5:15 PM - 7:15 PM
[SEM15-P02] Effect of mesh design with unstructured hexahedral elements on calculations of MT response functions at seafloor sites
Keywords:Magnetotellurics, Magnetotelluric method, Topography effects, Unstructured hexahedral elements, Finite element method
The magnetotelluric (MT) method is a geophysical exploration technique used to investigate subsurface resistivity structures and is particularly effective for understanding the distribution of underground fluids. In this method, the MT response function is calculated from observed electric and magnetic field data, and the resistivity structure is estimated based on this function. However, since the MT response function is affected by topography, improper representation of these effects can lead to errors in subsurface structure estimation. This issue is especially pronounced in seafloor environments, where low-resistivity seawater is directly contacted with high-resistivity subsurface materials.
To address this problem, a three-dimensional resistivity modeling code called FEMTIC has been developed to accurately represent topographic effects (Usui, 2015; Usui et al., 2018, 2024). FEMTIC computes MT response functions using the finite element method and supports two types of mesh discretization: tetrahedral and hexahedral elements. However, in the recently developed hexahedral mesh-based modeling, the degree of detail required to approximate topography for accurately evaluating its effects has not been thoroughly investigated.
In this study, we examined the impact of mesh design with hexahedral mesh on MT response function calculations by using a sinusoidal two-dimensional topographic model with a known analytical solution(Schwalenberg et al., 2004). The MT response functions caluculated with FEMTIC were compared with the theoretical solutions. Specifically, we investigated how different parameters—such as mesh resolution, computational domain size, refinement region near the observation point, and discretization in the X- and Z-directions—affect the response function.
The analysis suggests that, with appropriate mesh settings, the finite element method using hexahedral elements has the potential to adequately capture topographic effects. In particular, the refinement strategy for the mesh near the observation point appears to influence the accuracy of short-period components. Additionally, the resolution of mesh discretization in the X direction impacts the MT response function not only near the observation points but also across the entire calculation region.
To address this problem, a three-dimensional resistivity modeling code called FEMTIC has been developed to accurately represent topographic effects (Usui, 2015; Usui et al., 2018, 2024). FEMTIC computes MT response functions using the finite element method and supports two types of mesh discretization: tetrahedral and hexahedral elements. However, in the recently developed hexahedral mesh-based modeling, the degree of detail required to approximate topography for accurately evaluating its effects has not been thoroughly investigated.
In this study, we examined the impact of mesh design with hexahedral mesh on MT response function calculations by using a sinusoidal two-dimensional topographic model with a known analytical solution(Schwalenberg et al., 2004). The MT response functions caluculated with FEMTIC were compared with the theoretical solutions. Specifically, we investigated how different parameters—such as mesh resolution, computational domain size, refinement region near the observation point, and discretization in the X- and Z-directions—affect the response function.
The analysis suggests that, with appropriate mesh settings, the finite element method using hexahedral elements has the potential to adequately capture topographic effects. In particular, the refinement strategy for the mesh near the observation point appears to influence the accuracy of short-period components. Additionally, the resolution of mesh discretization in the X direction impacts the MT response function not only near the observation points but also across the entire calculation region.