By the 1970s, geophysicists had started to realize that information was contained in the amplitudes of the seismic reflections themselves. But this structural interpretation is ambiguous when it comes to identifying gas sands. The inserted curve is the P -wave sonic log. The high on the picked structure corresponds to the gas sand.

Such a structure can be seen in Figure 2, which shows a seismic line recorded over a gas-charged sand. The seismic reflection method was developed in the first quarter of the twentieth century and was used initially as a tool for identifying structures, such as anticlines, which could act as trapping mechanisms for hydrocarbon reservoirs. The effect of gas saturation on P and S-wave velocity. This talk will present both a history of seismic inversion and an overview of the inversion techniques themselves. In this figure, it can be noted that the P-wave velocity drops dramatically when gas is introduced into the reservoir whereas the S-wave velocity is largely unaffected by the introduction of the gas. The reason for this can be seen in Figure 1, which plots the P and S-wave velocities as a function of gas saturation.

To perform a less ambiguous interpretation of our inversion results, we must perform full elastic inversion, in which we estimate P-impedance, S-impedance (the product of density and S-wave velocity) and density. However, these predictions were somewhat ambiguous since P-impedance is sensitive to lithology, fluid and porosity effects, and it is difficult to separate the influence of each effect. Early inversion techniques transformed the seismic data into Pimpedance (the product of density and P-wave velocity), from which we were able to make predictions about lithology and porosity. Seismic inversion is a technique that has been in use by geophysicists for over forty years.

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