Decomposing the electromagnetic response of magnetic dipoles to determine the geometric parameters of a dipole conductor
Jacques K. Desmarais 1 3 Richard S. Smith 21 Earth Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, Canada, S7N 5E2.
2 Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 2C6.
3 Corresponding author. Email: jkd788@mail.usask.ca
Exploration Geophysics 47(1) 13-23 https://doi.org/10.1071/EG14070
Submitted: 17 July 2014 Accepted: 20 February 2015 Published: 25 March 2015
Abstract
A novel automatic data interpretation algorithm is presented for modelling airborne electromagnetic (AEM) data acquired over resistive environments, using a single-component (vertical) transmitter, where the position and orientation of a dipole conductor is allowed to vary in three dimensions. The algorithm assumes that the magnetic fields produced from compact vortex currents are expressed as a linear combinations of the fields arising from dipoles in the subsurface oriented parallel to the [1, 0, 0], [0, 1, 0], and [0, 0, 1], unit vectors. In this manner, AEM responses can be represented as 12 terms. The relative size of each term in the decomposition can be used to determine geometrical information about the orientation of the subsurface conductivity structure. The geometrical parameters of the dipole (location, depth, dip, strike) are estimated using a combination of a look-up table and a matrix inverted in a least-squares sense.
Tests on 703 synthetic models show that the algorithm is capable of extracting most of the correct geometrical parameters of a dipole conductor when three-component receiver data is included in the interpretation procedure. The algorithm is unstable when the target is perfectly horizontal, as the strike is undefined. Ambiguities may occur in predicting the orientation of the dipole conductor if y-component data is excluded from the analysis.
Application of our approach to an anomaly on line 15 of the Reid Mahaffy test site yields geometrical parameters in reasonable agreement with previous authors. However, our algorithm provides additional information on the strike and offset from the traverse line of the conductor. Disparities in the values of predicted dip and depth are within the range of numerical precision. The index of fit was better when strike and offset were included in the interpretation procedure. Tests on the data from line 15701 of the Chibougamau MEGATEM survey shows that the algorithm is applicable to situations where three-component AEM data is available.
Key words: airborne electromagnetic, conductor, dipole, discrete, interpretation.
References
Cooper, G. R. J., 2004, A semi-automatic procedure for the interpretation of geophysical data: Exploration Geophysics, 35, 182–187| A semi-automatic procedure for the interpretation of geophysical data:Crossref | GoogleScholarGoogle Scholar |
Cox, L. H., Wilson, G. A., and Zhdanov, M. S., 2010, 3D inversion of airborne electromagnetic data using a moving footprint: Exploration Geophysics, 41, 250–259
| 1:CAS:528:DC%2BC3cXhsFyjt7jN&md5=74793b40de54b147501e8ce64633db02CAS |
Hallof, P. G., 1992, Electrical IP and resistivity: grounded electrical methods in geophysical exploration, in R. Van Blaricom, ed., Practical Geophysics II: Northwest Mining Association, 39–138.
Hartman, R. R., Teskey, D. J., and Friedberg, J. L., 1971, A system for rapid digital aeromagnetic interpretation: Geophysics, 36, 891–918
| A system for rapid digital aeromagnetic interpretation:Crossref | GoogleScholarGoogle Scholar |
Liu, G., and Asten, M. W., 1993, Fast approximate solutions of transient EM response to a target buried beneath a conductive overburden: Geophysics, 58, 810–817
| Fast approximate solutions of transient EM response to a target buried beneath a conductive overburden:Crossref | GoogleScholarGoogle Scholar |
Lodha, G. S., and West, G. F., 1976, Practical airborne EM (AEM) interpretation using a sphere model: Geophysics, 41, 1157–1169
| Practical airborne EM (AEM) interpretation using a sphere model:Crossref | GoogleScholarGoogle Scholar |
Macnae, J., and Lamontagne, Y., 1987, Imaging quasi-layered conductive structures by simple processing of transient electromagnetic data: Geophysics, 52, 545–554
| Imaging quasi-layered conductive structures by simple processing of transient electromagnetic data:Crossref | GoogleScholarGoogle Scholar |
Macnae, J. C., Smith, R., Polzer, B. D., Lamontagne, Y., and Klinkert, P. S., 1991, Conductivity-depth imaging of airborne electromagnetic step-response data: Geophysics, 56, 102–114
| Conductivity-depth imaging of airborne electromagnetic step-response data:Crossref | GoogleScholarGoogle Scholar |
Naudy, H., 1971, Automatic determination of depth on aeromagnetic profiles: Geophysics, 36, 717–722
| Automatic determination of depth on aeromagnetic profiles:Crossref | GoogleScholarGoogle Scholar |
Paradis, S. J., 2010, Surficial geology, Chibougamau, Quebec/Geologie des formations superficielles, Chibougamau, Quebec: Geological Survey of Canada, Open File 6064, scale 1 : 250 000.
Reid, J., Fitzpatrick, A., and Godber, K., 2010, An overview of the SkyTEM airborne EM system with Australian examples: Preview, 145, 27–37
Sattel, D., and Reid, J., 2006, Modelling airborne EM anomalies with magnetic and electric dipoles buried inside a layered earth: Exploration Geophysics, 37, 254–260
| Modelling airborne EM anomalies with magnetic and electric dipoles buried inside a layered earth:Crossref | GoogleScholarGoogle Scholar |
Smith, R. S., and Chouteau, M. C., 2006, Combining airborne electromagnetic data from alternate flight directions to improve data interpretability: the virtual symmetric array: Geophysics, 71, G35–G41
| Combining airborne electromagnetic data from alternate flight directions to improve data interpretability: the virtual symmetric array:Crossref | GoogleScholarGoogle Scholar |
Smith, R. S., and Lee, T. J., 2001, The impulse-response moments of a conductive sphere in a uniform field, a versatile and efficient electromagnetic model: Exploration Geophysics, 32, 113–118
| The impulse-response moments of a conductive sphere in a uniform field, a versatile and efficient electromagnetic model:Crossref | GoogleScholarGoogle Scholar |
Smith, R. S., and Lee, T. J., 2002, The moments of the impulse response: a new paradigm for the interpretation of transient electromagnetic data: Geophysics, 67, 1095–1103
| The moments of the impulse response: a new paradigm for the interpretation of transient electromagnetic data:Crossref | GoogleScholarGoogle Scholar |
Smith, R. S., and Salem, A. S., 2007, A discrete conductor transformation of airborne electromagnetic data: Near Surface Geophysics, 5, 87–95
Thompson, D. T., 1982, EULDPH - a new technique for making computer-assisted depth estimates from magnetic data: Geophysics, 47, 31–37
| EULDPH - a new technique for making computer-assisted depth estimates from magnetic data:Crossref | GoogleScholarGoogle Scholar |
Wolfgram, P., and Karlik, G., 1995, Conductivity-depth transform of GEOTEM data: Exploration Geophysics, 26, 179–185
| Conductivity-depth transform of GEOTEM data:Crossref | GoogleScholarGoogle Scholar |
Wolfgram, P., Hyde, M., and Thompson, S., 1998, How to find localised conductors in GEOTEM data: Exploration Geophysics, 29, 665–670
| How to find localised conductors in GEOTEM data:Crossref | GoogleScholarGoogle Scholar |
Yang, D., Oldenburg, D. W., and Haber, E., 2014, 3-D inversion of airborne electromagnetic data parallelized and accelerated by local mesh and adaptive soundings: Geophysical Journal International, 196, 1492–1507
| 3-D inversion of airborne electromagnetic data parallelized and accelerated by local mesh and adaptive soundings:Crossref | GoogleScholarGoogle Scholar |
Zhdanov, M. S., 2002, Geophysical inverse theory and regularization problems (Methods in Geochemistry and Geophysics Series, Vol. 36): Elsevier, 62–69.
Zhdanov, M. S., 2009, Geophysical electromagnetic theory and methods (Methods in Geochemistry and Geophysics Series, Vol. 43): Elsevier, 643.