Longer windows tend to mix stratigraphy, which can complicate the geologic image. The analysis window chosen can have a significant effect of the results. Presented by Bednar (1998) and is based on a least-squares fit technique that fits a plane through a seismic data window, and is solved using an iterative technique. This method is sensitive to both waveform and lateral changes of the reflector amplitude. This method uses a larger analysis window, and tends to have lower lateral resolution. Gradient Structure Tensor-based Coherence This method only measures changes in the reflectors waveform (not amplitude). Then this wavelet is scaled to fit each input trace creating the 'coherent component'. Before this ratio can be calculated, the method calculates a wavelet that best represents all the wavelets in the analysis window. The Eigenstructure coherence is the ratio of the energy of the coherent component of the data to the energy of the original traces within an analysis window. Mathematically, it is similar to 1-semblance. Variance indicates how widely the individual points vary. The semblance measures the degree of similarity to each other of all of the traces along the selected dip within the defined 3D window. This method needs a 3D analysis window to be defined, as well as a dip and azimuth for each point in the seismic volume. This method takes into account only the shape of the waveform, and not the amplitude of the waveform. The simplest form, which typically used three neighboring seismic traces. A more detailed discussion of these methods can be found here. There are a variety of coherence methods. These attributes are very similar, with slight variations in the algorithms. A best practice is to always examine the colorbar, as some seismic softwares may show the inverse.Ĭoherence can also be referred to as 'semblance', or 'similarity'. Typically, high amplitudes shown with this attribute represent discontinuities is the data, while lower amplitudes represent continuous features. Structural features, like faults, are best seen on time (or depth) slices.Ĭoherence is also commonly referred to as 'discontinuity' and varies between 0 and 1. Stratigraphic features tend to display best on horizon or time slices, if the dip is not too large. The calculated coherence volumes dramatically enhance the ability of the interpreter to observe stractural and stratigraphic discontinuities. These lateral changes is what the coherency attribute measures. Since, acoustic impedance is affected by the lithology, porosity, density, and fluid type of the subsurface layers then strong lateral changes in impedance contrasts give rise to strong lateral changes in waveform character. That seismic response changes in terms of amplitude, frequency, and phase, depending on the acoustic-impedance contrast and thickness of the layers above and below the reflecting boundary. The seismic waveform is a response of the seismic wavelet convolved with the geology of the subsurface. Ĭoherence is a measure of similarity between waveforms or traces. Note how easily the salt dome and faults are seen in the coherence volume. An early example of coherence showing a (a) seismic time slice, and (b) cross-correlation coherence volume.
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