NAME
ampcor - compute and apply surface gain corrections
SYNOPSIS
ampcor [ -Nntap ] [ -Omtap ] [ -Totap ] [ -Stnam ] [
-tstrtstr ] [ -tstptstp ] [ -dspdamp1 ] [ -drpdamp2 ] [
-isedised ] [ -nitnit ] [ -scltisrcscl ] [ -A ] [ -R ] [ -M
] [ -V ] [ -? ]
DESCRIPTION
ampcor computes and applies a set of surface consistent gain
corrections for each shot and receiver point. The aim is to
provide corrections for source and receiver induced ampli-
tude distortions along a line.
ampcor operates on a set of traces sorted to CMP gathers.
Gain corrections are by computing the gains for each
gather, shot and receiver, and then applying this correction
to the gather. A new reference stack is then computed after
each pass. The user must specify the time window in which
the test stack is made. The algorithm is fully nonlinear in
the sense that traces in the common shot or receiver gather
are gain corrected using each newly computed gain factor,
and the test stack is then updated using these corrected
traces. An L1 (median) back projection algorithm is used to
find the updates for each of the gain corrections. The L1
norm is used since experience shows that the maximum lag
values of the crosscorrelation can have considerable
scatter. The L1 norm is robust agains the adverse affects of
these "outlying" lag values. The model parameters can be
constrained to give a spatially smooth solution. A penalty
function (which is the second spatial derivative of the shot
or receiver gain corrections) is used to impose this smooth-
ness constraint. A user adjustable trade-off parameter is
used to weight the affects of model smoothness versus minim-
ization of the least absolute deviation (minimum L1 norm) of
the crosscorrelation maximum lags. One iteration of the
algorithm is defined as operating on each shot and receiver
point one time.
The selection of the time window in which the test stack is
made can have an influence on the outcome of the calcula-
tion. One should pick a time window with the best data qual-
ity, both in the stack and prestack.
The use of the spatial smoothing constraint requires the
specification of two tradeoff parameters. These parameters
specify the relative amount of weight that is given to the
minimum absolute deviations of the maximum gain corrections
versus the smoothness of either the shot point or receiver
point gain corrections. The parameters specify the fraction
of the second derivative that is subtracted from the gain
correction computed from the gather. The following condi-
tions hold: 1) if the parameter is set to zero spatial
smoothness is neglected (This might be useful if shot or
receiver points were quite far apart, so that it is not rea-
sonable to assume that the gain corrections at neighboring
points are correlated), 2) if the parameters are set to 0.5
the weights are equal and 3) if the parameters are set to
1.0 the estimated gain corrections will be zero (The smooth-
ness is at a maximum). In general, smoothing parameters
should be between 0.0 and 0.5. ampcor. prints the stack
semblance at the start of the process and at each iteration,
as well as the gain corrections computed for each shot and
receiver point to printout file. This diagnostic information
is very important in assessing the performance of ampcor.
The affects of changing program parameters can be objec-
tively judged by noting the change in the stack semblence at
the end of the processing. A positive change in semblance
indicates that the processing was more successfull. If sem-
blance has remained close to constant for the last several
iterations of the algorithm this indicates convergence. If
convergence does not appear to have been achived, run ampcor
again specifying more iterations. Since the mapping from
phase shifts to stack power is quite nonlinear it is normal
that semblance may decrease at some iterations. If however,
semblance decreases throughout the entire process this indi-
cates that ampcor has failed entirely. Generaly the reason
for such a failure is that the data may be too noisey. Addi-
tional processing to remove coherent noise may help.
This program performs complex bookkeeping operations so that
common shot and receiver gathers can be extracted from the
CMP gather file. Because of this bookkeeping, piping out of
this program may lead to bizarre and unpredicatable
behavior. Further, if you do not want to work on a whole
line at once you should use editt to select the records and
traces to be used before the sort table is made with
presort. Operating on a full line should give a better solu-
tion since the number of traces at each shot and receiver
point will be the maximum possible. Further, there is a
chance that the gain corrections computed independently for
each segment of a line will not be compatible, resulting in
"hot spots" in the stack. This is especially true since the
gain corrections computed near the ends of lines (or line
segments) are less reliable since the number of traces at
each shot and receiver point will be fewer than the maximum.
Command line arguments
-N ntap
Enter the full path name of the file containing the
data set. The data must be sorted in common midpoint
gathers. NMO need not be applied. The program will
give an error message if the input file is read
protected. If no input file is given the program
assume input from a pipe. (default = No Default )
-O mtap
Enter the full path name for the output file. The out-
put file will be in the form of common midpoint gathers
with the estimated phase corrections corrections
applied. This file cannot be sent to standard output
since the traces are read in a random order as the gain
corrections are estimated and applied. NOTE: The
traces in the output file will have been RMS amplitude
normalized using the test stack time window. The pro-
gram will give an error message if the output file is
being written to a write protect directory. (default =
No Default )
-T otap
Enter the full path name of the file that is to contain
the test stack. The program will give an error message
if the output file is being written to a write protect
directory. (default = No Default )
-S tnam
Enter the full path name of the sort table produced by
the usp program presort. Presort must be run on the
file specified as input. Beware; the ability of the
program to detect errors in the sort table (ie. using a
table that was made for a file that is not in exactly
the same format as the one specified as input) is quite
limited. The program will give an error message if the
input file is read protected. If no input file is
given the program will hang. (default = No Default )
-tstr tstr
The start time of the test stack. This time must be at
least mshft (maximum time shift) from the start of the
input trace. (default = No Default )
-tstp tstp
The end time of the test stack. This time must be at
least mshft (maximum time shift) from the end of the
input trace. NOTE: Since each data set is different,
and since selection of the test stack time window is
critical to the success of this process, no defaults
are possible for tstr or tstp (default = No Default )
-dsp damp1
The spatial smoothness constraint parameter for shot
points. This parameter should generally be set between
0.0 and 0.5, and must always be less than 1.0. NOTE:
See the explanation of this parameter in the Descrip-
tion section above. (default = 0.0 )
-drp damp2
The spatial smoothness constraint parameter for
receiver points. This parameter should generally be set
between 0.0 and 0.5, and must always be less than 1.0.
NOTE: See the explanation of this parameter in the
Description section above. (default = 0.0 )
-ised ised
The seed for the random number generator used to deter-
mine the order in which shot and receiver points are
operated on. If ampcor is to be used on the same line
several times (say following velocity analysis), it is
a good idea to change ised so that serial correlation
of errors from one application of the process to
another will be reduced. (default = 1256 )
-nit nit
The number of iterations of gain corrections applied to
the data. (default = 2 )
-scl isrcscl
Enter the multiplier that was applied to the src point
#s in input trace header 109 (mnemonic SrcLoc). Usu-
ally it's been x10 to take care of fractional src point
numbers as integers, but it could be some other scale
factor (e.g. 2). The reason being is that with a x10
multiplier source point #s reach 32767 (the limit for
short integers) in long marine lines so one must use a
smaller multiplier to preserve fractional sp's.
Chances are if this limit was reached presort would
have failed with an appropriate message in the printout
file. (default = 10 )
-A If present on command line means the average absolute
amplitude will be used to compute the gain.. Note: for
most data it is recommended that the median option be
used.
-R If present on command line means the rms amplitude will
be used to compute the gain. If none of the above flags
are given the default is absolute maximum. Note: for
most data it is recommended that the median option be
used.
-M If present on command line means the median amplitude
will be used to compute the gain. If none of the above
flags are given the default is absolute maximum. Note:
for most data it is recommended that the median option
be used.
-V Verbose mode. All parameters from the command line and
line header and other diagnostic information will be
sent to standard error.
-? Query mode. With this flag, ampcor will send a
description of the command line arguments to the stan-
dard error output and stop.
BUGS
No trap for running program without an input file.
There is no way to change the range of records or traces
that are used in the processing. This would require very
complex bookkeeping operations. Use editt before presort to
change the configuration of the input data file.
There is no way to constrain the gain corrections at shot
and receiver points, since the bookkeeping to find adjacent
points is not in the program.
This program is fairly slow to run since random trace I/O
must be done on the test stack and output files.
AUTHORS
Paul Gutowski, APR, July, 1992) and based on the rstatic
program of Stephen F. Elston, Princeton University (July,
1991).
COPYRIGHT
copyright 2001, Amoco Production Company
All Rights Reserved
an affiliate of BP America Inc.
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