Investigating methods to transform acoustic impedance inversions into depth

Heather J. E. Lloyd and Roy Lindseth and Gary F. Margrave

ABSTRACT

The prime use of seismic reflection data is to map subsurface time structure. Attribute analysis seeks to extract or refine information about the nature of the subsurface. Seismic data can be used for attribute analysis but by converting it into impedance the inherent properties of rock layers can be analyzed. Seismic data, when converted into depth, becomes easier to interpret and easier for geologists and engineers to work with. Depth conversions are usually completed using sonic logs or check shots but impedance sections can also be used.

Impedance is the product of velocity and density so if density can be approximated then velocity remains. In 1974, Gardner et al. found a relationship between density and velocity which can be used to estimate the density from impedance using the original stationary parameters. Densities can also be provided from nearby wells and by creating non-stationary Gardner parameters. Once the velocity has been separated from the density it can be used to transform the section into depth. Formation tops from the Hussar seismic data were converted to depth and compared to the formation tops from nearby wells. This allowed each method to be evaluated for accuracy and precision. The standard Gardner method had a mean error of 131 ± 66 m averaged over all wells. The log density method had a mean error of a mean error of 115 ± 48 m. The time variant Gardner method had similar results of 86 ± 39 averaged over all wells.

Even though the time variant Gardner’s rule produced a depth conversion where the tops varied less with depth the top picks were different from the well picks ranging 50 to 120 meters. Further investigation must be done to limit these errors which could be reduced by better inversions in the overburden and better density estimation methods.

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