Tuning Phenomena in Reservoirs
Widess (1973) demonstrated the interaction of closely-spaced reflections. In his classic paper, “How thin is a thin bed?,” he discussed the effect of bed thickness on seismic signature. For a bed thickness of the order of a seismic wavelength or greater there is little or no interference between the wavelets from the top and the bottom of the bed and each is recorded without modification. For thinner beds these wavelets interfere both constructively and destructively. Considering wavelets of opposite polarity, the amplitude of the composite wavelet reaches a maximum for a bed thickness of one-quarter wavelength (one-half period) and this is known as the tuning thickness. For beds thinner than this the shape of the composite wavelet stays the same but its amplitude decreases. Clearly, the bed thicknesses at which these phenomena occur depend on the shape of the wavelet in the data and hence on its frequency content.
These tuning phenomena are of considerable importance to the stratigraphic interpreter. They must be recognized as effects of bed geometry as opposed to variations in the acoustic properties of the medium. Figure 6-1 shows a sedimentary pod. As the reflections from the top and the base come together (within the black square) the amplitude abruptly increases; this is interpreted as tuning between the top and base reflections.