Characteristics of different types of events and noise

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Problem 6.1

Classify the different types of events and noise in Table 6.1a on the basis of commonly observed characteristics on a seismic record.

Figure 6.1a.  Characteristics of events.
Figure 6.1b.  Characteristics of events.

Background

A wave is coherent when it appears on successive traces in a systematic way and with approximately the same waveshape. Apparent-velocity filtering, also called apparent-dip filtering, refers to discrimination (attenuation) based on the slope of a linear alignment of traces across a section (see problem 9.25). Frequency filtering refers to attenuation of certain frequencies relative to other frequencies (see problems 7.11 and Sheriff and Geldart, 1995, Section 9.5.10). Arrays, discussed in Sheriff and Geldart (1995, Section 8.3.5 to Section 8.3.9) discriminate on the basis of apparent wavelength. Common-midpoint (CMP) stacking is discussed in Sheriff and Geldart, 1995, Section 8.3.3. Muting involves setting to zero the parts of traces prior to a certain ‘mute schedule.’ Three-component recording uses geophones that respond to motion along three different orthogonal axes, e.g., north-south, east-west, and vertical.

Solution

In Table 6.1b we assume that the dimensions are those commonly used, that “distinctive apparent velocity” means apparent velocity within certain limits, that structural and other changes are generally small, that the 3-component phones are on bedrock rather than on top of a low-velocity layer. Usually only wind noise and other nearly random background noises are incoherent and all source-generated events are predictable trace-to-trace and repeatable. CMP stacking should attenuate events that do not have the same hyperbolic relationship (see problem 4.1a) and stacking velocity as the primary reflections. In Table 6.1b, (1) indicates that the attenuation will be mainly that of the diffraction tails, (2) indicates that reflected refractions from off to the side of the line may have hyperbolic curvature, and (3) indicates that Love waves are dispersive.

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