Figure shows the stacked section from the base survey, before any water injection. The plot is enlarged to focus on the reservoir reflections at 2 km depth. Stacked reflection amplitudes are roughly proportional to the P impedance contrast at a reflector, as long as no anomalous AVO is present. Before waterflood, there are no lateral variations in stacked reflection character along the reservoir, except those due to the added noise. Figure shows the stacked section from the survey after one time step of waterflood injection. There is some slight dimming in the stacked reflection amplitude centered about the well locations at 2 and 3 km distance along the line. Figure shows the stacked section from the survey generated after two time steps of waterflood. The dimming in reflection character is more apparent than after the first waterflood, since the water invasion zone has expanded to a greater distance, thereby creating a larger zone of lowered P impedance contrast. Figure shows a close-up of the stacked data at reservoir depth, showing more clearly the dimming and lateral spread of stacked reflection amplitudes due to the diffusive waterflood.
Figure shows the stacked difference section, obtained by subtracting the stacked pre-flood base section from the stacked waterflood section after one time step. The zones of water invasion are clearly evident in that they give rise to differential reflections, including diffusive diffraction tails at the diffuse edge of the water slug front. Figure shows the stacked difference section comparing the base pre-flood survey to the survey taken after two time steps of waterflood. The water invasion zone looks larger in spatial extent and therefore stronger in amplitude. Again, some diffuse diffractions are evident at the edge of the waterflood. We note that the stacked sections contain poor lateral resolution of the waterflood front because the stacked reflections are smeared laterally over a large Fresnel zone. Prestack wave-equation migration of the raw CMP data can collapse the Fresnel zone down to a spatial resolution on the order of a dominant seismic wavelength, i.e., from hundreds of meters unmigrated to tens of meters after migration.