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Travel Time Residual Calculation

Each iteration the travel time residual for every defining phase is calculated with respect to the last solution found.  This must done after phases have been reidentified in the light of the current source location (id_pha) and before weighting (calc_weight).  The function responsible for giving residuals to those phases that will contribute to the next solution is calc_resid .  (This function is also called in a different mode after a final convergence to give residuals to all possible phases, whether they were used or not.)  Function calc_resid steps through the phases calculating the observed travel time and calling function read_ttime to get the model travel time, it then subtracts the second from the first to get the residual.  To simplify the use of different methods to calculate residuals the function read_ttime is just an interface for a model specific travel time function, see the section on changing travel time models.  In addition to calling a travel time function function read_ttime calls a function correct_ttime that can be used to apply corrections based on station elevation or crustal model.  These corrections can be turned on or off in the configuration file.



Each phase structure has 11 members related to the calculation of the residual and derivatives used in each solution:

There are two external variables related to the calculation of travel times assigned in read_config and corresponding to lines in config.txt: