1. A concept from which one can deduce effects for comparison to observations; used to develop a better understanding of the observations. The ‘‘model’’ may be conceptual, physical, or mathematical; see Figure M-14. Models are essential in any interpretation or inversion. For example, one might calculate gravity, magnetic, or seismic effects for an assumed geological structure and compare these with actual data. Agreement between observations and effects derived from the model does not ‘‘prove’’ that the model represents the actual situation; geophysical interpretation problems almost always lack uniqueness. 2. Some simple mathematical models are shown in Figure M-15. The effects of more complicated models may be collected into a catalog of master curves or type curves for use in comparison with observed effects. See synthetic seismogram, numerical modeling, physical modeling, and also Fagan (1991).