f Unification of traveltime tomography methods

Traveltime tomography methods can be divided info two general classes: 1) methods which back-project travel time residuals along raypaths, and 2) methods which back-project traveltime residuals along wavepaths. In this paper 1 present a general formula for the back-projection of travel time residuals and, for special cases, show that it reduces to the back-projection term in wave-equation traveltime inversion (or WT in Luo and Schaster, 1991), wavepath eikonal traveltime inversion (WET in Schaster, 1991), ray tracing tomography (RT), and the Woodward-Rocca (WR in Woodward and Rocca. 1988) method. This unification provides for an understanding of the differences and similarities among these traveltime tomography methods.