1887
Volume 9 Number 5
  • ISSN: 1569-4445
  • E-ISSN: 1873-0604

Abstract

ABSTRACT

The very low‐frequency (VLF) electromagnetic method utilizes primary signals (field) transmitted from worldwide distant transmitters located in coastal areas. These transmitters are meant for long distance marine communication. VLF transmitters operate at a low communication frequency band (between 5–30 kHz) and the transmitted signal travels a long distance. Transmitted signals penetrate the Earth’s subsurface and produce electromagnetic induction in the subsurface even several thousands of kilometres away from the transmitters. The VLF method is quite simple and frequently used in the delineation of near‐surface conducting structures of various practical applications. Several conducting structures lying along a measured profile with different conductivities can be properly induced at distinct frequencies that yield the maximum response. Therefore, such conductors may not be identified or resolved well using single frequency VLF measurement.

A 2D numerical modelling study was carried out over a wide frequency range (1–500 kHz) to find the frequency that produces the maximum response for a given conductor. Results show that a particular frequency (focusing frequency) produces the maximum (peak) response for a conductor. When the measuring frequency either increases or decreases with respect to the focusing frequency, then the peak response always decreases. The focusing frequency remains almost similar with an increase in target depth and host resistivity. An increase in the overburden conductivity shows a decline in the focusing frequency. Two or more targets of different conductivity present in the subsurface yield peak responses at corresponding focusing frequencies. This shows that they will be resolved well at corresponding focusing frequencies. In such circumstances, inversion using single frequency VLF data yields inaccurate results. However, the use of multi‐frequency VLF data yields better results. Inversion of multi‐frequency VLF data is presented to show the efficacy of the approach. A field measurement is also presented and the effectiveness of multi‐frequency VLF measurement is highlighted. Since the numerical modelling study is performed over a broad frequency range covering the VLF and radiomagnetotelluric signal, the focusing study is valid for radiomagnetotelluric applications as well.

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2018-12-18
2020-07-08
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References

  1. BeamishD.1994. Two‐dimensional regularised inversion of VLF data. Journal of Applied Geophysics32, 357–374.
    [Google Scholar]
  2. BeamishD.1998. Three‐dimensional modeling of VLF data. Journal of Applied Geophysics39, 63–76.
    [Google Scholar]
  3. BeamishD.2000. Quantitative 2D VLF data interpretation. Journal of Applied Geophysics45, 33‐47.
    [Google Scholar]
  4. BensonA.K., PayneK.L. and StuddenM.A.1997. Mapping groundwater contamination using DC resistivity and VLF geophysical methods – A case study. Geophysics62, 80–86.
    [Google Scholar]
  5. BernardJ. and VallaP.1991. Groundwater exploration in fissured media with electrical and VLF methods. Geoexploration27, 81–91.
    [Google Scholar]
  6. BoschF.P. and MüllerI.2005. Improved karst exploration by VLF‐EM‐gradient survey: Comparison with other geophysical methods. Near Surface Geophysics3, 299–310.
    [Google Scholar]
  7. ColletL.S. and BackerA.1968. Radiohm method for earth resistivity mapping, Canadian patent 795919.
  8. FraserD.C.1969. Contouring of VLF‐EM data. Geophysics34, 958–967.
    [Google Scholar]
  9. GuerinR., TabbaghA. and AnderieuxP.1994. Field and/or resistivity mapping in MT‐VLF and implications for data processing. Geophysics59, 1695–1712.
    [Google Scholar]
  10. HaylesJ.G. and SinhaA.K.1986. A portable local loop VLF transmitter for geological fracture mapping. Geophysical Prospecting34, 873–896.
    [Google Scholar]
  11. JuppD.L.B. and VozoffK.1975. Stable iterative methods for the inversion of geophysical data. Geophysical Journal of the Royal Astronomical Society42, 957–976.
    [Google Scholar]
  12. KaikkonenP.1979. Numerical VLF modelling. Geophysical Prospecting27, 815–834.
    [Google Scholar]
  13. KaikkonenP. and SharmaS.P.1998. 2‐D nonlinear joint inversion of VLF and VLF‐R data using simulated annealing. Journal of Applied Geophysics39, 155–176.
    [Google Scholar]
  14. KaikkonenP. and SharmaS.P.2001. A comparison of performances of linearized and global nonlinear 2‐D inversions of VLF and VLF‐R electromagnetic data. Geophysics66, 462–475.
    [Google Scholar]
  15. KarousM. and HjeltS.E.1983. Linear‐filtering of VLF dip‐angle measurements. Geophysical Prospecting31, 782–894.
    [Google Scholar]
  16. McNeillJ.D. and LabsonV.F.1991. Geological mapping using VLF radio fields. In: Electromagnetic Methods in Applied Geophysics, Part B: Application (ed. M.N.Nabighian ), pp. 521–640. SEG.
    [Google Scholar]
  17. MohantyW.K., SharmaS.P. and GuptaS.2010. Integrated geological and geophysical study around Tangarapada area, Orissa, Final technical report IDCOL, Government of Orissa.
    [Google Scholar]
  18. OgilviR.D. and LeeA.C.1991. Interpretation of VLF‐EM in‐phase data using current density pseudo‐sections. Geophysical Prospecting39, 567–580.
    [Google Scholar]
  19. OlssonO.1980. VLF anomalies from a perfectly conducting half plane below an overburden. Geophysical Prospecting28, 415–434.
    [Google Scholar]
  20. OlssonO.1983. Computation of VLF response over half plane and wedge models. Geophysical Prospecting31, 171–191.
    [Google Scholar]
  21. OskooiB. and PedersenL.B.2006. Resolution of airborne VLF data. Journal of Applied Geophysics57, 227–241.
    [Google Scholar]
  22. PaalG.1965. Ore prospecting based on VLF radio signals. Geoexploration3, 139–147.
    [Google Scholar]
  23. PatersonN.R. and RonkaV.1971. Five years of surveying with the very low frequency electromagnetic method. Geoexploration9, 7–26.
    [Google Scholar]
  24. PoddarM.1982. Very low frequency response of a perfectly conducting half‐plane in a layered half space. Geophysics47, 1059–1067.
    [Google Scholar]
  25. PoddarM. and RathorB.S.1983. VLF survey of the weathered layer in southern India. Geophysical Prospecting31, 524–537.
    [Google Scholar]
  26. SaydamA.S.1981. Very low frequency electromagnetic interpretation using tilt angle and ellipticity measurements. Geophysics46, 1594–1604.
    [Google Scholar]
  27. SharmaS.P. and BaranwalV.C.2005. Delineation of groundwater‐bearing fracture zones in a hard rock area integrating very low frequency electromagnetic and resistivity data. Journal of Applied Geophysics57, 155–166.
    [Google Scholar]
  28. SharmaS.P. and KaikkonenP.1998a. An automatic finite element mesh generation and element coding in 2‐D electromagnetic inversion. Geophysica34, 93–114.
    [Google Scholar]
  29. SharmaS.P. and KaikkonenP.1998b. Two‐dimensional nonlinear inversion of VLF‐R data using simulated annealing. Geophysical Journal International133, 649–668.
    [Google Scholar]
  30. SmithB.D. and WardS.H.1974. On the computation of polarization ellipse parameters. Geophysics39, 867–869.
    [Google Scholar]
  31. TezkanB.1999. A review of environmental application of quasi‐stationary electromagnetic techniques. Surveys in Geophysics20, 279–308.
    [Google Scholar]
  32. TisleyJ.E.1973. A portable VLF‐EM source for use in geological mapping of veins and fault structures and conventional prospecting.Report by David S. Robertson and Associates, Toranto, Canada.
    [Google Scholar]
  33. WannamakerP.E. and StodtJ.A.1987. A stable finite element solution for two‐dimensional magnetotelluric modeling. Geophysical Journal of the Royal Astronomical Society88, 277–296.
    [Google Scholar]
  34. WonI.J.1980. A wide‐band electromagnetic exploration method – Some theoretical and experimental results. Geophysics15, 928–940.
    [Google Scholar]
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