7.1.8 Determining which travel time software is used

The parameter test(57) is used to determine whether a layered model or IASPEI91 software is used to calculate the travel times and their derivatives. For the initial starting location, the distances from each station are calculated and IASPEI91 is used if any of them exceed test(57). However, this can be overridden by the distance indicator in column 22 of the Nordic header record. If this is L, a crustal model is used regardless of distance, whereas if it is D, IASPEI91 is used, while R has no effect i.e., test(57) is still used. So if either a crustal model or IASPEI91 tables are wanted, use either L or D respectively.