Due to non-linear effects, the ground-force signal transmitted into the subsurface by vibrators is contaminated by harmonics. The lack of ideal rigidity of the baseplate contributes to the overall harmonic content of the vibrator output. The baseplate flexure exacerbates harmonic distortion, especially the 2nd harmonic distortion. Meanwhile, the ground-force approximated by the weighted-sum method is limited to a narrow frequency bandwidth due to the low baseplate rigidity. A new baseplate design significantly increases the stiffness of the baseplate and dramatically reduces its flexure. The new baseplate is 2.5 times stiffer than the standard baseplate while only suffering a 5% increase to its weight. Field testing demonstrates that with the new baseplate the validity of the weighted-sum ground force in a frequency range up to 160 Hz is achieved. Meantime, the suppression of the even order harmonics contained in the ground force is observed. Very significant harmonic distortion reduction is achieved as the vibrator is on hard and/or uneven ground.