NAME
taupspec - module to obtain time and rms velocity plot of
taup data set
SYNOPSIS
taupspec [ -Nntap ] [ -Ootap ] [ -reire ] [ -rsirs ] [ -nsns
] [ -nene ] [ -tminstart ] [ -tmaxstop ] [ -vminvmin ] [
-vmaxvmax ] [ -nvelnvel ] [ -igtigt ] [ -W ] [ tmultmul ] [
dmuldmul ] [ -A ] [ -radon ] [ -slnt ] [ -S ] [ -log ] [ -V
] [ -? ]
DESCRIPTION
A velocity spectrum is obtained by scanning forward tau-p
transform (radonf) of a CMP gather of traces along ellipti-
cal trajectories for signal coherence. These scans establish
a velocity function (i.e., velocity versus vertical seismic
time) that is used in calculating the NMO corrections prior
to the stacking of the tau-p'd CMP traces. The output is an
sis disk (or pipe) file of the semblance values.
taupspec takes a set of i traces and performs a time-
trajectory-scan operation to obtain stacking velocity as a
function of stacking time via a coherence measure (sem-
blance). The method used is the following. If there are N
traces in a tau-p transformed CMP gather at ray parameter
p(i), a hyperbolic time function
t(p(i)) = t(0)*sqrt[1 - (p(i)*V)**2],
is swept along the time-axis of t-p plane of traces for a
given set of test velocities, V. The velocity V that pro-
duces the maximum semblance represents the best-fitting
hyperbola 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(p(i),t) ]**2 ]/[ N SUMt SUMi
[A(p(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(p(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 plane wave
common midpoint. The velocity spectrum consists of an sis
work file of semblance values.
taupspec gets processing controls from the command line.
Reasonable defaults are set up.
Command line arguments
-N ntap
Enter the full path of the file continuing the data
set. If not specified, input is expected on the stan-
dard input. If standard input is not specified, and
there is no input, e.g., program run in background,
expect a very ungracious crash. (default standard
input)
-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.
-A The semblance is a quantity bounded above by +1. If -A
is invoked the contour plot, figure 2, plots the 0.5 ->
1.0 contour levels of the true semblance. If this flag
is not used, then the time trace of semblance is nor-
malized to a maximum of 1.0 at each velocity prior to
plotting the contours. This will in effect raise up
relatively lower peaks.
-S If present, calculate the velocity spectra in equal
increments of slowness squared, where
v=sqrt(slowness**2).
-log If present, the velocity (or slowness) scale is loga-
rithmic. This can often resul t in superior sampling
of the scans.
-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).
-vmin vmin
Minimum velocity value input in ft/sec (default =
1000.0).
-vmax vmax
Maximum velocity value input in ft/sec (default =
10,000.0).
-nvel nvel
Maximum number of velocities to use spaced between vmin
and vmax (default =101) There is maximum.
-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).
Within this gate the default is to apply a cosine bell
weighting. This has the effect of tightening the sem-
blance wraps over a simple box car (unit) weighting.
-W Include -W on the command line to turn off cosine
weighting within the time gate.
-dmul dmul
Multiply each distance, itr(117), 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.
-radon
This flag tells the program that the input data has
been written by program radon.
-slnt
This flag tells the program that the input data has
been written by program slnt.
-? Query mode. With this flag, taupspec 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.
SEE ALSO
radonf, velspec, pomega, velin
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
B. V. Nguyen and R. B. Herrmann, Saint Louis University,
April 1986, modified for tau-p data by Paul Gutowski, TRC,
x3146.
COPYRIGHT
copyright 2001, Amoco Production Company
All Rights Reserved
an affiliate of BP America Inc.
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