Marine magnetotelluric inversion with an unstructured tetrahedral mesh

Abstract

The finite element method using an unstructured tetrahedral mesh is one of the most effective methods for the 3-D modelling of marine magnetotelluric data that are strongly affected by bathymetry, because it enables us to incorporate both small-scale and regional-scale bathymetry into a computational mesh with a practical number of elements. The authors applied a 3-D inversion scheme using mesh of this type to marine magnetotelluric problems for the first time and verified its applicability. Forward calculations for two bathymetry models demonstrated that the results obtained with an unstructured tetrahedral mesh are close to the reference solutions. To evaluate the forward calculation results, we developed a general TM-mode analytical formulation for a 2-D sinusoidal topography. Moreover, synthetic inversion test results confirmed that a 3-D inversion scheme with an unstructured tetrahedral mesh enables us to recover subseafloor resistivity structure properly even for a model including a land-sea boundary as well as seafloor undulations. The verified inversion scheme was subsequently applied to a set of marine magnetotelluric data observed around the Iheya North Knoll, the middle Okinawa Trough. 3-D modelling using a mesh with precise bathymetry demonstrated that the data observed around the Iheya North Knoll are strongly affected by bathymetry, especially by the sea-depth differences between the depression of the trough and the shallow East China Sea. The estimated resistivity structure under the knoll is characterized by a conductive surface layer underlain by a resistive layer. The conductive layer implies permeable pelagic/hemipelagic sediments, which are consistent with a previous seismological study. Furthermore, the conductive layer has a resistive part immediately below the knoll, which is regarded as the consolidated magma intrusion that formed the knoll. Furthermore, at a depth of 10 km, we found that the resistor underneath the knoll extends to the southeast, implying that subseafloor resistivity under the Volcanic Arc Migration Phenomenon area is more resistive than the surroundings due to the presence of consolidated magma.

Enhanced and Asymmetric Melting Beneath the Southern Mariana Back‐Arc Spreading Center Under the Influence of Pacific Plate Subduction

“…We concluded that the 2-D inversion models obtained in the manner we applied to the observed data represent the main features of a possible 3-D electrical resistivity structure. Further, we carry out a 3-D inversion with the observed MT impedances using a program of Usui (2015) and Usui et al (2018), even though the data set may not be suitable for 3-D inversion because the MT impedances are available only along the transect (Figure 1). Details of the 3-D inversion are described in Supporting Information S1.…”

“…Further, we carry out a 3‐D inversion with the observed MT impedances using a program of Usui (2015) and Usui et al. (2018), even though the data set may not be suitable for 3‐D inversion because the MT impedances are available only along the transect (Figure 1). Details of the 3‐D inversion are described in Supporting Information S1.…”

Effect of Different Mineralogical Proportions on the Electrical Conductivity of Dry Hot-Pressed Sintering Gabbro at High Temperatures and Pressures

“…Results of the field geophysical observations from magnetotelluric (MT) and geomagnetic deep sounding (GDS) have confirmed that there is widespread existence of a large quantity of the anomalously high electrical conductivity phenomena in these typical regions of oceanic crust [1][2][3] and West African craton [4]. As a representative basic intrusive rock, it is well known that gabbro is widely outcropped in these global tectonic units of oceanic crust and craton regions, and its corresponding dominant mineralogical composition is constituted of clinopyroxene and plagioclase [5,6].…”

Hybrid finite difference–finite element method to incorporate topography and bathymetry for two-dimensional magnetotelluric modeling

“…Finite difference (FD) and finite element (FE) methods are mostly developed and applied as the main process of forward calculation for 2-D and 3-D inversions (Egbert and Kelbert 2012;Franke et al 2007;Key and Weiss 2006;Kordy et al 2016a, b;Lee et al 2009;Mackie et al 1993;Nam et al 2007;Ren et al 2013;Sarakorn 2017;Sharma and Kaikkonen 1998;Siripunvaraporn et al 2002;Usui 2015;Usui et al 2017Usui et al , 2018Wannamaker et al 1986Wannamaker et al , 1987. The FD method is accurate for simple modeling and, because of the limitation of the structured rectangular mesh, consumes less memory storage and computation time.…”

“…The FD method is accurate for simple modeling and, because of the limitation of the structured rectangular mesh, consumes less memory storage and computation time. The FE method is accurate for real-world complex modeling, especially topography and bathymetry, because of the greater flexibility of mesh schemes (Franke et al 2007;Grayver and Kolev 2015;Grayver 2015;Key and Weiss 2006;Kordy et al 2016a, b;Lee et al 2009;Nam et al 2007;Ren et al 2013;Sarakorn 2017;Sharma and Kaikkonen 1998;Usui 2015;Usui et al 2017Usui et al , 2018Wannamaker et al 1986Wannamaker et al , 1987. However, the disadvantages of FE are greater consumption of memory storage and longer computation time.…”