11.24 Semblance
Name
Semblance -- Attribute that returns a value (semblance coefficient) which is a measure of multichannel coherence.
Description
Semblance is, essentially, a measure of how similar a particular trace is to a group of traces within a user-defined correlation window. This attribute will calculate the semblance coefficient and return a value between 0 (completely dissimilar) and 1 (identical). This is done by summing the squares of the energies within the stack and dividing by the sum of those energies.
Whereas the Similarity attribute works by comparing pairs in given positions, Semblance uses all points within the window to create the score.
Input Parameters
The size of the correlation window is defined by the Time Gate and Stepout, with the 'Extension' defining its shape.
Steering may be used to enhance the results, if the Dip Steering plugin is installed (with a valid license.)
Time-Gate:
Defines the size of the vertical element in the correlation window (ie: the trace length to be used in the calculation).
Stepout:
Applies for 'Extensions' of shape Full block, Cross, All Directions and Diagonal. Defines the extent of the horizontal element in the correlation window (see 'Extension' below for more detail).
Applies for 'Extensions' of shape None, Mirror 90 degrees and Mirror 180 degrees. Defines the positions of the traces to be used in the correlation window (see 'Extension' below for more detail).
Extension shapes:
- None: Semblance is calculated using only the traces defined in 'Trace positions'.
- Mirror 90 degrees: Semblance is calculated using two additional traces: the defined trace pair (as in 'None') and the trace obtained by 90° rotation. (Not available for 2D data)
- Mirror 180 degrees: Semblance is calculated using two additional traces: the defined trace pair (as in 'None') and the trace obtained by 180° degree rotation.
- Full Block: Semblance is calculated using all possible traces in the column defined by the time-gate/step-out. Beware of potentially long processing times with large time-gates/stepouts.
- Cross: Semblance is calculated using all possible traces in the '+' -shaped column bounded by the time-gate/step-out.
- All Directions: Semblance is calculated using all possible traces in the '✴' -shape bounded by time-gate/step-out. 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).
- Diagonal: Semblance is calculated using all possible traces in the 'x' -shaped column bounded by the time-gate/step-out.
References
For a very concise explanation of semblance, please see: Dictionary: Semblance