Travel-time measurements form the common basis for all inverse techniques in ocean acoustics. These techniques rely on the assumption that the received signal consists of a train of pulses identical to that produced by the source. However, when few modes survive long-distance propagation, the validity of the travel-time measurement can be compromised by diffraction effects. A primary goal of this paper is to evaluate the restrictions caused by strong mode coupling in irregular sound channels. The results of numerical simulations of pulse propagation through a range-dependent medium with a single sound channel will be presented. The time-domain patterns were obtained by Fourier synthesis based on coupled-mode calculations for various inhomogeneities using a 100-Hz frequency band acoustic signal. The effects of modal dispersion and mode coupling on the accuracy of the pulse-travel-time measurements will be discussed.