(a) Velocity dispersion of collar waves in infinite liquids (V_f = 1500 m/s), a borehole surrounded by an elastic formation (V_p = 4000 m/s), and a rigid borehole, respectively. (b) Excitation intensities of collar waves in infinite liquids, borehole surrounded by elastic formations (V_p = 2000 m/s, 4000 m/s, 6000 m/s), and a rigid borehole, respectively. The receiver-to-source distance is 3 m. The excitation amplitudes are normalized with respect to the maximum value for the case in infinite liquids.