NAME
attsel - AVO Attributes with Selective Muting
SYNOPSIS
attsel [ -Nntap ] [ -Ootap ] [ -vRMS Velocity File ] [
-SShear Velocity File ] [ -rs ] [ -re ] [ -as ] [ -ae ] [ -c
] [ -at ] [ -md ] [ -M ] [ -E ] [ -D ] [ -rc ] [ -sa ] [
-abc ] [ -psc ] [ -? ] [ -h ]
DESCRIPTION
ATTSEL computes various AVA (Amplitude Versus Angle) attri-
butes from either CDP (X-T or X-Z) gathers or from angle-
dependent gathers (angle-T domain) created by program ANGST.
All of the attributes created are derived through least
squares inversion of time slices of amplitudes from the
gathers. Two forms of the linear Knott-Zoeppritz (KZ) P-
wave reflectivity equation (Aki and Richards, 1980) are
inverted. The first form is one in which the KZ equation is
modified by merging the fractional change in density with
the fractional change in Vp through Gardner's equation
(Thomsen and Hanson, 1985) and combining terms to produce
the familiar two-term reflectivity equation described below.
This equation is valid for all angles of incidence. Inver-
sion of the amplitudes with this equation produces the
intercept and gradient terms.
The second form, described below and also valid for all
angles of incidence, is the standard form of the Aki-
Richards equation modified as above but without further re-
combination of terms. Inversion of reflectivity with this
equation gives direct estimates of the fractional changes in
both Vp and Vs.
Computation of the fractional change in Vp, along with the
intercept and gradient, permits the estimation of the frac-
tional change in bulk density, bulk modulus, and shear
modulus (see description below). The latter two attributes
can be considered direct fluid and lithology indictors,
respectively.
The Vp/Vs ratio is an integral part of the inversion of
amplitudes for estimation of fractional changes in Vp and
Vs. Program ATTSEL permits the input of (interval) shear
velocities, which may be derived from any reasonable source.
A simple method for creating the shear velocity function is
to convert the RMS function to an interval function and
scale appropriately. If a spatially-variant Vp/Vs ratio can
be determined (from multiple wells on a line, for example),
the appropriate scaling should reflect this. The input
shear function, like the input RMS function, may contain a
single record or multiple records. If the file contains
multiple records, however, the number of records MUST be the
same as the number of input gathers. **NOTE**: If no shear
wave velocity is available, the user may omit the file and
the module will enable Castagna's Mudrock relationship.
Note: when running inside XIKP the 0 connector is for the
input data, the 1 connector is for the output data, the 3
connector is for the RMS P-velocity, and the 4 connector is
for the S-interval velocity.
The ratio between the intercept and gradient (see defini-
tions below) may be expected to have similar magnitudes. In
noisey data, this is not always the case. Program ATTSEL
allows for muting (by setting to zero) values of the inter-
cept and gradient which don't meet user-specified criteria
for ratio. Similar editing of the frational Vp and Vs out-
put can be performed.
The linearized KZ equation as normally written contains an
implicit average of incident and refraction angles at an
interface. Program ATTSEL performs this averaging, but pro-
vides the option to "turn off" the operation. Averaging or
not averaging will probly make little difference in the
results unless one of the "kinked" ray tracing techniques
(mode 2 or 3) are used.
All of the attributes listed below are computed and output
by program ATTSEL. The desired attributes can be selected
from the output for further processing and/or display. The
order of the traces is (using "D" to denoted "delta" or
change in):
Trace Number Attribute
1. Intercept (B0) from 2-term equation below
2. Slope (B1) from 2-term equation below
3. DVp/Vp from 2-term equation below
4. DVs/Vs from 2-term equation below
5. DRho/Rho (Bulk density - see Note 1 below)
6. DK/K (Bulk modulus - see Note 2 below)
7. DG/G (Shear modulus - see Note 2 below)
8. Standard deviations for intercept/gradient fit
9. Standard deviations for DVp/Vp / DVs/Vs fit
The two-component equation used for DVp/Vp and DVs/Vs compu-
tation is
R= A*DVp/Vp + B*DVs/Vs
with A= 0.5*[1.0+tan^2(theta)+ c - 4*c*X*sin^2(theta)] and
B= -X*sin^2(theta), where
X = (2*Vs/Vp)^2, Vp and Vs are the average P and S veloci-
ties at the interface, "^" denotes exponentiation, theta is
the angle of incidence, and c is the exponent for the
Gardner's relation (rho = const*Vp^c).
The two-component equation solved for the intercept and
slope is given by
R = A + B1*sin^2(theta), where
A = B0(1.0+((1.+c)^-1)*tan^2(theta)*sin^2(theta)),
B0 = 1/2(DVp/Vp + Drho/rho), and
B1 = 1/2(DVp/VP - X*DG/G) (X defined above)
Note: By setting c = 0, the program sets A to a constant.
Note 1: Given stable values of B0 and fractional change in
Vp, the fractional change in bulk density can be computed as
Drho/rho = 2*B0 - DVp/Vp
Note 2: Given stable values of B0, B1, and fractional
change in Vp, the fractional change in shear and bulk moduli
(G and K, respectively) can be computed as
DG/G = (DVp/Vp - B1)*(2/X) (X defined above)
DK/K = (3*B0 + B1 + DVp/Vp)*[2/(3-X)] (X defined above)
Program ATTSEL gets its data and parameters from command
line arguments. These arguments specify the input data and
velocity files, the output data set, and other computation
limitations.
Command line arguments
-N ntap
Enter the input data set name immediately after typing
-N. This input file should include the complete path
name if the file resides in a different directory.
Example -n/b/tsp/dummy tells the program to look for
file 'dummy' in directory '/b/tsp'. (Default = stdin).
Note: when running inside XIKP the 0 connector is for
the input data, the 1 connector is for the output data,
the 3 connector is for the RMS P-velocity, and the 4
connector is for the S-interval velocity.
-O otap
Enter the output data set name immediately after typing
-O. Specify the full path to write the file to a
directory other than the current working directory.
(Default = stdout ). Note: when running inside XIKP the
0 connector is for the input data, the 1 connector is
for the output data, the 3 connector is for the RMS P-
velocity, and the 4 connector is for the S-interval
velocity.
-v RMS velocity file
Enter the name of the file containing the RMS veloci-
ties corresponding to the input data set, one velocity
record for each CDP. This is typically the output from
program VELIN or program VOMIT. (Default = NONE. This
parameter is REQUIRED). Note: when running inside XIKP
the 0 connector is for the input data, the 1 connector
is for the output data, the 3 connector is for the RMS
P-velocity, and the 4 connector is for the S-interval
velocity. ***Important*** If the input is in depth,
this file must be interval velocities in depth. DO NOT
INPUT X-Z gathers AND X-T velocities!
-S S-wave (interval) velocity file
Enter the name of the file containing the S-wave inter-
val velocities corresponding to the input data set, one
velocity record for each CDP. (Default = Castagna's
Mudrock relationship). If the user does not use an
input file for this, then Castagna's Mudrock relation-
ship is enabled. Note: when running inside XIKP the 0
connector is for the input data, the 1 connector is for
the output data, the 3 connector is for the RMS P-
velocity, and the 4 connector is for the S-interval
velocity. ***Important*** If the input is in depth,
these interval velocites must also be in depth.
-rs start record
Enter the sequential number of the record on which to
begin processing. All data prior to this record will
be skipped and not output. (Default = first)
-re end record
Enter the sequential number of the record on which to
end processing. All data following this record will be
skipped and not output. (Default = last)
-as mininum angle
Enter the minimum incident angle, in degrees, to limit
the data values used in the weighted stacks. Data
corresponding to incident angles less than this value
are ignored. (Default = 0 degrees)
-ae maximum angle
Enter the maximum incident angle, in degrees, to limit
the data values used in the weighted stacks. Data
corresponding to incident angles greater than this
value are ignored. (Default = 45 degrees)
-c Gardner's Exponent
Enter the value to be used for exponent for Gardners
equation. The value may be set to 0.0 and the default
only applies if the parameter is not coded on the com-
mand line. (Default = 0.25, if the parameter is not
coded)
-at angle-T Domain flag
If this flag is present, the input data are assumed to
be in the angle-T domain and ray tracing will be used
to define the incident angle field for the regression
procedure. (Default = X-T, if -at is not present the
input data is assumed to be X-T domain.)
-M Input data is in Metric Units (Default). This flag is
ignored if Shear interval velocity is supplied.
-E Input data is in English Units. This flag is ignored
if Shear interval velocity is supplied.
-md Ray tracing mode
Enter the flag indicating the type of ray-tracing solu-
tion desired. A value of 0 gives a straight ray solu-
tion. A value of 1 gives a curved ray solution. A
value of 2 gives a perturbed curved ray solution
wherein the smooth curved ray solution is "perturbed"
in accordance with the input velocity function. A
value of 3 gives a "kinked" ray output consistent with
the Dix equation and used by most other external AVO
attribute generation codes. This parameter is ignored
if the -xt flat is not present. Otherwise, the default
is 1 (curved ray).
-rc Minimum number of points in inversion
Enter the minimum number of amplitude values to use in
the least square solutions. Technically, this number
should be on the order of 20 to ensure valid solutions.
However, as few as 3 may be used but is not recom-
mended.
-sa Flag to turn off averaging of incident angles
Supply this flag to tell the program to NOT average
angles across each interface. The default is to aver-
age the angles.
-abc B1/B0 ratio limit
Use this option to mute B0 and B1 values which produce
a B1/B0 ratio greater than the value of this parameter.
-psc DVs/Vs / DVp/Vp ratio limit
Use this option to mute DVp/Vp and DVs/Vs values which
produce a DVs/Vs / DVp/Vp ratio greater than the value
of this parameter.
-? Enter the command line argument '-?' to get online help.
The program terminates after the help screen is printed.
-H Same as -?, except a list of the output traces is also
printed. The program terminates after the help screen is
printed.
SEE ALSO
angst(1)
square(1)
sscale
sscaleu
REFERENCES AND FURTHER READING
Aki, K. and Richards, P.G., 1980, Quantitave Seismology:
Theory and Methods, Freeman, San Francisco.
Castagna, J.P., Batzle, M.L., and Eastwood, R.L., 1985,
Relationships Between Compressional-Wave and Shear-Wave
Velocities In Clastic Silicate Rocks, Geophysics, Vol. 50,
pp 571-581.
Castagna, J.P., Batzle, M.L., and Kan, T.K., 1992, Rock Phy-
sics - The Link Between Rock Properties and AVO Response in
Offset-Dependent Reflectivity - Theory and Practice of AVO
Analysis, J.P. Castagna and M.M. Backus (eds), Society of
Exploration Geophysics.
Castagna, J.P., and Smith,, S.W., 1994, Comparison of AVO
Indicators: A Modeling Study, Geophysics, Vol. 59, pp
1849-1855.
Simmons, James L., and Backus, Milo M., 1994, AVO inversion
and direct hydrocarbon detection, SEG Abstracts.
Smith, G.C. and Gidlow, P.M., 1987, Weighted Stacking for
Rock Property Estimation and Detection of Gas, Geophysical
Prospecting, Vol. 35, pp 993-1014.
Thomsen, Leon, 1992, Weak Anisotropic Reflections in
Offset-Dependent Reflectivity - Theory and Practice of AVO
Analysis, J.P. Castagna and M.M. Backus (eds), Society of
Exploration Geophysics.
Thomsen, L.P., 1982, Amplitude Vs. Range Attributes: Interim
Report, Amoco Production Company Research Department Report
T82-E-10.
Thomsen, L.A. and Hanson, K.E., 1985, Linear RDA: S and P,
Amoco Production Company Research Department Report T86-E-
44.
AUTHOR
Richard Crider Houston
Minor command line change (9-96) by James Gridley. Unit
flag and Castagna Mudrock line made 10-96 by James Gridley.
COPYRIGHT
copyright 2001, Amoco Production Company
All Rights Reserved
an affiliate of BP America Inc.
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