Three Axes To "Grind"

Three layer parameters can be obtained from an interpretation of seismic data, coupled with well log and core data.

Consequently, the seismic method may be viewed as a technique for estimating the co-ordinates of the layer properties of the sub-surface in the 3-D volume described by the axes of Vp, SIGMA and AAI, figure 3. Several factors make the estimation procedure imprecise, e.g. noise on the data, the limited bandwidth of the seismic data (approx. 10 to 50Hz) and the uncertainties in the interpretation used to obtain the SIGMA and AAI as layer parameters. The co-ordinates of a layer in the 3-D space cannot be defined precisely, there are associated uncertainty bars.

To distinguish between different lithologies by Vp, SIGMA and AAI estimation requires the lithologies to have different and distinct co-ordinates in the 3-D space, even after allowing for the uncertainty bars (figure 3). The dimensions of the uncertainty bars are typically 250m/s for Vp, 0.05 for SIGMA and 500(m/s)(gm/cc) for AAI. Ideally, lithology and fluid prediction would be concerned with defining the separation of the different lithologies in all three directions of the 3-D space. However, the separation of the co-ordinates of the different lithologies may be more significant in one or two directions. Therefore, the lithologies may be distinguished by the estimation of one or more of these parameters. The separation relative to the uncertainty bars determines the effectiveness of the estimation of the different parameters to distinguish lithology.

Figure 3 : Definition of Layer properties in terms of P-wave velocity (Vp), Poisson's ratio(SIGMA) and Absolute Acoustic Impedance (AAI). The co-ordinates of two different lithologies are indicated with the associated uncertainty bars of the estimates.

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