A gravity or magnetic structural model is a 2D or 2½D density and/or susceptibility model of given or assumed geology. The geology of an area can be modeled by representing lithologic layers as equi-density and/or equi-susceptibility layers and/or blocks. The layers are formed by contrast boundaries which may or may not correspond to specific geologic formation boundaries. Where high density or susceptibility contrasts exist in nature, the model may correspond closely to those geologic formations.

For 2-dimensional modeling, the density and susceptibility models of the geology and the observed gravity and magnetic anomalies for the model are assumed to be semi-infinite. For 2½-dimensional modeling, the third dimension y (in and out of the plane of the profile) is approximated by one or more given distances, thus providing a quasi-3D model.