GEOLOGICAL MAPS AND THEIR INTERPRETATION 57 only provide the quantitative information along the specified line, in the case of a cross-section, or, with the trigonometric approach, at the specified point Computer-generated visualisations can be deceptive in their visual attractiveness: as with traditional methods they involve assumptions and subjective interpretations (and simplified algorithms) Much more powerful is a third approach, using structure contours (Figure 7) Exactly analogous to topographic contours of the land surface, structure contours portray quantitatively the form of some specified surface below the ground The surface might be the upper boundary of a formation of particular interest, perhaps an aquifer or a mineralbearing horizon; it could be an unconformity or a fault surface The contour values can represent the depth below ground of a surface or, more usefully where the land is not flat, the elevation of the geological surface, with respect to sea-level or some local datum The numerical difference in altitudes between topographic contours for the land surface and the structure contours gives the depth of the contoured geological surface, at any desired location in the map area Values for locations intermediate between contour lines can be interpolated The numerical difference between contours for the upper and lower boundaries of a formation gives the vertical thickness of the material For dipping formations this value will differ from the true thickness, at right-angles to the bounding surfaces, but the trigonometric corrections are straightforward Contours that directly portray thicknesses are known as isopachytes Figure An example of a structure contour map: the Ekofisk oilfield, North Sea (A) Structure contour map of the top of the oil bearing formation (B) Oblique view of the form of the contoured surface (bottom level of drawing) and structure contours (drawn at an arbitrary level) and the sea bed (top level) Reproduced with modification from van der Bark and Thomas, 1980, American Association of Petroleum Geologists; used by permission