GA Record Template€¦ · Web viewFigure 8. A hypothetical event map of the anomalous vertical magnetic field associated with a unit magnetic induction (1 nT) linearly polarized

Electrical Conductivity Structures from Magnetotelluric Data in Cloncurry Region

GEOSCIENCE AUSTRALIA RECORD 2018/05

Liejun Wang1, Jingming Duan1, and Janelle Simpson 2

1. Geoscience Australia2. Geological Survey of Queensland, Department of Natural Resources, Mines and Energy

Department of Industry, Innovation and ScienceMinister for Resources, Energy and Northern Australia: Senator the Hon Matthew Canavan Secretary: Dr Heather Smith PSM

Geoscience AustraliaChief Executive Officer: Dr James JohnsonThis paper is published with the permission of the CEO, Geoscience Australia

© Commonwealth of Australia (Geoscience Australia) 2018

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ISSN 2201-702X (PDF)

ISBN 978-1-925297-75-1 (PDF)

GeoCat 116801

Version: 1709

Bibliographic reference: Wang, L., Duan, J, Simpson J., 2018. Electrical Conductivity Structures from Magnetotelluric Data in Cloncurry Region. Record 2018/05. Geoscience Australia, Canberra.http://dx.doi.org/10.11636/Record.2018.005

Contents

1 Introduction and Background...........................................................................................................51.1 Introduction................................................................................................................................ 51.2 Geological background..............................................................................................................71.3 Description of the MT method...................................................................................................91.4 Methods of MT data analysis.....................................................................................................9

2 Data Analysis................................................................................................................................. 112.1 Depth of Investigation.............................................................................................................112.2 Impedance Tensors................................................................................................................132.3 Vertical Magnetic Transfer Functions......................................................................................152.4 Dimensionality......................................................................................................................... 192.5 Directionality............................................................................................................................ 20

3 Two Dimensional Modelling and Inversion.....................................................................................223.1 Introduction............................................................................................................................. 223.2 Depth of Mount Isa Province and Near Surface Structures.....................................................233.3 Deep enhanced conductors within the Mount Isa Province.....................................................26

4 Three Dimensional Modelling and Inversion..................................................................................274.1 Model parameterisation...........................................................................................................274.2 Inversion of vertical magnetic transfer functions.....................................................................274.3 Inversion of impedance tensors..............................................................................................284.4 Sensitivity test of key features.................................................................................................34

5 Discussions.................................................................................................................................... 375.1 Comparison with aeromagnetic anomaly................................................................................375.2 Comparison with surface geological unit map.........................................................................385.3 Comparison with seismic reflection data.................................................................................395.4 Mineral prospectivity...............................................................................................................415.5 Relationship with Carpentaria Conductivity Anomaly..............................................................42

6 Conclusions................................................................................................................................... 44

Acknowledgements.............................................................................................................................. 45

References........................................................................................................................................... 46

Electrical Conductivity Structures from Magnetotelluric Data in Cloncurry Region iii

Executive Summary

Broadband and audio magnetotelluric (BBMT and AMT) data at 476 sites on a 2 km grid were acquired in the Cloncurry region between July and November 2016. The survey covers an area of approximately 40 km x 60 km on the eastern margin of the Mount Isa Province. The Cloncurry magnetotelluric (MT) project was funded by the Geological Survey of Queensland and is a collaborative project between the Geological Survey of Queensland and Geoscience Australia.Geoscience Australia managed the project and peformed data QA/QC, data analysis, and produced two-dimensional (2D) and three dimensional (3D) inverse models for both the BBMT and AMT data. This report outlines the field acquisition program and provides details of the methodologies used for processing, analysing, modelling and inverting the data.

The Mount Isa Province comprises the exposed cratonic rocks of the Mount Isa Inlier and their subsurface extensions from the Gulf of Carpentaria in the north, to the Diamantina Structure in the south. The eastern boundary of the Mount Isa Province is obscured by Mesozoic cover and has been mapped using potential field data. A 3D electrical resistivity model from this survey shows that a very resistive upper crust associated with the outcropping Mount Isa Province extends to east under the Carpentaria Basin. A prominent north-south trending conductor truncates the east extension of the resistive block. The Mount Isa Province shown in the 3D model consists of a resistive block (over 1000 Ωm) to a depth of 12 km, overlying a more conductive block. The thin conductive sedimentary basin cover is well resolved by AMT data. The cover thickness increases towards the north-east. 2D and 3D electrical resistivity models define an elongated conductor within the crust beneath the Carpentaria Basin. This structure is evident in the aeromagnetic and gravity data, which is interpreted as the eastern boundary of the Mount Isa Province. The location of the conductivity anomaly also coincides with the Carpentaria Conductivity Anomaly which is a major element in the electrical resistivity structure of the North Australian Craton. The other dominant features of the 3D model include two conductors in the Mount Isa Province on the locations of Ernest Henry and Mount Margaret mining sites. There are also other mineral prospects distributed along the elongated conductor on the eastern margin of the survey.

iv Electrical Conductivity Structures from Magnetotelluric Data in Cloncurry Region

1 Introduction and Background

1.1 Introduction

The Cloncurry Magnetotelluric Project is a collaborative project between Geoscience Australia and the Geological Survey of Queensland. The project aims to improve understanding of the thickness and nature of cover, the geology obscured by sedimentary cover or regolith, and the mineral systems potential of the region.

Audio frequency MT (AMT) and Broad-Band MT (BBMT) data were acquired for the project. Data were collected at 476 sites on a regular grid with a spacing of 2 km in an approximately 40 km x 60 km area north of Cloncurry (Figure 1). The survey area is bounded by longitudes 140.5°E and 141.0°E and latitudes 20.0°S and 20.7°S, and covers the area on the eastern margin of Mount Isa Province. Data quality and bandwiths vary for the 476 sites and can be summarised as:

223 sites (red circles in Figure 1) with maximum periods longer than 1000 s

53 sites (blue squares in Figure 1) with maximum periods between 500 s to 1000 s

111 sites (green triangles in Figure 1) with maximum periods between 100 s to 500 s

74 site (black cross in Figure 1) with maximum periods less than 100 s

The remaining 15 sites were of poor quality and not included in this report.

The details of field campaign, data acquisition, QA/QC processing, as well as derivation of impedance tensors, and magnetic transfer functions are reported in data acquisition report (Khrapov, 2016).Details of the data analysis and inversion results are presented in this report.

Electrical Conductivity Structures from Magnetotelluric Data in Cloncurry Region 5

Figure 1: Location of the Cloncurry MT Project sites on a map of the surface Geology in the eastern margin of the Mount Isa Province and the Carpentaria Basin in Queensland. The red star shows the Cloncurry town centre. The sites marked by the circles, blue squares, green triangles and black crosses indicate the maximum period in impedance data being over 1000 s, 500 -1000 s, 100 - 500 s and less 100 s respectively.

6 Electrical Conductivity Structures from Magnetotelluric Data in Cloncurry Region

1.2 Geological background

The Mount Isa Province is one of the Paleoproterozoic to early Mesoproterozoic orogenic belts in the North Australian Craton (NAC). The NAC consists of a series of Neoarchean to early Paleoproterozoic cratonic blocks on which are superimposed later Paleoproterozoic orogenic belts and basins. Younger Proterozoic and Phanerozoic basins obscure much of the NAC. Uplift has resulted in partial exposure of some of the cratonic rocks in the block, called the Mount Isa Inlier. The Mount Isa Province comprises all of the rocks exposed in the Mount Isa Inlier and their subsurface extensions from the Gulf of Carpentaria to the Diamantina Structure in the south. The eastern extension of the Mount Isa Province is obscured by sediments of the Carpentaria Basin, which have been mapped using potential field data (Figure 2). The dominantly north-south trend of the Mount Isa Province is evident in the gravity and magnetic data. The Diamantina Structure truncates this trend on the southern margin of the Mount Isa Province.


Figure 2. Aeromagnetic map (top left figure) and gravity map (top right figure) of the Mount Isa Province; showing the location of the survey area in white, outcrop extent (Mount Isa Inlier) in black and the location of the Diamantina Structure (dashed line). Airborne magnetic (bottom left figure) and gravity (bottom right figure) data are in the survey area with MT sites and the boundary of the Mount Isa Inlier (black line). Magnetic and gravity data were processed to emphasise shallow features.


𝑎𝑎

1.3 Description of the MT method

The MT method is a passive geophysical technique which uses natural variation in the Earth’s magnetic and electric fields to investigate subsurface variations in resistivity. A detailed description of the MT method, including mathematical derivation, can be found in Chave and Jones (2012) and Simpson and Bahr (2005).

Under the plane wave source assumption, the horizontal electric field (𝐸𝐸𝑥𝑥,, 𝐸𝐸𝑦𝑦) in mV/km and magnetic field (𝐵𝐵𝑥𝑥, 𝐵𝐵𝑦𝑦) in nT have a frequency-dependent linear relationship via MT tensors (M) following the notation of Weaver et al (2000):

𝐸𝐸𝑥𝑥 = 𝑀𝑀𝑥𝑥𝑥𝑥𝐵𝐵𝑥𝑥 + 𝑀𝑀𝑥𝑥𝑦𝑦𝐵𝐵𝑦𝑦, (1)

𝐸𝐸𝑦𝑦 = 𝑀𝑀𝑦𝑦𝑥𝑥𝐵𝐵𝑥𝑥 + 𝑀𝑀𝑦𝑦𝑦𝑦𝐵𝐵𝑦𝑦. (2)

The MT tensors are in units of mV/km.nT. The relation to impedance tensors in ohms is

𝑍𝑍 = 𝜇𝜇0𝑀 , (3)

where 𝜇𝜇0 is the permeability of free space.

The apparent resistivity 𝜌𝜌𝑎(𝜔𝜔) in the units of ohm-metres (Ωm) is related to the amplitude of the impedance tensor𝜌𝜌 (𝜔𝜔) = 1𝜔𝜔𝜇𝜇0 |𝑍(𝜔𝜔)|2. (4)

Phase angle ∅ in degrees is∅ = 𝑡𝑡𝑡𝑡𝑡𝑡−1 𝐼𝐼𝐼 𝐼 𝑍( 𝜔𝜔 ). (5)𝑅𝑅𝑅𝑅 𝑧𝑧(𝜔𝜔)

The vertical magnetic field transfer functions (VTFs) give the linear relationship between induced vertical magnetic component 𝐵𝐵𝑧𝑧 and horizontal components 𝐵𝐵𝑥𝑥 and 𝐵𝐵𝑦𝑦 of the source field in thefrequency domain (Simpson and Bahr, 2005) by

𝐵𝐵𝑧𝑧 = 𝑇𝑇𝑥𝑥𝐵𝐵𝑥𝑥 + 𝑇𝑇𝑦𝑦𝐵𝐵𝑦𝑦. (6)

VTFs are often represented as complex induction vectors (or induction arrows)

𝑇𝑇𝑧𝑧 = 𝑇𝑇𝑥𝑥𝒙𝒙 + 𝑇𝑇𝑦𝑦𝒚𝒚, (7)

where 𝒙𝒙 and 𝒚𝒚 are the unit vectors in the geographic north and east direction respectively.

1.4 Methods of MT data analysis

A number of methods have been developed for analysing the dimensionality, directionality, galvanic distortions of MT data (e.g. Swift, 1967; Bahr, 1988; Groom, 1989; Caldwell et al., 2004; Bibby et al., 2005; Booker, 2014). Knowledge of these attributes plays an important role in appropriately parameterising inversion and informing interpretation.

In this report, we adopt Swift (1967) and phase tensor (Caldwell et al., 2004; Bibby et al., 2005; Booker, 2014) methods to determine dimensionality and directionality of the MT data acquired in Cloncurry.


Swift (1967) maximizes the sum of the off-diagonal components by rotating the impedance tensor to determine the regional strike angle. The ratio of the sum of the diagonal components and the sum of off-diagonal components (Skew or Skewness) determines dimensionality.

Caldwell et al. (2004) introduced the MT phase tensor, which makes no assumptions about regional structure, and is unaffected by a static shift. A detailed discussion of phase tensor can be found in Booker (2014).

Geoelectric dimensionally is a description of how electrical resistivity varies in the Earth. It is an inherent property of acquired MT data and is often frequency dependent, with high-frequency data generally displaying 1D character, while lower frequency data are more commonly 2D or 3D (Rodi and Mackie, 2012). Where data are 2D, a preferred orientation of current flow exists; this orientation is the directionality, or strike of the dataset. Similar to dimensionality, the directionality of a dataset may vary with period.

The phase tensor representation of dimensionally can be summarised as:

1-dimensional (1D), where the same resistivity layering extends in effectively infinite directions all around the site, the phase tensor forms a circle;

2-dimensional (2D), where the resistivity varies along a line away from the station but is effectively infinite perpendicular to this direction. In a graphical representation, the phase tensor forms an ellipse in reference to a unit circle axis. The major and minor axes of the ellipse correspond to phase maximum and phase minimum values respectively and often are used as a colour legend to fill the ellipses, to indicate the conductivity variations with depth. The angle between the major axis of the ellipse and the unit circle axis is called a skew. Although strictly 2D structure requires skew ѱ be zero, a practical definition might be that where ѱ is less than 6° (0.1 radians) it can be treated as a quasi-2D case.

3-dimensional (3D), where the resistivity structures vary in all directions. The phase ellipse is skewed in reference to a unit circle axis. The skew ѱ is greater than 6°.


2∅(𝑇𝑇)

2 Data Analysis

2.1 Depth of Investigation

Depth of investigation (DOI) for MT data is an important consideration during interpretation. In general, the depth of investigation is defined as skin depth (δ) in a half-space Earth (Chave and Jones 2012), at which the amplitude of the source electromagnetic field decreases to about 35% (1/e) of its value at the Earth surface. The reduction of field strength with depth is due to the energy diffusion processes, and is related to the subsurface conductance. The conductivity of Earth varies over many orders of magnitude across large areas due to crustal and mantle conductivity variations and the effective depth of investigation can vary significantly across a region.

In the Cloncurry region, the variation in depth of investigation can be appreciated by plotting an estimate of the depth of penetration for various periods (Figure 3). Depths are derived from the Niblett–Bostick transform of the rotationally invariant Berdichevsky arithmetic average of the off- diagonal components of the impedance tensor defined as (Jones 1983).

ℎ𝑛𝑛𝑛𝑛 = 𝜌𝜌𝑎( 𝑇𝑇 ) 𝑇𝑇 2𝜋𝜋𝜇𝜇0, (8)

𝜌𝜌𝑛𝑛𝑛𝑛(ℎ𝑛𝑛𝑛𝑛) = 𝜌𝜌𝑎𝑎(𝑇𝑇)( 𝜋𝜋 − 1) , (9)

where 𝜌𝜌𝑛𝑛𝑛𝑛(ℎ𝑛𝑛𝑛𝑛) is approximate resistivity, and ℎ𝑛𝑛𝑛𝑛 is approximate depth which is 1/√2 𝛿𝛿, where δ is skin depth.

The depth of investigation has a similar trend at all periods with reducing penetration in a north- easterly direction (Figure 3). This trend corresponds to thickening cover to the north-east. At 10 s period, average penetration is 10 -15 km in southwest, whereas it is about 5 km in the north-east. At 100 s period, the penetration depth for the south-west corner where has Mount Isa Province outcrops is over 40 km. In contrast, the depth of penetration is less than 25 km in the east section of the survey. At 505 s periods, penetration depth is somewhat more uniform and is greater than 40 km in south- west, is about 25 ~30 km in the north-east. One outstanding feature is the shallower penetration depth of less than 15 km in an isolated NW-SE elongated narrow area in the south-west.

Broadly these trends are interpreted to be due to the presence of more resistive rocks associated with the Mount Isa Province in south-west of the survey and more conductive packages in the east and north-east of the survey area. The reduced DOI in the east and north-east at shorter periods may be due to the presence of highly conductive sedimentary cover in the Carpentaria Basin. At longer periods, the reduced signal penetration is consistent with the presence of conducting elements in the middle to lower crust.

From practical experience, the effective depth of investigation is a fraction (1/3) of the penetration depth as displayed in the Figure 3.


Figure 3. Penetration depths of the averaged impedance at 10 s, 50 s, 100 s and 505 s. Note that at 505 s (bottom right), the most outstanding feature is the shallower penetration depth of 15 km in the NW-SE elongated narrow area on the Mount Isa Province. DOI maps of 50 s, 100 s and 505 s show a persistent NNW strike.


2.2 Impedance Tensors

In a uniform medium the source field has an exponential decay characterised by the skin depth (penetration depth) which is dependent on the frequency and conductivity. If we assume the frequency is a proxy for the penetration depth as described in section 2.1, a high frequency or the short period corresponds to a shallower depth, while a lower frequency or longer period is for a greater depth.Impedance tensors at different frequencies act as an aid to visualization of variations in conductivity with depth.

Qualitative insights into variations in the subsurface conductivity structure for the survey area can be obtained from phase tensor ellipses filled with phase minimum values (Figures 4 and 5). Ellipses with blue colour show phase min values less than 45º, which indicate decreasing conductivity with depth. Ellipses with warm colours show phase minimum values great than 45º, which indicate increasing conductivity with depth.

At a short period of 0.002 s, ellipses are blue only on the south-west corner of the survey area indicating that the Earth material becomes less conductive (or more resistive) below the relatively shallow penetration depth of these short-period variations. In the remainder of the survey area the warm-coloured ellipses indicate that the Earth material is becoming more conductive (or less resistive) below the penetration depth. Warm colours across the rest of the survey area are associated with sedimentary cover. The patterns of the phase ellipses correspond well with known surface geology in which the relatively resistive Mount Isa Province crops out in the south-west corner of the survey area while the remainder of the area is covered by more-conductive sediments.

At greater phase ellipse periods the effective investigation depth increases. As the phase ellipse periods increase from 0.002 s to 0.02 s (Figure 4 and Figure 5), the portion of the survey area that has warm coloured ellipses recedes towards the north-east, indicating that with increasing penetration depths these survey sites continue to detect more-conductive underlying geology, implying that the conductive sedimentary cover thickens towards the north-east of the survey area. Phase tensor map for 0.02 s shows a line striking NNW at longitude of 140.82º, and the line divides the survey area into two parts with distinct physical properties. To the west of the line, the physical properties of the earth are electrically resistive, highly magnetic and relatively dense, while to the east, the physical properties of the earth are electrically conductive, lower magnetic and less dense (Figure 2).


Figure 4. Phase tensor map at periods of 0.002 s ( left) and 0.005 s (right), coloured by phase minimum.

Figure 5. Phase tensor map at periods of 0.008 s (left) and 0.02 s (right), coloured by phase minimum.


2.3 Vertical Magnetic Transfer Functions

The induced vertical magnetic field is assumed to be entirely the result of the local deviation from a layered electrical resistivity structure. It is produced by perturbation in the current flow created by horizontal contrasts in conductivity. The vertical magnetic transfer function Tx is associated with currents induced by a north-polarized magnetic field while Ty is associated with an east-polarised magnetic field (Equation 7).

In a period range from 0.001 s to 0.01 s (0.01 s is shown on Figure 6), neither the northerly (Tx) nor easterly (Ty) polarizations of the horizontal magnetic field gives rise to detectable induced vertical fields as both Tx and Ty are within the relatively small range -0.1 to 0.1. This small range indicates there are no strong near-surface perturbation in the horizontal current flow, and that the near-surface conductivity structure can be approximated as 1D. Hence the conductivity within these shallower depths dominantly changes in the vertical direction.

For periods from 0.1 s to 10 s, induced vertical magnetic fields arise from structures at depths between a few hundred metres to a few kilometres (1/3 investigation depth as shown in Figure 3). In figure 7 (period 9.3 s), the strongest pattern is evident in the consistently negative values to the west of 140.82º exhibited by the real part of Ty, while the imaginary part of Ty is positive. Both real and imaginary components of Tx have weak anomalies over the survey area. This evidence in real Ty suggests that below a few hundred metres, electric currents are induced in structures predominantly oriented south-north, and evidently deeper conductivity structures are more complex than those near the surface.

When the periods increase to 100 s, induction is most strongly developed in the easterly (Ty) polarization and also in-phase with the source field. The weak north-polarisation response evident in Tx and strong east-polarisation response evident in Ty indicate that the anomalous vertical field induced in the survey area primarily arises from the interaction of east-west oriented magnetic field variations with electrically different north-south oriented geological structures. This character is more strongly developed as the period increases to 500 s (Figure 8). It is unlikely that eddy currents within sedimentary basins can influence data with periods of 500 s. Instead, the regional (continental)-scale induced currents at a greater depths are likely drawn into a highly conductive structure within the basement, possibly a fault-like structure forming conductive conduits in the basement.


Figure 6. A hypothetical event map of the anomalous vertical magnetic field associated with a unit magnetic induction (1 nT) linearly polarized in a northerly direction (top two figures) and easterly directions (bottom two figures) at a period of 0.01s.


Figure 7. A hypothetical event map of the anomalous vertical magnetic field associated with a unit magnetic induction (1 nT) linearly polarized in a northerly direction (top two figures) and easterly directions (bottom two figures) at a period of 9.3 s.


Figure 8. A hypothetical event map of the anomalous vertical magnetic field associated with a unit magnetic induction (1 nT) linearly polarized in a northerly direction (top two figures) and easterly directions (bottom two figures) at a period of 500 s.


2.4 Dimensionality

The dimensionality of the MT data was assessed using the phase tensor analysis. The phase tensor ellipses are plotted at a short period of 0.002 s (Figure 9) and a long period of 100 s (Figure 10). The ellipses are coloured by their normalised skew angle ѱ. The dimensionality of the MT response shown in the phase tensor ellipses differs between sites in the north-east and the sites in the south-west. At short period of 0.002 s the ellipses at sites in the north-east are nearly circular in shape. This is a typical 1D response of a layer structure, and is associated with conductive flat-laying sedimentary rocks. This area corresponds to the location of the Carpentaria Basin cover sequences. In contrast, the ellipses on the exposed Mount Isa Province are polarised in variable directions with some sites having skew values outside the range of +/-6°, indicating the resistivity structure is a mix of 2D and 3D.

At the period of 100 s, the majority of ellipses have a skew value within a range of +/-6° so that resistivity structures can be approximated as quasi-2D and 3D.

Figure 9: Phase tensor ellipses at periods of 0.002 s. To normalise the ellipses to the same size for comparison purposes, the long axis of the ellipse is normalised by the minor axis. The ellipse colour represents the normalised skew angle (ѱ). Inconsistent induction arrow directions or near-zero induction arrows indicate 1D structure of the basin and more complex structure on the Mount Isa Province.


Figure 10: Phase tensor ellipses at periods of 100 s. To normalise the ellipses to the same size for comparison purposes, the long axis of the ellipse is normalised by the minor axis. The ellipse colour represents the normalised skew angle (ѱ).

2.5 Directionality

The Impedance tensors (Equation 1 and 2) are sensitive to the directions of conductivity features and depend on a coordinate system where the north-south and east-west components of the electrical and magnetic fields are measured. Figure 11a presents rose-diagram of the directions at which two principal components Zxy and Zyx are maximised followed Swift method (1967) reference to a south- north and west-east coordinate. Figure11b presents rose-diagram of the phase tensor major axis directions using the Caldwell et al. (2004) method. Both methods indicate a dominant geoelectric strike direction either NNW or ENE with a 90 ambiguity in the periods 10 s to 100 s. DOI maps (Figure 3), VTFs (Figure 8) and potential field data are used to resolve this ambiguity.


Accordingly, a value of 0° (NS direction) is adopted as the regional strike for 2D inversion since we focus on very shallow structures. The regional strike of 0° means we define the TE mode impedance as corresponding to electric current flowing parallel to an approximately north-south regional strike.

Figure11: A diagram (left figure) of the geoelectric strike directions at which two principal components Zxy and Zyx are maximised follow Swift method (1967) on these sites. A rose-diagram (right figure) of the phase tensors shows major axis directions.


∑𝑖𝑖 ,

3 Two Dimensional Modelling and Inversion

3.1 Introduction

All measurement sites from the observed profiles were projected onto approximately straight lines prior to 2D MT inversion. Data were projected in such a way as to maintain the 2D assumptions of directionality of electrical structure both parallel and perpendicular to these projected profiles.Apparent resistivity and phase data of both the transverse electric (TE) and the transverse magnetic (TM) modes were inverted using the WinGLinkTM software package. The inversions were parameterised to fit static shift during the inversion process. The initial model was a uniform half space of resistivity 100 Ωm. Standard deviation was used as the default data error, which is otherwise set to 10% for apparent resistivity and phase. An error floor of 20% in apparent resistivity and an equivalent value for phase (5.8°) were applied in the inversion. A range of weighting functions and regularization parameters were chosen as suggested in Adetunji et al (2015).

The inversion stopped when the maximum number of iterations 300 was reached or the normalised RMS model misfit equalled one or no longer decreased.

The normalised model misfit is defined as

𝑛𝑛𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑡𝑡𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 𝑅𝑅𝑀𝑀𝑅𝑅 = 𝑖𝑖=1 (𝑑𝑑𝑖𝑖−𝐼𝐼𝑖𝑖)2/𝜎𝜎2 𝑛𝑛(10)

where 𝑁𝑁𝑖𝑖 is the measured data, 𝑁𝑁𝑖𝑖 is the modelled data, and 𝜎𝜎𝑖𝑖 is the data error.

Inversions were carried out using the apparent resistivity and phase data in the audio frequency range of 0.001 s to 10 s, and broad frequency range 0.01 s to 1000 s separately to emphasise the near surface structures and the deeper structures. The AMT data inversions have achieved consistent RMS misfit across profiles with an average of 2.1. RMS misfits for BBMT data were more spread across the profiles and bigger than AMT data, in particular between lines 10 to 15 (Figure 12). This spread indicates the deeper structures are more complex than the 2D approximation.


5

4.5

4

3.5

3

2.5

2

RMS_AMTRMS_BBMT

1.5

1

0.5

0 0 5 10 15 20 25 30

Figure 12. RMS misfits for AMT data and BBMT data. Horizontal axis is the line number (lines are east-west oriented with line number increasing to the north).

3.2 Depth of Mount Isa Province and Near Surface Structures

The Mount Isa Province hosts rich mineral deposits and there is significant mineral potential within the sedimentary basins and basement beneath the cover. In the survey area the well-known Ernest Henry Cu-Au deposit (between sites 12 - 14 on Line 14 in Figure 1) is concealed by 35 m to 60 m of Phanerozoic cover (Lilly 2015). This depth set a benchmark for interpretation of the cover thickness in 2D electrical resistivity models.

To investigate the depth of the sedimentary cover and the near surface structures, the apparent resistivity and phase data in the audio frequency range of 0.001 s to 10 s were inverted to produce 2D models from Line 5 to Line 24 with a fine mesh from the surface to a depth of 1 km.


Figure 13. Left hand side, shallow 2D models (with topography considered) of line 13 (top) and line 14 (bottom), and, right hand side, resistivity versus depth curves for the 5 selected sites on line 13 and line 14. Vertical to horizontal ratio (V:H) is 28:1.

The resistivity versus depth (with topography considered) curve (Figure 13), at the selected 5 sites were extracted from the 2D inversion models on lines 13 and 14. One type of resistivity versus depth curves is that the resistivity reduces from surface to a depth of a few tens of metres, and then increases with increasing depth (the left panel in Figure 13). Turning points of resistivity depth curves occur at depths from 20 m to 60 m, which is close to the depth of the known sedimentary cover at the Ernest Henry deposit (35 m to 60 m) (Lilly 2015). The turning point of the resistivity depth curves may serve as the indicating depth of the sedimentary cover overlying the crystalline basement rock of the Mount Isa Province. Another type of resistivity versus depth curves shows resistivity steadily increases with depth at these sites located on more complex 2D structures.

Figure 14 provides a visualization of the 2D inversion models between lines 12 and 14. Sedimentary cover is visible along the tops of these profiles. The warm colour to depth of around 100 m shows the sedimentary basin which is interpreted to overlie the basement. The basement is imaged as a resistive body (R1, in blue colour) of over 1000 Ωm. In general, the sedimentary cover thickens towards the east. The west-dipping basement conductor (C2) directly underlies the location of the Ernest Henry


Mine. The east boundary of Mount Isa Province lies between site 16 and site 17 (48000 on horizontal axis).

The conductor C1 at the east end of the profiles is a significant feature. The conductor C1 dips west in the north (Line 21) and gradually becomes sub-vertical before becoming to steeply east-dipping in the south (Line 12). The boundary between the warm colour C1 and blue colour R1 from the profile L21 to profile L12 strikes NNW.

Figure 14. 2D electrical resistivity models from Line 12 (south) to Line 21 (north) to a depth of 600 m (topography is included). Vertical to horizontal ratio (V:H) is 5:1.


3.3 Deep enhanced conductors within the Mount Isa Province

The apparent resistivity and phase data in the frequency range of 0.01 s to 1000 s were inverted to derive 2D models for the deep part of the electrical resistivity structures (Figure 15). The major features of these models include a thick and highly resistive region (R1) in the west, and a conductive region (C1) in the east. The conductor associated with the Ernest Henry deposit in Figure 14 now is clearly shown as a west-dipping structure (C2) to a depth of over 10 km.

Two basement domains of consistent resistivity character can be recognised from the inversion results; a highly resistive basement to the west and relative conductive basement to the east. This macro-scale difference implies that the basement in the east might have different geological characteristics to the Mount Isa Province. In general, Carpentaria Basin is imaged as a good conductor across all lines. Based on phase tensor analysis (Figure 10) and RMS misfit (Figure 12), the deep structures require 3D inversion.

Figure 15. 2D electrical resistivity models of all profiles from Line 12 (south) to Line 21 (north) to a depth of 10 km. Vertical to horizontal ratio (V:H) is 3:10.


4 Three Dimensional Modelling and Inversion

4.1 Model parameterisation

Three-dimensional inversions were performed using the ModEM algorithm of Egbert and Kelbert (2012, 2014). Many inversions were run to investigate the effect of varying parameters such as the model covariance, background resistivity (see section 4.4 Sensitivity Test), model dimensions, cell size, number of layers and the data error. The features of the preferred model in the report are robust features that have withstood this testing.

In this report, an initial model of resistivity of 100 Ωm was used with a model co-variance factor of 0.2 in all directions. 1000 m grid spacing was used in the model and there were two cells between adjacent sites on average. In the south-north direction, the model consisted of 76 cells and 12 padding cells (increases by a factor of 1.5) on both sides. The total length was 848 km. In the west- east direction, the model consisted of 50 cells and 12 padding cells (increases by a factor of 1.5) on both sides. The total length of the model was 822 km. In the vertical direction, the first layer thickness was 20 m, and subsequent layer thickness was increased by a factor of 1.2 until the depth of the model reaches 758 km.

At period 0.001 s, the skin depth in a 100 Ωm half space is 158 m. The first layer thickness is 1/8 of the skin depth for the highest frequency. At period 1000 s, the skin depth in a 100 Ωm half space is 158 km. The model depth and width are about 4 times the skin depth of the lowest frequency.Therefore the model satisfies the boundary conduction that is the source field decays to near zero at the bottom of the model.

4.2 Inversion of vertical magnetic transfer functions

The magnetic transfer functions were inverted to attempt to resolve the simplest structures of the model. Error floors were set to be 10% for Tx and 5% for Ty in the initial run to allow the inversionprocess to search conductivity value for each cell within a relatively wide range, and then reduced to 5% and 3% in the second run to refine the model. When the data error exceeded the error floor, the data error was used. The error weighs were chosen to reflect the relative importance of each component; assed during data analysis in section 2.3, namely that the Ty data contains more information about the structures present.

Figure 16 has three depth slices from the resulting model at depths of 1933 m, 3414 m and 7187 m. Dominant feature of the 3D model is a division into the two domains observed in the 2D inversion results. A NNW striking conductive anomaly is present on the eastern margin of the survey, while the Mount Isa Province is shown as a more resistive block. There is a curvilinear of approximately north- south orientation which separates the conductive block and the resistive block.


Figure 16. Results from inversion of vertical magnetic transfer function. From left to right, the depth slices are at 1933 m, 3414 m and 7187 m. A NNW striking conductive anomaly is present on the eastern margin of the survey.

4.3 Inversion of impedance tensors

The full impedance tensors in the period range of 0.01 s to 2000 s were inverted; the resulting 3D model is shown in Figures 17, 18, 19 and 20. Error floors were set to be 10% for Zxx and Zyy and 5% for Zxy and Zyx in the initial run to allow the inversion process to search the conductivity value for each cell within a relatively wide range, and then reduced to 5% and 3% in the subsequent runs to refine the model. RMS misfit is 2.85 at RMS difference being 0.001 between the current iteration minus the previous iteration.

To a first order, the 3D electrical resistivity model shows common features with the 2D models discussed in section 3. One of these features is the spatial relationship of the resistive block and the conductive block on the east of survey area. The mapped surface geology is reflected in the 20 m depth slice for the 3D model; the survey area is broadly conductive, consistent with the presence of conductive units in the Carpentaria Basin. There is a small resistive area to the south west of the survey where conductive sediments of the Carpentaria Basin thin onto the outcropping Mount Isa Province.

At the depths of ~200 m and ~7000 m (Figures 17 and 18), the model is dominated by a very resistive crust (R1) on the western side and a strong crustal conductor in the east (C1). Both profile A-A’ and C- C’ (Figures 19 and 20) in the 3D model reveals that the Mount Isa Province is relatively resistive (>1000 Ωm) to a depth of 15 km, becoming more conductive below this depth (Figure 18, depth slice of 15010 m). Figure 21 shows fits between observed and model responses, as represented by sounding curves, for two representative observation sites (L19S12 and L11S05) on R1 in the profile A- A’ and C-C’ of Figure 19. It clearly shows the feature R1 fits the observed data well.

Figure 22 shows fits between observed and model responses for two sites (L19S19 and L11S19) on Feature C1. The apparent resistivity across the periods 0.01 s to 1000 s are in the range of 1 Ωm to


20 Ωm in two anti-diagonal components of impedance tensors Zxy and Zyx. The data themself have defined an enhanced conductive zone beneath these sites. The feature C1 fits anti-diagonal components impedance tensors Zxy and Zyx fairly well, but Zxx and Zyy relatively poor because these two diagonal components are very small; in particular apparent resistivity of Zyy is below 1 Ω.m.

The B-B’ profile in Figure 20 intersects two conductors (C2 and C3) within the resistive crust (R1). A westerly dipping conductor (C2) in the vicinity of the Ernest Henry deposit is a major feature. The B-B’ profile reveals that the conductor C2 dips toward the south-west with its conductive root extending to a depth of beyond 20 km. A shallower conductor (C3) at site L16S15 appears at a depth of 2 km and is close to Mount Margaret deposit location. The conductor C4 at a depth of 16 km could be a deeper part of the C2, but considering the profiles A-A’ and C-C’, C4 appears a narrow and steep-westerly dipping conductor. Figure 23 shows fits between observed and model responses for two sites (L14S10 and L16S14) on these conductive features. The observed data themselves displayed more complex structures on these sites in comparison with the observed data shown in Figure 21.


Figure 17. Resistivity model slices at depths of 32 m, 228 m and 1312 m from left to right.

Figure 18. Resistivity model slices at depths of 1933 m, 7187 m and 15010 m from left to right.


Figure 19. Cross sections (A,B,C,D,E) overlaying the resistivity slice at depth of 2340 m of the 3D electrical resistivity model. Profiles D and E are extracted from the 3D model in the directions which parallel to the 07GA- IG1 (Henson et al. 2009) and 06GA-M4 seismic lines (Gibson et al. 2015) respectively. Seismic lines are annotated with CDP for reference 4000 to 7000, and 1200 to 16000 for 07GA-IG1 and 06GA-M4 respectively.


Figure 20. Cross sections (AA’, BB’ and CC’) of the 3D electrical resistivity model along three profiles (A, B and C) shown in Figure 19. The letters R and C indicate the resisitor and the conductor discussed in the text.


Figure 21. Apparent resistivity and phase of observed data from site L19S12 on A-A’ cross section (Left) and site L11S05 on C-C’ cross-section (right) of the 3D model presented in Figure 20. Lines are the model responses for each of Zxx, Zxy, Zyx, Zyy of the 3D electrical resistivity model. Data errors are not plotted for a clear view of fitting.

Figure 22. Apparent resistivity and phase of observed data from site L19S19 (Left, A-A’ section) and site L11S19 (right, C-C’ section) in the feature C1 of Figure 20. Lines are the model responses for each of Zxx, Zxy, Zyx, Zyy of the 3D electrical resistivity model. Data errors are not plotted for a clear view of fitting.


Figure 23. Apparent resistivity and phase of the observed data from site L14S10 and site L16S14 on B-B’ cross section of Figure 20. Both sites are on the feature C2, C3 and C4. Lines are the model response for each of Zxx, Zxy, Zyx, Zyy of the 3D electrical resistivity model. Data errors are not plotted for a clear view of fitting.

4.4 Sensitivity test of key features

The sensitivity and resolvability of key features were tested using perturbations of the resistivity of the reference model. The standard deviation (5% Zxx and Zyy, 3% Zyx and Zyy), and other parameters were set the same for inversion of input data. The only difference is the resistivity value in the reference models or the starting models named A, B, C, D in Figure 24 (left pictures). The RMS misfit for the half space reference models of 1 Ωm (model A), 10 Ωm (model B), 100 Ωm (model C) and 1000 Ωm (model D) were 13.3, 13.0, 38.0 and 139.9 respectively. The inversion reached RMS misfitof 4.07, 3.81, 3.86 and 4.74 respectively when RMS difference reached 0.05 for the sensitivity test run. Figure 24 shows a comparison between resistivity structures generated from starting models of 1 Ωm, 10 Ωm,100 Ωm and 1000 Ωm in the cross section B-B’ as it intersects all major features.

Feature R1 in the Mount Isa Province was shown a resistivity over 1000 Ωm in the models of the starting models of A, B and C. In the model inverted from a 1000 Ωm starting model D, the area of R1 was little perturbation from the starting model. Consistent presence of the R1 resistor in all tested models (right column in Figure 24) demonstrates that it is a necessary structure for fitting the data.

Feature C1 in the east was shown a resistivity 1 Ωm in the models of the starting models of 10 Ωm (B), 100 Ωm (C) and 1000 Ωm (D); while there was little perturbation for the 1 Ωm starting model (A) for this conductor. The C1 shows clearly a west-dipping feature, and extends to lower crust in Figure 24 B and C. The geometry of C1 at a depth of below 16 km is not well defined in Figure 24 A as the feature has about 1 Ωm which is the same value with the starting model. On other hand, the testing confirms a resistivity value of 1 Ωm for C1 is a conductivity anomaly required by the data. In particular C1 was inverted from 1000 Ωm starting model, in such highly resistive background; the high conductivity C1 would not be inverted unless it is a necessary structure required by the observed data.


Features C2, C3, and C4 are the conductors contained within the resistor R1. C2 is a dominant feature in all test models, and dips westwards to the lower crust. C3 is a shallower feature. C4 remains an inconsistent feature. Especially, it is an unresolved feature in the 1000 Ωm starting model D.

This sensitivity testing also indicate the depth resolution of this dataset is limited to about 30 km. Below this depth, inverted features show a strong dependence on the resistivity of the starting half- space models.


Figure 24. Sensitivity testing using starting models with resistivity of 1 Ωm (A), 10 Ωm (B), 100 Ωm (C) and 1000 Ωm (D) to examine whether the major features R1, C1, C2, C3, and C4 are necessary for fitting the data.


5 Discussions

5.1 Comparison with aeromagnetic anomaly

Figure 25 shows resistivity depth slice at 1932 m overlain on the aeromagnetic image for this area. The Mount Isa Province is highly electrically resistive and exhibits a strong magnetic signature. In contrast, there are highly conductive anomalies with a magnetically quiet signature in the eastern area. The change to a low magnetic response has a prominent north-south strike, and then turns north-northeast at S20°30”. This trend is correlated with the location and orientation of the conductivity anomaly in the 3D model and regional geological structures.

Figure 25. A resistivity depth slice at 1932 m from the 3D model overlain on the aeromagnetic image for the Mount Isa Province, Carpentaria Basin and Eromanga Basin area.


5.2 Comparison with surface geological unit map

Comparison between the 32 m depth slice of the 3D model and the mapped surface geology supports that the AMT data can map cover in the survey area (Figure 26). The resistivity slice contains a resistive feature in the south-west, consistent with the location of the outcropping, or shallowly buried Mount Isa Province. The rest of the survey area is broadly conductive at this depth and interpreted to represent the sediments of the Carpentaria Basin. The 2D models also accurately delineate the depth extent of these sedimentary packages (See Figure 14 and Section 3). This indicates AMT is a powerful tool to map the surface and subsurface geological units in this region.

Figure 26. Comparison of shallow resistivity structures at 32 m depth from the 3D model to mapped surface geology. The thick black line roughly separates the Mount Isa Inlier and the Carpentaria Basin.


5.3 Comparison with seismic reflection data

07GA-IG1:

Deep seismic reflection data and magnetotelluric data were acquired along 07GA-IG1 of Isa- Georgetown profile by Geoscience Australia in 2007. The seismic data revealed three seismic provinces (Figure 27 top) that have discrete geological features but are not evident at the surface (Henson et al. 2009). These three seismic provinces occur beneath the exposed Mount Isa Province and Etheridge Province. A major west dipping Paleoproterozoic (or older) continental suture zone has been interpreted from the seismic data on the eastern margin of the Mount Isa Province (Korsch et al. 2012). This suture separates the Numil Seismic Province from the Mount Isa Province and extends to the Moho. A cross section parallel to the seismic line was extracted from the 3D model for comparison (D-D’ profile). The extracted profile shows the interpreted suture zone corresponds to a west-dipping conductor C1 separating the resistive Mount Isa Province (R1) to the east and Numil Seismic Province to the west. It highlights the presence of detectable resistivity contrast of the faults zone between Mount Isa Province and Numil Seismic Province.

A 2D resistivity model inverted from 2007 (07GA-IG1) MT survey (Figure 27 middle) and the D-D’ cross section of the current 3D resistivity model have similar features. The consistency between these models suggests that a geological contrast exists between the electrically resistive rocks of the upper most crust in the Mount Isa Province and the upper crustal rock of the Numil Seismic Province.


Figure 27. From top to bottom, a) Images of the interpreted seismic section; b) un-interpreted seismic section; c)

2D resistivity model of the traverse 07GA-IG1; d) Cross section D of 3D resistivity model from this survey; e) Conductivity slice at depth of 2340 m of the 3D model and location of seismic line and cross-section DD’.

06GA-M4:

The cross section E-E’ of the 3D resistivity model parallels the section of CDP 12000 to 1500 of 06GA- M4 seismic line with about 25 km offset in the horizontal direction. The reflection seismic image is in poor quality and faults are ill-defined, but interpreted to be sub-vertical to steeply east-dipping (Gibson et al. 2015). The sub-vertical conductor on the E-E section at site L07S16 extends from the surface to more than 10 km with an eastward dip of 40° to 60°. The conductor could be associated with faults causing enhanced conductivity.


Figure 28. Deep reflection seismic section with interpretation (top), and Cross section E-E’ of the 3D resistivity model (bottom left) from this survey, and the conductivity slice at depth of 2340 m of the 3D model and location of seismic line 06GA-M4 and cross-section EE’ (bottom right). E and E’ correspond to CDP14000 and 12000.

5.4 Mineral prospectivity

There are two prominent conductors (C2 and C3) in the upper crust of the resistive Mount Isa Province. The C2 conductor is beneath the Ernest Henry deposit, while the C3 conductor is coincident with the Mount Margaret deposit. The presence of elevated conductivity might indicate the presence of hydrothermal alteration zones associated with mineral deposition in the Ernest Henry and Mount Margaret mine sites. A number of known mineral occurrences are also distributed along the prominent conductor C1 (Figure 29). The results of the Cloncurry MT model suggest MT data may be a valuable tool in assessing mineral prospectivity


Figure 29. Known mineral occurrence sites (source: Geological Survey of Queensland) plotted on top of 1932 m depth slice of the 3D resistivity model.

5.5 Relationship with Carpentaria Conductivity AnomalyThe Carpentaria Conductivity Anomaly (CCA) is a major deep electrical conductivity structure across Queensland (Chamalaun et al., 1990, 1993 and 1999; Lilley et al., 2003). This conductive anomaly stretches over 1000 km from the Gulf of Carpentaria in the north, to Birdsville in the south west of Queensland (Figure 30). The CCA was discovered by a continent-wide magnetometer array study with station spacing over 270 km (Chamalaun and Barton 1993), and then further studied by a more detailed magnetometer array with station spacing of 100 km (Chamalaun et al 1999). Higher resolution MT data, collected by Hanekop (2006) also support the existence of the CCA. These studies indicated a broad path of the north to south orientated CCA, which broadly corresponds with the eastern edge of the Mount Isa Province, leading Hanekop (2006) and Wang et al. (2014) to speculate that the elevated conductivity may be the result of a collisional event along the margin of the Mount Isa Province.

The Cloncurry MT survey is located on the western flank of the CCA. The observed real induction arrow data of this MT survey presented in Figure 10 clearly show two distinctive areas where induction arrows consistently point to the north-east on the Mount Isa Province and reverse to west on Carpentaria Basin. Comparison of the induction arrow data from the 2 km x 2 km grid MT survey with


the induction arrow data of 100 km x 100 km grid magnetometer arrays (Chamalaun et al 1999) indicates a good agreement between two datasets.

The electrical resistivity models inverted from the VTFs data (Figure 16) and impedance data separately (Figure 18) show a common feature: a NNW striking conductive anomaly is present on the eastern margin of Mount Isa Province. This curvilinear conductor named as C1 of approximately north- south orientation separates the conductive Carpentaria Basin and the resistive Mount Isa Province, and is on the path of Gidyea suture zone interpreted by 07GA-IG1 seismic reflection data. C1 is interpreted as a section of the CCA running through Mount Isa Province, and defines the western boundary of the CCA.

These evidences support that electrical conductivity anomaly (Figures 17 and 18) on the eastern margin of this survey is part of CCA mapped by the magnetometer arrays. This suggests that the CCA is complex in nature and is present from shallow crustal levels to upper mantle. However the small size of the Cloncurry MT survey makes interpretation of such large scale conductivity features difficult.

Figure 30. Location of the Cloncurry MT survey (black box) compared the CCA. Resistivity model is the 52 km depth slice from the Wang et al. (2014). Real component of induction vectors (Parkinson (1959) convention, vectors point towards elevated conductivity) plotted from available historic data at 3000 s. Data originally collected by Chamalaun and Barton (1990), Hanekop (2006).


6 Conclusion

Broadband and audio magnetotelluric (BBMT and AMT) data at 476 sites with a grid spacing of 2000 m were acquired from July to November 2016 in the Cloncurry region. Three component magnetic field and two component electric field time series data were recorded for average of 24 hours per station. Reliable magnetic transfer function and impedance tensor data were derived from the time series data in the period range from 0.001 s to 2000 s. The effective penetration depths of the impedance tensors were estimated to be approximately 10 km on the conductive eastern margin of the survey and to 40 km in the resistive Mount Isa Province. Major outcomes of the analysis and modelling of these data can be summarised as:

a) The AMT data mapped the cover sequences in higher spatial resolution on the Mount Isa Province. The cover thickness is a few metres from the exposed basement in the west to up to 100 m to the east across the survey area.

b) The Mount Isa Province was well imaged under the thin conductive cover. The Province shown in the 3D electrical resistivity model consists of very resistive block (over 1000 Ωm) to a depth of 12 km, overlying a more conductive block beneath. The depth extent of conductive part of basement is beyond 40 km and is not resolved by this dataset.

c) There are two prominent conductors (C2 and C3) in the upper crust of the resistive Mount Isa Province which are associated with mineral deposits in the Ernest Henry and Mount Margaret sites. A number of existing mineral occurrence sites are distributed along the prominent conductor C1.

d) The conductor C1 forms a continuous conductive block from the ground surface to below 40 km. This westerly dipping structure is coincident with the eastern margin of the Mount Isa Province and may represent a prominent fault. The C1 conductor is also coincident with the location of the Carpentaria Conductivity Anomaly and may be related to this feature.


Acknowledgement

The authors acknowledge the traditional owners and landholders within the Cloncurry region, without whose cooperation these data could not have been collected. The survey was funded by the Geological Survey of Queensland. We owe a debt of gratitude to Paul Donchak from Geological Survey of Queensland who provided advice on geology and help on survey design and land access.

Thanks Naser Meqbel for proving 3D grid software and help to work on the model.

Adrian Hitchman and Wenping Jiang from Geoscience Australia are thanked for reviewing this record.

This record is published with the permission of the CEO, Geoscience Australia.


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GA Record Template€¦ · Web viewFigure 8. A hypothetical event map of the anomalous vertical magnetic field associated with a unit magnetic induction (1 nT) linearly polarized