Frequency-domain acoustic-elastic coupled waveform inversion using the Gauss-Newton conjugate gradient method

We developed a frequency-domain acoustic-elastic coupled waveform inversion based on the Gauss-Newton conjugate gradient method. Despite the use of a high-performance computer system and a state-of-the-art parallel computation algorithm, it remained computationally prohibitive to calculate the approximate Hessian explicitly for a large-scale inverse problem. Therefore, we adopted the conjugate gradient least-squares algorithm, which is frequently used for geophysical inverse problems, to implement the Gauss-Newton method so that the approximate Hessian is calculated implicitly. Thus, there was no need to store the Hessian matrix. By simultaneously back-propagating multi-components consisting of the pressure and displacements, we could efficiently extract information on the subsurface structures. To verify our algorithm, we applied it to synthetic data sets generated from the Marmousi-2 model and the modified SEG/EAGE salt model. We also extended our algorithm to the ocean-bottom cable environment and verified it using ocean-bottom cable data generated from the Marmousi-2 model. With the assumption of a hard seafloor, we recovered both the P-wave velocity of complicated subsurface structures as well as the S-wave velocity. Although the inversion of the S-wave velocity is not feasible for the high Poisson's ratios used to simulate a soft seafloor, several strategies exist to treat this problem. Our example using multi-component data showed some promise in mitigating the soft seafloor effect. However, this issue still remains open.