- An isopach is a line that connects points of equal true thickness (i.e. measured perpendicular to bedding), whereas...
- an isochore is a line that connects points of equal vertical thickness.
|Fig. 1.- A constant thickness layer is drilled at different angles by three wells|
If we would do an isopach map of the yellow unit, it would show a constant value of 500 m, because this unit doesn't show any thickness (true) variation... It would be a pretty boring map :-). But it would be telling us quite a lot already; for starting, we could think we are dealing with a parallel fold formed by flexural flow (for example).
|Fig. 2-. Isochore map for the yellow unit in figure 1.|
We can see that the difference between an isopach map and an isochore map is quite obvious and simple to understand. Unfortunately, some people interchange both terms, and too many times we can see isochore maps refered to as "isopach maps"
If we would get the map from figure 2, and somebody would tell us that this is a isopach map over and anticline, we could wrongly conclude that the real thickness around the hinge line of the fold is smaller than in the flanks, and we would probably think that we have in front of us as growth anticline, for example.
Another error may come from working with seismic surveys and not realising that if you take a surface representing a top unit, and you substract another surface representing a bottom unit, the result is a isochore map, not a isopach map.
Hence... how many structures have been wrongly studied, how many wells haven't reached a target because somebody used wrong a simple word? Better not even to know it! And this is considering that everytime we read a vertical thickness in a well log the well was actually vertical and not deviated at all. If you take a surface representing a top unit, and you substract another surface representing a bottom unit, the result is a isochore map, not a isopach map.
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