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Adapting reverse-time migration (RTM) and demigration (RTDM) as the migration and modeling operators to maximize the cross-correlation between the simulated and the acquired seismic data, we introduce a new practical least-squares RTM (LSRTM) scheme and derive a steepest descent method in seeking the optimal image. Through both synthetic and real data experiments, we demonstrate that the proposed LSRTM provides high quality images with balanced amplitudes, improved focusing and enhanced resolution. The method is also capable of removing free surface ghosts caused by towed streamer acquisition, filling the structures at the acquisition holes. Together with ghost compensation and impedance perturbation techniques, the proposed method can be a useful tool for both seismic imaging and inversion.