ModEM is a modular system of parallel computer codes for inversion of electromagnetic (EM) geophysical data, developed over the past decade at Oregon State University. The code has been structured as a flexible system, adaptable to a range of EM geophysical data types, and a range of inverse problem solution strategies, and regularization models. The organization of the ModEM code is summarized schematically in the figure. General theory is described in Egbert and Kelbert (2012) and more specific implementation details are given in Kelbert et al. (2014). ModEM is parallelized using standard MPI libraries, as described in Meqbel (2009).

ModEM has so far been applied most extensively to 3D magnetotelluric (MT) data, using a non-linear conjugate gradient (NLCG) scheme for parameter search. A stable version of this code, tailored to 3D MT inversion has been publicly released to the academic community. There are now over 120 registered academic users worldwide, and already at least 20 peer-reviewed publications, and a significantly larger number of conference presentations, describe use of ModEM in basic research applications. A version of this by now well-tested 3D MT code can be licensed for commercial use, with full support and training. We can also provide custom inversion and interpretation services.

Some initial development and application of ModEM for frequency domain controlled source EM (CSEM) data, and for joint inversion of multiple-method EM datasets have recently been completed. We can work with potential customers to implement these extensions, or similar specialized inversion capabilities (e.g., specialized data types), if desired. Ongoing developments which we expect to complete soon include additional options for regularization, and implementation of alternative optimization strategies.

The code is parallelized with MPI, and requires a modest-sized PC-cluster (approximately 30-50 cores, typically running the linux operating system) for effective use. The only external software components required are standard MPI libraries. ModEM can be used on smaller computers (e.g. a multi-core desktop) but will be significantly slower. Further details on computing requirements can be provided by.

Alternative licensing arrangements (e.g., royalty based, reduced cost for limited duration use) may be arranged in some circumstances. Reduced licensing costs may be available in for commercially funded research, by not-for-profit institution, e.g., governmental agencies or universities, engaged in commercially funded research that benefits the enterprise funding the work. Contact us for further details, and quotes for licensing fees.

All commercial licenses include training and support for the first year. Details (e.g., total hours of support and consulting provided) may depend on specific license arrangements. In general support includes:

• Assistance with installation and initial testing of software on client computer.
• Initial training, by phone, email, or other online media in use of the code. In person training sessions can be arranged for additional fees.
• Bug fixes and updates to 3D-MT code for one year. Verified bugs will be fixed within 2 weeks of report by client.
• Support for one year, including consulting and advice on program usage by phone, email or other online media: allowing multiple contacts for both quick and more involved questions (e.g., debugging input files).
• User Interface for model and data setup, and for display of inversion results, are included with the license, with training and support for these tools included as part of the basic training and support package described above.

Support beyond the first year, including any updates and improvements to the code, will be available, typically for 20% of initial license price. Continuing support includes installation and training on use of any upgrades, including consulting on new aspects of code use by an expert from ModEM Geophysics. Customer may resume support (and receive updated code versions) after a lapse in coverage, by remitting support payments for all years missed, or by purchase of a new license.

Bedrosian, P. A., & Feucht, D. W. (2013). Structure and tectonics of the northwestern United States from EarthScope USArray magnetotelluric data. Earth and Planetary Science Letters.

Booker, J. R. (2014). The magnetotelluric phase tensor: a critical review. Surveys in Geophysics, 35(1), 7-40.

Cherevatova, M., Smirnov, M., Korja, T., Kaikkonen, P., Pedersen, L. B., Hübert, J., ... & Kalscheuer, T. (2014). Crustal structure beneath southern Norway imaged by magnetotellurics. Tectonophysics.

Egbert, G. D., & Kelbert, A. (2012). Computational recipes for electromagnetic inverse problems. Geophysical Journal International, 189(1), 251-267.

Egbert, G. D. (2012). Hybrid conjugate gradient-Occam algorithms for inversion of multifrequency and multitransmitter EM data. Geophysical Journal International, 190(1), 255-266.

Kelbert, A., & Egbert, G. D. (2012). Crust and upper mantle electrical conductivity beneath the Yellowstone Hotspot Track. Geology, 40(5), 447-450.

Kelbert, A., Kuvshinov, A., Velímský, J., Koyama, T., Ribaudo, J., Sun, J., ... & Weiss, C. J. (2014). Global 3-D electromagnetic forward modelling: a benchmark study. Geophysical Journal International, ggu028.

Kelbert, A., Meqbel, N., Egbert, G. D., & Tandon, K. (2014). ModEM: A Modular System for Inversion of Electromagnetic Geophysical Data. Computers & Geosciences.

Kühn, C., Küster, J., & Brasse, H. (2014). Three-dimensional inversion of magnetotelluric data from the Central Andean continental margin. Earth, Planets and Space, 66(1), 1-13.

Kiyan, Duygu, Alan G. Jones, and Jan Vozar. (2013). "The inability of magnetotelluric off-diagonal impedance tensor elements to sense oblique conductors in three-dimensional inversion." Geophysical Journal International. ggt470.

Khoza, T. D., Jones, A. G., Muller, M. R., Evans, R. L., Miensopust, M. P., & Webb, S. J. (2013). Lithospheric structure of an Archean craton and adjacent mobile belt revealed from 2-D and 3-D inversion of magnetotelluric data: Example from southern Congo craton in northern Namibia. Journal of Geophysical Research: Solid Earth, 118(8), 4378-4397.

Khoza, D., Jones, A. G., Muller, M. R., Evans, R. L., Webb, S. J., & Miensopust, M. (2013). Tectonic model of the Limpopo belt: Constraints from magnetotelluric data. Precambrian Research, 226, 143-156.

Meqbel, N., Egbert, G. D., Wannamaker, P. E., Kelbert, A., & Schultz, A. (2014). Deep electrical resistivity stSucture of the northwestern US derived from 3-D inversion of USArray magnetotelluric data. Earth and Planetary Science Letters.

Miensopust, M. P., Queralt, P., & Jones, A. G. (2013). Magnetotelluric 3-D inversionء review of two successful workshops on forward and inversion code testing and comparison. Geophysical Journal International, 193(3), 1216-1238.

Ogaya, X., Queralt, P., Ledo, J., Marcuello, Á., & Jones, A. G. (2014). Geoelectrical baseline model of the subsurface of the Hontomín site (Spain) for CO< sub> 2 geological storage in a deep saline aquifer: A 3D magnetotelluric characterisation. International Journal of Greenhouse Gas Control, 27, 120-138.

Piña-Varas, P., Ledo, J., Queralt, P., Marcuello, A., Bellmunt, F., Hidalgo, R., & Messeiller, M. (2014). 3-D Magnetotelluric Exploration of Tenerife Geothermal System (Canary Islands, Spain). Surveys in Geophysics, 1-20.

Rao, C. K., Jones, A. G., Moorkamp, M., & Weckmann, U. (2014). Implications for the lithospheric geometry of the Iapetus suture beneath Ireland based on electrical resistivity models from deep-probing magnetotellurics. Geophysical Journal International, ggu136.

Ritter, O., Spitzer, K., Afanasjew, M., Becken, M., Börner, R. U., Eckhofer, F., ... & Weißflog, J. (2014). Three-Dimensional Multi-Scale and Multi-Method Inversion to Determine the Electrical Conductivity Distribution of the Subsurface (Multi-EM). In Tomography of the Earth's Crust: From Geophysical Sounding to Real-Time Monitoring (pp. 83-93). Springer International Publishing.

Sass, P., Ritter, O., Ratschbacher, L., Tympel, J., Matiukov, V. E., Rybin, A. K., & Batalev, V. Y. (2014). Resistivity structure underneath the Pamir and Southern Tian Shan. Geophysical Journal International, ggu146.

Tietze, K., & Ritter, O. (2013). Three-dimensional magnetotelluric inversion in practice--the electrical conductivity structure of the San Andreas Fault in Central California. Geophysical Journal International, 195(1), 130-147.

Conference Abstracts Citing ModEM (Very Incomplete)

Bologna, M. S., Egbert, G., Kelbert, A., Pádua, M. B., VITORELLO, I., PADILHA, A., & SAMPAIO, E. (2013). Geoelectrical structure of the Archean Serrinha Block and surroundings, Northeastern Brazil, obtained from 3D modeling of magnetotelluric measurements. In Thirteenth International Congress of the Brazilian Geophysical Society, Rio de Janeiro, Brazil.

Egbert, G., Meqbel, N., & Strack, K. Cabled marine magnetotellurics: denser data at lower cost and higher information content, In 2013 SEG Annual Meeting. Society of Exploration Geophysicists.

Egbert, G., Meqbel, N., & Ritter, O. (2014, October). Implementing Novel Schemes for Inversiono 3D EM Data in Modem, the OSU Modular EM Inversion System. In 2014 SEG Annual Meeting. Society of Exploration Geophysicists.

Kiyan, D., Jones, A. G., Fullea, J., Ledo, J., Siniscalchi, A., & Romano, G. (2013, April). Three-dimensional magnetotelluric imaging of crustal and uppermost mantle structures of the Atlas Mountains of Morocco. In EGU General Assembly Conference Abstracts (Vol. 15, p. 12295).

Kiyan, D., Jones, A. G., Fullea, J., Ledo, J., Siniscalchi, A., & Romano, G. (2013, December). Crustal and Uppermost Mantle Structure of the Atlas Mountains of Morocco Revealed from 3-D Inversion of Magnetotelluric Data. In AGU Fall Meeting Abstracts (Vol. 1, p. 2384).

Meqbel, N., & Ritter, O. (2014, June). New Advances for a joint 3D inversion of multiple EM methods. In 76th EAGE Conference and Exhibition-Workshops.

Milligan, P. (2013). 3D modelling of magnetotelluric data acquired along the Youanmi deep seismic reflection transects in the Yilgarn, Western Australia. ASEG Extended Abstracts, 2013(1), 1-4.

Neukirch, M., Garcia, X., Galiana, S., & Roux, E. (2012, April). GarField-A MT Survey of the Crustal Structure of the Intraplate Volcanic Region La Garrotxa, Northwestern Spain. In EGU General Assembly Conference Abstracts (Vol. 14, p. 9377).

Pethick, A. M., & Harris, B. D. (2014, October). Structural Constraints in Joint Inversion of Seismic And EM Data: Analysis and Visualization. In 2014 SEG Annual Meeting. Society of Exploration Geophysicists.

Piña-Varas, P., Ledo, J., Queralt, P., Marcuello, A., Bellmunt, F., & Hidalgo, R. (2013, April). Three-dimensional magnetotelluric exploration of Tenerife geothermal field (Canary Islands, Spain). In EGU General Assembly Conference Abstracts (Vol. 15, p. 5490).

Sass, P., Ritter, O., Rybin, A., & Batalev, V. (2013, April). 3D and 2D inversion of magnetotelluric data from the continental collision zone in the Pamirs and Tien Shan, Central Asia. In EGU General Assembly Conference Abstracts (Vol. 15, p. 1474).

Slezak, K., Brasse, H., Józwiak, W., & Nowozynski, K. (2014, May). 3-D MT modelling and HMT analysis for the north-west part of Poland. In EGU General Assembly Conference Abstracts (Vol. 16, p. 4028).

Weckmann, U., Meqbel, N., Kapinos, G., Jegen-Kulcsar, M., & Ritter, O. (2014, May). Electrical conductivity images across the Namibian passive margin: Implications for tectonic processes along the Kaoko Belt, the western Kongo Craton and the Walvis Ridge. In EGU General Assembly Conference Abstracts (Vol. 16, p. 10323).