Coherence Variance Attributes
Figure 1: shows the comparison of original seismic section, variance with plain default before structural smoothing is applied and variance with plain default after structural smoothing is applied for inline 732.
In Figure 2, the visibility of the geological features such as the fault and the bright spot differ between original seismic data, variance before and after structural smoothing. Although the fault can already be seen in the original seismic data, after the variance attribute is applied, the structure of the fault is becoming more obvious and can be easily interpreted. The edge is sharpened and can be easily detected rather than in the original seismic data. In addition, the continuity of the seismic line is better enhanced after the variance attribute is applied on the original seismic data. On the other hand, the structural smoothing that is being applied on the variance attribute has removed the unwanted noises. However, as the structural smoothing removed the anomalies, it somehow reduces the valuable data as well. Even so there are some parts of the seismic section is quite chaotic before structural smoothing is done, it still gives better display of the structural features compared to after the structural smoothing is being applied. From our variance seismic sections, the resolution is somehow better before structural smoothing is done. Some of the seismic lines are no longer seen as continuous after the structural smoothing is applied. Overall, the geological features that can be seen in variance seismic section is similar as the ones that can be seen in the original seismic section even though they differ in opacity and visibility. In fact, the presence of bright spot can act as Direct Hydrocarbon Indicator (DHI).
Figure 2: shows the comparison between the cross seismic section at Inline 732 for variance attribute with and without structural smoothing.
The variance attribute is calculated in three dimensions and represent the trace to trace variability over a particular sample interval and therefore produces interpretable lateral changes in acoustic impedance. Similar traces (continuity) are represented by low variance coefficients whereas the discontinuity have high variance coefficients. As channel may cause discontinuities in the neighboring lithologies and subsequently in the trace to trace variability, they somehow become detectable in the three dimensional seismic volume. In our seismic volume, a channel can be observed in the original seismic section as well as the before smoothing variance seismic section. The channel can be seen at timeslice value range from -438 to -368. The figure shows the channel displayed at timeslice -398. At deeper depth, where the timeslice value is greater, width of the channel is smaller. As it goes up to the higher range, the width is increasing gradually before it completely disappeared at the timeslice of -348. Within this timeslice range where the channel is observed, according to the legend, it shows that the channel is in the state of medium to high positive amplitude. High amplitude means that the channel is filled with sand.
Figure 3: shows a channel that can be observed in both original seismic section and variance seismic section.