  The detection of Earth-skimming tau neutrinos has turned into a very
promising strategy for the observation of UHE cosmic neutrinos. The sensitivity
of this channel crucially depends on the parameters of the propagation of the
tau neutrino (and the tau lepton) through the terrestrial crust, which governs
the flux of emerging tau leptons that can be detected. This propagation problem
is usually treated in a simplified framework where several effects are
neglected, e.g. the possibility of multiple regenerations of the tau neutrino,
the weak interactions of the tau lepton, as well as the stochastic nature of
its energy losses. We discuss here the validity of such approximations by
studying the propagation in standard rock of tau leptons and neutrinos with
both mono-energetic and power-law spectra. We also investigate the impact of
such simplifications in non-standard scenarios for the neutrino-nucleon
interactions as well as for the tau energy losses.
