Identification of events

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Problem 6.13

Figure 6.13a shows events from a high-velocity layer 1.5 wave-lengths thick embedded in lower velocity media; they have been corrected for the normal moveout of the reflection from the top of the layer. Discuss the events and their characteristics.


A wide-angle reflection is one reflected at an angle greater than the critical angle.


By inspection of Figure 6.13a we note that the embedded wavelet (see Sheriff and Geldart, 1995, p. 284) is approximately symmetrical (zero phase, see Sheriff and Geldart, 1995, p. 553) and apparently has SEG standard polarity (see Sheriff and Geldart, 1995, Figure 6.49) with a central peak for positive reflectivity.

Figure 6.13a.  Source gather (Sheriff and Geldart, 1995, 173) after NMO correction.

The reflection from the top of the layer first decreases in amplitude with offset until the vicinity of is reached, then it increases in amplitude and becomes a wide-angle reflection and the head wave peels off. The phase of the wide-angle reflection begins to change beyond the critical angle and finally is out-of-phase with the zero-offset reflection.

The head wave has about the same waveshape as the subcritical reflection and it falls off in amplitude rather rapidly. The reflection from the base of the layer is a negative reflection. It converges on the reflection from the top as the offset increases and its raypath in the high-velocity layer lengthens. Its normal moveout is not hyperbolic. It contributes to the amplitude and phase changes in the reflection from the top of the layer as the two converge.

The converted reflection from the base of the layer involves S-wave travel on either the down-going or up-going legs. They have zero amplitude at zero offset and increase in amplitude with offset; they have the same traveltime and polarity and so reinforce each other.

The converted head wave travels along the interface at the S-wave velocity in the high-velocity layer.

The unidentified event and an associated head wave that project back to zero offset at about 0.71 s may be a reflection from the base of the plate that converts at the top of the plate and travels as an S-wave for both legs in the layer and a head wave that it generates. These would have zero amplitude at zero offset and be weaker than the converted reflection referred to above.

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