A seismic source of the present disclosure includes a first rotation member and a second rotation member having a common axis of rotation and rotated at a same rotational rate in mutually opposite directions. In addition, the seismic source may include a first mass that is eccentrically coupled to the first rotation member and rotates along with the first rotation member. A second mass may be eccentrically coupled to the second rotation member and configured to rotate along with that member. The second mass may be configured such that the center of gravity of the second mass is located radially external to the center of gravity of the first mass relative to the common axis of rotation. The center of gravities of the first and second mass may be located at the same location along a length of the common axis of rotation.