5:15 PM - 6:30 PM
[SEM14-P02] 1-D forward calculation of tidally-induced magnetic variation in the Lau Basin: Comparison of calculation and seafloor magnetic data and sensitivity analysis
Keywords:ocean tide, resistivity
Electrically conductive seawater moving in the geomagnetic main field causes electric currents in the ocean. In recent years, researches aiming at using this phenomenon to infer resistivity structures (Schnepf et al. 2014; 2015, Zhang et al. 2019) attract interests of researchers. It is known that the conventional MT method reduces estimation accuracy of its response function over periods around half a day to a day. The periods of ocean tides can complement this period band and the sounding depth corresponds to the upper mantle. Therefore, combination of MT and subseafloor sounding by the tidally induced magnetic variation possibly improves accuracy of resistivity values of the upper mantle compared to previous studies. Although Grayver et al. (2016) performed inversions of satellite tidally-induced magnetic data observed at the satellite altitude, there have been no reports of inversions of tidally-induced magnetic data observed at the seafloor. At the satellite altitude, one can observe only the poloidal component of the tidal magnetic variation. On the other hand, the seafloor magnetic data can observe both toroidal and poloidal components with shorter horizontal wave lengths compared to the satellite altitude. This implies that the usage of seafloor tidal magnetic variation may provide more accurate resistivity images with higher spatial resolution.
In this study, we developed a one-dimensional (1-D) forward calculation code for the tidally-induced magnetic variation, compared calculation results with seafloor magnetic data observed at the Lau Basin, and further investigated the sensitivity of the tidal magnetic variation to the resistivity structure, toward future inversions of seafloor tidal magnetic data. We focus only on the M2 component in this study. The tidal components of magnetic data were estimated by the least square method for sinusoidal functions. In our 1-D forward calculation, we used the analytical solution from Chave and Luther (1990), the tidal current model from Egbert and Erofeeva (2002), and the geomagnetic main field from IGRF-13 (Alken et al. in print). We prepared a 1-D resistivity structure from the two-dimensional structure in the Lau Basin inferred by Matsukura (2014, Master thesis of Kobe University). Comparison of the vertical component of the tidal magnetic field between the numerical and observation results shows discrepancy apparently correlated with the bathymetry, which implies the effect of bathymetry neglected in our 1-D calculation. On the other hand, the comparison of the horizontal components shows significantly larger amplitudes of the observed magnetic variation compared to the 1-D calculation. This could be due to toroidal magnetic component excited by the baroclinic current due to the internal tidal wave, which was not included in the adopted barotropic tidal model. We further investigated the sensitivity of the seafloor tidal magnetic variation in the Lau Basin to changes of the adopted 1-D resistivity structure. From shallow to deep part, we multiplied the original resistivity values by three within a 50 km thick zone and checked the change of amplitude of the vertical component of the tidal magnetic variation at the seafloor. As a result, we found that tidal magnetic variation in the Lau Basin is most sensitive to the depth of 250 to 300 km, where the amplitude enhanced by 2.5 %. In the presentation, we will explain details of our 1-D forward calculation, and report the results of comparison between the observation and numerical calculation and those of sensitivity analysis mentioned above.
In this study, we developed a one-dimensional (1-D) forward calculation code for the tidally-induced magnetic variation, compared calculation results with seafloor magnetic data observed at the Lau Basin, and further investigated the sensitivity of the tidal magnetic variation to the resistivity structure, toward future inversions of seafloor tidal magnetic data. We focus only on the M2 component in this study. The tidal components of magnetic data were estimated by the least square method for sinusoidal functions. In our 1-D forward calculation, we used the analytical solution from Chave and Luther (1990), the tidal current model from Egbert and Erofeeva (2002), and the geomagnetic main field from IGRF-13 (Alken et al. in print). We prepared a 1-D resistivity structure from the two-dimensional structure in the Lau Basin inferred by Matsukura (2014, Master thesis of Kobe University). Comparison of the vertical component of the tidal magnetic field between the numerical and observation results shows discrepancy apparently correlated with the bathymetry, which implies the effect of bathymetry neglected in our 1-D calculation. On the other hand, the comparison of the horizontal components shows significantly larger amplitudes of the observed magnetic variation compared to the 1-D calculation. This could be due to toroidal magnetic component excited by the baroclinic current due to the internal tidal wave, which was not included in the adopted barotropic tidal model. We further investigated the sensitivity of the seafloor tidal magnetic variation in the Lau Basin to changes of the adopted 1-D resistivity structure. From shallow to deep part, we multiplied the original resistivity values by three within a 50 km thick zone and checked the change of amplitude of the vertical component of the tidal magnetic variation at the seafloor. As a result, we found that tidal magnetic variation in the Lau Basin is most sensitive to the depth of 250 to 300 km, where the amplitude enhanced by 2.5 %. In the presentation, we will explain details of our 1-D forward calculation, and report the results of comparison between the observation and numerical calculation and those of sensitivity analysis mentioned above.