NAME
filt - filter traces and decimate samples
SYNOPSIS
filt [ -Nntap ] [ -Ootap ] [
-bias ] [ [ -flfl ] [ -fhfh ] [ -nornorder ] ] [ [ -ormsby
] [ -f1f1 ] [ -f2f2 ] [ -f3f3 ] [ -f4f4 ] [ -powpow ] [
-conv ] [ -dbdb ] [ -wtwt ] ] [ [ -bess ] [ -iaint ] [
-llsf ] [ -wex ] ] [ -nsns ] [ -nene ] [ -rsirs ] [ -reire ]
[ -didec ] [ tmultmul ] [ -C ] [ -M ] [ -? ]
DESCRIPTION
filt Apply an either an n'th order (nor) Butterworth filter
(default) or a trapezoidal (ormsby) filter or a bessel
filter to the specified traces in the input file or simply
remove a DC bias from the input traces. The Butterworth
filter is a lowpass one converted to bandpass with digital
corner frequencies at fl,fh and can either be minimum phase
or zero phase. The ormsby is done in the frequency domain
and is always zero phase. The Bessel filters do not allow
you to filter specific frequency bands but they have beauti-
fully behaved responses such that if you just want to trim
off some high frequency noise without otherwise affecting
the data with artifacts this is the option for you. The
output is zerophase if -C option is not used in the command
line. Large orders should be avoided because of the prob-
lems with numerical errors. Note that filt will handle very
long traces (up to 16000 samples).
filt gets processing controls from the command line. Reason-
able defaults are set up.
Command line arguments
-N ntap
Enter the full path of the file containing the data
set.
-O otap
Enter the output file name for the filtered traces
-rs irs
(integer) First record to process (default = 1)
-re ire
(integer) Last record to process (default = all)
-ns ns
First trace in record to process (default = 1)
-ne ne
Last trace in record to process (default = all)
-nor norder
The Butterworth filter order (default = 2)
-flfl
The lower corner frequency of the filter. For a lowpass
filter set=0.0 (default = 10.0 Hz).
-fhfh
The upper corner frequency of the filter. (default =
40.0 Hz).
-ormsby
If present on command line the filter applied is a tra-
pezoid in the frequency domain with corner frequencies
specified below:
-ormsby
If present on command line the filter applied is a tra-
pezoid in the frequency domain with corner frequencies
specified below:
-f1f1, -f2f2, -f3f3, -f4f4
The four corner frequencies of the trapezoid starting
from the lowest (f1, f2, f3, f4 must be in ascending
order). If either f1,f2 and/or f3,f4 pairs are missing
then an out-out on that side is assumed.
-pow pow
Enter the exponent with which to power the trapezoidal
filter. Default is 1.0
-conv
If present on command line the ormsby filter applied is
a convolutional operation in the time domain rather
than a frequency domain operation. This is slower but
gives better control over the magnitude of the rejec-
tion (see -db[] below).
-db db
Ormsby convolution option: enter the decibel rejection
outside the passband of the amplitude spectrum. Default
is 40.
-wt wt
Ormsby convolution option: enter the bessel weighting
for the filter design. Default is 1.0
-bess
If present on command line the filter applied is a
bessel filter with notch paramter, filter length, and
ross weight described below.
-iaint, -llsf, -wex
Enter the intercept (values from .01 - narrow to .09 -
wide), filter length, and ross weight. Intercept
default is .05; filter length default is 3; ross weight
default is 0. Narrow filters chop off more of the high
frequencies in the data; wide wifters leve more high
frequencies in the data. Long filters chop off more of
the high frequencies; short filters leave more highs in
(-l3 gets you to about 1/2 nyquist).
-tmul tmul
Multiply the sampling interval, dt, by this factor
(default = 1.0). This is useful if the sampling inter-
val is in different units than expected.
-d idec
Enter the data decimation factor: -d2 means throw away
every other sample. Default = 1.
-bias
Remove DC bias only from input data. With this option
flagged there will be no other filtering of any kind.
Any decimation will be ignored and there will be no
restore mute.
-C Output is for a minimum delay filter rather than zero-
phase (default zero phase if not specified)
-M If present on command line do not restore early mute.
Default is to restore early mute.
-? Query mode. With this flag, filt will give a descrip-
tion of the command line arguments and stop the pro-
gram.
DISCUSSION
The Butterworth option of the program makes use of a bil-
inear transformation of an analog log pass Butterworth
filter of order norder to a digital bandpass filter with a
corner fh. If fl=0.0, the bandpass filter reduces to a
lowpass filter. If fh equals a number greater than the
Nyquist frequency, the filter reduces to a high pass filter
with corner at fl. Note that the filter order is than of
the analog lowpass causal filter. The zerophase filter
response varies as f^(2 norder) and f(-2 norder) at fre-
quencies less than fl and greater than fh, respectively.
The trapezoidal option allows the user to specify four
corners of a trapezoid in the frequency domain. The two
pairs of frequencies define the low side ramp and the high
side ramp. Bweteen the ramps the trapezoid has unit ampli-
tude; on either side of the ramps the amplitude response is
zero. Application is in the frequency domain by vector mul-
tiplecation.
BUGS
The response for very high filter orders may be in error due
to numerical noise propagation.
AUTHORS
R. B. Herrmann and T. Mokhtar, Saint Louis University, 1986.
COPYRIGHT
copyright 2001, Amoco Production Company
All Rights Reserved
an affiliate of BP America Inc.
Man(1) output converted with
man2html