Exploration Geophysics Exploration Geophysics Society
Journal of the Australian Society of Exploration Geophysicists
RESEARCH ARTICLE

3D numerical modelling of negative apparent conductivity anomalies in loop-loop electromagnetic measurements: a case study at a dacite intrusion in Sugisawa, Akita Prefecture, Japan

Ame Thato Selepeng 1 Shin’ya Sakanaka 2 3 Tadashi Nishitani 2
+ Author Affiliations
- Author Affiliations

1 Graduate School of Engineering and Resource Science, Akita University, 1-1 Tegatagakuen-machi, Akita City 010-8502, Japan.

2 Faculty of International Resource Sciences, Akita University, 1-1 Tegatagakuen-machi, Akita City 010-8502, Japan.

3 Corresponding author. Email: sakanaka@gipc.akita-u.ac.jp

Exploration Geophysics 48(3) 177-191 https://doi.org/10.1071/EG16027
Submitted: 8 March 2016  Accepted: 11 March 2016   Published: 20 April 2016
Originally submitted to SEGJ 30 March 2015, accepted 25 January 2016  

Abstract

Under certain geological conditions, low induction number electromagnetic (LIN-EM) instruments are known to produce negative apparent conductivity (σa) responses. This is particularly the case when the shallow subsurface is characterised by highly conductive bodies, however little attention has been given to this issue in the research literature. To analyse negative σa anomalies and their causative structures, we make use of a 3D integral equation forward modelling technique based on a 3D weighting function. We present 3D numerical modelling results over a volcanic tuff body intruded by several dacite dikes, in Sugisawa, Akita Prefecture, Japan. Apparent conductivity data were acquired using a Geonics EM-34–3 system in the horizontal magnetic dipole (HMD) and vertical magnetic dipole (VMD) operating modes. Our 3D model resolved the horizontal and vertical extent of the dacite dikes and also delineated a high conductive zone between the volcanic tuff and the intrusive dacite dikes. This zone is the causative structure for negative σa responses in the VMD data, and is interpreted to be an alteration zone. Interestingly, the negative σa response was absent when the instrument alignment azimuth was changed, implying an anisotropic effect on the EM signature in the study area. The true conductivity model achieved by 3D forward modelling is shown to compare favourably with the DC resistivity data acquired in the same area.

Key words: electromagnetics, forward modelling, negative apparent conductivity, weighting function.


References

Beamish, D., 2011, Low induction number, ground conductivity meters: a correction procedure in the absence of magnetic effects: Journal of Applied Geophysics, 75, 244–253
Low induction number, ground conductivity meters: a correction procedure in the absence of magnetic effects:CrossRef |

Callegary, J. B., Ferré, T. P. A., and Groom, R. W., 2007, Vertical spatial sensitivity and exploration depth of low-induction-number electromagnetic-induction instruments: Vadose Zone Journal, 6, 158–167
Vertical spatial sensitivity and exploration depth of low-induction-number electromagnetic-induction instruments:CrossRef |

Caminha-Maciel, G., and Figueiredo, I., 2013, Error analysis in measured conductivity under low induction number approximation for electromagnetic methods: ISRN Geophysics, ,
Error analysis in measured conductivity under low induction number approximation for electromagnetic methods:CrossRef |

Dalan, R. A., 1995, Geophysical surveys for archaeological research electromagnetic conductivity surveys: Technical Report (unpublished), Illinois, USA.

deGroot-Hedlin, C., and Constable, S., 1990, Occam’s inversion to generate smooth, two dimensional models from magnetotelluric data: Geophysics, 55, 1613–1624
Occam’s inversion to generate smooth, two dimensional models from magnetotelluric data:CrossRef |

Everett, M. E., 2013, Near-surface applied geophysics: Cambridge University Press.

Gómez-Treviño, E., 1987, Nonlinear integral equations for electromagnetic inverse problems: Geophysics, 52, 1297–1302
Nonlinear integral equations for electromagnetic inverse problems:CrossRef |

Gómez-Treviño, E., Esparza, F. J., and Méndez-Delgado, S., 2002, New theoretical and practical aspects of electromagnetic soundings at low induction numbers: Geophysics, 67, 1441–1451
New theoretical and practical aspects of electromagnetic soundings at low induction numbers:CrossRef |

Hayles Geoscience Surveys Ltd, 2004, Report: EM-31 & EM-34 surveys near Springs Hill - September 1 to 5, 2004: Technical Report (unpublished), Manitoba, Canada.

Kamm, J., Becken, M., and Pedersen, L. B., 2013, Inversion of slingram electromagnetic induction data using a Born approximation: Geophysics, 78, E201–E212
Inversion of slingram electromagnetic induction data using a Born approximation:CrossRef |

Keller, G. V., and Frischknecht, F. C., 1966, Electrical methods in geophysical prospecting: Pergamon Press, Inc.

Loke, M. H., 2001, Electrical imaging surveys for environmental and engineering studies: a practical guide to 2D and 3D surveys. [Web document]. Available at http://www.geotomosoft.com/downloads.php (accessed 20 January 2016).

McNeill, J. D., 1980, Electromagnetic terrain conductivity measurement at low induction numbers: Technical Note TN-6 (unpublished), Geonics Limited, Canada.

McNeill, J. D., 1983, EM 34–3 survey interpretation techniques: Technical Note TN-8 (unpublished), Geonics Limited, Canada.

Méndez-Delgado, S., Gómez-Treviño, E., and Pérez-Flores, M. A., 1999, Forward modeling of direct current and low frequency electromagnetic fields using integral equations: Geophysical Journal International, 137, 336–352
Forward modeling of direct current and low frequency electromagnetic fields using integral equations:CrossRef |

Mester, A., van der Kruk, J., Zimmermann, E., and Vereecken, H., 2011, Quantitative two-layer conductivity inversion of multi-configuration electromagnetic induction measurements: Vadose Zone Journal, 10, 1319–1330
Quantitative two-layer conductivity inversion of multi-configuration electromagnetic induction measurements:CrossRef |

Meulenbeld, P. M., 2007, Establishing geobotanical geophysical correlations in the North-Eastern parts of South Africa for improving efficient borehole siting in difficult terrain: Ph.D. thesis, University of the Free State, Bloemfontein, South Africa.

Monteiro Santos, F. A., 2004, 1D laterally constrained inversion of EM-34 profiling data: Journal of Applied Geophysics, 56, 123–134
1D laterally constrained inversion of EM-34 profiling data:CrossRef |

Pérez-Flores, M. A., Méndez-Delgado, S., and Gómez-Treviño, E., 2001, Imaging of low-frequency and DC electromagnetic fields using a simple linear approximation: Geophysics, 66, 1067–1081
Imaging of low-frequency and DC electromagnetic fields using a simple linear approximation:CrossRef |

Pérez-Flores, M. A., Antonio-Carpo, R. G., Gómez-Treviño, E., Ferguson, I., and Méndez-Delgado, S., 2012, Imaging of 3D electromagnetic data at low-induction numbers: Geophysics, 77, WB47–WB57
Imaging of 3D electromagnetic data at low-induction numbers:CrossRef |

Press, W. H., Teukolsky, S. A., Vetterling, W. T., and Flannery, B. P., 1992, Numerical recipes in C: the art of scientific computing (2nd edition): Cambridge University Press.

Romberg, W., 1955, Vereinfachte numerische Integration: Det Kongelige Norske Videnskabers Selskab Forhandlinger (Trondheim), 28, 30–36

Sasaki, Y., 1989, Two-dimensional joint inversion of magnetotelluric and dipole-dipole resistivity data: Geophysics, 54, 254–262
Two-dimensional joint inversion of magnetotelluric and dipole-dipole resistivity data:CrossRef |

Sasaki, Y., 2001, Full 3D inversion of electromagnetic data on PC: Journal of Applied Geophysics, 46, 45–54
Full 3D inversion of electromagnetic data on PC:CrossRef |

Sasaki, D., 2007, Electrical conductivity structure analysis by electromagnetic survey: M.Sc. thesis, Akita University [in Japanese with English abstract].

Sasaki, Y., and Meju, M. A., 2006, A multidimensional horizontal-loop controlled-source electromagnetic inversion method and its use to characterize heterogeneity in aquiferous fractured crystalline rocks: Geophysical Journal International, 166, 59–66
A multidimensional horizontal-loop controlled-source electromagnetic inversion method and its use to characterize heterogeneity in aquiferous fractured crystalline rocks:CrossRef |

Song, Y., and Kim, J. H., 2008, An efficient 2.5D inversion of loop-loop electromagnetic data: Exploration Geophysics, 39, 68–77
An efficient 2.5D inversion of loop-loop electromagnetic data:CrossRef |

Spies, B. R., 1989, Depth of investigation in electromagnetic sounding methods: Geophysics, 54, 872–888
Depth of investigation in electromagnetic sounding methods:CrossRef |

Wait, J. R., 1982, Geo-electromagnetism: Academic Press.

Wessel, P., Smith, W. H. F., Scharroo, R., Luis, J. F., and Wobbe, F., 2013, Generic mapping tools: improved version released: EOS, 94, 409–410
Generic mapping tools: improved version released:CrossRef |

Zhdanov, M. S., and Fang, S., 1996, Quasi-linear approximation in 3D electromagnetic modeling: Geophysics, 61, 646–665
Quasi-linear approximation in 3D electromagnetic modeling:CrossRef |



Export Citation