Figure 1. Comparison of an AI curve with a 30°EI curve for the Foinaven discovery well 204/24a-2.

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.

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.

A method of compensating for the presence of discrete overburden velocity anomalies during depth conversion of time horizons interpreted on conventional, post‐stack time‐migrated seismic data is presented. Positive and negative time delays are estimated either from the push‐down or pull‐up of reflectors directly beneath the anomalies or from interpreted time thickness of the anomalous body and interval velocities estimated from well data. The critical steps are pre‐stack simulation of seismic acquisition across the velocity anomalies, incorporating the effects of a Fresnel volume which changes its width as a function of depth, and simulation of common‐midpoint (CMP) stacking using a linear regression of time delay, Δt, versus offset‐squared, X2. The time‐correction method predicts the time distortion for any target horizon and the distortion is removed as a correction in time. Depth conversion is then performed using a background velocity function. The final average velocity map is calculated from the resulting depth structure and the raw times at the target horizon. The method is implemented by manipulating time grids within an industry‐standard mapping package. The final average velocity map shows steep lateral velocity gradients which are constrained by the interpreted boundaries of the velocity anomalies.

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