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Seismic amplitude alone is therefore a fairly good indicator that something in the subsurface is anomalous (porosity, hydrocarbon, etc), but is an ambiguous indicator of hydrocarbons. Almost due east of the A1 well are two deviated wells also containing gas but this is not evident from the seismic amplitudes. Notice that all three of these wells correlate with amplitude anomaly trends (shown by the brown colour on the map), but only two of the wells are producing gas wells. Well A5 is the downdip delineation well, and encountered no gas sand and 140 ft (33 m) of porous water sand at the reservoir level. Well A6 is the updip delineation well, and encountered 80 ft (25 m) of gas sand and over 60 ft (20 m) of water sand at the reservoir level. Well A1 is the initial discovery well, and encountered 150 ft (40 m) of gas at the reservoir level. Three wells are indicated on the map in Figure 1: A1, A5, and A6.
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An amplitude map over the Marlin field in the Gulf of Mexico, where well A1 is the original discovery, well A6 is the updip delineation well, and well A5 is the downdip delineation well. For example, Figure 1 shows an extracted seismic amplitude map from a recent 3D survey over the Marlin field from the Gulf of Mexico.
#HAMPSON RUSSELL INVERSION FULL#
With the advent of 3D seismic recording in the 1980s, we could map the seismic amplitude change over the full extent of a prospect. This information could be correlated with porosity changes, lithology changes, or even fluid changes within the subsurface of the earth. However, by the 1970s, geophysicists had begun to realize that information was contained in the amplitudes of the seismic reflections themselves. This was done by simply identifying the continuity of the reflections seen on the seismic sections. 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.
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This article presents both a history of seismic inversion and an overview of the techniques themselves, illustrated by a case study from the Gulf of Mexico. We have now progressed to the point where inversion for P-impedance, S-impedance and density is feasible. The reason for this is that the P and S-wave response of the subsurface is sufficiently different to allow us to see the difference between fluid and lithology effects. 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 P-impedance (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 almost forty years.