Although not as common as other types of wind turbine damage, foundation damage can compromise the stability of the wind turbine, limit availability, and lead to high repair costs. This work focuses on a specific type of damage identified in a large number of existing onshore concrete foundations in which gaps develop between the concrete and the steel tower. A wind turbine with this damage was analyzed using LVDTs on the foundation to measure the gaps and two accelerometers on the tower. Natural frequencies were determined using the covariance-based stochastic subspace identification algorithm (SSI-COV), and vibration data was analyzed. The foundation was then repaired with injections of epoxy resin to fill the existing gaps, and the results obtained before and after the repair are presented in this work.