1. A method of determining time-dependent changes in sub-surface density of a natural resource reservoir, comprising the steps of: obtaining a set of time-lapse gravity gradient data for a sub-surface natural resource deposit characterized by a change in a density characteristic over time; creating a model of the change in density of the reservoir having a plurality of volume elements therein, including constraints on the model; establishing a set of quantized mathematically related parameters defining the density model and computing at least gravity gradients for that quantized model and computing a corresponding figure of merit therefor; perturbing at least one parameter of the model and recalculating the figure of merit for the perturbed model;evaluating the figure of merit for the perturbed model relative to that of the immediately preceding model and accepting the perturbed model if more optimal relative to the immediately preceding model and accepting the perturbed model if less optimal in accordance with a probability function that varies in accordance with a control parameter; and repeating the perturbing and evaluation steps while the control parameter decreases the probability function with successive repetitions.