FEMTIC is a 3-D magnetotelluric inversion code based on the following studies.

Y. Usui, 3-D inversion of magnetotelluric data using unstructured tetrahedral elements: applicability to data affected by topography, Geophys. J. Int., 202 (2): 828-849, doi.org/10.1093/gji/ggv186, 2015.

Y. Usui, Y. Ogawa, K. Aizawa, W. Kanda, T. Hashimoto, T. Koyama, Y. Yamaya and T. Kagiyama, Three-dimensional resistivity structure of Asama Volcano revealed by data-space magnetotelluric inversion using unstructured tetrahedral elements, Geophys. J. Int., 208 (3): 1359-1372, doi.org/10.1093/gji/ggw459, 2017.

Y. Usui, T. Kasaya, Y. Ogawa and H. Iwamoto, Marine magnetotelluric inversion with an unstructured tetrahedral mesh, Geophys. J. Int., 214(2): 952-974, doi.org/10.1093/gji/ggy171, 2018.

Y. Usui, M. Uyeshima, H. Hase, H. Ichihara, K. Aizawa, T. Koyama, et al. Three-dimensional electrical resistivity structure beneath a strain concentration area in the back-arc side of the northeastern Japan arc. Journal of Geophysical Research: Solid Earth, 129, e2023JB028522. http://doi.org/10.1029/2023JB028522, 2024

Y. Usui, M. Uyeshima. Three-dimensional combined inversion method of the MT and Network-MT response functions. Earth Planets Space, 77, 135. doi.org/10.1186/s40623-025-02266-x , 2025.

Y. Usui, M. Uyeshima, S. Sakanaka, and Y. Yamaya. Three-dimensional magnetotelluric inversion code for transverse isotropic electrical resistivity structure. Computers and Geosciences. Submitted, 2026.

FEMTIC enables us to incorporate topography and bathymetry into an inversion model.

FEMTIC is applicable to land magnetotelluric survey data and ocean bottom magnetotelluric survey data.

Functional overview

FEMTIC gives a three-dimensional electrical resistivity structure from the response functions at observation points on the Earth's surface.

Mesh type: Tetrahedral mesh / Hexahedral brick mesh / Non-conforming deformed hexahedral mesh
Data type: Impedance tensor / Vertical magnetic transfer function / Inter-station horizontal magnetic transfer function / Phase tensor / Apparent resistivity & Phase / Network-MT response function / Impedance tensor obtained from the Network-MT response functions / Apparent resistivity & Phase obtained from the Network-MT response functions
Inversion algorithm: Model-space Gauss-Newton method / Data-space Gauss-Newton method
Parallel computation: Multiple processes parallel computation with MPI / Multiple threads parallel computation with OpenMP / MPI & OpenMP hybrid parallel computation
Model parameter: Subsurface electrical resistivity, including anisotropic resistivity / Distortion matrix of galvanic distortion

GitHub repository

Source codes, manuals, and sample input files of FEMTIC can be downloaded from the following GitHub repository.

github.com/yoshiya-usui/femtic.git

Release note

v5.0 Jun. 19, 2026: Added anisotropic resistivity inversion mode.

v4.3 Feb. 6, 2025: The directory for the out-of-core files of the sensitivity matrix becomes changeable.

v4.2 Mar. 25, 2024: I modified some parts to allow the use of large-scale models and large datasets.

v4.1 Nov. 9, 2021: This new version supports difference filter. The error calculation of log10(apparent resistivity) is modified. Rotation angles of distortion matrix are limited to from -90 to 90 (deg.) when gains and rotations of galvanic distortion are estimated.

v4.0 Jun. 3, 2021: This new version supports non-conforming deformed hexahedral mesh.

v3.5 Jan. 11, 2021: This new version supports observed data of apparent resistivity and phase and unsymmetric roughening matrix.

v3.4.7 Sep. 4, 2020: The integer indices into the multiple right-hand-side vectors and solution vectors were changed from 32-bit to 64-bit.

v3.4.6 Sep. 2, 2020: This version allows us to make resistivity of every individual subsurface element to be a different model parameter, in analogy with other 3-D inversion code.

Pre/post-processing tools for FEMTIC

Some pre/post-processing tools for FEMTIC, including meshing tools, and their manuals can be downloaded from GitHub. Results of FEMTIC can be visualized by ParaView.

makeDHexaMesh : Tool for making non-conforming deformed hexahedral mesh for FEMTIC

makeTetraMesh : By using this tool, you can make a surface mesh for creating a tetrahedral mesh.

makeMtr : This tool output .mtr file of TetGen by reading node and .ele files of TetGen.

TetGen2Femtic : This program converts output files of TetGen to FEMTIC.

makeHexaMesh : Tool for making hexahedral brick mesh for FEMTIC

mergeResultOfFEMTIC : By this program, you can merge result files (.csv) of FEMTIC.

makeCutawayForGMT : By using this program, you can make a file needed to draw a cross-section of a resistivity structure by GMT.

changeResistivityForFemtic : By this program, you can change resistivity values of a specified area for sensitivity tests of FEMTIC inversion results

Mesh for modeling around Amami Oshima island, Japan

Mesh for modeling around Asama volcano, Japan

Non-conforming deformed hexahedral mesh representing Izu Oshima Island, Japan