During the second half of the 1970s, a controversial debate took place regarding a fundamental concept of reflection seismology, the geological meaning of a continuous reflector. We know that a seismic section represents a view of the contrasts in the seismic velocities and densities. It is also well known that these two physical attributes are controlled by the lithology, degree of compaction, porosity, cementation, fluid content, pattern of micro-cracks, and other, less important factors. Because most of these factors are to some degree related to the lithology and depth, the lithostratigraphic school interprets a seismic horizon (a reflector) as an expression of the boundary between different lithofacies (Harms and Tackenberg, 1972). However, with the support of empirical evidence from bore-hole data (Vail et al., 1977), the chronostratigraphic school interprets the seismic horizon as an isochrone or, equivalently, as the boundary between sediments deposited at different geological times (independently of the lithology). The simple example that follows shows how these two apparently contradictory interpretations can be reconciled.
Figure (center) shows a typical fluvial deposition system, with the transition from coarse-grained to fine-grained sandstone and then to shale, as we move away from the sediment source. A closer look at this transitional layer (bottom) reveals that it includes a thinly-stratified sequence caused by short-period oscillations in the energy level of the transport mechanism. A schematic representation of the velocity profiles in different positions shows that, although the velocities change laterally, their relative vertical changes (which cause the reflections) form a continuous lateral pattern. While locally the reflector represents a lithologic transition, in a more extended view it represents time transitions. If we then consider the same layer from the perspective of a longer time-span (top), we can see that it belongs to a transgressive sequence. The low-frequency components of the velocity profiles clearly delineate different lithofacies, that is, isolated bodies with roughly the same lithology. While the seismic section is an expression of isochrone units, an interval velocity section (derived from tomography or other velocity estimation methods) is a representation of lithologic units. These are complementary descriptions of the subsurface geology. A good illustration of this complementarity can be found in a comparison among a seismic section, a reversed VSP and a cross-well tomographic section presented by Chen et al. (1990). Although the authors emphasize the higher resolution of the tomographic imaging compared to that of the seismic imaging, it is clear from the figures that they are images of completely different geological attributes. They resolve different aspects (derivatives or integrals) of the velocity field.
Making an analogy with the concepts of quantum mechanics, where particles and waves are considered as complementary descriptions of the same phenomenon (light, or subatomic entities), the seismic horizon can simultaneously represent a time boundary or a lithological boundary, depending on the way we look at it.