In GOCAD Mallet (1993), a model is a set of domains bounded by interfaces (triangulated surfaces) and in which some properties are defined such as velocities, porosities, permeabilities, and densities. These properties can be constant and attached to a whole domain or variable and computed at each point inside the domain.
Figure 1 shows a 2-D slice of such a model composed of six domains delimited by their boundaries. Each boundary is characterized by an interface and a side (+ or -). On the same page lies an excerpt of an example of a file used in GOCAD to describe the partition of the 3-D space associated with the set of interfaces. For example, the domain named Domain_4 is defined by the boundary S2-, F-, and S3+, and the properties available in this domain are a constant velocity (2700.0 m/s) and a function named density( input1, output1) giving the density of the layer (ouput1) with respect to some parameters (input1). In the GOCAD library, there are functions to retrieve the properties attached to a domain from the location of a point in the 3-D space.