  Geoneutrino is a new channel of information about geochemical composition of
the Earth. We alnalysed here the following problem. What statistics do we need
to distinguish between predictions of Bulk Silicate Earth model and Hydridic
Earth model for Th/U signal ratio? We obtained the simple formula for
estimation of error of Th/U signal ratio. Our calculations show that we need
more than $22 kt \cdot year$ exposition for Gran-Sasso underground laboratory
and Sudbury Neutrino Observatory. We need more than $27 kt \cdot year$
exposition for Kamioka site in the case of stopping of all Japanese nuclear
power plants.
