Network ray tracing program package NET
(general description)
                                                                 
Version: 3.10
Date: 1998, October 24

Authors:
    Ludek Klimes
            Department of Geophysics, Charles University Prague,
            Ke Karlovu 3, 121 16 Praha 2, Czech Republic,
            E-mail: klimes@seis.karlov.mff.cuni.cz
    Michal Kvasnicka
            Department of Geophysics, Charles University Prague,
            Ke Karlovu 3, 121 16 Praha 2, Czech Republic,
            E-mail: qasnicka@seis.karlov.mff.cuni.cz

This is just a general overview, the detailed description of input
data, procedures, and other important topics is included within the
individual FORTRAN77 source code files.

References:
    Klimes L. and Kvasnicka M. (1994):  3-D network ray tracing.
            Geophys.J.int., 116, 726-738.
    Klimes L. (1996):  Grid travel-time tracing: second-order method
            for the first arrivals in smooth media.
            PAGEOPH, 148, 539-563.

Purpose:
    Network ray tracing is designed to calculate first-arrival travel
    times and the corresponding rays.  For your application, please,
    do not confuse the first-arrival travel times with the ray-theory
    body-wave travel times.  The first-arrival travel times from the
    given source of any shape are calculated at all grid points of a
    given rectangular grid of points.  Together with the first-arrival
    travel times, also their maximum errors may be automatically
    estimated.

Model specification:
    For network ray tracing, the seismic model is specified in terms
    of grid velocities at a rectangular grid.  The density of the grid
    should correspond to the accuracy required and to the computer
    memory available.  In a layered or blocky medium, also the indices
    of geological blocks at gridpoints are recommended to be specified
    in addition to the velocities, in order to improve accuracy
    considerably.
    The model specification subroutine package MODEL is recommended
    to describe the models and to generate the grid values, unless the
    user prefers his own model specification system.

Attention: Array dimensions MIND, MGRID, MICB, MNFS, MPRED, MFS, MSRC,
    and MREC in 'net.for' should be adjusted according to a particular
    computer memory and particular application.  In the basic version
    delivered, arrays in 'net.for' are roughly dimensioned for a
    computer with 16MB of RAM (see 'net.inc'), assuming no indexing of
    Fresnel volumes, and assuming all other options like forward-star
    optimization and geological block indexing enabled.  If having
    adjusted the array dimensions in 'net.inc' and going to use
    'netind.for', it is strongly recommended to adjust array
    dimensions MPOS and MTT in 'netind.for' according to MIND and
    MGRID in 'net.for'.

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Application of the network shortest path ray tracing program NET in
the models described in terms of model specification subroutines of
the package MODEL, version 4.00 or later (MODEL ver. 5.10 or later is
strongly recommended):

(1) Compile and link the program 'grid.for' of the package MODEL.
    See the package MODEL for details.  Compile and link the program
    'net.for' of this package.
(2) Prepare the input data for the package MODEL, describing the
    seismic model of the medium.
    Example: In the demo data (E) of this package, the demo model data
            file 'model.dat' of the package MODEL is assumed.
(3) Program 'grid.for' of the package MODEL generates the velocities
    and indices of geological blocks in the given rectangular grid of
    points.
    Example: In the demo data (E), the main input data file for
            'grid.for' is called 'len-grid.dat' (thus, this name,
            embedded in apostrophes, has to be submitted manually to
            the program when it starts and asks for the filename).
            The input model file is 'model.dat' and the grid is
            described within the input data 'len-net.dat'.  The output
            files with velocities and indices of geological blocks are
            generated under filenames 'vel.out' and 'icb.out',
            respectively.
(4) The program 'net.for' of this package performs the network
    shortest path calculation of the first-arrival travel times in the
    generated discretized (raster) representation of the model.
    Example: In the demo data (E), the main input data file for
            'net.for' is called 'len-net.dat'.  This filename, in
            apostrophes, has to be submitted manually to the program
            when it starts and asks for the filename.  Other input
            files are 'lenn-src.dat', 'lenn-rec.dat', 'vel.out', and
            'icb.out'.
            The generated output files 'tt.out' and 'err.out' contain
            first-arrival travel times and their estimated maximum
            errors.

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Acknowledgements:
    The development of the NET package and the related algorithms has
    been partially supported by:
    Faculty of Mathematics and Physics, Charles University, Prague.
    Institute of Geotechnics, Czechoslovak Academy of Sciences,
            Prague.
    Grant agency of the Academy of Sciences of the Czech Republic
            under Contracts 31223 and 346110.
    Grant Agency of the Charles University under Contracts 8/94 and
            38/94.
    Grant Agency of the Czech Republic under Contract 205/95/1465.
    European Commission within the framework of the JOULE II Project
            'Integrated Structural Imaging of Seismic Data'.
    Members of consortium 'Seismic Waves in Complex 3-D Structures'.

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