1. A computer-implemented method for updating simulation models of a subterranean reservoir, the method comprising: providing an ensemble of reservoir models representing a subterranean reservoir; providing reservoir data from the subterranean reservoir; and updating, using a computer processor, the ensemble of reservoir models using the reservoir data and a subspace ensemble Kalman filter that constrains each ensemble member of the ensemble of reservoir models to a different subspace of a full space, wherein an equation is used to update the ensemble of reservoir models, wherein the equation is: m j u = g j ( ξ j f + ⅆ g j ⁡ ( ξ ) ⅆ ξ j f ⁢ K g ⁡ ( d o , j - d j ) ⁢ ❘ 1 : N m ) ∀ j = 1 , … ⁢ , M wherein M refers to number of ensemble members of the ensemble of reservoir models; wherein m refers to a random field; wherein g is a parameterization function relating a small number of random variables ξ to the random field m; wherein K g refers to a Kalman Gain; wherein d o refers to the reservoir data with perturbation; and wherein d refers to simulated reservoir data.