Profiling electromagnetic distributed acquisition systems (DAS), such as MIMDAS and Titan-24, are commonly used for near-surface exploration. Such systems allow more rapid acquisition compared to standard magnetotelluric (MT) approaches. Instead of recording horizontal electric and magnetic fields at every site, DAS acquire a single (along-profile) component of the electric field at every site, but the perpendicular component is measured at every second or so site, and the magnetic field sensors are normally positioned only at a couple of locations within the study area. It is common practice to apply standard MT inversion algorithms to invert such DAS data, despite the lack of full four component measurements at each site. This is only valid in the strictly two-dimensional (2D) case with the geoelectric strike perpendicular to the acquisition profile and for the transverse magnetic (TM) mode, and also under certain assumptions on frequency range and resistivities. In case of the 2D transverse electric (TE) mode and in the general 3D case, employing standard MT inversion software will lead to erroneous results. Therefore, we have developed a new 3D inversion algorithm that considers the correct positioning of all sensors. We investigate the ability of the new inversion to recover the subsurface resistivity and compare the results to standard MT inversion of DAS data. The newly developed inversion code is also useful for conventional MT surveys when data from some channels is lost. With the new approach missing fields can be easily substituted by fields from another site. Presentation Date: Wednesday, October 17, 2018 Start Time: 1:50:00 PM Location: 213A (Anaheim Convention Center) Presentation Type: Oral

An integrated interpretation of the east Kimberley, northern Western Australia was completed to determine mineral prospectivity, and was centred on a portion of a magnetotelluric (MT) survey conducted across the entire Kimberley Craton and surrounding orogens. A structural geophysical interpretation used potential field data, and was constrained by geological field observations, petrophysics, remote sensing and understanding of the tectonic history of the region. Potential field forward modelling located along the same survey traverse as the MT data allowed comparison between the two datasets and their interpretations revealing interesting features suggesting the presence of large-scale structures, the presence of mineralization deep in the crust, and where mineralization may be at or near the surface. The King River Fault is shown from both the MT inversion and potential field modelling as a crustal-scale, west-dipping structure, the footwall of which bounds the western side of a large resistive body. A conductive anomaly is also located on the hanging wall of the King River Fault. Our assessment suggests that graphitic rocks, most likely with some sulphide content, contribute to the strength of this anomaly, and highlights the potential of the east Kimberley to host graphite and base metal deposits.

An integrated interpretation of potential field and magnetotelluric (MT) data was performed in the east Kimberley, northern Western Australia Structural interpretation of potential field data was constrained by geological field observations, petrophysics, remote-sensing and an understanding of the tectonic history of the region. Forward modelling of the potential field data located along the same survey traverse as the magnetotelluric data allowed comparison between the two datasets to assess complementarity of images and assist interpretation. Interpreted features include the presence of large-scale structures and associated electrical anomalies that indicate the presence of mineralisation deep in the crust, and guide prediction of mineralisation at or near the surface. The King River Fault is revealed to be a crustal-scale, west-dipping structure, which footwall bounds the western side of a large resistive body. A conductive anomaly is also located on the hanging wall of the King River Fault. A number of scenarios are discussed to the source of conductivity, including the presence of sulphides, saline water and graphite. Our assessment suggests that graphitic rocks, most likely with some sulphide content, contribute to the strength of this anomaly, and highlights the known potential of the east Kimberley to host graphite deposits. The conductive anomaly has a spatial and geometric correlation to Speewah Dome, a known prospective region. The depth of the conductor (c. 5km) precludes mining, but does indicate King River Fault is likely to form a mineralising conduit, and may contribute to possible Pb-Zn mineralisation where the fault reaches the surface.

Working closely with the Geological Survey of Western Australia (GSWA) and funded by the WA State Government’s Exploration Incentives Scheme, a team of reseachers at CET has recently completed a major study of the prospectivity of key ‘greenfields’ terrains in Western Australia (Fig.1). The targeted regions – the west Arunta Orogen, west Musgrave Province, Gasgoyne Province and King Leopold Orogen – were selected on the basis of their geology being poorly understood and their being considered to be ‘under explored’ in regards to potential economic mineral endowment. The GSWA funded this work seeking to provide explorationrelevant precompetitive geoscience datasets to attract investment to WA as an exploration destination, and thereby maintain a pipeline of mineral deposit discoveries that will underpin the revenue base of the State Government into the future.

Previous No AccessInternational Workshop and Gravity, Electrical & Magnetic Methods and their Applications, Chenghu, China, 19-22 April 2015Determining mineral prospectivity through integrated geological and geophysical interpretation: Riding the gravity high in the east KimberleyAuthors: M. D. LindsayA. R. A. AitkenS. A. OcchipintiM. D. DentithJ. SprattS. EvansJ. A. HollisM. D. LindsayCentre for Exploration Targeting, The University of Western Australia, Crawley, AustraliaSearch for more papers by this author, A. R. A. AitkenCentre for Exploration Targeting, The University of Western Australia, Crawley, AustraliaSearch for more papers by this author, S. A. OcchipintiCentre for Exploration Targeting, The University of Western Australia, Crawley, AustraliaSearch for more papers by this author, M. D. DentithCentre for Exploration Targeting, The University of Western Australia, Crawley, AustraliaSearch for more papers by this author, J. SprattConsultant, Wakefield, Quebec, CanadaSearch for more papers by this author, S. EvansMoombarriga Geoscience, Perth, AustraliaSearch for more papers by this author, and J. A. HollisGeological Survey of Western Australia, East Perth, AustraliaSearch for more papers by this authorhttps://doi.org/10.1190/GEM2015-134 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract An integrated geophysical investigation of geological structure in the east Kimberley, northern Western Australia, was performed to identify structures and features important for the mineral potential of the region. Subsurface structure was constrained through the use of magnetic, gravity and magnetotelluric (MT) data along an E-W transect. Significant crustal-scale structures were interpreted and investigated to determine their influence on the development of regional structure, the emplacement of magma, and circulation of hydrothermal fluids. Some newly interpreted features include a north-trending structure that intersects the region, orogen-normal structures and a large mafic magma chamber at 20km depth. Keywords: interpretation, magnetotelluric, magnetic, data setsPermalink: https://doi.org/10.1190/GEM2015-134FiguresReferencesRelatedDetails International Workshop and Gravity, Electrical & Magnetic Methods and their Applications, Chenghu, China, 19-22 April 2015ISSN (online):2159-6832Copyright: 2015 Pages: 520 publication data© 2015 Published in electronic format with permission by the Society of Exploration Geophysicists and the Chinese Geophysical SocietyPublisher:Society of Exploration Geophysicists HistoryPublished Online: 22 Apr 2015 CITATION INFORMATION M. D. Lindsay, A. R. A. Aitken, S. A. Occhipinti, M. D. Dentith, J. Spratt, S. Evans, and J. A. Hollis, (2015), "Determining mineral prospectivity through integrated geological and geophysical interpretation: Riding the gravity high in the east Kimberley," SEG Global Meeting Abstracts : 517-520. https://doi.org/10.1190/GEM2015-134 Plain-Language Summary Keywordsinterpretationmagnetotelluricmagneticdata setsPDF DownloadLoading ...

New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of >300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45-53 degrees, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15-35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3Ga) and Trans-Hudson orogenies (1.85Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from similar to 3000 Omega m in the northeast (beneath Baffin Island and the Melville Peninsula) to similar to 10,000 Omega m beneath the central Rae craton, to > 50,000 Omega m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of similar to 500 Omega m at depths of 180-220km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary.

Over the last 30 years, through Lithoprobe and other programmes, modern, high-quality magnetotelluric (MT) measurements probing deep into the lithosphere and underlying asthenosphere have been made at over 6000 sites across Canada in all provinces and territories, except Nova Scotia. Some regions are well covered, particularly Alberta, southern British Columbia, and western Ontario, whereas others remain poorly covered, such as Quebec and large swaths of Nunavut. Prior publications from individual studies have added significantly to the wealth of Canada’s geoscience knowledge, and have demonstrated that MT can contribute significantly to understanding of the tectonic processes that have shaped Canada. However, to date no continent-scale maps of lithospheric electrical parameters have been constructed from the extensive MT database. Herein we review the contributions made by the MT components of Lithoprobe, and present new continental-scale maps of various electrical parameters at crustal and upper mantle depths for the whole of Canada. From those maps, combined with regional estimates of temperature, we develop derivative information on petrological–geophysical properties, including predictions of temperature and water content. We find that at 100 km depth the Canadian Shield is cold and dry, and the Cordillera is warmer but mostly dry, i.e., little water is present in the peridotite. Exceptions are beneath the Prairies, the Wopmay Orogen, and northeast Nunavut where there does appear to be water in the nominally anhydrous minerals. Also, southwest British Columbia appears colder than the rest of the Cordillera due to the subducting Juan de Fuca plate. In contrast, at 200 km depth almost all of Canada is dry.

Source field effects in magnetotelluric data acquired at high geomagnetic latitudes can result in erroneous interpretations of Earth conductivity structure deep within the mantle. This paper describes a simple technique most appropriate for a region that is dominantly one-dimensional (1-D) and uses the vertical magnetic field variations for identifying intervals of likely low contamination by non-uniform sources. Times are chosen when the variations stay within prescribed limits defined on the basis of a histogram of the variations for the whole recording interval. An example is given showing application of the method for data from a site under the auroral oval at a time when solar activity was at its lowest for the last solar cycle. A model derived from the responses obtained by processing all available data implies a decrease in resistivity at about 350 km to about 100 Ω.m. In contrast, the model obtained from low activity interval responses shows a less rapid decrease in resistivity, without a change at around the 410 km phase boundary. The responses obtained from all data can be explained by the influence of a source with an average wavelength of 3,000 km.

As part of a multidisciplinary geoscience project, the conductivity structure of the lithosphere beneath the Melville Peninsula, Nunavut, has been imaged using lithosphere-probing magnetotelluric methods. Two-dimensional resistivity models demonstrate a strong correlation between the resistivity structure and features mapped at the surface. An analysis of distortion effects and structural dimensionality show that, toward the north at crustal depths, the geoelectric strike angle is 099° azimuth, consistent with abundant east-trending faults; however toward the south and at mantle depths the geoelectric strike direction is 034°, similar to the regional structural trend. The data reveal near-vertical, less resistive features that extend through the highly resistive Archean-age Prince Albert crustal block to the base of the crust and correlate with surface fault traces. Paleoproterozoic metasedimentary rocks of the Penrhyn Group are characteristic of extremely low resistivities associated with graphite-bearing metapelites. A near-vertical low resistivity zone is interpreted to represent a shear zone that marks the northern extent of the Archean Repulse Bay Block. Variations in the resistivity structure of the mantle lithosphere suggest changes in structure or composition between the Repulse Bay Block to the south and the Prince Albert Block to the north. Resumé : Dans le cadre d’un projet géoscientifique multidisciplinaire, une représentation de la structure de conductivité de la lithosphère sous la presqu’île Melville (Nunavut) a été obtenue à l’aide de méthodes magnétotelluriques (MT) de sondage de la lithosphère. Des modèles bidimensionnels de la résistivité indiquent une forte corrélation entre la structure de résistivité et les entités cartographiées à la surface. Une analyse des effets de distorsion et de la dimensionnalité structurale montre que, vers le nord à des profondeurs crustales, l’azimut de la direction géoélectrique est de 099°, ce qui est cohérent avec l’abondance de failles orientées est-ouest. Cependant, vers le sud et aux profondeurs du manteau, l’azimut de la direction géoélectrique est de 034°, ce qui est similaire à celui de la direction structurale à l’échelle régionale. Les données montrent des entités moins résistives, presque verticales, qui traversent le bloc crustal hautement résistif de Prince Albert de l’Archéen jusqu’à la base de la croûte et qui peuvent être mises en corrélation avec les lignes de faille en surface. Les roches métasédimentaires du Groupe de Penrhyn du Paléoprotérozoïque sont caractéristiques des résistivités extrêmement faibles associées à des métapélites graphiteuses. Selon les interprétations, une zone presque verticale de faible résistivité représenterait une zone de cisaillement marquant la limite nord du bloc de Repulse Bay de l’Archéen. Des variations de la structure de résistivité de la lithosphère mantellique laissent supposer des changements dans la structure ou la composition entre le bloc de Repulse Bay au sud et le bloc de Prince Albert au nord.

30 Project INDEPTH (International Deep Profiling of Tibet and the Himalaya) 31 magnetotelluric (MT) surveys crossed the India-Asia collision zone, the Yarlung-Zangbo 32 suture (YZS, also known as the Indus-Tsangpo suture), at ~90 oE longitude in 1995, and at 33 ~92 oE in 2001. Our re-analyses and re-modeling of these data provide new images of the YZS 34 that show significant variation in conductivity structure along the profiles, across the orogen. 35 Focused inversions of the MT data in the vicinity of the YZS show more structure within the 36 upper crust than previous smoother models, and the near-surface conductors help locate the 37 South Tibetan Detachment, Renbu-Zedong thrust and Gangdese thrust. 38 Comparison of the two parallel profiles shows remarkable along-strike similarity of 39 several conductors and resistors, attesting to the regional nature of the processes of 40 conductivity enhancement within the collision. On both profiles, in contrast to prior models, 41 the upper-crustal conductors south of the YZS are spatially isolated and are not connected to 42 the mid-crustal conductors observed at and north of the YZS. Consistent with prior 43 interpretations, the strong mid-crustal conductor north of the YZS likely represents partial melt 44 and indicates a weak mid-to-lower crust that greatly affects rheology and deformation 45 processes of Tibet. However, this inferred partial melt cannot continue unbroken beneath and 46 south of the YZS. No continuous melt-weakened layer, as expected for ongoing channel-flow 47 extrusion of the Himalaya, is continuously present from the Lhasa Block of southernmost Tibet 48 into the Tethyan Himalaya. Geodynamic models have to be re-assessed to accommodate this 49 new observation. 50 Spratt et al. India-Asia collision zone from INDEPTH MT Page 3

In 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic-covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8–12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that ∼1700 and ∼2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin.

Two-dimensional (2-D) models of audio and broadband magnetotelluric (MT) data collected throughout the southern Nechako basin in south central British Columbia, Canada, provide electrical resistivity images of the Cretaceous to Oligocene volcanic and sedimentary packages and underlying crustal-scale features. Analysis of distortion effects and structural dimensionality indicate that the MT responses are primarily one-dimensional (1-D) at periods less than 0.1 s. Departures from a 1-D response occur with maximum phase differences occurring between 0.1 and 10 s. The upper crustal resistivity models reveal a low resistivity layer at near surface depths, interpreted as Chilcotin basalts, that blankets portions of the region to depths less than 50 m, but locally thicken up to 200 m. Cretaceous sedimentary units (those showing the highest potential for hydrocarbons) are characterized by moderately low resistivities that are laterally variable. More uniform, lower resistivities appear to be associated with the Eocene volcaniclastic groups, suggesting that the MT method can distinguish between these units and may be useful in targeting areas that are more prospective for hydrocarbon exploration. In general midcrustal resistivity values are high, consistent with values of typical volcanic terranes; however, a low-resistivity zone is identified at 8-10 km depth that correlates closely with the location of seismic reflectors as well as recent microseismic activity. This low-resistivity zone is interpreted as a midcrustal reservoir of magma, the top of which marks the upper limit of fluids migrating from lower crust depths. Additionally, several crustal-scale faults are imaged.