1. A method for creating a modified realization of a geostatistical model of a subterranean hydrocarbon reservoir, wherein the modified realization is based on a current realization which is a function of a first uniform random number field, the method comprising the steps of: a) creating at least one further uniform random number field U i ; b) linearly combining the first uniform random number field and said at least one further uniform random number field U i together with combination coefficients r i to derive a modified non-uniform random number field V; c) performing a uniform score transformation procedure on the modified non-uniform number field V to derive a modified uniform random number field U mod ; and d) deriving a modified realization of the model from the uniform random number field U mod .