OpendTect User Documentation version 4.6 | ||
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In OpendTect, we favor a different approach. We first try to find the direction of best match at every position, which is a result by itself: the dip. By using this dip we can then calculate the best Similarity between adjacent traces. Similarity is based on fundamental mathematics: the samples of the trace are seen as components of a vector, and the Similarity is defined in terms of distance in hyperspace.

The point about using the Similarity is that it's mathematically simple; it is very clear what is going on. Then, by combining different kinds of similarities and other attributes, you can always get much better results with lots less computing time.

Consider the trace segments to be vectors in hyperspace. Similarity is then defined as one minus the Euclidean distance between the vectors, normalized over the vector lengths.

The trace segments is defined by the

- None: Only the similarity between the pair of traces defined in 'Trace positions' is calculated.
- Mirror 90 degrees: Two similarities are computed: one for the defined trace pair (as in 'None') and one for the pair obtained by 90 degree rotation. (Not available for 2D data)
- Mirror 180 degrees: Two similarities are computed: one for the defined trace pair (as in 'None') and one for the pair obtained by 180 degree rotation.
- Full Block: Similarities between all possible trace pairs in the rectangle defined by the step-out are computed.
- Cross: Similarities between all possible trace pairs in the '+' -shape defined by the step-out are computed.
- Diagonal: Similarities between all possible trace pairs in the 'x' -shape defined by the step-out are computed.
- All Directions: Similarities between all possible trace pairs in the '' -shape defined by step-out are computed. This is the extension found to be most useful: it gives a degree of accuracy almost equal to that of 'Full Block' but with significantly less processing time (depending on the step-out, up to a factor of 10).

Similarity "None" Steering: This option is used when non steeringcube algorithm is used. This is ok in the case the layering is mainly horizontal (with less dip).

However, in very complex geology, the similarity result using "None" as steering option will deteriorate. Full steering should be used instead. The dip steering plugin is required.

Another steering option to use is the "Browse dip". This is a similarity feature acting as a 'Coherency' attribute.

It enables the calculation of 'Similarity' by comparing one trace with the next trace.

Then a value between 0 (not similar at all) to 1 (completely similar) is awarded. In order to compare traces, two variables should be specified:

The 'Maximum dip' represents the maximum dip in microseconds per metre (μs/m), relative to an event in one trace, in which the algorithm will look for similar events along the neighbouring trace. Default is 250.

The "Delta Dip" is a variable which represents the window in microseconds per metre (μs/m) which is shifted along the neighbouring trace to detect similar events whithin the earlier specified 'Maximum Dip'. The closer the value to 1 the more precise the results will be. The default value is 10. Using this value will result in a good balance between calculation time and quality of the results, this also depends on the quality of the data itself.

Xi, i=1,15

Yi, i=1,15

The similarity is 1 minus the Euclidean distance between the vectors divided by the sum of the length of each vector. Please note that the length of a vector is its L2 norm, also called RMS value:

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