CLASSIFYING

 

classes
Figure 3 The three classes of reflectivity. The upper layer is a sand layer and the lower layer is a shale layer. Plotted to the right is impedance and depth is down. The shallow sections of the well exhibit Class III behavior. At 2.4 km the well exhibits Class II behavior and the deepest sections display Class I behavior.

The three classes of reflectivity are shown in Figure 3. The V_p, V_s and $\rho$ (density) of shale as a function of depth were all fitted to straight lines. Subsequently, the same three properties (V_p, V_s and $\rho$) of sand were fitted to straight lines. I picked three different depths at which to calculate these six well properties. I then used these values to construct simplistic two-layer models. The depths used were two, 2.4 and four kilometers below the surface. The medium depth (2.4 km) was chosen specifically because it is the crossover point (V_p(sand)=V_p(shale)) and therefore, a Class II reflection. The shallow and deep depths were simply chosen to be depths significantly far from 2.4 kilometers and on opposing sides. Looking at Table 1, it is obvious that the depth closest to the surface is an example of Class III reflectivity as V_p(shale)>V_p(sand). For similar reasons, the deepest portion is a class I well as V_p(shale)<V_p(sand). This is obviously an example of Class II reflectivity.