C Files 'prem*.*':  B-spline approximation of PREM by Dziewonski and
C Anderson (1981).
C Guide to the input data for the complete ray tracing program.
C
C By Ludek Klimes
C Date: 1997, September 26
C
C The spherically symmetric Earth model is a B-spline approximation to
C PREM.  The model consists of 13 complex blocks (layers) limited by
C Earth's surface (surface 1), and separated by 9 interfaces of the
C first order and 3 interfaces of the second order.  The values of
C parameters at grid points are taken from the table II of the paper by
C Dziewonski and Anderson (1981).  Some values, specified in the table
C II of the paper, which will likely be accurately fitted by cubic
C splines are left out.  Between the given points within each complex
C block, the parameters are interpolated using natural cubic splines.
C The model is specified in geographic spherical coordinates:
C X1=longitude (in radians), X2=latitude (in radians), X3=radius.  Four
C elementary waves passing through the core are computed for the source
C in the depth of 200 km.  To change the source depth, see the note in
C input data file 'prem-crt.dat'.  Two-point rays, coming to the
C receivers at the Earth's surface in the epicentral distances from 110
C to 178 degrees with the step of some 2 degrees, are determined.
C
C Files 'prem-*.dat' listed below contain the sample input data for the
C complete ray tracing program.  The input data consist of the 7 data
C sets (1) to (7) listed in file 'crtdoc.htm' (corresponding to lines
C (2) to (8) of input data for 'crtin.for').  Note that several data
C sets may be located in a single file.
C     'prem-crt.dat'... file containing the data sets:
C             (1) CRT
C             (3) DCRT
C             (4) INIT
C     'prem-mod.dat'... File containing the data set:
C             (2) MODEL
C     'prem-cod.dat'... File containing the data set:
C             (5) CODE
C     'prem-rpa.dat'... File containing the data set:
C             (6) RPAR
C     'prem-wri.dat'... File containing the data set:
C             (7) WRIT
C     'prem-src.dat'... File containing the source coordinates.
C     'prem-rec.dat'... File containing the receiver coordinates.
C
C Files listed below contain the sample batch files to run the complete
C ray tracing program with the above data:
C     'prem-crt.bat'... MS-DOS,
C     'prem-crt'... Unix.
C
C Remarks within the input data files:
C     The comments in brackets situated at the ends of lines, after the
C             input items, are not the part of the input data.  They are
C             just remarks placed so as to be skipped when reading the
C             input data.
C     The last input line of a data file is usually followed by
C             (a) Empty line, line '====' and empty line, or by
C             (b) Empty line, line '----', empty line, text describing
C                 the input data, empty line, line '====' and empty
C                 line.
C
C Reference:
C     Dziewonski A.M., Anderson D.L.(1981): Preliminary reference Earth
C            model.  Phys. Earth Planet. Inter., 25, 297.
C
C=======================================================================
C