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.

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 / Distortion matrix of galvanic distortion
Regularization: L2 regularization with Laplacian filter / L2 regularization with difference filter / L1 regularization with difference filter

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

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