GCChronostratigaphic Surfaces and Seismic Reflections*
Bob A. Hardage1, Randy L. Remington1, and Paul E. Murray1
Search and Discovery Article #40212 (2006)
Posted October 5, 2006
*Adapted from the Geophysical Corner column, prepared by the authors and entitled, “Reflections Have a ‘Tipper Point’,” in AAPG Explorer, September, 2006. Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.
1Bureau of Economic Geology, Austin, Texas ( )
A fundamental premise of seismic stratigraphy is that seismic reflections follow chronostratigraphic surfaces, not lithostratigraphic surfaces.
In 1993, Tipper published an intriguing paper (Geological Magazine, v. 130, no. 1, p. 47-55) in which the following question was posed: “Do seismic reflection events necessarily follow chronostratigraphic surfaces?” Simple earth models and forward seismic modeling were used to illustrate basic and important interpretation principles.
The editor of this monthly column (Hardage) has observed increased interest in seismic interpretation among graduate students when they have been asked to analyze this Tipper paper, so repeating some of its concepts here seems appropriate.
We use the stratigraphic model in Figure 1 as a demonstration. This model shows five units deposited at five different geologic times -- T1 through T5. These five chronostratigraphic bodies are shown in the top panels of Figures 2, 3, and 4 as stacked, overlapping targets that are to be imaged. This five-layer stack is then illuminated with seismic wavelets having varying resolution properties.
In these figures, the left column shows the illumination created by a high-resolution wavelet; the center column uses a moderate-resolution wavelet for the imaging; and the right column documents the image produced by a low-resolution wavelet. The illuminating wavelet is shown beside each five-layer model for easy comparison of wavelet length with bed thicknesses and bed spacings.
Modeling calculations are done in a dimensionless way in which all aspects of the model (bed thickness, bed spacing, bed overlap) are defined in terms of the dominant wavelength of the illuminating wavelet. This approach allows one person to think of the analysis as “the wavelet is the same in all cases, but the stratigraphic units have different thicknesses and spacings,” while another person can view the picture as “the unit thicknesses and spacings are always the same, but the wavelet varies.” Either view is correct. Use the one that is less taxing to the brain.
What does this modeling exercise tell us? With l to represent the dominant wavelength of the illuminating wavelet, some key points are:
In this case, the seismic reflection response is a diachronous event, not a chronostratigraphic event. We lose the ability to analyze the internal architecture of the layered system, and seismic reflections no longer follow chronostratigraphic surfaces.
Even though the imaging is not 100 percent correct, there is a reflection event for each chronostratigraphic surface. In this case, we can say that each image in the center columns consists of chronostratigraphic, but incomplete, seismic reflections.
Whether seismic reflections follow chronostratigraphic surfaces depends on:
Probably all reflection possibilities illustrated in these models occur within any single 3-D seismic volume. Our recommendation is that the premise that seismic reflections follow chronostratigraphic surfaces is sound and should be applied as a first principle of seismic interpretation.
However, in critical prospect areas, modeling similar to what is illustrated here should be done to determine whether the assumption that seismic reflections are chronostratigraphic needs to be abandoned in a few local areas, even though the concept is correct in a general sense.
Tipper, John C., 1993, Do seismic reflections necessarily have chronostratigraphic significance?: Geological Magazine, v. 130, p. 47-55.
Documenting principles of elastic-wavefield seismic stratigraphy such as this example has been funded by DOE/NETL.