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46 lines
1.6 KiB
Plaintext
46 lines
1.6 KiB
Plaintext
1 year ago
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.. _global_fx-dfx:
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######
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FX-DFX
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######
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Implementation of the CTBTO/IDC polarization analysis for
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three-component stations.
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Description
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===========
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The feature extraction as implemented at CTBTO IDC for single three-component
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stations determines back azimuth (station to origin) and slowness, including the
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uncertainties for both of these. In the IDC source code and database, the back
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azimuth is referred to as only azimuth.
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Algorithm
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=========
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The algorithm computes polarization attributes for a three-component station using
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a modification to the :cite:t:`jurkevic-1988` algorithm. Some of these attributes are
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then used to determine detection azimuth (seazp = P-type azimuth in degrees),
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detection slowness and azimuth/slowness uncertainties (inang1 = emergence (incidence)
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angle and rect = rectilinearity).
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A fixed noise window of 9.5 seconds ([-30s;-20.5s] with respect to trigger time)
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and a signal window of 5.5 seconds ([-4s;1.5s] with respect to trigger time)
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is used. The signal window is subdivided into intervals of 1.5s length which
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overlap by 50%.
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1. De-mean data according to mean of noise window.
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2. Apply cosine ramp to noise data and filter the entire data window.
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3. Rotate three components into ZNE space.
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4. Compute 3x3 covariance matrix for each interval.
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5. Extract eigenvalues and compute parameters including rectilinearity.
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6. Choose the result set with the largest rectilinearity.
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Picks
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=====
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In addition to the extracted back azimuth and slowness values the rectilinearity
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is added as a comment to the resulting pick. The comment ID is
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``DFX:rectilinearity`` and the comment is the value in string representation.
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