# ADMM-based multi-parameter wavefield reconstruction inversion in VTI acoustic media with TV regularization

## TL;DR

This paper extends wavefield reconstruction inversion (WRI) with ADMM to multi-parameter VTI acoustic media, incorporating TV regularization to improve subsurface parameter estimation and mitigate cycle skipping in full waveform inversion.

## Contribution

It introduces a novel ADMM-based IR-WRI framework for multi-parameter VTI acoustic media, including new bilinear wave equations and regularization strategies.

## Key findings

- IR-WRI effectively reconstructs vertical wavespeed in VTI media.
- TV regularization reduces parameter cross-talk and artifacts.
- Method performs well on realistic North Sea case study.

## Abstract

Full waveform inversion (FWI) is a nonlinear waveform matching procedure, which suffers from cycle skipping when the initial model is not kinematically-accurate enough. To mitigate cycle skipping, wavefield reconstruction inversion (WRI) extends the inversion search space by computing wavefields with a relaxation of the wave equation in order to fit the data from the first iteration. Then, the subsurface parameters are updated by minimizing the source residuals the relaxation generated. Capitalizing on the wave-equation bilinearity, performing wavefield reconstruction and parameter estimation in alternating mode decomposes WRI into two linear subproblems, which can solved efficiently with the alternating-direction method of multiplier (ADMM), leading to the so-called IR-WRI. Moreover, ADMM provides a suitable framework to implement bound constraints and different types of regularizations and their mixture in IR-WRI. Here, IR-WRI is extended to multiparameter reconstruction for VTI acoustic media. To achieve this goal, we first propose different forms of bilinear VTI acoustic wave equation. We develop more specifically IR-WRI for the one that relies on a parametrisation involving vertical wavespeed and Thomsen's parameters delta and epsilon. With a toy numerical example, we first show that the radiation patterns of the virtual sources generate similar wavenumber filtering and parameter cross-talks in classical FWI and IR-WRI. Bound constraints and TV regularization in IR-WRI fully remove these undesired effects for an idealized piecewise constant target. We show with a more realistic long-offset case study representative of the North Sea that anisotropic IR-WRI successfully reconstruct the vertical wavespeed starting from a laterally homogeneous model and update the long-wavelengths of the starting epsilon model, while a smooth delta model is used as a passive background model.