Crossplots are commonly used in the geosciences to gain qualitative insight about relationships between different variables, typically three (for two-dimensional colored crossplots). On rare occasions, the relationships among four variables are explored by using three-dimensional colored crossplots. The variable used to color the crossplot is usually related to the property of interest, sand or pay for instance. In these cases, crossplots can be used in a quantitative sense by selecting (drawing) a region in the crossplot where most of the property of interest “lives.” Drawing a polygon in a 2D crossplot to separate “good” from “bad” areas is the extension to 2D of simple cutoffs commonly applied to 1D well-log data to separate scenarios of interest. One drawback of this approach is that it works best only when there is no overlap between the region occupied by the property of interest and the region occupied by the background. Another drawback is that it is difficult to extend to three-dimensional crossplots and impossible to apply for dimensions higher than three.

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