NAME
opstk - extract the CDP stack and (optionally) the velocity
field from the output from opstf
SYNOPSIS
opstk [ -Nntap ] [ -Ootap ] [ -O2vtap ] [ -pkpkfile ] [
-mfmutef ] [ -rsrs ] [ -rere ] [ -riri ] [ -ththresh ] [
-stosto ] [ -nstnst ] [ -vmvmin ] [ -vxvmax ] [ -nsmnsm ] [
-W ] [ -? ]
DESCRIPTION
opstk extracts the CDP stack and (optionally) the velocity
field from the multiplexed stacked data and semblance panels
created by program OPSTF. All picking is automatic and no
user intervention is required. This program is one of six
programs in a suite of programs for creating, processing,
and/or analyzing optical stack results. The other programs
in the suite, and their functions, are:
opstf - create forward Tp scans and semblance panels
opstd - Demultiplexes the OPSTF output to create separate
optical stack and semblance panels for analysis.
opstcv - Resamples semblance panels created by program OPSTF
from constant Tp traces to constant (stacking) velocity
traces for analysis.
opstr - do inverse of forward Tp program opstf
XOS - Provides graphical analysis of optical stack or
semblance panels created by OPSTF.
Command line arguments
-N ntap
Enter the input data set name or file immediately after
typing -N unless the input
is from a pipe in which case the -N entry must be
omitted. The input to the stack/velocity field extrac-
tion procedure OPSTK is the output from program OPSTF,
in its multiplexed form. An ASCII file containing mute
functions to be applied to the Tp scans and semblance
data before extracting the stack and velocity data may
also be input. This file may be obtained through the
digitizing procedures in program XOS.
-O otap
Enter the output data set name or file immediately
after typing -O. This output file is not required when
piping the output to another process. The output from
program OPSTK is an SIS formatted data set contains the
stack for each CDP gather input to program OPSTF. A
second (optional) data set contains stacking velocities
for each time sample for each of the CDP's processed.
The output also consists of two ASCII files. The first
of these is automatically created and is a standard
output file containing a summary of data set charac-
teristics, input parameters, and any error messages
generated by the program. This file name of this data
set has the form OP.XXXXX.YYYYY, where the X's and Y's
represent a series of numbers which are unique for each
execution of the program. The user is responsible for
deleting these files when they are no longer needed.
-O2 vtap
The second ASCII file output by program OPSTK is
optional. It contains the record, trace, and time coor-
dinates of results from the automatic semblance-based
picking procedure. If this file is requested, it
should be requested during the analysis phase only.
This file may be read and displayed by either the opti-
cal stack analysis program XOS or by program XSD.
-pk pkfile
Name of the ASCII file containing XSD- and XOS-
compatible picks extracted from the Tp scan data.
Default: No pick file output.
-mf mutef
Name of the ASCII file containing mute functions to be
applied to the Tp scans and semblance data before
extracting the stack and velocity data. This file is
useful for attenuating undesireable events, such as
multiples. This file is created in either the optical
stack analysis program XOS or in program XSD. Note:
since the output of opstf are essentially seismic
traces the user can also use the regular seismic muting
program bdmute which allows the user to pick "on" and
"off" mutes using xsd. The procedure would be to pick
an "off" mute and then create another pick file for the
"on" mute and pipe the two bdmute runs together.
Default: No mute file input.
-sto sto
Enter the number corresponding to the desired procedure
for extracting the stack and velocity data from the Tp
scans and semblance panels. Possible values are:
0 - Isotime average of nst Tp scan values.
1 - Use semblance-based picks, with interpolation
between
semblance maxima. Default: 0 (Isotime aver-
age).
-nst nst
Enter the number of Tp scan samples to sum for stacking
option 0. Default: 3.
-vm vmin
Enter the slowest stacking velocity expected in the
data. This value is used to define a mute to be applied
(prior to and in addition to any mute function sup-
plied) to the Tp scan and semblance data prior to
extracting the stacked data and velocity information.
Application of this mute is necessary to put the Tp-
domain data into the proper velocity regime. Default:
No minimum velocity mute applied.
-vx vmax
Enter the fastest stacking velocity expected in the
data. This value is used to define a mute to be applied
(prior to and in addition to any mute function sup-
plied) to the Tp scan and semblance data prior to
extracting the stacked data and velocity information.
Application of this mute is necessary to put the Tp-
domain data into the proper velocity regime. Default:
No maximum velocity mute applied.
-rs nrst
Enter start record number. Default value is the first
record.
-re nred
Enter end record number. Default value is last record.
-nsm nsm
Enter the number of points in the smoothing (cos**2)
function for the semblance. This number corresponds to
the semblance gate for other programs, such as velspec.
The smoothing function is applied to the semblance
before computing the envelope and picking the stacked
amplitudes and velocities. Default: 0 (No smoothing =
semblance gate of 1 sample)
-W Enter the parameter "-W" to cause the Tp scan data to
multiplied by the corresponding semblance data before
extracting the stacked data and velocity information.
Default: No.
-? Enter the command line argument '-?' to get online
help. The program terminates after the help screen is
printed.
DISCUSSION
Stacked Data and Stacking Velocity Extraction
The stacked data output by program OPST is extracted from
the Tp scan panels output by program OPSTF by first identi-
fying the maximum stacked amplitude for each T0 and then
determining the Tp for which the amplitude occurs. Once the
Tp is identified, the stacking velocity can be computed in
the manner described in the OPSTF documentation (Equation
3). Two methods for identifying the appropriate Tp are pro-
vided and are discussed below.
1. Sum of N largest amplitudes for each T0
In this method (Stack Option 0), the scan values for each T0
are sorted into ascending order, according to magnitude. A
user- defined number (-nst option) of values which have both
the largest magnitude and have corresponding semblance
greater than or equal to the threshold limit are summed and
the Tp is defined as the average of the Tp's corresponding
to the values used in the summation. The number of averaged
values may range from one to the number of Tp scan traces
generated in program OPSTF. A reasonable value is one that
agrees with the width of the "wrap- ups" on the Tp scan
panels. Use program XOS to view these panels and determine
the appropriate value to be used.
2. Extraction according to maximum semblance
This method corresponds to a automatic picking method fre-
quently used in conventional velocity analysis. In this
method, an envelope of semblance maxima is assembled from a
semblance panel by scanning for grouping of maxima which
meet or exceed a user-defined threshold limit. The maxima
in this envelope then define the location (trace or Tp
value) of the corresponding stacked values in the Tp scan
panel. Linear interpolation is used to define Tp values,
and from this the stacked data values, at points between
semblance maxima. The velocity output for each time sample
is computed from the picked and interpolated Tp values.
Mute Applications
Two types of mutes may be applied to the input Tp scan and
semblance panels before extracting stacked data and velocity
information. These mute types are velocity mute and mute by
input function. All mutes are applied in an isotime sense.
Velocity Mute
The velocity mute helps to insure than any non-zero values
on the Tp scan and semblance panels correspond to the summa-
tion of valid corrected moveout hyperbola and not to low
velocity or high velocity noise bursts nor to the summation
of the "tails" of two or more hyperbolas. It is defined by
the minimum and maximum velocities you supply and the equa-
tion
where Vmute is the minimum or maximum mute velocity, V0 is
the initial velocity used in program OPSTF, T0 is the zero-
offset time, and the notation "**2" means squared. When
applied, this mute sets to zero any Tp scan and semblance
values corresponding to velocities greater than the maximum
velocity or less than the minimum velocity.
Mute by Input Function
This mute is useful for attenuating undesirable events, such
as multiple reflections, in the Tp scan and semblance data.
The mute is defined by mute functions created in program XOS
(see documentation for program XOS of a detailed description
of how to pick the mute function(s)) and input to the pro-
gram as an ASCII file.
When a file containing mute functions at various records
across the data set being processed is input, the program
uses linear interpolation, both temporily and spatially, to
compute a mute function for each CDP. It then applies the
appropriate function to each scan and semblance panel before
extracting the stacked data and velocity information. If
only one function is supplied, it is used for all CDP's.
Semblance Weighting
Multiplying (weighting) the Tp scan data by the correspond-
ing semblance data has the effect of accentuating the larger
stacked amplitudes and improving the overall signal-to-noise
ratio of the extracted stack. The effects of this weighting
should be carefully examined before choosing it for applica-
tion to the entire data set.
Pick File
The location of the semblance maxima, for each CDP, deter-
mined in the "Extraction according to maximum semblance"
method of stacked data and velocity information extraction
may be saved to an ASCII file and overlain on the Tp scan or
semblance data in the analysis program XOS. This file may
be saved whether this method is used to created the actual
stacked data and velocity output or not (i.e, the maximum
semblance groups are always automatically picked). Display-
ing these picks overlain on the data is a good way to get an
approximation of what either stack extraction method will
"see" in the data and, thereby, an approximation of the
stack to be output. Furthermore, by using the pick editting
and format conversion features available in the analysis
program, these picks may be saved as time-velocity sets to
be used in conventional normal moveout correction programs
(either DISCO or USP). Extraction of this file is recom-
mended for the analysis stage of this process, but NOT
RECOMMENDED for the actual processing phase, since this
ASCII file can use up a large amount of disk space.
REFERENCES
de Bazelarie, E., 1988, Normal moveout revisited: Inhomo-
geneous media and curved interfaces, Geophysics, Vol. 53,
143-157.
Arnold, Richard H. and Semaan, Mars E., 1990, Implementation
of the Optical Stack Method, SEG Expanded Abstracts, Vol II,
San Francisco.
BUGS
unknown
SEE ALSO
opstr, opstk, opstd, opstcv
AUTHOR
Richard Crider, ES&S
COPYRIGHT
copyright 2001, Amoco Production Company
All Rights Reserved
an affiliate of BP America Inc.
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