Vertical profile of turbidity and coda Q

SUMMARY The well‐known decrease of scattering capability of the Earth medium with increasing depth is parametrized as a power‐law decay of turbidity, or scattering coefficient. The single isotropic scattering approximation for body waves including scattering loss is derived for this profile. An asymptotic case is analysed where the upper constant‐turbidity layer is thin, i.e. coda lapse time is much larger than the traveltime across this layer. In this important case, power‐law coda shapes are obtained. Predictions of this model are tested against relatively abundant data on coda Q and its frequency dependence, and also against some coda shapes. For filtered or spectrally analysed data, agreement is reasonable, explaining in particular the lapse‐time dependence of coda Q , and, to a lesser extent, the exponent of the Q versus frequency power law. The data can be fit by turbidity decay as the inverse square or cube of depth and intrinsic Q of the order of 2000.