3:30 PM - 3:45 PM
[SEM16-07] Development of 3D forward calculation code of tidally-induced magnetic variation.
Keywords:resistivity, tidal
Electrically conductive seawater moving in the geomagnetic main field causes electric currents in the ocean. The ocean tides, which cause periodic seawater motion, generate periodic electromagnetic field variation (e.g., Minami 2017). It is expected that the electromagnetic field can be used to estimate the resistivity structure beneath the seafloor.
Grayver et al. (2016) has already performed inversion of tidally-induced magnetic data observed at the satellite altitude. We aim to use tidally-induced magnetic fields observed at the seafloor to estimate the resistivity structure beneath the seafloor. Magnetic data observed at the seafloor enable us to use toroidal magnetic fields that are not observed at satellite altitudes. We have previously developed a one-dimensional forward calculation code for tidally induced magnetic fields using an analytical solution (Chave and Luther 1990) and have compared the forward calculation values with the observed data in the Lau Basin. Our one-dimensional calculation, however, could not reproduce large differences of 4nT in observed magnetic field among actual observation points located at about 30km intervals in east-north direction in the Lau Basin. This is probably because the one-dimensional calculation is not capable of including the effects of topography and three-dimensional resistivity structure. Therefore, we developed a three-dimensional forward calculation code.
In this study, we have developed a forward calculation code to calculate the magnetic field due to the motionally-induced current consisting of the seawater conductivity, the earth's main magnetic field, and the tidal seawater velocity field. The mesh used in the calculation is a tetrahedral mesh including the seafloor topography as in Minami et al. (2017). A finite element method solver of Minami et al. (2018) is used to calculate the magnetic field. The flow field was obtained from the TPXO model (Egbert and Erofeeva 2002) and the earth's main magnetic field was from IGRF-13 (Alken et al. 2021). In order to confirm the accuracy of the calculation, we calculated the magnetic field produced by a plane long wave with a wavelength of 100 km and compared it with an analytical solution (Tyler 2005). The discrepancy between them was 5% in amplitude and 3 degrees in phase.
In the future, we will compare results of our forward calculation with the observed data in the Lau Basin, and proceed to implementation of inversion using our forward code. In the presentation, we report the details of the developed forward calculation code and the results of the forward calculation.
Grayver et al. (2016) has already performed inversion of tidally-induced magnetic data observed at the satellite altitude. We aim to use tidally-induced magnetic fields observed at the seafloor to estimate the resistivity structure beneath the seafloor. Magnetic data observed at the seafloor enable us to use toroidal magnetic fields that are not observed at satellite altitudes. We have previously developed a one-dimensional forward calculation code for tidally induced magnetic fields using an analytical solution (Chave and Luther 1990) and have compared the forward calculation values with the observed data in the Lau Basin. Our one-dimensional calculation, however, could not reproduce large differences of 4nT in observed magnetic field among actual observation points located at about 30km intervals in east-north direction in the Lau Basin. This is probably because the one-dimensional calculation is not capable of including the effects of topography and three-dimensional resistivity structure. Therefore, we developed a three-dimensional forward calculation code.
In this study, we have developed a forward calculation code to calculate the magnetic field due to the motionally-induced current consisting of the seawater conductivity, the earth's main magnetic field, and the tidal seawater velocity field. The mesh used in the calculation is a tetrahedral mesh including the seafloor topography as in Minami et al. (2017). A finite element method solver of Minami et al. (2018) is used to calculate the magnetic field. The flow field was obtained from the TPXO model (Egbert and Erofeeva 2002) and the earth's main magnetic field was from IGRF-13 (Alken et al. 2021). In order to confirm the accuracy of the calculation, we calculated the magnetic field produced by a plane long wave with a wavelength of 100 km and compared it with an analytical solution (Tyler 2005). The discrepancy between them was 5% in amplitude and 3 degrees in phase.
In the future, we will compare results of our forward calculation with the observed data in the Lau Basin, and proceed to implementation of inversion using our forward code. In the presentation, we report the details of the developed forward calculation code and the results of the forward calculation.