wiki:ShmDocFileTravelTimeTables
Last modified 14 years ago Last modified on 02/04/2009 03:35:57 PM

The travel time tables

For several options SH/SHM use its own travel time tables. All the file must reside in $SH_INPUTS and have an extension .TTT. For historic reasons (the VAX/VMS origin of the program) the travel time table filenames use all uppercase letters. To translate a phasename with lowercase letters in it to the corresponding filename convert the lowercase letter to uppercase and precede it by an uppercase V. E.g. the phase pPcP translates into the filename VPPVCP.TTT. The format of the files is described below. The travel time tables supplied in the distribution are based on the IASP91 velocity model (for teleseismic phases). They were created using the program ttimes of Ray Buland. You may replace the existing files by your own version (but keep a copy of your files on a private directory, so you can restore them after making a complete update of SH/SHM) or add travel times tables of new phases to the directory. SHM makes no restrictions on using arbitrary phasenames, except phases starting with lowercase b and phases with V or v in the name (see above).

The format of the files:

  • Lines at the beginning of the file starting with an exclamation sign ! are regarded as comment and ignored.
  • The first data line contains the three characters TTT.
  • The next line is ignored. It usually contains the distance bounds.
  • The next line contains two floating point numbers, separated by blank(s), specifying the interval of epicentral distance (lower and upper bound in degrees) where this table has travel time values.
  • The next line is ignored. It usually contains the depth steps of the travel times lines.
  • The next line starts with an integer N, specifying the number of depths steps following in this line. Appended are N floating point numbers giving the depth grid points of the following table. They need not to be equidistant, but most of them are. All numbers must be separated by blanks.
  • The following lines hold the travel time information for the phase. The first (floating point) number in each line tells the distance value, followed by N travel times (floating point, in s), one for each depth specified in the depth line above. A value of 0 indicates that there could be no travel time computed for this distance/depth combination. The distance grid needs not to be equidistant, however it should span the distance interval specified in the beginning of the file.

When a travel time is computed, SHM reads the table of the phase and interpolates linearly in both variables, distance and depth.


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