3D numerical model for investigating the effect of relative magnetic permeability in magnetotellurics

This work concerns the construction and testing of a 3D numerical model suitable for the simulation of magnetotelluric measurements using COMSOL Multiphysics. The aim of the study is to inspect the effect of varying relative magnetic permeability of rocks (different from the general approximation of 1) and integrate it to geology based numerical models.

Experiences from field magnetotelluric (MT) measurements indicate that in the vicinity of igneous rocks field MT data shows increased resistivities at lower frequencies. This effect may be connected to the rocks’ magnetic susceptibility/permeability which differ from these parameters of vacuum and are generally neglected (e.g. Li & Cao 2005, Kiss et al. 2023). The presence of media with relative magnetic permeability higher than 1 may affect the depth estimation during data processing and interpretation (Kiss et al. 2023). In this study, we investigate these effects in detail applying a model to a real geological research area in Hungary to find possible explanations of the behaviour of measured MT data.

First, 1-, 2-, and 3-layer synthetic models were created to test several model parameters and verify numerical results with the analytical solutions of the respective models. Parameter tests included the size of the model, the frequency range, the resistivity and thickness of the “air” domain, the resistivity contrast of the layers, and the mesh resolution. As a result, numerically accurate models were gained: the difference between the numerical results and the analytical ones were less than 0.05% for the apparent resistivity and less than 0.3% for the phase.

Second, the effect of the relative magnetic permeability was studied. Values were chosen to range from 1 to 10. Larger values allowed us to examine the effect of the physical phenomenon, while smaller values provided information on the potential extent of the effect in reality. In the model 11 simulated measurement points were distributed along a line, 1 km apart from each other. Several models were created where the boundary of the regions with different permeability values was perpendicular to the “measurement line” or was located at an angle to the line.

Third, a model was built based on the real geology of a study area near Székesfehérvár, Hungary, where the Pre-Cenozoic basement consists of granitoid plutons and siliciclastic and carbonate formations covered by Miocene sedimentary rocks. Preliminary results of real magnetotelluric field measurements and the modelled ones were compared. It was established that the numerical model results harmonize with field observations, but further refinements are needed.

 

References:

X.M. Li & J.X. Cao (2005): A study on the influence of magnetic susceptibility on MT response. Chinese Journal of Geophysics, 48(4):1017-1021

Kiss, L. Szarka & E. Prácser (2023): Magnetic distortions in magnetotellurics: Predictable distortions in classical processing MT procedures in presence of a magnetic medium based on 2D direct modelling results (in Hungarian). Hungarian Geophysics, 64(1):43-57