Fluid substitution

A method of fluid substitution comprises providing an initial data set, a substituted data set and a rock physics model is provided. The initial data set comprises initial data of a geophysical parameter e.g. p-wave velocity and initial fluid data e.g. saturation and bulk modulus and/or density of an initial fluid obtained from well logs. The substituted data set comprises data for a fluid that a user would like to substitute into the rock. The model and the initial data set are used to calculate first calculated data of the geophysical parameter. The model and the substituted data set are then used to calculate second calculated data of the geophysical parameter. The difference between the first and second calculated data of the geophysical parameter is calculated, and said difference is applied to the initial data of the geophysical parameter to produce substituted data of the geophysical parameter. The substituted data of the geophysical parameter can be used to prospect for hydrocarbons,