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→Applications of dipmeters
==Applications of dipmeters==
==Applications of dipmeters==
<gallery mode=packed heights=300px widths=300px>
file:dipmeters_fig5.png|{{figure number|5}}Simple dip model for the description of a normal fault with drag.
file:dipmeters_fig6.png|{{figure number|6}}Model of a tilted plunging anticline as it would appear on an arrow plot.
file:dipmeters_fig7.png|{{figure number|7}}Field example of a detailed dip computation through a sequence of interrupted meandering stream point bars.
</gallery>
===Structural applications===
===Structural applications===
Superficially, the determination of structural dip from a dipmeter seems simple and straightforward. In practice, it may be tricky. Difficulties arise from questions of scale and perspective. ''Structural dip'' by definition is the dip of recognizable lithological unit boundaries in the general vicinity of the borehole. However, a sharp change in lithology, such as from a shale to a sandstone, is the signature of a catastrophic event in geological history. Therefore, such a contact is liable to be highly irregular over the extremely short section exposed by the borehole. On an outcrop, a field geologist would astutely measure the dip of an eyeball average of the contact. The stringent confines of the borehole offer no such luxury of perspective, and the section of the contact exposed is liable to be highly unrepresentative of the average structural dip. Outcrop perspectives cannot be extrapolated to the borehole, so a different approach is needed.
Superficially, the determination of structural dip from a dipmeter seems simple and straightforward. In practice, it may be tricky. Difficulties arise from questions of scale and perspective. ''Structural dip'' by definition is the dip of recognizable lithological unit boundaries in the general vicinity of the borehole. However, a sharp change in lithology, such as from a shale to a sandstone, is the signature of a catastrophic event in geological history. Therefore, such a contact is liable to be highly irregular over the extremely short section exposed by the borehole. On an outcrop, a field geologist would astutely measure the dip of an eyeball average of the contact. The stringent confines of the borehole offer no such luxury of perspective, and the section of the contact exposed is liable to be highly unrepresentative of the average structural dip. Outcrop perspectives cannot be extrapolated to the borehole, so a different approach is needed.
Given computation approaches tailored for structural applications, structural dip can then be defined by looking for a consistent trend on the arrow plot. The most repetitive dip should be the structural dip.
Given computation approaches tailored for structural applications, structural dip can then be defined by looking for a consistent trend on the arrow plot. The most repetitive dip should be the structural dip.
The interpretation of structural anomalies is best accomplished by comparison to a set of models—the simpler the model, the better. A simple model, such as the one shown in [[:file:dipmeters_fig5.png|Figure 5]] for a normal fault with drag, is adequate to describe the geometry of such a fault. [[:file:dipmeters_fig6.png|Figure 6]] shows a more complicated arrow plot of low angle dips, reducing to a minimum then increasing to a high angle with the azimuth changing continuously with the dip angle. This is the signature of a tilted plunging anticline. A cross-sectional sketch of the anticline can be produced using the rule of interchangability of perspectives in which the horizontal geometry is interpretable from the vertical pattern of dip. (The application of dipmeter data to solving structural problems is covered in “[[Evaluating structurally complex reservoirs]]”.)
The interpretation of structural anomalies is best accomplished by comparison to a set of models—the simpler the model, the better. A simple model, such as the one shown in [[:file:dipmeters_fig5.png|Figure 5]] for a normal fault with drag, is adequate to describe the geometry of such a fault. [[:file:dipmeters_fig6.png|Figure 6] shows a more complicated arrow plot of low angle dips, reducing to a minimum then increasing to a high angle with the azimuth changing continuously with the dip angle. This is the signature of a tilted plunging anticline. A cross-sectional sketch of the anticline can be produced using the rule of interchangability of perspectives in which the horizontal geometry is interpretable from the vertical pattern of dip. (The application of dipmeter data to solving structural problems is covered in “[[Evaluating structurally complex reservoirs]]”.)
===Stratigraphic applications===
===Stratigraphic applications===