Constant angle projections of seismic sections can be designed to provide maximum discrimination between fluids or lithologies. The optimum projection for a noise‐free, isotropic environment can be obtained using an extension to the elastic impedance concept, which itself is an extension of acoustic impedance (AI) to nonzero angles of incidence. To achieve this, we modify the definition of elastic impedance (EI) beyond the range of physically meaningful angles by substituting tanχ for sin2 θ in the two‐term reflectivity equation. The primary variable now becomes χ rather than θ. We allow it to vary between −90° and +90°, which gives an extension of EI for any combination of intercept and gradient. We refer to this form of elastic impedance as extended elastic impedance (EEI). In this paper we demonstrate that EEI can be tuned using different χ values to be approximately proportional to a number of elastic parameters, and we give EEI expressions for shear impedance (SI), bulk modulus, shear modulus, Lamé's parameter, and Vp/Vs. This leads to the identification of different areas of EEI space that tend to be optimum for fluid and lithology imaging. Having identified an appropriate χ value, the equivalent seismic section can be obtained from combinations of intercept and gradient stacks from routine AVO processing.

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Constant angle seismic sections can be designed to provide maximum discrimination between either fluids or lithologies. The optimum projection, for a noise free, isotropic environment, can be obtained through the use of an extension to Elastic Impedance theory (Connolly, 1999). To achieve this we modify the definition of Elastic Impedance, EI, to allow arbitrarily large positive or negative values of sin 2 θ which now becomes the primary variable rather than θ. We refer to this form of Elastic Impedance, which includes an additional normalization term, as Extended Elastic Impedance or EEI.We demonstrate that several elastic parameters, including bulk modulus, shear modulus and Lame's parameter, can be approximated by Extended Elastic Impedance functions with different sin 2 θ values. This leads to the identification of different areas of EEI space which tend to be good for fluid and lithology imaging. Appropriate seismic for each case can then be obtained by inverting the appropriate sin 2 θ projection from routine AVO processing.

This work provides explorationists with simple procedures to perform depth conversion more accurately than can be achieved with simple vertical layer cake depth conversion. The use of image rays, which are inadequate in structurally complex areas, is avoided. Migrated time interpretations are still used and are "demigrated" using the Kirchhoff time migration equations. This backs out the effect of the time migration prior to a ray depth migration and enables the lateral shifts between the time migrated image and a depth migrated image to be quantified. These shifts can be separated into a mismigration component and a refraction component. The relative size of the components define whether time or depth migration is required and may be used to justify a remigration of the seismic image. Furthermore, the tedious layer by layer approach to ray depth migration may be avoided by using the velocity depth model from the vertical layer cake depth conversion of the time‐migrated data for ray depth migration of the unmigrated data for all horizons in a single step. A satisfactory result is usually achieved without the need to iterate. These methods are illustrated with both a synthetic example and a real 3-D data set from the Norwegian North Sea.