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The Dip and Coherence process estimate the surface normal G=[Gx,Gy,Gt] at each sample in the seismic from an image a statistical analysis of the samples in a window around the sample in question.

If there happen to be a minima or maxima at the sample; this surface normal will point in the same direction as the normal to the interpreted horizon going trough the sample. If not one can still imagine an implicit surface; where the seismic amplitude is constant.

From the surface normal: the following are calculated:

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CrlDip=Gy/Gt 

The InlDip in a point (balck black circle) is the ratio of the inline, x, component of the surface normal to the t component of the surface normal, G (black vector) and therefore a measure of how much the implicit surface (green line) is dipping relative to the horizontal (blue line).  

 

 

 

Estimate surface normal ├ G┴⇀=[Gx,Gy,Gt ] directly from seismic for every seismic sample.

The Coherence attribute is a byproduct of the dip telling us how reliable the InlDip and CrlDip estimates are: Since the dip is calculated over a window the result is some form of average over the window and as a byproduct we therefore get Coherence which is a measure of how much the samples in the window are in accordance with this average, or how reliable the dip estimates are: 

0 <= Coherence <=1, where 1: absolute certain.

But it Coherence can also be viewed as a measure of "order" in the seismic. A non chaotic region of the seismic will have a high coherence, whereas a chaotic region, e.g. inside a salt, will have a low coherence. 

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