Thickness maps are a fundamental tool in structural geology. They represent thickness variations and thickness trends of a given unit.
Isochore and
isopach maps are two different types of thickness map. To make it clear from the beginning:
- 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.
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| Fig. 1.- A constant thickness layer is drilled at different angles by three wells |
Figure 1 depicts a perfect
cilindrical fold, where the western flank dips more than the eastern flank.
Well 1 cuts the yellow bed at a higher angle than
well 3, and therefore, the vertical thickness found in well 1 is larger than in
well 3.
Well 2 cuts the bed where is horizontal, and therefore the vertical thickness equals the real thickness. The
real thickness of the yellow bed is constant: 500 m.
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).
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| Fig. 2-. Isochore map for the yellow unit in figure 1. |
Figure 2 shows the
isochore map -contours for equal vertical thickness- for the yellow bed depicted in
figure 1. Red colour indicates minimum vertical thickness, whilst purple shows maximum value. You can see that it shows what you can see in the cross section: we would measure increasing vertical thickness as we move away from the hinge of the fold, as the dip of the bed increases and therefore we would cut the yellow bed at a higher angles.
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.
If you have any question, just ask in the comments. If you have any suggestion for a new article, just let me know. You are also welcomed to write some contribution!
The Geological Society of London organises this week the Industrial Structural Geology: Principles, Techniques and Integration conference, from the 28th to the 30th of November.
, which he coathors together with John Allen. This must-have book that any student has used since it appeared some 20 years ago is a solid and strong summary of what we know in terms of basin evolution. I think I consult it twice a week, and we have at home two different editions (first and second editions... I am not aware of a third edition!)