3-D Projected $ L_{1}$ inversion of gravity data

TL;DR

This paper presents a novel 3-D gravity data inversion method using $L_1$ regularization and projected spectrum techniques, improving accuracy in reconstructing subsurface models from noisy data.

Contribution

It introduces an efficient approach combining $L_1$ regularization with spectrum truncation and projection methods for gravity data inversion.

Findings

The method accurately reconstructs the Mobrun ore body model.

Projected spectrum regularization improves inversion accuracy.

Synthetic tests validate the approach with noisy data.

Abstract

Sparse inversion of gravity data based on $L_1$-norm regularization is discussed. An iteratively reweighted least squares algorithm is used to solve the problem. At each iteration the solution of a linear system of equations and the determination of a suitable regularization parameter are considered. The LSQR iteration is used to project the system of equations onto a smaller subspace that inherits the ill-conditioning of the full space problem. We show that the gravity kernel is only mildly to moderately ill-conditioned. Thus, while the dominant spectrum of the projected problem accurately approximates the dominant spectrum of the full space problem, the entire spectrum of the projected problem inherits the ill-conditioning of the full problem. Consequently, determining the regularization parameter based on the entire spectrum of the projected problem necessarily over compensates for the non-dominant portion of the spectrum and leads to inaccurate approximations for the full-space solution. In contrast, finding the regularization parameter using a truncated singular space of the projected operator is efficient and effective. Simulations for synthetic examples with noise demonstrate the approach using the method of unbiased predictive risk estimation for the truncated projected spectrum. The method is used on gravity data from the Mobrun ore body, northeast of Noranda, Quebec, Canada. The $3$-D reconstructed model is in agreement with known drill-hole information.