1. A method for modeling the spatial distribution of a continuous primary petrophysical variable over a regular array of a large number of pixels representative of secondary observation stations, comprising: a) assembling in a programmed computer including a data processor, a first set of measurements of a primary variable at a small number of control pixels and a second set of measurements of an associated secondary variable at all pixels of said array; b) estimating the mean and variance of the Gaussian prior distribution of the primary variable from the first measurement set; c) estimating the joint probability distribution of the primary and secondary variables from a scatter diagram of the first and second measurement sets at the control pixels; d) selecting an as-yet unsimulated pixel at random; e) estimating the Gaussian conditional probability distribution of the primary variable at said selected pixel by kriging the primary data at control pixels within a selected search region encompassing said selected pixel; f) in the data processor, calculating the probability ratio between said conditional probability distribution and the prior distribution at said selected pixel; g) extracting a one-dimensional slice through the joint probability distribution corresponding to a measured value of the secondary data at said selected pixel; h) calculating, in the data processor, a posterior probability distribution for the primary variable at said selected pixel by taking the product of the probability ratio and the extracted one-dimensional slice; i) drawing a simulated value for said primary variable at said selected pixel by sampling at random from said posterior probability distribution; j) entering the simulated value of said primary variable into said array at said selected pixel as an additional control pixel; k) repeating steps d) through j) until the primary variable is simulated for all pixels of the array; l) with the aid of a computer graphics program, mapping the distribution of the simulated primary petrophysical variable over the array of pixels.