5:15 PM - 7:15 PM
[SEM15-P01] Forward Modeling of Geomagnetic Observatory Tippers Considering Real Terrain Effects in Continental-Scale 3D Resistivity Models Beneath China
Keywords:Geomagnetic observatories, Tippers, Nested mesh, Topography, Forward modeling accuracy
Minute-means geomagnetic observatories capture short-period geomagnetic variations. The tippers' response, by analyzing the relationship between vertical and horizontal magnetic-field variations, provides insights into the heterogeneous electrical conductivity structure of the crust and upper mantle. A long-time series of minute-means data from 61 geomagnetic observatories (approximately 75–135°E, 15–55°N) in China was collected and analyzed. The estimated tippers were derived for sixteen periods, ranging from 240.48s to 9078.12s.
We propose a cost-effective 3-D resistivity modeling that explicitly incorporates topography, bathymetry, and shoreline data from the ETOPO1 Global Relief Model to enhance the accuracy of electrical structure recovery beneath China. While modeling using a finer mesh is expected to yield more accurate model responses, it also increases model complexity and computational cost. To balance the grid resolution and computational resources, we apply the FEMTIC package (Usui, 2015), which utilizes a non-conforming deformed hexahedral mesh, to construct the model with a nested mesh consisting of a global mesh and sub-mesh (cuboids). After comparing the geomagnetic observatory tippers' responses for different meshes, we found that sub-mesh around geomagnetic observatories plays a crucial role. Provided that the sub-mesh resolution is sufficiently high, the precision of the global mesh has minimal impact on the results. Based on this analysis, we selected the most practical mesh design for modeling topography and coastlines as follows. The study area, covering 5000 × 5600 km², is represented using a global mesh with 50 × 56 equal-sized grids (each 100 × 100 km²). Around each geomagnetic observatory, a sub-mesh covering 200 × 200 km² is implemented with unequal-sized grids, refined to 3.125 × 3.125 km² at the center.
Our forward modeling results for geomagnetic observatory tippers' responses are validated using two test models—one with topography and one without. Coastal geomagnetic observatory tippers are primarily influenced by the bathymetry effect, while some inland geomagnetic observatory tippers are affected beyond the typical observational errors by undulating surface topography. The verification shows the necessity of incorporating topography in the modeling process. For quantitative evaluation of the accuracy of the tippers calculated to the resistivity model including topography, we employed a simple method based on the arbitrary selection of horizontal coordinate systems in 3D topography over 100 Ohm-m half-space. This method calculates the mean and standard deviation of tipper responses for each geomagnetic observatory, each frequency, and each component by randomly rotating the forward model across ten different azimuthal coordinate systems, including the traditional (XN,YE) coordinate system. The standard deviation is at most 0.027, indicating that the misfit between the observed and modeled tipper within this range is statistically negligible. Moreover, we found that the forward modeling accuracy is negatively correlated with the roughness of local topography.
We propose a cost-effective 3-D resistivity modeling that explicitly incorporates topography, bathymetry, and shoreline data from the ETOPO1 Global Relief Model to enhance the accuracy of electrical structure recovery beneath China. While modeling using a finer mesh is expected to yield more accurate model responses, it also increases model complexity and computational cost. To balance the grid resolution and computational resources, we apply the FEMTIC package (Usui, 2015), which utilizes a non-conforming deformed hexahedral mesh, to construct the model with a nested mesh consisting of a global mesh and sub-mesh (cuboids). After comparing the geomagnetic observatory tippers' responses for different meshes, we found that sub-mesh around geomagnetic observatories plays a crucial role. Provided that the sub-mesh resolution is sufficiently high, the precision of the global mesh has minimal impact on the results. Based on this analysis, we selected the most practical mesh design for modeling topography and coastlines as follows. The study area, covering 5000 × 5600 km², is represented using a global mesh with 50 × 56 equal-sized grids (each 100 × 100 km²). Around each geomagnetic observatory, a sub-mesh covering 200 × 200 km² is implemented with unequal-sized grids, refined to 3.125 × 3.125 km² at the center.
Our forward modeling results for geomagnetic observatory tippers' responses are validated using two test models—one with topography and one without. Coastal geomagnetic observatory tippers are primarily influenced by the bathymetry effect, while some inland geomagnetic observatory tippers are affected beyond the typical observational errors by undulating surface topography. The verification shows the necessity of incorporating topography in the modeling process. For quantitative evaluation of the accuracy of the tippers calculated to the resistivity model including topography, we employed a simple method based on the arbitrary selection of horizontal coordinate systems in 3D topography over 100 Ohm-m half-space. This method calculates the mean and standard deviation of tipper responses for each geomagnetic observatory, each frequency, and each component by randomly rotating the forward model across ten different azimuthal coordinate systems, including the traditional (XN,YE) coordinate system. The standard deviation is at most 0.027, indicating that the misfit between the observed and modeled tipper within this range is statistically negligible. Moreover, we found that the forward modeling accuracy is negatively correlated with the roughness of local topography.