Multiscale strategies are very important in the successful application of waveform-based velocity inversion.The strategy that sequentially preceeds from long to short scale of velocity model,has been well developed in full waveform inversion(FWI) to solve the local mininum problem.In contrast,it's not well understood in the image-domain waveform tomography(IWT),which back-projects incoherent waveform components of the common image gather into velocity updates.IWT is less prone to local minimum problem but tends to build long-scale model with low resolution.In order to build both long-and short-scale model by IWT,we discuss several multiscale strategies restricted in the image domain.The strategies include model reparameterization,objective function switching and gradient rescaling.Numerical tests on Marmsousi model and real data demonstrate that our proposed multiscale IWT is effective in buidling velocity model with wide wavenumber spectrum.
