Method of determining boundary interface changes in a natural resource deposit

A method of identifying the boundary or interface between a driveout fluid and to-be-recovered oil in a sub-surface oil reservoir undergoing secondary oil recovery, comprising the steps of:

• measuring the gravity gradient at a plurality of observation sites associated with the oil reservoir over a period of time to obtain time-displaced gravity gradients associated with each observation site;
• creating a change of density model of the oil 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.