SeismicHandler - Waveform Analysis Tool

Menu entry Filter

Applies filter to traces on display. The filter dialog box appears to select which filter to apply. A description of the elements of the filter dialog box follows below. After clicking on OK the filtering is done (may take some time, depending on number and length of traces and on the CPU power of the host). The resulting traces are displayed on screen. The original traces remain in memory but are not visible any more. When the filter is changed, the previous filter output traces are deleted and a new filter operation is started on the original traces. Deselecting the filters (filter None) restores the original traces on screen. Once a filter is selected in remains active until it is explicitly switched off in the filter dialog box or a Read New is executed (see command Read). Read Again keeps the filter, that means the traces read in are filtered automatically.

SHM knows two kinds of filters, FFT filters and recursive filters, In principle all filter oprations can be performed with either of these types, practically there are some contraints. In general the recursive filters are faster in computation time, in particular for long traces. The computation time for recursive filters grows linearly with the number of sample points, N, while the FFT filters need time proportional to N log(N), due to the FFT operation. In some cases the FFT filters turn out to be numerically more stable than recursive ones. FFT filters have one filter file for all possible sample rates of the input traces, recursive filters need a separate file for each rate (see filter files of SHM). The program package supplies utility programs to create FFT/recursive filters for several standard filters, special filters have to be created manually by the user. In general, creating FFT filter files is much easier than computing recursive filter coefficients.

The filter dialog box

This dialog box has the following elements

List of available filters

Whether SHM uses FFT or recursive filters may be set (temporarily) in Setup (parameter filter type) or (permanently) in Configure (parameter number 30, values true or false). The Read New option of command Read can apply a default filter after reading a new data set automatically. This also may be changed in Setup (parameter default filter) or Configure (parameter number 38, example filter s+standard_bp, no filter means here ;;).

For further information please see the topics

This page last modified 13-Jul-2000