GSLIB Help Page: SGSIM

Description:

Sequential Gaussian simulation program

Parameters:

datafl: the input data in a simplified GeoEAS formatted file.
If this file does not exist then an unconditional simulation will be
generated.

icolx, icoly, icolvr, icolwt and icolsec: the column
numbers for the x,y and z coordinates, the variable to be
simulated, the declustering weight, and the secondary variable (e.g.,
for external drift if used). One or two of the coordinate column
numbers can be set to zero which indicates that the simulation is
2D or 1D. For equal weighting, set icolwt to zero.

tmin and tmax: all values strictly less than tmin
and strictly greater than tmax are ignored.

itrans: if set to 0 then no transformation will be performed;
the variable is assumed already standard normal (the simulation
results will also be left unchanged). If itrans=1,
transformations are performed.

transfl: output file for the transformation table if
transformation is required (igauss=0).

ismooth: if set to 0, then the data histogram, possibly with
declustering weights is used for transformation, if set to 1, then the
data are transformed according to the values in another file
(perhaps from histogram smoothing).

smthfl: file with the values to use for transformation to normal
scores (if ismooth is set to 0).

icolvr and icolwt: columns in smthfl for the
variable and the declustering weight (set to 1 and 2 if smthfl
is the output from histsmth).

zmin and zmax the minimum and maximum allowable data
values. These are used in the back transformation procedure.

ltail and ltpar specify the back transformation
implementation in the lower tail of the distribution:
ltail=1 implements linear interpolation to
the lower limit zmin, and ltail=2 implements power model
interpolation, with w=ltpar, to the lower limit
zmin.

The middle class interpolation is linear.

utail and utpar specify the back transformation
implementation in the upper tail of the distribution:
utail=1 implements linear interpolation to the upper
limit zmax, utail=2 implements power model interpolation,
with w=utpar, to the upper limit zmax, and
utail=4 implements hyperbolic model extrapolation with
w=utpar. The hyperbolic tail extrapolation is limited by
zmax.

idbg: an integer debugging level between 0 and 3. The larger the
debugging level the more information written out.

dbgfl: the file for the debugging output.

outfl: the output grid is written to this file. The output
file will contain the results, cycling fastest on x then y
then
z then simulation by simulation.

nsim: the number of simulations to generate.

nx, xmn, xsiz: definition of the grid system (x axis).

ny, ymn, ysiz: definition of the grid system (y axis).

nz, zmn, zsiz: definition of the grid system (z axis).

seed: random number seed (a large odd integer).

ndmin and ndmax: the minimum and maximum number of
original data that should be used to simulate a grid node. If there are
fewer than ndmin data points the node is not simulated.

ncnode: the maximum number of previously simulated nodes to use
for the simulation of another node.

sstrat: if set to 0, the data and previously simulated grid nodes
are searched separately: the data are searched with a super block
search and the previously simulated nodes are searched with a spiral
search (see section II.4). If set to 1, the data are relocated to grid
nodes and a spiral search is used and the parameters ndmin
and ndmax are not considered.

multgrid: a multiple grid simulation will be performed if this is
set to 1 (otherwise a standard spiral search for previously simulated
nodes is considered).

nmult: the number of multiple grid refinements to consider
(used only if multgrid is set to 1).

noct: the number of original data to use per octant. If this
parameter is set less than or equal to 0, then it is not used;
otherwise, it
overrides the ndmax parameter and the data is partitioned into
octants and the closest noct data in each octant is retained
for the simulation of a grid node.

radius_hmax, radius_hmin and radius_vert:
the search radii in the maximum horizontal direction, minimum
horizontal direction, and vertical direction (see angles below).

sang1, sang2 and sang3: the angle parameters that
describe the orientation of the search ellipsoid. See the discussion
on anisotropy specification associated with Figure II.4.

ktype: the kriging type (0 = simple kriging, 1 = ordinary
kriging, 2 = simple kriging with a locally varying mean, 3 = kriging
with an external drift, or 4 = collocated cokriging with one secondary
variable) used throughout the loop over all nodes. SK is required by
theory; only in cases where the number of original data found in the
neighborhood is large enough can OK be used without the risk of
spreading data values beyond their range of influence

rho: correlation coefficient to use for collocated cokriging
(used only if ktype = 4).

secfl: the file for the locally varying mean, the external drift
variable,
or the secondary variable for collocated cokriging (the secondary
variable must be gridded at the same resolution as the model being
constructed by sgsim).

nst and c0: the number of semivariogram structures and the
isotropic nugget constant.

For each of the nst nested structures one must define it,
the type of structure; cc, the c parameter;
ang1,ang2,ang3, the angles defining the geometric anisotropy;
aa_hmax, the maximum horizontal range; aa_hmin,
the minimum horizontal range; and aa_vert, the
vertical range.

Application notes:

This program requires standard normal data and writes standard normal
simulated values. Normal score transform and back transform are to be
performed outside of this program

Recall that the power model is not a legitimate model for a
multiGaussian phenomenon and it is not allowed in sgsim

The semivariogram model is that of the normal scores.
The kriging variance is directly interpreted as the variance of the
conditional distribution; consequently, the nugget constant c0
and c (sill) parameters should add to 1.0.