New methods for interpretation of magnetic vector and gradient tensor data II: application to the Mount Leyshon anomaly, Queensland, Australia
David A. ClarkCSIRO Materials Science and Engineering, and CSIRO Earth Science and Resource Engineering, PO Box 218, Lindfield, NSW 2070, Australia. Email: David.Clark@csiro.au
Exploration Geophysics 44(2) 114-127 https://doi.org/10.1071/EG12066
Submitted: 28 October 2012 Accepted: 6 March 2013 Published: 2 April 2013
Abstract
Acquisition of magnetic gradient tensor data is anticipated to become routine in the near future. In the meantime, modern ultrahigh resolution conventional magnetic data can be used, with certain important caveats, to calculate magnetic vector components and gradient tensor elements from total magnetic intensity (TMI) or TMI gradient surveys. An accompanying paper presented new methods for inverting gradient tensor data to obtain source parameters for several elementary, but useful, models. These include point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models. A key simplification is the use of eigenvalues and associated eigenvectors of the tensor. The normalised source strength (NSS), calculated from the eigenvalues, is a particularly useful rotational invariant that peaks directly over 3D compact sources, 2D compact sources, thin sheets, and contacts, independent of magnetisation direction. Source locations can be inverted directly from the NSS and its vector gradient.
Some of these new methods have been applied to analysis of the magnetic signature of the Early Permian Mount Leyshon gold-mineralised system, Queensland. The Mount Leyshon magnetic anomaly is a prominent TMI low that is produced by rock units with strong reversed remanence acquired during the Late Palaeozoic Reverse Superchron. The inferred magnetic moment for the source zone of the Mount Leyshon magnetic anomaly is ~1010 Am2. Its direction is consistent with petrophysical measurements. Given estimated magnetisation from samples and geological information, this suggests a volume of ~1.5 km × 1.5 km × 2 km (vertical). The inferred depth of the centre of magnetisation is ~900 m below surface, suggesting that the depth extent of the magnetic zone is ~1800 m. Some of the deeper, undrilled portion of the magnetic zone could be a mafic intrusion similar to the nearby coeval Fenian Diorite, representing part of the parent magma chamber beneath the Mount Leyshon Intrusive Complex.
Key words: dipole localisation, eigenvalues, eigenvectors, magnetic field vector, magnetic gradient tensor, magnetic moments, Mount Leyshon, normalised source strength.
References
Baranov, W., 1975, Potential fields and their transformations in applied geophysics: Geoexploration Monographs Series 1- Number 6, Gebrüder Borntraeger.Beiki, M., Clark, D. A., Austin, J. R., and Foss, C. A., 2012a, Estimating source location using normalized magnetic source strength calculated from magnetic gradient tensor data: Geophysics, 77, J23–J37
| Estimating source location using normalized magnetic source strength calculated from magnetic gradient tensor data:Crossref | GoogleScholarGoogle Scholar |
Beiki, M., Keating, P., and Clark, D. A., 2012b, Depth estimation of magnetic and gravity sources using normalized source strength calculated from gradient tensor: Society of Exploration Geophysicists Annual Meeting, Las Vegas, Expanded Abstracts.
Blakely, R. J., 1996, Potential theory in gravity and magnetic applications: Cambridge University Press.
Caratori Tontini, F., and Pedersen, L. B., 2008, Interpreting magnetic data by integral moments: Geophysical Journal International, 174, 815–824
| Interpreting magnetic data by integral moments:Crossref | GoogleScholarGoogle Scholar |
Christensen, A., and Rajagopalan, S., 2000, The magnetic vector and gradient tensor in mineral and oil exploration: Preview, 84, 77
Clark, D. A., 2010, Method and apparatus for detection using magnetic gradient tensor. US Patent Application Pub. No.: US 2010/0211337 Al, Pub. Date: 19 August 2010 (US patent 8,229,688 B2 granted 24 July 2012).
Clark, D. A., 2012a, New methods for interpretation of magnetic vector and gradient tensor data I: eigenvector analysis and the normalised source strength: Exploration Geophysics, 43, 267–282
| New methods for interpretation of magnetic vector and gradient tensor data I: eigenvector analysis and the normalised source strength:Crossref | GoogleScholarGoogle Scholar |
Clark, D. A., 2012b, New methods for interpretation of magnetic gradient tensor data: ASEG 22nd International Geophysical Conference and Exhibition, 26–29 February 2012, Brisbane, Australia, Expanded Abstracts, CD-ROM, 11 pp.
Clark, D. A., 2012c, Interpretation of the magnetic gradient tensor and normalized source strength applied to the Tallawang magnetite skarn deposit, New South Wales, Australia: Society of Exploration Geophysicists Annual Meeting, Las Vegas, Expanded Abstracts.
Clark, D. A., and Lackie, M. A., 2003, Palaeomagnetism of the Early Permian Mount Leyshon Intrusive Complex and Tuckers Igneous Complex, North Queensland, Australia: Geophysical Journal International, 153, 523–547
| Palaeomagnetism of the Early Permian Mount Leyshon Intrusive Complex and Tuckers Igneous Complex, North Queensland, Australia:Crossref | GoogleScholarGoogle Scholar |
Clark, D. A., Geuna, S. E., and Schmidt, P. W., 2004, Predictive magnetic exploration models for porphyry, epithermal and iron oxide Cu-Au deposits: implications for exploration: CSIRO Exploration and Mining Report 1073R, 398 pp. + CD Atlas of Geophysical Signatures + Relational Database of Porphyry, Epithermal and Iron Oxide Cu-Au Deposits (AMIRA International P700 Final Report CD-ROM).
Clark, D. A., Young, J. A., and Schmidt, P. W., 2009, Magnetic tensor gradiometry in the marine environment: correction of electric and magnetic field and gradient measurements in a conductive medium and improved methods for magnetic target location using the magnetic gradient tensor: MARELEC (Marine Electromagnetics) Conference, Stockholm, July 2009, CD-ROM, 10 pp.
Emerson, D. W., Clark, D. A., and Saul, S. J., 1985, Magnetic exploration models incorporating remanence, demagnetization and anisotropy: HP 41C handheld computer algorithms: Exploration Geophysics, 16, 1–122
| Magnetic exploration models incorporating remanence, demagnetization and anisotropy: HP 41C handheld computer algorithms:Crossref | GoogleScholarGoogle Scholar |
Foss, C., 2006, The improvements in source resolution that can be expected from inversion of magnetic field tensor data: The Leading Edge, 25, 81–84
| The improvements in source resolution that can be expected from inversion of magnetic field tensor data:Crossref | GoogleScholarGoogle Scholar |
Gordin, V. M., Tikhotskii, S. A., and Shur, D. Y., 2006, Reconstruction of the harmonic component of the magnetic field modulus anomalies: Izvestiya, Physics of the Solid Earth, 42, 334–343
| Reconstruction of the harmonic component of the magnetic field modulus anomalies:Crossref | GoogleScholarGoogle Scholar |
Helbig, K., 1963, Some integrals of magnetic anomalies and their relation to the parameters of the disturbing body: Zeitschrift für Geophysik, 29, 83–96
Hughes, D. S., and Pondrom, W. L., 1947, Computation of vertical magnetic anomalies from total field measurements: Transactions - American Geophysical Union, 28, 193–197
| Computation of vertical magnetic anomalies from total field measurements:Crossref | GoogleScholarGoogle Scholar |
Lackie, M. A., Clark, D. A., and French, D. H., 1991, Rock magnetism and palaeomagnetism of the Mount Leyshon gold mine in northeast Queensland: CSIRO Exploration Geoscience Report 217R, 108 pp.
Lima, E. A., Irimia, A., and Wikswo, J. P., 2006, The magnetic inverse problem, in J. Clarke, and A.I. Braginski, eds., The SQUID handbook, Vol. II: applications of SQUIDs and SQUID systems: Wiley-VCH, 139–267.
Lourenço, J. S., and Morrison, H. F., 1973, Vector magnetic anomalies derived from measurements of a single component of the field: Geophysics, 38, 359–368
| Vector magnetic anomalies derived from measurements of a single component of the field:Crossref | GoogleScholarGoogle Scholar |
Munschy, M., and Fleury, S., 2011, Scalar, vector, tensor magnetic anomalies: measurement or computation?: Geophysical Prospecting, 59, 1035–1045
| Scalar, vector, tensor magnetic anomalies: measurement or computation?:Crossref | GoogleScholarGoogle Scholar |
Nara, T., Suzuki, S., and Ando, S., 2006, A closed-form formula for magnetic dipole localization by measurement of its magnetic field and spatial gradients: IEEE Transactions on Magnetics, 42, 3291–3293
| A closed-form formula for magnetic dipole localization by measurement of its magnetic field and spatial gradients:Crossref | GoogleScholarGoogle Scholar |
Nelson, J. B., 1988, Calculation of the magnetic gradient tensor from total field gradient measurements and its application to geophysical interpretation: Geophysics, 53, 957–966
| Calculation of the magnetic gradient tensor from total field gradient measurements and its application to geophysical interpretation:Crossref | GoogleScholarGoogle Scholar |
Pedersen, L. B., and Rasmussen, T. M., 1990, The gradient tensor of potential field anomalies: some implications on data collection and data processing of maps: Geophysics, 55, 1558–1566
| The gradient tensor of potential field anomalies: some implications on data collection and data processing of maps:Crossref | GoogleScholarGoogle Scholar |
Pedersen, L. B., Rasmussen, T. M., and Dyrelius, D., 1990, Construction of component maps from aeromagnetic total field anomaly maps: Geophysical Prospecting, 38, 795–804
| Construction of component maps from aeromagnetic total field anomaly maps:Crossref | GoogleScholarGoogle Scholar |
Phillips, J. D., Nabighian, M. N., Smith, D. V., and Li, Y., 2007, Estimating locations and total magnetization vectors of compact magnetic sources through combined Helbig and Euler analysis: Society of Exploration Geophysicists Annual Meeting Expanded Abstracts, 26, 770–774.
Pilkington, M., and Beiki, M., 2012, Mitigating remanent magnetization effects in magnetic data using the normalized source strength: Society of Exploration Geophysicists Annual Meeting, Las Vegas, Expanded Abstracts.
Reid, A. B., 1980, Aeromagnetic survey design: Geophysics, 45, 973–976
| Aeromagnetic survey design:Crossref | GoogleScholarGoogle Scholar |
Schmidt, P. W., and Clark, D. A., 1998, The calculation of magnetic components and moments from TMI: a case study from the Tuckers igneous complex, Queensland: Exploration Geophysics, 29, 609–614
| The calculation of magnetic components and moments from TMI: a case study from the Tuckers igneous complex, Queensland:Crossref | GoogleScholarGoogle Scholar |
Schmidt, P. W., and Clark, D. A., 2006, The magnetic gradient tensor: its properties and uses in source characterization: The Leading Edge, 25, 75–78
| The magnetic gradient tensor: its properties and uses in source characterization:Crossref | GoogleScholarGoogle Scholar |
Sexton, M. A., Morrison, G. W., Orr, T. O. H., Foley, A. M., and Wormald, P. J., 1995, The Mount Leyshon magnetic anomaly: Exploration Geophysics, 26, 84–91
| The Mount Leyshon magnetic anomaly:Crossref | GoogleScholarGoogle Scholar |
Vestine, E. H., and Davids, N., 1945, Analysis and interpretation of geomagnetic anomalies: Terrestrial Magnetism and Atmospheric Electricity, 50, 1–36
| Analysis and interpretation of geomagnetic anomalies:Crossref | GoogleScholarGoogle Scholar |