Small scale velocity fluctuations

The presence of such fluctuations can be modeled by adding small random perturbations to the travel times. These random shifts decrease the semblance values, especially for high-frequency signals. The particular results will depend on different circumstances (shape of the seismic array, frequency content of the signal, etc.), so here I show the effect of adding random velocity fluctuations to the data described above by adding time shifts to each trace. Figure  shows the decrease of semblance that accompanies an increase in the standard deviation of the time perturbations. Such time shifts can sometimes be caused by the uppermost layer of the medium. In this (favorable) case, they can be treated as a function of geophone location and hopefully can be compensated for (static corrections). Also, some kind of static corrections can be made using crosscorrelations. Steve Cole created an iterative algorithm for applying such corrections to passive seismic data.

 

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Figure 5 vran

Effect on semblance computation of small random velocity fluctuations. The horizontal axis is the standard deviation of travel times divided by the period of the signal.