Semblance is a measurement of waveform similarity used to highlight discontinuity between traces. This process helps to reveal abrupt structural changes such as faults, and stratigraphic features such as channels. At each sample, a semblance calculation is performed over the analysis window to determine the similarity of an individual trace to surrounding traces.
The resulting volumes are best reviewed as time slices in the Map View.
Semblance is calculated by taking the ratio of the energy of a trace to the energy of neighbouring traces within an analysis window. Semblance values near or equal to zero indicate high similarity of a sample trace to its surrounding traces while values closer to one indicate high dissimilarity.
Semblance values change with variations in amplitude and waveform. Compare this with correlation and incoherence, which change with waveform, but are insensitive to amplitude. This amplitude sensitivity makes semblance useful for displaying subtle depositional features, as well as creating cleaner images in noisy data.
Note: Reverse output selected in image above. Values closer to one indicate higher dissimilarity, while values closer to zero indicate higher similarity.
- In the Control Panel, open the Process tab.
- Click the Add icon, select New Process, and double-click Semblance (Similarity).
- Type a name for the process and click OK.
- Select the seismic volume for which you want to calculate semblance.
- At Maximum Dip (ms/m) or (m/m), enter the maximum dip of reflection events that Insight needs to allow. For example, an event with a dip of 4ms/m or greater will not be recognised as a "similar" event with a Maximum Dip parameter of 3ms/m. In this case, the event will appear as having high dissimilarity along its length.
Note: A greater maximum dip angle will incur more processing time to generate the output volume.
- The Trace Half-Window (ms or m) defines the vertical calculation window. In general, the half-window length should be comparable to the feature of interest, but large enough to ensure it encompasses the feature. If the window is too small, the result will become noisy. If the window is too large, the result will have insufficient resolution. Any changes to this parameter is immediately reflected in the views.
Note: Smaller windows display faster in map view, but take longer in section view, as more iterations are made.
- Checking Reverse output will alter display so that high similarity values will be closer to zero, while low similarity values will be closer to one.
- As a result of this process, a new volume is available in the Volume tab.
- It may be useful to display the "Semblance" volume with the seismic volume it represents. To do this, you can group the "Semblance" volume and the seismic volume to display them collectively (see Creating a Group).
Tip: To quickly open the output volume, right-click the process item in the Process tab and click Open the output volume. Vice versa, you can also open the process from the Volume tab directly. To do this, right-click the volume which was created from a process (indicated in italics) and click Open the Process.