Ambient noise

Random noise was added to the zero-offset section of the horizontal reflector in Figure 2, where the medium was assumed to have a constant velocity equal to 2000 m/s. The noisy section is shown in Figure 15. The corresponding migrated section in Figure 16 shows that the noise in the section has decreased in amplitude as a result of migration. Note that a constant scaling factor was applied to the section so that the noise could actually be seen, hence the very large amplitudes of the reflector. It can be seen from the figure that noise amplitudes become lower down the section. This time-variant reduction of noise is due to the time factor in the spherical spreading term of Kirchhoff summation, as was explained earlier on.

 

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Figure 15 Zero-offset seismic response of a horizontal reflector (shown in Figure 2) with random noise added to the section.

 

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Figure 16 Migration (constant velocity medium).

The zero-offset seismic response of the same reflector was generated using a time-variant velocity function, where velocity increases with increasing traveltime. Its output migrated section is shown in Figure 17. Here the variable velocity factor results in a greater reduction of noise towards the bottom of the section, where the velocities are higher. However the background noise looks more coherent as its amplitude is decreased, that is, in the deeper parts of the section.

 

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Figure 17 Migration (medium velocity increases with increasing time).