NAME
rnmospec - semblance scan for residual moveout
SYNOPSIS
rnmospec [ -Nntap ] [ -Ootap ] [ -reire ] [ -rsirs ] [ -nsns
] [ -nene ] [ -tminstart ] [ -tmaxstop ] [ -rminrmin ] [
-rmaxrmax ] [ -unitiscl ] [ -nvelnvel ] [ -igtigt ] [ tmult-
mul ] [ dmuldmul ] [ -V ] [ -? ]
DESCRIPTION
A velocity spectrum is obtained by scanning the initially
moved out CMP gather of traces along parabolic trajectories
for signal coherence. These scans establish a velocity devi-
ation function (i.e., delta velocity about some initial
input velocity-time function versus vertical seismic time)
that is used in calculating the RNMO corrections prior to
the stacking of the CMP traces. The output is an sis disk
(or pipe) file of the semblance values. rnmospec takes a
set of i traces and performs a time-trajectory-scan opera-
tion to obtain rnmo velocity as a function of stacking time
via a coherence measure (semblance). The method used is the
following. If there are N traces in a CMP gather at dis-
tances x, a hyperbolic time function
t(r(i)) = sqrt [ t0 **2 + (gamma **2 - 1) * (x/2) **2 ]
(where t0 is the zero offset time or depth) is swept along
the time-axis of t-x plane of traces as a function of gamma.
The delta moveout that produces the maximum semblance
represents the best-fitting parabola and thus produces the
best stack at stacking time t(0). The semblance (Taner and
Koehler, 1969) is defined as
CN(V,t) = [ SUMt [SUMi A(r(i),t) ]**2 ]/[ N SUMt SUMi
[A(r(i),t)]**2 ]
where the SUMi is from i = 1 to N; SUMt is the time-gate sum
from t-T/2 to t+T/2 at time t(0); and A(r(i),t) is the trace
amplitude at distance r and time t.
The procedure is repeated for a collection of zero-offset
(vertical) times t(0), at equally spaced time increments.
The result is called the velocity spectrum at the common
midpoint. The velocity spectrum consists of an sis work file
of semblance values between 0.0 and 1.0.
The resulting semblance gathers can be automatically picked
under horizon control using either horvel (2D case) or
horvel3d (3D case). The output of these auto-pickers will be
TDFN function of time-gamma pairs (with gamma scaled by an
appropriate value, say 1000). These function are then input
to of of the velocity tape programs, e.g. velin, slvr, vi3d.
Once the gamma seismic format traces have been generated
they can be used by rnmo to apply the residual corrections
to the input seismic gathers.
rnmospec gets processing controls from the command line.
Reasonable defaults are set up.
Command line arguments
-N ntap
Enter the full path of the file containing the data set
which has already had an initial nmo applied. If not
specified, input is expected on the standard input. If
standard input is not specified, and there is no input,
e.g., program run in background, expect a very ungra-
cious crash. Default is standard input (stdin)
-O otap
Enter the full path of the output disk file containing
velocity-time semblance scans. Default is pipe out
(stdout)
-rs irs
First record for processing (default = 1)
-re ire
Last record for processing (default = last)
-ns ns
Define the starting trace for analysis (default = 1)
-ne ne
Define the last trace for analysis in each record
(default = ns). The use of the ns and ne pair are use-
ful to be able to skip dead traces or to select just a
portion of the data set for analysis which is loga-
rithmic scale of 2 cycle length.
-V Flag for verbose output on the standard output. This
lists salient trace header information as well as all
command values used for processing, the default ones
given if not overridden.
-tmin start
Starting time of traces in milliseconds (default = 0,
beginning of trace).
-tmax stop
End time in each trace for analysis in milliseconds
(default = end of trace).
-rmin rmin
Minimum residual moveout gamma value (scaled by iscl
below) on low side of the input RNMO function. If this
value drops below 0 no semblance scan will be done and
0 will be output. Default = 950
-rmax rmax
Maximum residual moveout gamma value (scaled by iscl
below) on the high side of the input RNMO function.
Default = 1050
-unit iscl
Scale factor for gamma values. Since gamma values are
in the range of .9 to 1.1 integer representation in our
velocity tools presents a problem. The scale factor
here is to boost the value to a level where integer
representation is possible. Default = 1000
-nvel nvel
Maximum number of RNMO scans. Default = 100
-igt igt
An integer value (default = 8) to give the time-gate
window T centered at t at time t(0). The half-time
gate T/2 is related to igt by T/2=igt*dt, where dt is
the sample interval in ms. Time gate is small (Robin-
son, 1983). Time gate is usually tens of milliseconds
and generally is approximately equal to the period of
the dominant frequency in the data (Schultz, 1984).
-dmul dmul
Multiply each distance, DstSgn, by this factor
(default=1.0). This is useful when the header has dif-
ferent units than expected.
-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.
-? Query mode. With this flag, rnmospec will give a
description of the command line arguments and stop the
program.
BUGS
It is assumed that the record(s) to be analyzed should be in
a CMP gather and already has primary nmo applied. Also care
should be taken to match the input data gather sequence to
the traces on the velocity tape. Ideally, there should be
one velocity trace/gather. If the velocity trace is not as
long as the input data the last velocity will be repeated to
pad out the trace; if it is longer it will be truncated.
SEE ALSO
vomit, velin, rnmo, horvel, horvel3d
REFERENCES CITED
Hubral, P. and T. Krey (Larner, K. L., Ed.) (1980). Interval
velocities from seismic reflection time measurements,
Society of Exploration Geophysicists, 203pp.
Robinson, E. A. (1983). Seismic velocity analysis and the
convolution model, IHRDC, 290pp.
Schultz, P. S. (1984). Seismic velocity estimation, in
Proceedings of the IEEE, Oct. 1984, 1330-1339.
Taner, M. T. and F. Koehler (1969). Velocity spectra--Digital
computer derivation and applications of velocity functions,
Geophysics, 34, 859-881.
AUTHOR
Paul Gutowski
COPYRIGHT
copyright 2001, Amoco Production Company
All Rights Reserved
an affiliate of BP America Inc.
Man(1) output converted with
man2html