  The cosmic ray spectrum extends to energies above 10^20 eV. In direct
production or acceleration models, as well as by photo-pion interaction high
energy cosmic ray flux must contain neutrinos and photons. The latter are
absorbed by cosmic radiations while neutrinos are not. The need of a Neutrino
Astronomy is compelling. In this paper a study of a detector array designed to
measure horizontal tau air-showers emerging from the ground, produced by nu_tau
interactions with the Earth's crust, is presented. Each array unit is composed
of a pair of scintillator tiles mounted on a frame with a front field of view
of about 0.1 sr, optimized to distinguish between up-going and down-going
crossing particles by their time of flight. The detector array sensitivity, the
size of the array and the tau shower identification are discussed. Because of
the almost complete mixing of nu_mu to nu_tau the ultrahigh energy neutrino tau
and its minimal consequent tau-airshower rate is estimated; assuming that the
neutrino energy spectrum follows a Fermi-like power law E^-2, the sensitivity
with 3 years of observation is estimated to be about 60 eV cm^-2s^-1sr^-1 in
the energy range 10^{17-20} eV. This value would provide competitive upper
limit with present and future experiments. We found also that, in the same
time, this system can observe about one GZK neutrino event per km^2.
