The Longest Conductivity Anomaly in the World Explained: Sulphides in Fold Hinges Causing Very High Electrical Anisotropy

Jones, Alan G.; Katsube, T J; Schwann, P L

doi:10.5636/jgg.49.1619

Cited by 80 publications

(48 citation statements)

References 20 publications

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“…The rocks of the Wapassini sheet are derived from highly migmatized pelitic gneisses and pelite diatextite protoliths and provide a probable source of conductive rocks. Analysis of pelitic rocks from the Wapassini sheet confirms the presence of conducting sulphides (Jones et al 1997a). It is not possible to exclude a contribution to the conductor from graphitic rocks that occur in the Guncoat thrust (Katsube et al 1996).…”

Section: Nacp Conductormentioning

confidence: 83%

“…As noted by , the NACP conductor lies within westward dipping reflectivity and tracing of the reflections updip to the surface indicates that the conductor lies within the rocks of the La Ronge domain. More detailed interpretations of the NACP conductor are given in and Jones et al (1997aJones et al ( , 2005. The NACP conductor is interpreted to be bounded above by the Birch Rapid straight zone and below by the Guncoat thrust, with its western extent limited by the Needle Falls shear zone.…”

Section: Comparison Of Resistivity Models and Seismic Resultsmentioning

confidence: 99%

“…North of the present survey location it turns east and follows the rocks of the THO beneath Hudson Bay. In more recent studies done as part of the Lithoprobe project, Jones et al ( , 1997a have analysed data from the Glennie domain, La Ronge domain, and Rottenstone domain to image the conductor. Additional petrophysical studies have suggested that the cause of the enhanced conductivity is the presence of sulphides that have been anisotropically concentrated in fold hinges (Katsube et al 1996;Jones et al 1997a).…”

Section: Previous Magnetotelluric and Electromagnetic Studiesmentioning

confidence: 99%

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Electrical-resistivity imaging of the central Trans-Hudson orogen

Ferguson¹,

Stevens²,

Jones³

2005

Can. J. Earth Sci.

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Magnetotelluric (MT) measurements were made on a profile across the Trans-Hudson orogen in 1992 as part of the Lithoprobe transect. The present study includes analysis of results from a 300 km-long section of the profile in which allocthonous Paleoproterozoic juvenile terranes and arc rocks of the western Trans-Hudson orogen have been juxtaposed against the Archean Sask craton. Impedance tensor decomposition of data from the 40 MT sites in the area indicates a geoelectric strike of N28°E. Two-dimensional inversion of the data using a non-linear conjugate gradient algorithm provided images of the resistivity structure. Resistivity images reveal that the crust of the Sask craton is relatively resistive

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Section: Nacp Conductormentioning

confidence: 83%

Section: Comparison Of Resistivity Models and Seismic Resultsmentioning

confidence: 99%

Section: Previous Magnetotelluric and Electromagnetic Studiesmentioning

confidence: 99%

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Electrical-resistivity imaging of the central Trans-Hudson orogen

Ferguson¹,

Stevens²,

Jones³

2005

Can. J. Earth Sci.

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“…Where AMT signal levels are low, e.g., in the dead band at $ 800-3000 Hz (Garcia and Jones 2002), the controlled-source audiomagnetotelluric method (CSAMT, f = 1-10000 Hz, Zonge and Hughes 1991) is routinely applied. Consequently, MT methods have a long-standing history in the investigation of mineral deposits (Strangway et al 1973;Meju 2002;Jones 2017) with field cases reported from, for instance, copper, gold, lead, silver and zinc deposits (Kellett et al 1993;Garcia Juanatey et al 2013a, b;Hübert et al 2013;Hu et al 2013), copper, gold and iron deposits (Heinson et al 2006), copper and iron deposits (Chouteau et al 1997;Jones and Garcia 2003), copper, iron and zinc deposits (Basokur et al 1997), copper, lead and zinc deposits (Sasaki et al 1992;Bastani et al 2009), copper and nickel deposits (Lakanen 1986;Livelybrooks et al 1996;Jones et al 1997;Balch et al 1998;Stevens and McNeice 1998;Zhang et al 1998;Watts and Balch 2000;King 2007;Xiao et al 2011;Varentsov et al 2013; Le et al 2016a), copper, silver and zinc deposits (Gordon 2007), gold deposits (Jones et al 1997;Liu et al 2006;Howe et al 2014;Takam Takougang et al 2015;Hübert et al 2016;Le et al 2016b), and uranium deposits (Leppin and Goldak 2005;Tuncer et al 2006;Farquharson and Craven 2009;Goldak et al 2010;…”

Section: Mt Methods In Exploring Deep Ore Depositsmentioning

confidence: 99%

Two-Dimensional Magnetotelluric Modelling of Ore Deposits: Improvements in Model Constraints by Inclusion of Borehole Measurements

Kalscheuer

Juhojuntti

Vaittinen³

2017

Surv Geophys

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A combination of magnetotelluric (MT) measurements on the surface and in boreholes (without metal casing) can be expected to enhance resolution and reduce the ambiguity in models of electrical resistivity derived from MT surface measurements alone. In order to quantify potential improvement in inversion models and to aid design of electromagnetic (EM) borehole sensors, we considered two synthetic 2D models containing ore bodies down to 3000 m depth (the first with two dipping conductors in resistive crystalline host rock and the second with three mineralisation zones in a sedimentary succession exhibiting only moderate resistivity contrasts). We computed 2D inversion models from the forward responses based on combinations of surface impedance measurements and borehole measurements such as (1) skin-effect transfer functions relating horizontal magnetic fields at depth to those on the surface, (2) vertical magnetic transfer functions relating vertical magnetic fields at depth to horizontal magnetic fields on the surface and (3) vertical electric transfer functions relating vertical electric fields at depth to horizontal magnetic fields on the surface. Whereas skin-effect transfer functions are sensitive to the resistivity of the background medium and 2D anomalies, the vertical magnetic and electric field transfer functions have the disadvantage that they are comparatively insensitive to the resistivity of the layered background medium. This insensitivity introduces convergence problems in the inversion of data from structures with strong 2D resistivity contrasts. Hence, we adjusted the inversion approach to a three-step procedure, where (1) (2) this inversion model from surface impedances is used as the initial model for a joint inversion of surface impedances and skin-effect transfer functions and (3) the joint inversion model derived from the surface impedances and skin-effect transfer functions is used as the initial model for the inversion of the surface impedances, skin-effect transfer functions and vertical magnetic and electric transfer functions. For both synthetic examples, the inversion models resulting from surface and borehole measurements have higher similarity to the true models than models computed exclusively from surface measurements. However, the most prominent improvements were obtained for the first example, in which a deep small-sized ore body is more easily distinguished from a shallow main ore body penetrated by a borehole and the extent of the shadow zone (a conductive artefact) underneath the main conductor is strongly reduced. Formal model error and resolution analysis demonstrated that predominantly the skin-effect transfer functions improve model resolution at depth below the sensors and at distance of $ 300-1000 m laterally off a borehole, whereas the vertical electric and magnetic transfer functions improve resolution along the borehole and in its immediate vicinity. Furthermore, we studied the signal levels at depth and provided specifications of borehole magnetic and elect...

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“…tectonic, geodynamic and petrophysical meaning) must be attempted. Special attention must be put on the petrophysical interpretation of the data and its correlation with laboratory and physical studies (Duba and Shankland, 1982;Constable et al, 1992;Jones et al, 1997;Bahr and Duba, 2000) this is the kind of information which in many cases it is feasible to obtain only with EM methods (Jones, 1992). It is evident that the geological information obtained from MT data can be incorporated into the first stage of this set of recommendations.…”

Section: Geological Interpretationmentioning

confidence: 99%

2-D Versus 3-D Magnetotelluric Data Interpretation

Ledo¹

2006

Surv Geophys

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Abstract. In recent years, the number of publications dealing with the mathematical and physical 3-D aspects of the magnetotelluric method has increased drastically. However, field experiments on a grid are often impractical and surveys are frequently restricted to single or widely separated profiles. So, in many cases we find ourselves with the following question: is the applicability of the 2-D hypothesis valid to extract geoelectric and geological information from real 3-D environments? The aim of this paper is to explore a few instructive but general situations to understand the basics of a 2-D interpretation of 3-D magnetotelluric data and to determine which data subset (TE-mode or TM-mode) is best for obtaining the electrical conductivity distribution of the subsurface using 2-D techniques. A review of the mathematical and physical fundamentals of the electromagnetic fields generated by a simple 3-D structure allows us to prioritise the choice of modes in a 2-D interpretation of responses influenced by 3-D structures. This analysis is corroborated by numerical results from synthetic models and by real data acquired by other authors. One important result of this analysis is that the mode most unaffected by 3-D effects depends on the position of the 3-D structure with respect to the regional 2-D strike direction. When the 3-D body is normal to the regional strike, the TE-mode is affected mainly by galvanic effects, while the TM-mode is affected by galvanic and inductive effects. In this case, a 2-D interpretation of the TM-mode is prone to error. When the 3-D body is parallel to the regional 2-D strike the TE-mode is affected by galvanic and inductive effects and the TM-mode is affected mainly by galvanic effects, making it more suitable for 2-D interpretation. In general, a wise 2-D interpretation of 3-D magnetotelluric data can be a guide to a reasonable geological interpretation.

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The Longest Conductivity Anomaly in the World Explained: Sulphides in Fold Hinges Causing Very High Electrical Anisotropy

Cited by 80 publications

References 20 publications

Electrical-resistivity imaging of the central Trans-Hudson orogen

Electrical-resistivity imaging of the central Trans-Hudson orogen

Two-Dimensional Magnetotelluric Modelling of Ore Deposits: Improvements in Model Constraints by Inclusion of Borehole Measurements

2-D Versus 3-D Magnetotelluric Data Interpretation

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