Continuous monitoring by ambient-seismic noise tomography |

corrcoefVt-shots-si
Correlation coefficients as a function of start time comparing a 3 hour partial window with the full stack. The top panel shows the spectragram with the correlation coefficients overlayed and the bottom panel shows only the curve of correlation coefficients.
Figure 7. |
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We want to analyse the cross-correlation convergence rate as a function of inter-station distance for various frequency ranges. We compute the correlation coefficient between two virtual sources after bandpassing for a certain central frequency, and bin the coefficients as a function of inter-station distance. One virtual sources is computed by cross-correlating all data while the second is computed cross-correlating a partial recording. Furthermore, to eliminate sensitivity to a particular starting time of the partial recording, we slid the partial recording window across the complete recording. We repeat the whole procedure for two virtual sources and average the mean correlation coefficients for each distance bin and partial recording length. The frequency range Hz is shown in 8a, Hz in 8b, Hz in 8c, Hz in 8d, Hz in 8e, Hz in 8f, Hz in 8f. Dotted and dashed lines are and contours. The contour lines are not very smooth. This can probably be improved by averaging the computations for more virtual sources. These figures show a trend of faster convergence rate for lower frequencies. This trend is especially apparent with inter-station distances larger than meters. This trend is expected, because low frequencies have a larger Fresnel zone than do higher frequencies, and therefore require fewer sources surrounding the stations. Since the same sources excite the entire frequency regime, the background correlation fluctuations will be dominated by higher frequencies, and the correlation will stabilize faster for lower frequencies. Correlations between stations at shorter distances converge much faster, but a trend for different frequencies at shorter distances is not clear. This behavior explains the results in Figure 7, where a strong presence of high frequencies actually deteriorates the convergence rate.

coef-high,coef-middle,coef-low,coef-lower,coef-verylow,coef-verylower
Correlation coefficient of partial versus total cross-correlated signal time. Dotted and dashed lines are
and
contours. Analysis made for frequency ranges;
Hz in (a),
Hz in (b),
Hz in (c),
Hz in (d),
Hz in (e),
Hz in (f),
Hz in (f).
Figure 8. |
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Continuous monitoring by ambient-seismic noise tomography |

2012-05-10