Seismic reflections come from interfaces where the acoustic properties of the rocks change, and this fact is the basis of our understanding of the nature of seismic data. Acoustic impedance of a rock layer is the product of the density and the velocity of that layer, and strictly a reflection is generated by a contrast in acoustic impedance. In fact impedance and lithology normally follow each other, so that impedance boundaries and lithologic boundaries normally concur.
Consider a sand encased in shale, perhaps the most common situation forming a hydrocarbon reservoir. The shale-sand interface at the top generates a reflection, and the sand-shale interface at the base generates a reflection (Figure 1-1). Thus a sand has a reflection from the top and another from the base. These two reflections should be considered together in all studies of the reservoir sand.
At one location the sands normally have one impedance and the shales have another impedance. (Typically, sands have a lower impedance than shales in younger rocks and a higher impedance than shales in older rocks.) Thus the interfaces at top and base of a sand reservoir will almost always have impedance contrasts in the opposite sense. The sense of the impedance contrast determines the polarity of the seismic reflection, so that the top and base reflections for a sand reservoir encased in shale are opposite polarity from each other. This is a very significant piece of information used in the identification of reservoir reflections. Numerous figures in this book illustrate the pairing of top and base reflections — for example, Figure 1-1 many figures in Chapters 2 and 5.
Tying of geologic data and seismic data together involves some knowledge of velocity, but the depth-to-time tie is not sufficient. We must identify seismic reflections on the basis of the character expected from the geologic interfaces and the fact that the layer of interest will normally have a top reflection and a base reflection. Consideration of the top and base reflections together involves the topics of natural pairing, choice of color schemes, data phase, data polarity, seismic resolution, and tuning, all of which are subjects of this book.