Source wavefield reconstruction based on an implicit staggered-grid finite-difference operator for seismic imaging

Abstract

Reverse time migration and full waveform inversion involve the crosscorrelation of two wavefields, propagated in the forward- and reverse-time directions, respectively. As a result, the forward-propagated wavefield needs to be stored, and then accessed to compute the correlation with the backward-propagated wavefield. Boundary-value methods reconstruct the source wavefield using saved boundary wavefields and can significantly reduce the storage requirements. However, the existing boundary-value methods are based on the explicit finite-difference (FD) approximations of the spatial derivatives. Implicit FD methods exhibit greater accuracy and thus allow for a smaller operator length. We develop two (an accuracy-preserving and a memory-efficient) wavefield reconstruction schemes based on an implicit staggered-grid FD (SFD) operator. The former uses boundary wavefields at M layers of grid points and the spatial derivatives of wavefields at one layer of grid points to reconstruct the source wavefield for a (2 M + 2)th-order implicit SFD operator. The latter applies boundary wavefields at N layers of grid points, a linear combination of wavefields at M – N layers of grid points, and the spatial derivatives of wavefields at one layer of grid points to reconstruct the source wavefield (0 ≤ N < M ). The required memory of accuracy-preserving and memory-efficient schemes is ( M +1)/ M and ( N +2)/ M times, respectively, that of the explicit reconstruction scheme. Numerical results reveal that the accuracy-preserving scheme can achieve accurate reconstruction at the cost of storage. The memory-efficient scheme with N = 2 can obtain plausible reconstructed wavefields and images, and the storage amount is 4/( M +1) of the accuracy-preserving scheme.