Densities in formations are usually assumed to be constant for gravity model calculations. This constancy implies that formations are homogeneous and isotropic. However, formations are usually heterogeneous, and densities vary depending on heterogeneity. For this reason, densities should be considered variables. Some scientists consider densities as variables in their model calculations of each formation. In other words, density is defined as a variable of the required parameters. In fact, variable change is regular, whereas density is an irregular variable that depends on the change boundaries of seismic velocity. This study aimed to take density into account as a variable by using determined seismic velocity boundaries at which the seismic velocity changes for each formation. The change boundaries of seismic velocity are an indication of the change of density in the formation. It is not possible for the respective description of this change to be variable for each point. In addition to defining the main formations in model geometry by using 2D inversion calculations, this study defined another formation, which is described with a combination of all of the change boundaries of seismic velocity that are present in each formation in a specific order. The reliability of the results of the method depends on the reliability of the seismic velocity boundaries. Moreover, an increasing number of seismic velocity boundaries lead to a higher resolution of density variations.