Abstract: Dispersion curves and synthetic waveforms which excited by dipole sources in a fluid-filled borehole surrounded by infinite soil are calculated. It shows the flexural modal waves which propagate at shear wave velocity may be excited at very low frequencies. At this require, a dipole transducer which works at very low frequencies has been designed. Measurements have been made in a soil well by this transducer, and it proves this method is correct and feasible. It has great importance to shear wave velocity logging in soil formation.