Near Surface Variations and its Impact on Seismic Repeatability at Naylor Field, Australia

The micro-array investigation of the near-surface layers at Naylor site indicated that the velocities and Q-factor have same directional variations at this site. The first 0.5m layer of the agricultural soil (elasto-plastic zone) has a low velocity and low Q-factor, hence this zone significantly attenuated seismic energy. Micro-VSP measurements show a consistence increase in velocity with depth and no seismic anisotropy. The change in the water saturation in near surface can cause the changes in velocity and attenuation. Around 30% of RMS amplitude difference can be measured at the reservoir level from the simulated models and that is by simulating the measured parameters of the near surface properties of wet and dry conditions of the near surface with no change in the reservoir properties.