DETERMINING A PARAMETER IN A PHYSICAL SYSTEM

1. A method of determining a parameter of a physical model representing the electrical behavior of an earth formation the method comprising: interpreting logging measurements of a plurality of the electrical properties of the earth formation, including a first electrical property and a plurality of physical variables of the earth formation; defining said model by a relationship between at least one electrical property of the earth formation, a plurality of physical variables of the earth formation and said parameter; selecting an incoherence function defining a difference between the measured electrical properties and the electrical properties as calculated from said relationship, said incoherence function being such that measurements determining a weighting factor reflective of the accuracy of the measurements; determining said parameter by minimization of said incoherence function; characterized in that measuring a second electrical property of the core sample, typical for the earth formation, for various magnitudes of at least one of said physical variables, wherein the incoherence function includes the first and the second electrical property of the earth formation and the core sample, typical for the earth formation. 2. The method of claim 1 wherein the step of determining said parameter by minimization of said incoherence function includes an iterative process. 3. The method of claim 1 or 2, wherein a plurality of said parameters are determined simultaneously by minimizing the incoherence function, each parameter being represented in said relationship. 4. The method of any claims 1 to 3, wherein said incoherence function is selected such that independent measurements of substantially equal accuracy are substantially equally represented therein. 5. The method of any claims 1 to 4, wherein the electrical properties of the earth formation as defined in said relationship and the measured electrical property of the sample are selected from the conductivity, the resistivity and the membrane potential. 6. The method of any claims 1 to 5, wherein the measured electrical property of the composition is selected from the conductivity and the resistivity. 7. The method of any claims 1 to 6, wherein said composition includes a fluid and the model forms a saturation model for the fluid in the composition. 8. The method of claim 7, wherein said composition comprises an earth formation, and the model forms a saturation model for a selected one of brine and hydrocarb