An orbital vibrator source (OVS), a fluid-coupled shear-wave source, has many properties useful for crosswell, single-well, and borehole-to-surface imaging of both P- (compressional) and S- (shear) wave velocities of reservoir rocks. To this day, however, only a limited number of quantitative models have been developed to explain its properties. In this article, we develop both 2D and 3D models of an OVS, allowing us to examine source characteristics such as radiation patterns, frequency dependence of wave amplitudes, and guided-wave generation. These models are developed in the frequency-wavenumber domain using the partial wave expansion of the wavefield within and outside the borehole. The models predict many unique characteristics of an OVS, including formation-property-dependent vibrator amplitudes, uniform isotropic S-wave radiation pattern, and small tube-wave generation.