Seismic interferometry versus spatial auto-correlation method on the regional coda of the NPE |
The spectrum at each station was estimated using a multitaper spectral-estimation technique. This provides several statistically independent estimates of the spectrum and decreases spectral leakage (Prieto et al., 2007,2008b). In this procedure, a time record with samples is first multiplied with a set of orthogonal Slepian tapers (Thomson, 1982). Second, the discrete Fourier transformation is computed for each tapered trace as follows:
(12) |
(13) |
We first study the result from processing the records in the time domain, as is common in seismic interferometry practices. A common-midpoint section at is given in Figure 4, and a common-offset section for is given in Figure 5. de Ridder (2008) did not observe any event, besides the surface-wave events intersecting each other at that is coherent across different midpoints. When we study the common-offset section in Figure 5, we can see the arrival time of the surface wave event at slightly varying with offset. If we neglect this and assume the earth is horizontally layered, the recovered gathers can be stacked over common-offsets as shown in Figure 6. Estimating the slope of the event visible in Figures 4 and 6, de Ridder (2008) found a velocity of . The subscript refers to a Rayleigh wave, which is the dominant surface-wave type recorded in the vertical component of particle velocity in groundroll. It is difficult to extract more information from the time-domain images. Additional analysis of the retrieved gathers can be performed in the frequency-domain, by inverting for phase velocity and attenuation factors. The frequency-domain equivalents of the time-domain gathers in Figures 4 and 6 are shown respectively in Figures 7 and 9.
si375m
Figure 4. Interferometric common-midpoint gather gather, at ; a) retrieved from recording 1, b) retrieved from recording 2. [ER] |
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si375o
Figure 5. Interferometric common-offset gather, for ; a) retrieved from recording 1, b) retrieved from recording 2. [ER] |
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SIHS
Figure 6. Common-offset stack for the interferometric gathers; a) retrieved from recording 1, b) retrieved from recording 2. [ER] |
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SPACHS
Figure 7. Frequency-domain common-offset stack; a) retrieved from recording 1, b) retrieved from recording 2. [ER] |
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Seismic interferometry versus spatial auto-correlation method on the regional coda of the NPE |