Comparison of Different Coupling Methods for Joint Inversion of Geophysical Data: A Case Study for the Namibian Continental Margin

C Berndt,W Rabbel,G Franz,M Jegen,M Moorkamp

doi:10.1029/2021jb022092

Abstract

AbstractIntegration of multiple geophysical data is a key practice to reduce model uncertainties and enhance geological interpretations. Electrical resistivity models resulting from inversion of marine magnetotelluric (MT) data, often lack depth resolution of lithological boundaries and distinct information for shallow model parts. This is due to the diffusive nature of electromagnetic fields, enhanced by deficient data sampling and model regularization during inversion. Thus, integrating data or models to constrain layer thicknesses or structural boundaries is an effective approach to derive better constrained and more detailed resistivity models. We investigate the different impacts of three cross‐gradient coupled constraints on 3D MT inversion of data from the Namibian passive continental margin. The three constraints are (a) coupling with a fixed structural density model; (b) coupling with satellite gravity data; (c) coupling with a fixed gradient velocity model. Here, we show that coupling with a fixed model (a and c) improves the resistivity model the most. Shallow conductors imaging sediment cover are confined to a thinner layer in the resulting resistivity models compared to the MT‐only model. Additionally, these constraints help to suppress vertical smearing of a conductive anomaly attributed to a fracture zone, and clearly show that the seismically imaged Moho is not accompanied by a change in electrical resistivity. All of these observations help to derive an Earth model, which will form the basis for future interpretation of the processes that lead to continental break‐up during the early Cretaceous.

Highlights

Different geophysical data generally result from different physical properties of the Earth that are not necessarily dependent on each other
Our study aims to achieve two main goals: (a) to improve the Namibian Margin Earth model regarding the geological features related to continental break-up, and (b) to investigate the impacts of different joint inversion coupling constraints on the resulting inversion models
This study focuses on three main objectives: 1. How can we improve the 3D resistivity model from single method MT inversion to gain new insights and which inversion constraints can benefit geological interpretations?

Summary

Introduction

Different geophysical data generally result from different physical properties of the Earth that are not necessarily dependent on each other. Electromagnetic data depend on the electrical resistivity of the subsurface, gravity data on density variations, and seismic data on seismic elastic properties and density variations. There are many physical property distributions that fit the observed data, because there are fewer measurements than necessary to derive a unique model, that is, the problem is under-constrained and information is further deteriorated by the associated errors. The governing physics may limit the various geophysical data, for example, diffusive methods such as electromagnetic measurements cannot resolve sharp boundaries, gravity data have a limited depth resolution, and seismic data provide limited velocity information for short observation distances. As a result, simplified (1D, 2D, and/or smoothed) earth models are commonly used both as a starting and end FRANZ ET AL.

Results

Discussion

Conclusion