Sign Changes in DHEM Surveys for Cindered Coal in the Sydney Basin

Michael W. Asten; Alan King; John Peacock

doi:10.1071/EG987319

ISSN: 0812-3985
E-ISSN: 1834-7533

Sign Changes in DHEM Surveys for Cindered Coal in the Sydney Basin
Authors Michael W. Asten^a, Alan King^b and John Peacock^b
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^a Exploration DepartmentBHP Minerals Ltd, PO Box 559 Camberwell, , Vic, 3124, Australia ^b Geoterrex Pty Ltd, 13 Whiting Street Artarmon, , NSW, 2064, Australia
Publisher: Australian Society of Exploration Geophysicists
Source: Exploration Geophysics, Volume 18, Issue 3, Jun 1987, p. 319 - 323
DOI: https://doi.org/10.1071/EG987319
- Published online: 01 Jun 1987

Abstract

Coal seams adjacent to post-depositional igneous intrusives may show very high conductivities due to presence of graphite in the cindered (metamorphosed) coal. A downhole TEM survey through the Bulli coal seam, in the south Sydney basin shows the conductance of a 4 metre intersection of cindered coal to be about 50 S, indicating that the TEM method, both surface and downhole, has applications in the identification of areas of cindered coal. The anomaly due to the highly conductive cindered coal shows a sign-reversal at intermediate times which is attributable to the influence of overlying conductive sediments and which demonstrates the importance of considering the influence of conductive overburden or host rock before attempting interpretation of target geometry using free-space models.

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References

Asten, M. W. (1986)—‘Full transmitter waveform TEM modelling and inversion’, Geophysics52, 279-288.
Eaton, P. A. & Hohmann, G. W. (1984)—‘The influence of a conducting host on two-dimensional borehole transient electromagnetic responses’, Geophysics49, 861-869.
Lamb, P. (1985)—Personal communication.
Martin, D. J. (1986)—Sedimentary depositional environments and some igneous intrusions in the Sydney-Gunnedah Basin. Australian Coal Geology 6, Geol. Soc. Aust. special publication.
McNeill, J. D., Edwards, R. N. & Levy, G. M. (1984)—‘Approximate calculations of the TEM response from buried conductors in a conductive halfspace’, Geophysics49, 918-924.
Nabighian, M. (1979)—‘Quasi-static transient response of a conducting halfspace —‘ an approximate representation’, Geophysics44, 1700-1705.
Peacock, J. & King, A. (1985)—‘Central loop transient electromagnetic soundings’, Explor. Geophys.16, 261-265.
West, G. F., Macnae, J. C. & Lamontagne, Y. (1979)—‘A time-domain electromagnetic system measuring the step response of the ground’, Geophysics49, 1010-1026.
Woods, D. V. (1975)—‘A model study of the Crone borehole pulse electromagnetic (PEM) system’, M.Sc. Thesis, Queen’s University, Kingston.

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Article Type: Research Article

Keyword(s): Coal; Downhole TEM; drive delay; sign change; Sydney Basin

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Sign Changes in DHEM Surveys for Cindered Coal in the Sydney Basin

Abstract

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