polarisation filter
command: POLFIL <ZNE-list> [<coherence-lth>] [<lo-bound> <hi-bound>]
Applies a polarization filter to a 3-dimensional trace. Suppresses parts of a seismogram which are not linearly polarized or not polarized in a given direction.
- Default operation is a linear polarization filter:
Determines polarisation of a 3-dim seismogram given by the three traces in <ZNE-list> in each data point and suppresses that parts which are not linearly polarized. The polarisation is determined by evaluation of eigenvalues of the covariance matrix, computed within a time window of width <coherence-wdw> and centered at the sample point to be filtered. Let the (real and non-negative) eigenvalues of the covariance matrix be L1, L2 and L3 in descendent order. Then the filter factor of the filtered sample point is given by the formula
f = (1 - L2/L1) ^ p,where p is an arbitrary exponent. The default value of p is 1.0. It can be changed to <p> by the qualifier "/POWER=<p>". In this mode the <coherence-lth> parameter is required.
- With qualifier "/POLVEC=<Z>,<N>,<E>":
Takes each 3-dim data point (from <ZNE-List>) and computes scalar product with the given polarization vector. Both vectors are normalized to one before. The result is used to rescale the 3 input traces. If "/POWER=<p>" is specified the scaling factor is taken to the power of <p>. In this mode the <coherence-lth> parameter is ignored.
- With qualifier "/BAZDIR=<baz>":
Computes a 2-dim scalar product of the N,E vector and the given back-azimuth <baz> (in deg). Both vectors are normalized to 1 before. The result is used to rescale the 3 input traces. If "/POWER=<p>" is specified the scaling factor is taken to the power of <p>. In this mode the <coherence-lth> parameter is ignored.
The input traces remain unchanged. There are created output traces containing the filtered result.
parameters
- <ZNE-list> parameter type: trace list
List of three traces containing Z, N and E component of seismogram to be polarisation filtered.
- <coherence-lth> parameter type: real
Length of time window in seconds where to compute the covariance matrix of each sample point.
- <lo-bound>, <hi-bound> parameter type: real
Time window on input traces to restrict the filtering.
qualifiers
- /power=<p>
Exponent of filter factor (see above description). Maybe any positive real value.
- /cmreset=<n>
Only valid for default operation (no /POLVEC and no /BAZDIR). For numerical stability the covariance matrix is recomputed completely every <n> steps and not computed from the difference dyads to the preceding one. <n> defaults to 100.
- /polvec=<Z>,<N>,<E>
See above.
- /bazdir=<baz>
See above.
examples
- polfil/power=10. 1-3 1. 0. 100.
- filters traces 1-3 (Z,N,E-traces) uses exponent of 10.
- polfil/power=6. 1,3,5
- filters whole traces 1,3,5 (Z,N,E)
- polfil/power=3. 1-3 *
- filters traces 1-3 (Z,N,E) within time windows which can be selected by graphic cursor
