Page 279 - Acquisition and Processing of Marine Seismic Data

270 5. PREPROCESSING

FIG. 5.28 Example filter panels for a stack strip with variable low-frequency cut-off value. Band-pass filter outputs for
(A) 6–250 Hz, (B) 30–250 Hz, (C) 50–250 Hz, (D) 70–250 Hz, (E) 100–250 Hz, (F) 150–250 Hz, and (G) 200–250 Hz pass-bands.

The time domain length of the filter operator Because narrowing in the frequency domain
is one of the most important factors affecting the results in widening in the time domain
filter performance. Operators with smaller time (Section 4.5), definition of a narrow pass-band
lengths do not have an amplitude spectrum of in the frequency domain requires a significantly
ideal trapezoid shape (Fig. 5.20), so the length wide filter operator in the time domain, consist-
of the operator should be long enough to prop- ing of several nonzero time samples (Bracewell,
erly suppress undesired frequency components. 1965). Application of such a filter operator after
Fig. 5.29 shows outputs of the filtering applica- truncating both ends will significantly reduce
tions using filter operators with different time the filter performance since several nonzero
lengths and their mean amplitude spectra. Short samples of the operator are excluded. In order
filter operators fail in removing all of the swell to avoid this, much longer time domain filter
noise, leaving some residual undesired swell operator length should be considered when
noise amplitudes around 0 Hz in the output the required pass-band of the filter is narrow,
amplitude spectra. These residual amplitudes, e.g., when applying a single frequency notch
however, vanish with increasing filter operator filter. Longer operator lengths, however, require
length and completely disappear in the output more computational time, and they may not be
shot filtered with 400-ms operator length. If economical, especially for large data volumes.
the swell noise cannot be removed by a band- Fig. 5.30 schematically shows the relationship
pass filter, longer operator lengths must be con- between the width of the pass-band in the fre-
sidered. An effective f-k filter, or two successive quency domain and the operator length in the
band-pass filters with the same parameters, can time domain. Wider pass-bands always corre-
also be used. spond to narrow time series (the filter operator)