The time-domain forward-backward time-stepping (FBTS) inversion method is a reconstruction technique being capable of reconstructing large objects. However its efficiency is affected by saving great amount of simulation data to hard disk and reading them from it. This paper proposes a method which need not save simulation data. In the proposed method, the simulation data is obtained by finite-difference time-domain (FDTD) method with perfect matched layer (PML) absorbing boundaries, and the simulation of the adjoint field is calculated using FDTD too, but random boundaries are used. In the random boundaries, the permittivity is randomly distributed and the adjoint fields caused by the random boundaries are uncorrelated. The adjoint field is calculated reverse in time, and the wave fields at 0 and Ai/2 are saved to hard disk. The adjoint field at any time step is calculated from the saved initial conditions. As a result, the computation sequence of both fields is kept in the same forward time direction. A numerical reconstruction example is illustrated to compare inversion results using the conventional and the suggested methods. The reconstructed results by using these methods are almost the same, however, the calculation time of the suggested method is about 78% of the conventional method.
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