  We study the feasibility of detecting preshower initiated by ultra-high
energy photons using Monte-Carlo simulations of nearly horizontal air showers
for the example of the La Palma site of the Cherenkov Telescope Array. We
investigate the efficiency of multivariate analysis in correctly identifying
preshower events initiated by 40 EeV photons and cosmic-ray dominated
background simulated in the energy range 10 TeV -- 10 EeV. The effective areas
for such kind of events are also investigated and event rate predictions
related to different ultra-high energy photons production models are presented.
While the expected number of preshowers from diffuse emission of UHE photon for
30 hours of observation is estimated around $3.3\times10^{-5}$ based on the
upper limits put by the Pierre Auger Observatory, this value is at the level of
$2.7\times10^{-4}$ ($5.7\times 10^{-5}$) when considering the upper limits of
the Pierre Auger Observatory (Telescope Array) on UHE photon point sources.
However, UHE photon emission may undergo possible "boosting" due to gamma-ray
burst, increasing the expected number of preshower events up to 0.17 and
yielding a minimum required flux of $\sim 0.2$ $\mathrm{km^{-2}yr^{-1}}$ to
obtain one preshower event, which is about a factor 10 higher than upper limits
put by the Pierre Auger Observatory and Telescope Array (0.034 and 0.019
$\mathrm{km^{-2}yr^{-1}}$, respectively).
