Identification of events

Problems in Exploration Seismology and their Solutions

Series	Geophysical References Series
Title	Problems in Exploration Seismology and their Solutions
Author	Lloyd P. Geldart and Robert E. Sheriff
Chapter	6
Pages	181 - 220
DOI	http://dx.doi.org/10.1190/1.9781560801733
ISBN	ISBN 9781560801153
Store	SEG Online Store

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.

Background

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

Solution

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 $\theta _{c}$ 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 $180^{\circ }$ 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 $({\rm {P}}_{1}{\rm {S}}_{2}{\rm {P}}_{2}{\rm {P}}_{1})$ or up-going $({\rm {P}}_{1}{\rm {P}}_{2}{\rm {S}}_{2}{\rm {P}}_{1})$ 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 $({\rm {P}}_{1}{\rm {S}}_{2}{\rm {S}}_{2}{\rm {P}}_{1})$ 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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Table of Contents (book)

Also in this chapter

• Characteristics of different types of events and noise
• Horizontal resolution
• Reflection and refraction laws and Fermat’s principle
• Effect of reflector curvature on a plane wave
• Diffraction traveltime curves
• Amplitude variation with offset for seafloor multiples
• Ghost amplitude and energy
• Directivity of a source plus its ghost
• Directivity of a harmonic source plus ghost
• Differential moveout between primary and multiple
• Suppressing multiples by NMO differences
• Distinguishing horizontal/vertical discontinuities
• Identification of events
• Traveltime curves for various events
• Reflections/diffractions from refractor terminations
• Refractions and refraction multiples
• Destructive and constructive interference for a wedge
• Dependence of resolvable limit on frequency
• Vertical resolution
• Causes of high-frequency losses
• Ricker wavelet relations
• Improvement of signal/noise ratio by stacking

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