effects on the coherences of diffuse wavefields

In the context of the classical real-frequency and complex-wavenumber (k) integral representation of elastodynamic Green's function for a plane layered elastic medium, leaky modes are defined as those waves associated with complex-k poles in the P-SV integrands representing continuations of the higher Rayleigh modes below their cutoff frequencies. Nevertheless, for some types of models, the path of the leaky-mode poles through the complex plane (for varying frequency) can intercept the real k axis at particular frequencies, canceling the complex character which was conferring to these wave the exponential decay as the horizontal distance increases. Starting from the Haskell-Harkrider formulation, the characteristics of these slowly-attenuating leaky waves and their excitation by surface forces are investigated. The conditions for existence and their frequency are evaluated for the particular case of an elastic layer over a halfspace. Some numerical simulations point to the detectability of these waves around the fundamental resonance of vertical S waves fS0 in standard frequency-domain observables defined for random elastic wavefields. These results provide new insights into the behavior of coherences of ambient seismic vibrations when a high velocity contrast exists.