This Letter provides an advanced time-frequency analysis method for quickly and precisely estimating dispersion curves of low-frequency impulsive sounds propagated in shallow water. First, the modal chirplet transform is derived based on an approximated variation law of the modal instantaneous frequency, which can omit the modal filtering step of warping-based methods. Then an extension modal chirplet transform (EMCT) is developed to further improve its potential for automation, together with a trick for fast computation. EMCT is applied to a recorded signal to demonstrate that the method can achieve high time-frequency concentration for modal components in a significantly decreased run time.