In a soft sand, the presence of gas will lower the impedance significantly, making a negative amplitude even more negative and therefore brighter (Figure 1). If we have an extensive reservoir sand, perhaps a sheet flood, beneath a shale, then the amplitude might vary laterally due to porosity variation or changes in reservoir quality by as much as 10–20% and these amplitude changes may be interpretable in terms of property changes. The amplitude response could be positive, negative or negligible depending on the relative hardness of the sands to the encasing shales. The rock property contrast between a sand and shale varies with factors such as depth of burial, compaction, porosity and lithological composition.
Note that seismic cannot tell us about the absolute change, only about relative changes. If the data are converted to relative impedance then seismic data observe the relative change of the rock property impedance (= hardness) as we move from one layer to the next. In a seismic survey, seismic amplitude is a measure of the contrast in properties between two layers. A Simple Guide to Seismic Amplitudes and Detuning Seismic Amplitudesĭata processing such as detuning reveals the true sweetspots in your data.
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