How much trust should we place in seismic amplitudes and the interpretations we make from them? In a textbook world, seismic amplitude interpretation will tell us not just about the structure of the earth but also provide additional information about the rocks in the subsurface such as fluid-fill, porosity, and lithology. We use quantitative interpretation tools such as amplitude versus offset, frequency decomposition, and seismic inversion to extract a great deal of information from 3D and 4D seismic surveys.
In the real world, seismic amplitudes show significant complexities that challenge textbook assumptions. Transmission through the earth creates multiple raypaths and distortions that impact the phase and amplitudes of the seismic waves. This complexity is evident when we examine first arrival amplitudes recorded by downhole receivers or look at the offset dependence of reflection seismic images. It is common that these amplitude distortions are larger than signals predicted from changes in lithology and/or fluids from idealized rock physics models.
In the time-lapse seismic arena, we try to measure typically small changes in 3D seismic data acquired before and after the production of oil and gas. If we compare two 3D seismic surveys that are acquired using dissimilar acquisition geometries, we find large differences (up to 40%) between the surveys that have nothing to do with the production effects we are looking for. This isn’t because of defects in the individual acquisition designs but rather the sensitivity of the seismic amplitudes and phase to the raypaths used to generate each seismic image. If we repeat either of these surveys using near identical geometries, we can reduce the difference to less than 10%. One of the key learnings from time-lapse seismic is that the transmission noises are repeatable.
So how much do you trust your favorite 3D seismic survey?
Paul Hatchell joined Shell in 1989 after receiving his PhD in theoretical physics from the University of Wisconsin. He began his career at Shell’s Technology Center in Houston and worked on a variety of research topics including shear-wave logging, quantitative seismic amplitude analysis, and 3D AVO applications. Following a four-year oil and gas exploration assignment in Shell’s New Orleans office, Paul returned to Shell’s technology centers in Rijswijk and Houston where he is currently a member of the Areal Field Monitoring team and Shell’s principal technical expert for 4D reservoir surveillance. His current activities include developing improved 4D seismic acquisition and interpretation techniques, seafloor deformation monitoring, and training the next generation of geoscientists.