Least-squares reverse time migration using the most energetic source wavefields based on excitation amplitude imaging condition

Least-squares reverse time migration, a linearized inversion problem, can provide high-quality migration image by minimizing the misfit function, which is defined by predicted and observed data. According to the theory of data-domain least-squares reverse time migration, a forward source wavefield that is simulated with a fixed background velocity does not change during iterations. However, storing the forward source wavefield directly into computer memory involves substantial memory consumption. Although a source wavefield reconstruction technique can be applied during least-squares reverse time migration iterations, this approach can increase the computational cost because of the need for additional wavefield simulations. To alleviate this computational issue in storing the forward source wavefield, we propose an efficient least-squares reverse time migration scheme based on an excitation amplitude method. Unlike conventional excitation amplitude imaging conditions, the proposed least-squares reverse time migration scheme enables one to reconstruct the forward source wavefield by convolving a source wavelet with the excitation amplitude of Green's function at the excitation time. With this excitation amplitude method, the forward source wavefield can be efficiently stored in the computer memory because the sizes of the excitation amplitude and excitation time maps are equal to the size of one snapshot. To validate the feasibility of our least-squares reverse time migration scheme, we perform dot-product tests and compare forward source wavefields, demigrated data and gradient vectors obtained by conventional least-squares reverse time migration and our proposed least-squares reverse time migration. Using numerical tests on synthetic data, we confirm that our least-squares reverse time migration can produce high-quality migration results with a significant improvement in computational efficiency with respect to performance time and memory consumption.