A method for determining a radial profile of sonic shear velocity of formations surrounding a fluid-containing borehole, including: suspending a logging device in the borehole; transmitting sonic energy from the logging device to establish flexural waves in the formation; receiving sonic energy from the flexural waves and producing, from the received sonic energy, measurement signals at a number of frequencies; determining, at each frequency, the flexural wave velocity in the formation; deriving sonic compressional and shear velocities of the substantially undisturbed formation; deriving sonic compressional velocity of the borehole fluid; and determining the radial profile of sonic shear velocity from the derived compressional and shear velocities of the substantially undisturbed formation, the derived compressional velocity of the borehole fluid, and the flexural wave velocities.