A defect grating in a silicon on insulator waveguide is simulated. We consider spectral changes in the optical transmission when a thin silicon nitride or silicon tip is scanned across the defect. The tip perturbs the resonance field, moving its peak wavelength andpossibly changing its shape and quality factor. For the nitride tip, the influence is mostly a spectral shift; for silicon, the change of the resonance shape is pronounced. In particular for the nitride tip we observe a close correspondence between the wavelength shift as a function of tip position, and the local intensity in the unperturbed structure
