Direct inversion of data-space Hessian for efficient time-domain extended-source waveform inversion using the multiplier method

TL;DR

This paper introduces a novel method for efficient time-domain extended-source waveform inversion by directly constructing and inverting the data-space Hessian matrices, significantly reducing computational costs.

Contribution

The paper presents a new approach to compute and invert the data-space Hessian matrices efficiently, enabling faster extended-source FWI in the time domain.

Findings

Substantial reduction in computational cost for Hessian inversion.

Effective use of receiver-side Green functions in the time domain.

Numerical experiments confirm improved efficiency and accuracy.

Abstract

The augmented Lagrangian (AL) method has been successfully applied for solving the full waveform inversion (FWI) problem. In AL-based FWI, the Lagrange multipliers serve as source extensions, offering several advantages to the inversion, such as improved robustness to cycle skipping, faster convergence, and simplified penalty parameter tuning. Time-domain applications of this method have been enabled by reformulating the optimization problem in the data space, significantly reducing memory requirements by projecting source-side multipliers into the data space. These data-side multipliers act as data extensions, effectively expanding the data space. A key challenge in these methods lies in computing the data-side multipliers, which involves solving a linear system to deblur the data residuals using the data-space Hessian matrix before it serves as the adjoint source. This Hessian matrix is prohibitively large to construct and invert explicitly. Iterative Krylov methods can be applied to solve this system as inner iterations, but they require two PDE solves per inner iteration per source, leading to significant computational costs. In this work, we present a key improvement to extended waveform inversion based on multiplier methods. We propose a novel approach that significantly reduces the computational cost of Hessian inversion. The method computes receiver-side Green functions in the time domain and directly constructs frequency-domain Hessian matrices for all required frequencies. These Hessian matrices, with dimensions equal to the number of receivers, can be computed, inverted, and stored in memory. Once constructed, they can be used simultaneously for all sources, further enhancing efficiency. Numerical experiments demonstrate the substantial computational gains achieved by the proposed method, highlighting its effectiveness for extended-source FWI in the time domain.