Resonant frequencies in a circular third sound resonator are shifted by DC persistent current flow fields present in the resonator. Each mode is shifted differently according its ''overlap'' with the flow. Measurements of these shifts consequently reflect the flow field, and hence the distribution of vortices. In addition, large amplitude circularly polarized third sound modes are used to create and destroy the flow states. Several conclusions can be drawn from the experiments: 1) Vortex densities of hundreds of thousands per square centimeter exist in quasi-equilibrium; 2) Vortex distributions include large densities of both circulation senses polarized to different radii within the cell; 3) Pinning strengths are consistent with the bulk pinning model of Schwarz1 applied to a film; and 4) Third sound amplitudes with AC flows approaching the Feynman critical velocity create new vorticity. These results offer validity to the hypothesis that pinned vortices are a pervasive feature of superfluid films at low temperatures.
