Waveform tomography, or full waveform inversion (FWI), is in general better than the ray tomography method, however, a reliable initial model is usually required to ensure the success of seismic inversion. In this work we designe a cascade-like hybrid tomography technology to solve the crosswell seismic inversion problem. The new method is a combination of several widely used tomography technologies. We start from the Radon-transform based back projection (BP) method, which produces a smooth initial velocity model. Next, the Linear Iterative Reconstruction (LIR) method is adopted to update this initial model, then it is further improved by the nonlinear Gradient-based Eikonal Traveltime Tomography (GETT) method. The velocity model reconstructed from the previous multi ray tomography methods is sufficiently reliable to serve as the initial model for the computationally intensive waveform tomography method, from which the accurate velocity model is obtained. The numerical example shows that this hybrid tomography method has great potential in reconstructing accurate acoustic velocities or other high-resolution reservoir characterization for crosswell seismic inversion. With this hybrid tomography method, the inversion result of three cascade ray tomography methods is used as the initial model for FWI that provide a perfect initial velocity model, which leads to a significantly reduced computation time and number of iterations. It is noticeable that this hybrid tomography method is able to obtain an accurate velocity even when the recorded seismic data suffers from poor spatial coverage and the signal-to-noise ratio is low.
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