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Seeing negative values in a 2D isopach

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Why does my 2D isopach map of a single formation show negative values?

The Stratigraphy > Stratigraphic Thickness > 2D (Isopach) tool creates thickness models by creating a grid model of the formation's upper surface, a grid model of the formation's lower surface, subtracting the two, and storing the difference as an isopach grid model. Depending on your data and the algorithm used, it is not impossible for the upper surface to dip below the lower, resulting in a negative thickness.

You may want to double-check your data by using a different gridding algorithm to create the isopach model.

And, you can verify the observed data with some "hand-contouring": First, export an XYZ file of the observed thickness values using the Stratigraphy > Export > ASCII XYZ File tool, choosing the name of the formation in question and specifying Thickness as the data type. This will represent thickness as entered: formation top minus formation base, right from the Stratigraphy data tabs. Copy the exported XYZ (thickness) data to the Geological Utilities datasheet. Then, use the Geological Utilities Map > EZ Map to draw contours of the observed thicknesses, and compare this map to the grid-based map(s).

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