Least-squares Reverse Time Migration Method Research

As exploration object becomes increasingly complex, the high resolution imaging of deep reservoir requirements gradually increase. At present, based on the two-way wave equation numerical solution method of reverse time migration(RTM) technology is considered a good method for complex structure imaging. RTM has the advantages of turning wave, multiple wave and prisms imaging. And this method does not affected by the velocity variation. But it also has the limition of high wavenumber artifacts, limited resolution and unbalanced amplitudes of the reflectors from shallow to deep part.Least-squres migration has been shown to have the following advantages:(1) it can reduce migration artifacts from a limited recording aperture and/or coarse source and receiver sampling;(2) it can balance the amplitudes of the reflectors; and(3) it can improve the resolution of the migration images. When lesast-squares migration is implemented with the reverse time migration method, it can reduce not the acquisition footprint but also the artifacts in the RTM image, while enhancing the image resolution.However, least-squares migration is usually considered to be too expensive for pratical use. In this dissertation, I propose a new algorithm to combine the blended sources processing technique with least-squares migration to increase its computational efficiency. The multisource least-squares reverse time migration algorithm is proposed to invert waveforms linearly based on the Born approximation and increase the computational efficiency by more than an order-of-magnitude compared to conventional LSRTM.A drawback of the multisource least-squares reverse time migration algorithm is that its convergence is sensitive to the accuracy of the velocity model. To remedy this problem I propose incorporating a regulation term into the LSRTM method that penalizes misfits between the images in the plane-wave domain. The plane-wave prestack least-squares migration method where the migration image of each shot is updated separately and an ensemble of prestack images is produced with common image gathers. The advantage over conventional LSRTM where all the shot gathers are explained by single migration image is that it is relatively less sensitive to bulk errors in the migration velocity. The plane-wave encoding technique can significantly reduce the computional an input/output(I/O) cost. In contrast to conventional multisource least-suqares migration with phase-encoded supergathers, it can be applied to marine data.