NAME
vtispec - obtain time and eta (vertically anisotropic,
nonhyperbolic) semblance of data set
SYNOPSIS
vtispec [ -Nntap ] [ -Ootap ] [ -vvtap ] [ -Ffpik ] [ -reire
] [ -rsirs ] [ -nsns ] [ -nene ] [ -tminstart ] [ -tmaxstop
] [ -eminemin ] [ -emaxemax ] [ -nvelnvel ] [ -igtigt ] [
-unitiscl ] [ tmultmul ] [ dmuldmul ] [ thldthresh ] [ -nmo
] [ -W ] [ -V ] [ -? ]
DESCRIPTION
A semblance spectrum over the nonhyperbolic parameter eta is
obtained by scanning the CMP gather of traces along trajec-
tories for signal coherence. These scans establish an eta
function (i.e., eta versus vertical seismic time) that is
used in calculating the fourth order NMO corrections prior
to the stacking of the CMP traces. The output is an usp
disk (or pipe) file of the semblance values. vtispec takes
a set of i traces and performs a time-trajectory-scan opera-
tion to obtain stacking 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
distances x, a nonhyperbolic time function
t(x) = sqrt{ t0**2 + (x/V)**2
- 2eta*x**4 / [V**2(t0**2*V**2 + (1+2eta)x**2] }
is swept along the time-axis of t-x plane of traces for a
given normal incidence time, t0. The offset dependent time,
t(x), is comprised of two terms: a sort offset based stan-
dard moveout term and a long offset nonhyperbolic eta depen-
dent moveout term. Here x is the trace offset, t0 is the
normal incidence time, V is the RMS stacking velocity deter-
mined before hand using standard short offset velocity
analysis techniques, and eta is the nonhyperbolic fourth
order parameter related to the true anisotropic parameters
by
eta = ( epsilon - delta ) / (1 + 2*delta)
(see Tsvankin and Thomsen, 1994).
The parameter eta that produces the maximum semblance
represents the best-fitting fourth order hyperbola and thus
produces the best stack at stacking time t(0). The sem-
blance (Taner and Koehler, 1969) is defined as
CN(eta,t) = [ SUMt [SUMi A(x,t) ]**2 ]/[ N SUMt SUMi
[A(x,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 t0; and A(x,t) is the trace
amplitude at distance x and time t.
The procedure is repeated for a collection of zero-offset
(vertical) times t0, at equally spaced time increments. The
result is called the eta spectrum at the common midpoint.
The velocity spectrum consists of an usp work file of sem-
blance values.
A eta fairway can be input in the form of two xsd pick seg-
ments (made presumably from a typical semblance spectrum) to
limit the semblance calculations. If no fairway is provided
the entire semblance from emin to emax is calculated.
vtispec gets processing controls from the command line. Rea-
sonable 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)
-O otap
Enter the full path of the file to which the output
semblance is to be written. Default is stdout (pipe
out).
-v vtap
Enter the name of the file containing the corresponding
velocity field. There must be one velocity trace for
each input gather. In the XIKP vtispec menu panel the
input socket 3 is for velocity input.
-F fpik
(Optional) Enter the full path of the file containing 2
xsd pick segments made on a typical semblance spectrum,
i.e. the user should run a sample record of his data
through vtispec and output the entire semblance spec-
trum (no fairway pick file). Then in xsd pick a lower
eta-time function and an upper eta-time function to
form a fairway. When this is input in the major
vtispec run no semblance calculations will be made out-
side this fairway significantly reducing run time.
When saving the picks be sure to fill out the eta units
and offset for the trace fields (e.g. for a vtispec run
using 101 etas and a emin=-500 and a emax=+500, i.e. a
eta increment of 5, the trace units will be 5 and the
offset will be -505 [so the first trace will come out
to be -500]).
-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
-nmo If present a standard RMS normal moveout based on short
offset velocity analysis has already been applied, so
scan using only the fourth order term.
-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).
-emin emin
Minimum eta value input (default = -500.0). The eta
values are nominally scaled up by a factor of 1000.
-emax emax
Maximum eta value input (default = +500.0). The eta
values are nominally scaled up by a factor of 1000.
-nvel nvel
Maximum number of eta's to use spaced between emin and
emax (default =201)
-unit iscl
Scale factor (integer) applied to eta values. Default =
1000 This value must be put into the line header slot
T_Unit of the eta data input to the nonhyperbolic
moveout program vtinmo
-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.
-thld thresh
Semblance threshold: semblances less than this value
will be zeroed. Default = 0.0 (no thresholding). With
the -fI-A option (max semblances scaled to 1.0) this
represents a relative thresholding.
-nmo Enter the command line argument '-nmo' to signal that
standard normal moveout (based on short offset velocity
analysis) has already been applied and that only the
forth order term need be used in 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.
-? Query mode. With this flag, vtispec will give a
description of the command line arguments and stop the
program.
NOTE 1
The eta functions determined from the semblance spectra must
be translated into an eta field (usp traces) for use by
vtinmo. The eta data set must have the eta scale factor
inserted into the line header T_Unit slot prior to input
into the moveout code. SH BUGS It is assumed that the
record(s) to be analyzed should be in a CMP gather but
there's no automatic way of determining the type of gather.
SEE ALSO
vtinmo, vomit, velin, vi3d, slvr
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.
Tsvankin, I. and Thomsen, L., (1994). Nonhyperbolic reflec-
tion moveout in anisotropic media, Geophysics, 59, 1290-
1304.
AUTHOR
B. V. Nguyen and R. B. Herrmann, Saint Louis University,
April 1986.
COPYRIGHT
copyright 2001, Amoco Production Company
All Rights Reserved
an affiliate of BP America Inc.
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