International scientific journal

ISSN: 2663-0419 (electronic version)

ISSN: 2218-8754 (print version)

International scientific journal

ISSN: 2663-0419 (electronic version)

ISSN: 2218-8754 (print version)


VLF-method of geophysical prospecting: a non-conventional system of processing and interpretation (implementation in the Caucasian ore deposits)

Eppelbaum L.V.

1 – Department of Earth Sciences, Faculty of Exact Sciences, Tel Aviv University Ramat Aviv 6997801, Tel Aviv, Israel

2 – Azerbaijan State Oil and Industry University 20 Azadlig Ave., Baku, AZ1010, Azerbaijan


 Investigation of the electromagnetic (EM) fields from distant VLF (very low friquency) military transmitters is one of the fastest and low-expensive geophysical methods. At present, it finds frequent application in prospecting for various deposits, search of subsurface underground water, archaeogeophysical studies and various types of geological mapping. For geophysical investigation can be utilized a few dozens of the VLF transmitters disposed in various countries. The different frequencies and angles of registered EM radiation enable to obtain additional preferences by interpretation. A depth of investigation depends on the radiowave frequency and averaged resistivity of the host medium and usually ranges from several tens to several hundred meters (last values – under very favorable conditions). Both the electric and magnetic components of EM field are used in investigation by the VLF method. Generally only the magnetic field (H) is employed. A wide using of the VLF-technique was limited by absence of reliable methods for elimination of the EM field time variations, rugged relief influence and procedures for quantitative interpretation of the VLF-anomalies. These problems are successfully solved and the unified methodological system is developed. For elimination of the temporal variations a special procedure based on the direct continuous filtering is proposed. The correlation technique enables to significantly reduce the rugged relief influence. For quantitative interpretation is proved a possibility to use the modern interpreting methods elaborated in magnetic prospecting for complex geological-geophysical conditions. Finally, for revealing hidden objects against the high-intensive geological noise background, an application of non-conventional statistical, informational and wavelet algorithms is suggested. The main components of the developed system were successfully tested in the Caucasian polymetallic and copper deposits.

Keywords: VLF-transmitters, temporal variations, rugged relief influence, advanced quantitative analysis, ore deposits, geological-geophysical mapping, earthquake precursors



Abtahi S., Pedersen L., Kamm J., Kalscheuer T. Extracting geoelectrical maps from vintage very- low-frequency airborne data, tipper inversion, and interpretation: a case study from northern Sweden. Geophysics, Vol. 81, No. 5, 2016, pp. B135-B147.

Alperovich L., Eppelbaum L., Zheludev V., Dumoulin J., Soldovieri F., Proto M., Bavusi M., Loperte A. A new combined wavelet methodology applied to GPR and ERT data in the Montagnole experiment (French Alps). Journal of Geophysics and Engineer-ing, Vol. 10, No. 2, 025017, 2013, pp. 1-17.

Alpin L.M., Dayev D.S., Karinsky A.D. Theory of fields employed in geophysical prospecting. Nedra. Moscow, 1985, 407 p. (in Russian).

Al-Tarazi E., Abu-Rajab J.A., Al-Naqa A., El-Waheidi M. Detecting leachate plumes and groundwater pollution at Ruseifa municipal landfill utilizing VLF-EM method. Jour. of Applied Geophysics, Vol. 65, 2008, pp. 121-131.

Baker H.A., Myers S.O. A topographic correction for VLF-EM profiles based on model studies. Geoexploration, Vol. 18, 1980, pp. 135-144.

Barr R., Llanwyn Jones D., Rodger C.J. ELF and VLF radio waves. Journal of Atmospheric and Solar-Terrestrial Phys-ics, Vol. 62, 2000, pp. 1689-1718.

Basokur A.T., Candansayar M.E. Enhancing VLF data for qualitative interpretation: An example of massive chalcopyrite exploration. The Leading Edge, Vol. 22, No. 6, 2003, pp. 568-570.

Bayrak M. Exploration of chrome ore in Southwestern Turkey by VLF-EM. Journal of the Balkan Geophysical Society, Vol. 5, No. 2, 2002, pp. 35-46.

Beamish D. Quantitative 2D VLF data interpretation. Journal of Applied Geophysics, Vol. 45, No. 1, 2000, pp. 33-47.

Bosch F.P. and Müller I. Improved karst exploration by VLF-EM-gradient survey: comparison with other geophysical methods. Near Surface Geophysics, Vol. 3, No. 4, 2005, pp. 299-310.

Bozzo E., Lombardo S., Merlanti F. VLF prospecting: observations about field experiments. Annali di Geofisica, Vol. 37, No. 5, 1994, pp. 1215-1227.

Darnet M., Sailhac P., Marquis G. Geophysical investigation of antique iron furnaces: insights from modelling magnetic and VLF data. Near Surface Geophysics, Vol. 2, 2004, pp. 93-99.

Djeddi M., Baker H.A., Tabbagh A. Interpretation of VLF-EM anomalies of 3D structures by using linear filtering techniques. Annali Di Geofisica, Vol. 41, No. 2, 1998, pp. 151-163.

Dmitriyev V.I. (Ed.). Computing mathematics and computer methods in exploration geophysics. Geophysicist’s manual. Nedra. Moscow, 1982, 222 p. (in Russian).

Dmitriyev V.I., Baryshnikova I.A., Zakharov E.V. Anomalous electromagnetic fields of geological bodies. Nedra. Leningrad, 1977, 168 p. (in Russian).

Drahor M.G. Integrated geophysical studies in the upper part of Sardis archaeological site, Turkey. Jour. of Applied Geophysics, Vol. 59, No. 3, 2006, pp. 205-223.

Drahor M.G., Berge M.A. Geophysical investigations of the Seferihisar geothermal area, Western Anatolia, Turkey. Geothermics, Vol. 35, No. 3, 2006, pp. 302-320.

Eberle D. A method of reducing terrain relief effects from VLF-EM data. Geoexploration, Vol.19, No. 2, 1981, pp. 103-114.

Electrical prospecting instruction. USSR Ministry of Geology. Nedra. Moscow, 1984, 134 p. (in Russian).

Eppelbaum L.V. Examples of terrain corrections in the VLF-method in the Caucasian region, USSR. Geoexploration, Vol. 28, 1991, pp. 67-75.

Eppelbaum L.V. Localization of ring structures in Earth’s environments. Jour. of the Archaeological Soc. of the Slovakian Acad. of Sci., Spec. Issue: ‘Archaeological. Prospecting’, Vol. XLI, 2007a, pp. 145-148.

Eppelbaum L.V. Revealing of subterranean karst using modern analysis of potential and quasi-potential fields. Proceed. of the 2007 SAGEEP Conference, Vol. 20, Denver, USA, 2007b, pp. 797-810.

Eppelbaum L.V. Study of magnetic anomalies over archaeological targets in urban conditions. Physics and Chemistry of the Earth, Vol. 36, No. 16, 2011, pp. 1318-1330.

Eppelbaum L.V. Four color theorem and applied geophysics. Applied Mathematics, Vol. 5, No. 4, 2014a, pp. 358-366.

Eppelbaum L.V. Estimating informational content in geophysical observations on example of searching economic minerals in Azerbaijan. ANAS Proceedings. The sciences of Earth, Nos. 3-4, 2014b, pp. 31-40.

Eppelbaum L.V. Quantitative interpretation of magnetic anomalies from bodies approximated by thick bed models in complex environments. Environmental Earth Sciences, Vol. 74, No. 7, 2015, pp. 5971-5988.

Eppelbaum L.V. Remote Operated Vehicles geophysical surveys in air, land (underground) and submarine archaeology: General peculiarities of processing and interpretation. Trans. of the 12th EGU Meet., Geophysical Research Abstracts, Vol. 18, EGU2016-10055, Vienna, Austria, 2016, pp. 1-7.

Eppelbaum L.V. Geophysical potential fields: geological and environmental applications. Elsevier. Amsterdam – N.Y., 2019, 465 p.

Eppelbaum L. Theories of probability, information and graphs in applied geophysics. In: (K. Kyamakya, Ed.) Prime archives in applied mathematics, Vide Leaf. 2020, pp. 1-35.

Eppelbaum L.V., Alperovich L., Zheludev V., Pechersky A. Application of informational and wavelet approaches for integrated processing of geophysical data in complex environments. Proceed. of the 2011 SAGEEP Conference, Charleston, South Carolina, USA, Vol. 24, 2011, pp. 24-60.

Eppelbaum L.V. and Finkelstein M.I. Radon emanation, magnetic and VLF temporary variations: removing components not associated with dynamic processes. Collection of Selected Papers of the XXVI General Assembly of the European Seismological Commission (Tel Aviv, Israel), 1998, pp. 122-126.

Eppelbaum L.V., Khesin B.E. VLF-method: elimination of noises and quantitative interpretation. Collection of papers. Regional Symposium on Electromagnetic Compatibility “1992 – From a Unified Region to a Unified World”, Section “LF to ULF” Electromagnetics and the Earth, 5.2.1, IEEE Publ., Tel Aviv, 1992, pp. 1-6.

Eppelbaum L.V., Khesin B.E. Geophysical studies in the Caucasus. Springer. Dordrecht-N.Y., 2012, 411 p.

Eppelbaum L.V., Khesin B.E., Itkis S.E. Prompt magnetic investigations of archaeological remains in areas of infrastructure development: Israeli experience. Archaeological Prospection, Vol. 8, No. 3, 2001, pp. 163-185.

Eppelbaum L.V., Livshits Ya., Flexer A., Ben-Avraham Z. Integrated geological-geophysical analysis of Ring Structures phenomenon in the Eastern Mediterranean. Trans. of the Conf. of Israel Geological Soc., Annual Meet., Mizpe-Ramon, Israel, 1998, p.25.

Eppelbaum L.V., Mishne L.R. Some theoretical aspects of optimal filtration of geophysical signals with dependent noises. Deposited in VINITI, USSR Academy of Sciences, No. 6034-B88, 1988, pp. 1-10 (in Russian).

Eppelbaum L.V., Mishne A.R. Unmanned airborne magnetic and VLF investigations: effective geophysical methodology of the near future. Positioning, Vol. 2, No. 3, 2011, pp. 112- 133.

Eppelbaum L.V., Zheludev V., Averbuch A. Diffusion maps as a powerful tool for integrated geophysical field analysis to detecting hidden karst terranes. ANAS Proceedings. The sciences of Earth, Nos. 1-2, 2014, pp. 36-46.

Fischer G., Le Quang B.V., Muller L. VLF ground surveys, a powerful tool for the study of shallow two-dimensional structures. Geophysical Prospecting, Vol. 31, 1983, pp. 977-991.

Fraser D.C. Contouring of VLF-EM data. Geophysics, Vol. 34, No. 6, 1969, pp. 958-967.

Ginzburg S.N. VLF-method for searching and prospecting of pyrite-polymetallic deposits. Prospecting and protection of mineral resources, No. 9, 1982, pp. 39-44 (in Russian).

Ginzburg S. et al. Underground geophysical investigations in ore deposits of Belokan-Zakatala ore field. Unpublished Report of TzNIGRI (Central Research Inst. of Geol. Prospecting for Base and Precious Metals), The funds of TzNIGRI, Moscow, 1981, 238 p. (in Russian).

Gokhberg M.B., Gufeld I.L., Rozhnoy A.A., Marenko V F., Yampolsky V.S., Ponomarev E.A. Study of seismic influence on the ionosphere by super long wave probing of the Earth-ionosphere waveguide. Physics of Earth and Planet. Inter., Vol. 57, 1989, pp. 64-67.

Gordeyev S.G. On the problem of relief influence in the VLF-method. Proceed. of TSNIGRI, Vol. 89, 1970, pp. 188-195 (in Russian).

Gordeyev S.G. Method of accounting for the relief influence on magnetic fields of remote transmitters. Proceed. of TsNIGRI, Vol. 179, 1983, pp. 56-60 (in Russian).

Gordeyev S.G., Sedelnikov E.S. On the interpretation of the VLF method results on the basis of the conductive bed modeling. Proceedings of TSNIGRI, Vol. 116, 1974, pp. 88-99 (in Russian).

Gordeyev S.G., Sedelnikov E.S., Tarkhov A.G. Electric prospecting by the radio comparing and bearing method (VLF). Nedra. Moscow, 1981, 132 p. (in Russian).

Guerin R., Benderitter Y. Shallow karst exploration using MT-VLF and DC resistivity methods. Geophysical Prospecting, Vol. 43, No. 5, 1995, pp. 635-653.

Gürer A., Bayrak M., Gürer O.F. A VLF survey using current gathering phenomena for tracing buried faults of Fethiye–Burdur Fault Zone, Turkey. Journal of Applied Geophysics, Vol. 68, No. 3, 2009, pp. 437-447.

Hamdan H., Kritikakis G., Andronikidis N., Economou N., Manoutsoglou M., Vafidis A. Integrated geophysical methods for imaging saline karst aquifers. A case study of Stylos, Chania, Greece. Jour. of the Balkan Geophysical Soc., Vol. 13, No. 1, 2010, pp. 1-8.

Hänl H., Maue A.W., Westpfahl K. Theory of diffraction. Springer-Verlag. Berlin, 1961, 218 p. (in German).

Harrison R.G., Aplin K.L., Rycroft M.J. Atmospheric electricity coupling between earthquake regions and the ionosphere. Jour. Atmos. Sol. Terrest. Phys., Vol. 72, No. 5-6, 2010, pp. 376-381.

Hayakawa M., Kasahara Y., Nakamura T., Muto F., Horie T., Maekawa S., Hobara Y., Rozhnoi A.A., Solovieva M., Molchanov O.A. A statistical study on the correlation between lower ionospheric perturbations as seen by subionospheric VLF/LF propagation and earthquakes. Jour. of Geophys. Research, Vol. 115, A9, 2010, pp. 1-9.

Ishimaru A. Electromagnetic wave propagation, radiation, and scattering: from fundamentals to applications. 2nd Ed. Wiley. USA, 2017, 940 p.

Jeng Y., Lin M.J., Chen C.S., Wang Y.H. Noise reduction and data recovery for a VLF-EM survey using a nonlinear decomposition method. Geophysics, Vol. 72, No. 5, 2007, pp. F223-F235.

Kachakhidze N., Kachakhidze M., Kereselidze Z., Ramishvili G. Specific variations of the atmospheric electric field potential gradient as a possible precursor of Caucasus earthquakes. Nat. Hazards Earth Syst. Sci., Vol. 9, 2009, pp. 1221-1226.

Kaikkonen P., Sharma S.P. A comparison of performances of linearized and global nonlinear 2-D inversion of VLF and VLF-R electromagnetic data. Geophysics, Vol. 66, No. 2, 2001, pp. 462-475.
Karous M. Effect of relief in EM-data methods with very dis-tant source. Geoexploration, Vol. 17, 1979, pp. 33-42.

Karous M., Hjelt S.E. Determination of apparent current density from VLF measurements. Dept. of Geophys., University of Gulu, No. 89, 1979, 1-9 p.

Karous M., Hjelt S.E. Linear filtering of VLF dip-angle measurements. Geophysical Prospecting, Vol. 31, No. 5, 1983, pp. 782-794.

Kaufman A.A., Keller G.V. The magnetotelluric method. Elsevier. Amsterdam, 1981, 596 p.

Khain V.E. Main problems of the modern geology. Nauka. Moscow, 1995, 348 p. (in Russian).

Khalil M.A., Abbas A.M., Santos F.A.M., Mesbah H.S.A., Massoud U. VLF-EM study for archaeological investigation of the labyrinth mortuary temple complex at Hawara area, Egypt. Near Surface Geophysics, Vol. 8, 2010, pp. 203-212.

Khesin B.E., Alexeyev V.V., Eppelbaum L.V. Interpretation of geophysical fields in complicated environments. Kluwer Academic Publisher (Springer). Ser.: Advanced Approaches in Geophysics, Dordrecht – London – Boston, 1996, 368 p.

Kushida Y., Kushida R. Possibility of earthquake forecast by radio observations in the VHF band. Jour. of Atmos. Elect., Vol. 22, 2002, pp. 239-255.

Landau L.D., Lifshitz E.M., Pitaevskii L.P. Electrodynamics of Continuous Media. Pergamon Press. Oxford, 1984, 460 p.

Liu H., Liu J., Yu C., Ye J., Zeng Q. Integrated geological and geophysical exploration for concealed ores beneath cover in the Chaihulanzi goldfield, northern China. Geophysical Pro-specting, Vol. 54, No. 5, 2006, pp. 605-621.

McNeill J.D., Labson V.F. Geological mapping using VLF radio fields. In: (Nabighian, M.C., Ed.) Geotechnical and Environmental Geophysics, Review and Tutorial. Vol. 1. Society of Exploration, Tulsa, 1991, pp. 191-218.

Michael A.J. Testing prediction methods: Earthquake clustering versus the Poisson model. Geophysical Research Lett., Vol. 24, No. 15, 1997, pp. 1891-1894.

Miecznik S. Conducting cylinder in the field of a plane electro-magnetic wave. Techn. Poszuk. Geol., 25, No. 3, 1986, 16-22 (in Polish).

Mohanty W.K., Mandal A., Sharma S.P., Gupta S., Misra S. Integrated geological and geophysical studies for delineation of chromite deposits: a case study from Tangarparha, Orissa, India Chromite exploration at Tangarparha. Geophysics, Vol. 76, No. 5, 2011, pp. B173-B185.

Moriya T., Mogi T., Takada M. Anomalous pre-seismic transmission of VHF-band radio waves resulting from large earthquakes, and its statistical relationship to magnitude of impending earthquakes. Geophysical Jour. Intern., Vol. 180, 2010, pp. 858-870.

Ogilvy R.D., Cuadra A., Jackson P.D., Monte J.L. Detection of an air-filled drainage gallery by the VLF resistivity method. Geophysical Prospecting, Vol. 39, No. 6, 1991, pp. 845-859.

Olsson O. VLF-Anomalies from a perfectly conducting half-plane below an overburden. Geophysical Prospecting, 28, No. 3, 1980, pp. 415-434.

Olsson O. Computation of VLF response over half-plane and wedge models. Geophysical Prospecting, Vol. 31, No. 1, 1983, pp. 171-191.

Oskooi B., Pedersen L.B. Resolution of airborne VLF data. Jour. of Applied Geophysics, Vol. 58, No. 2, 2006, pp. 158-175.

Pazzi V., Tapete D., Cappuccini L., Fanti R. An electric and electromagnetic geophysical approach for subsurface investigation of anthropogenic mounds in an urban environment. Geomorphology, Vol. 273, 2016, pp. 335-347.

Pedersen L.B., Oskooi B. Airborne VLF measurements and variations of ground conductivity. Surveys in Geophysics, Vol. 25, 2004, pp. 151-181.

Pierce E.T. Atmospheric electricity and earthquake prediction. Geophysical Research Lett., Vol. 3, No. 3, 1976, pp. 185-188.

Pinel N., Boulier C. Electromagnetic wave scattering from random rough surfaces: asymptotic models. Wiley. USA, 2013, 265 p.

Poddar M. Very low-frequency electromagnetic response of a perfectly conducting half-plane in a layered half-space. Geophysics, Vol. 47, 1982, pp. 1059-1067.

Poikonen A., Suppala I. On modeling airborne very low-frequency measurements. Geophysics, Vol. 54, No. 12, 1989, pp. 1596-1606.

Pulinets S. and Boyarchuk K. Ionospheric precursors of earthquakes. Springer. Berlin – Heidelberg, 2004, 315 p.

Rajab J.A. Mapping the near-surface geoelectrical structure of the Mottled Zone using the very low frequency–electromagnetic method. Jour. of Applied Geophysics, Vol. 184 (104240), 2021, pp. 1-12.

Rozhnoi A., Solovieva M., Hayakawa M. VLF/LF signals method for searching of electromagnetic earthquake precursors. In: (M. Hayakawa, Ed.) Earthquake Prediction Studies: Seismo Electromagnetics, TERRAPUB. Tokyo, 2013, pp. 31-48.

Sandrin A., Elming S.A. Geophysical and petrophysical study of an iron oxide copper gold deposit in northern Sweden. Ore Geology Reviews, Vol. 29, No. 1, 2006, pp. 1-18.

Santos F.A.M., Mateus A., Figueiras J., Goncalves M.A. Map-ping groundwater contamination around a landfill facility using the VLF-EM method – A case study. Journal of Applied Geophysics, Vol. 60, No. 2, 2006, pp. 115-125.

Sedelnikov E.S. The effect of topography on the field of a distant source of alternating electromagnetic field. Izvestiya of the USSR Acad. of Sci., Ser: Physics of the Solid Earth, No.7, 1983, pp. 102-106 (in Russian).

Sharma S.P., Kaikkonen P. Two-dimensional non-linear inver-sion of VLF-R data using simulated annealing. Geophysical Journal Inter., Vol. 133, No. 3, 1998, pp. 649-668.

Sharma S.P., Biswas A., Baranwal V.C. Very low-frequency electromagnetic method: a shallow subsurface investigation technique for geophysical applications. In: (Sengupta D., Ed.) Recent trends in modelling of environmental contaminants”, Springer. 2014, 119-141.

Shendi E.-A., Aziz A., Mamoun K., Gamal M. The effectiveness of the very low frequency electromagnetic method (VLF‑EM) in the exploration of sulphide mineralization in arid environments, case study from South Sinai Peninsula, Egypt. Environmental Earth Sciences, Vol. 76, No. 22, 2017, pp. 1-11.

Singh A., Maurya S.K., Sharma, S.P. Forward modeling and in-version of very low frequency electromagnetic data over rugged topography using 2d triangular elements. In: (Biswas, A. and Sharma, S.P., Eds.) Advances in modeling and interpretation in near surface geophysics, 2020, pp. 97-120.

Singh A., Sharma S.P. Interpretation of very low frequency electromagnetic measurements in terms of normalized current density over variable topography. Jour. of Applied Ge-ophysics, Vol. 133, 2016, pp. 82-91.

Sinha A.K. Recent developments in quantitative interpretation of VLF-EM data. In: (Eds. Roy K.K. et al.) Deep electro-magnetic exploration, Narosa Publ. House. New Delhi, India, 1998, pp. 599-606.

Simon F.-X., Tabbagh A., Donati J.S., Sarris A. Permittivity mapping in the VLF–LF range using a multi-frequency EMI device: first tests in archaeological prospection. Near Sur-face Geophysics, Vol. 17, No. 1, 2019, pp. 27-41.

Smirnov S. Negative anomalies of the Earth’s electric field as earthquake precursors. Geosciences, Vol. 10, No. 10, 2019, pp. 1-7.

Tarkhov A.G. Principles of geophysical prospecting by radio comparing and bearing method (VLF). Gosgeoltekhizdat. Leningrad, 1961, 216 p. (in Russian).

Tawfik M., Farag K.S., Ibraheem M. The efficiency of (VLF-EM) method in detecting buried old tunnels in the Egyptian Nile Delta. Trans. Of the 73rd EAGE Conf. & Exhib., Vienna, Austria, SP04, 2011, pp. 1-3.

Tesmull F.A., Crossley D.I. Inversion of VLF-data for simple lateral inhomogeneties. Geological Survey of Canada, No. 81-15, 1981, pp. 79-86.

Timur E. Magnetic susceptibility and VLF-R investigations for determining geothermal blowout contaminated area: a case study from Alaşehir (Manisa/Turkey). Environmental Earth Sciences, Vol. 72, 2014, pp. 2497-2510.

Xu L.-B. Application of the very low frequency (VLF) electro-magnetic method in the Zhuyuan copper deposit, Henan Province. Chinese Geology, Vol. 28, No. 11, 2001, pp. 25-28 (in Chinese with English abstract).

Villee M.A., Chouteau M., Palacky G.J. Effect of temporal and spatial variations of the primary signal on VLF total-field surveys. Geophysics, Vol. 57, No.1, 1992, pp. 97-105.

Wait J.R. Introduction to the theory of VLF propagation. Proceedings of the Inst. Of Radio Engineers (IEEE), Vol. 50, 1962, pp. 1624-1647.

Zaborovsky A.I. Electric prospecting. Gostoptekhizdat. Moscow, 1963, 423 p. (in Russian).

Zakharov E.V., Pimenov Y.V. Numerical analysis of radiowave diffraction. Radio and Communication. Moscow, 1982, 184 p. (in Russian).

Zhdanov M.S. Integral transforms in geophysics. Springer. Berlin – Heidelberg, 1988.

Zhdanov M.S. The analogues of the Cauchy type integral for the theory of geophysical fields. Nauka. Moscow, 1984, 328 p. (in Russian).

Zhdanov M.S., Keller G.V. The geoelectrical methods in geophysical exploration, Elsevier. Amsterdam, 1994, 873 p.

Zlotnicki J., Vargemezis G., Mille A., Bruère F., Hammouya G. State of the hydrothermal activity of Soufriere of Guadeloupe volcano inferred by VLF surveys. Jour. of Applied Geophysics, 2006, Vol. 58, No. 4, pp. 265-279.

DOI: 10.33677/ggianas20210200060