Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling. Record 2017/03 ecat Liejun Wang, Richard Chopping and Jingming Duan

Transcription

1 Record 2017/03 ecat Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling Liejun Wang, Richard Chopping and Jingming Duan APPLYING GEOSCIENCE TO AUSTRALIA S MOST IMPORTANT CHALLENGES

2 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling GEOSCIENCE AUSTRALIA RECORD 2017/03 L. Wang, R. Chopping and J. Duan

3 Department of Industry, Innovation and Science Minister for Resources and Northern Australia: Senator the Hon Matthew Canavan Assistant Minister for Industry, Innovation and Science: The Hon Craig Laundy MP Secretary: Ms Glenys Beauchamp PSM Geoscience Australia Chief Executive Officer: Dr Chris Pigram This paper is published with the permission of the CEO, Geoscience Australia Commonwealth of Australia (Geoscience Australia) 2017 With the exception of the Commonwealth Coat of Arms and where otherwise noted, this product is provided under a Creative Commons Attribution 4.0 International Licence. ( Geoscience Australia has tried to make the information in this product as accurate as possible. However, it does not guarantee that the information is totally accurate or complete. Therefore, you should not solely rely on this information when making a commercial decision. Geoscience Australia is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document please clientservices@ga.gov.au. ISSN X (PDF) ISBN (PDF) ecat Bibliographic reference: Wang, L., Chopping, R., and Duan, J., Southern Thomson magnetotelluric (MT) survey report and data modelling. Record 2017/03. Geoscience Australia, Canberra. Version: 1701

4 Contents Executive Summary... iv 1 Introduction, data acquisition and processing Introduction Data acquisition parameters and site/traverse locations Data processing and quality assurance Depth of investigation Data analysis Dimensionality Directionality Apparent resistivity structure GA GA GA3 (BBMT) GA GA6 & GA7a GA7b GA Two dimensional (2D) modelling Introduction D models GA GA GA GA6 & GA7a GA7b GA Conclusions Acknowledgements...48 References...49 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling iii

5 Executive Summary Broadband magnetotelluric (BBMT) and audio magnetotelluric (AMT) data were acquired in 2014 in the southern Thomson Orogen region, on the Queensland and NSW borders in the western half of these states. The data were acquired by Zonge Engineering and Research Organisation (Zonge) on behalf of Geoscience Australia and the Geological Surveys of Queensland and New South Wales. Quality Assurance and Quality Control (QA/QC) were performed on the data, which were then analysed for dimensionality and directionality. Two-dimensional (2D) inverse models were also generated for both the BBMT and AMT data. This report details the field acquisition program and the methodologies used for processing, analysing and modelling these data. These data have the potential to be utilised to understand the basement geology and the thickness and nature of the cover in this greenfields exploration region. iv Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

6 1 Introduction, data acquisition and processing 1.1 Introduction The Southern Thomson Project is a collaborative project between Geoscience Australia (GA), the Geological Survey of New South Wales (GSNSW) and the Geological Survey of Queensland (GSQ). This is a multi-disciplinary project with the aim of characterising the largely unexplored southern Thomson Orogen region (Figure 1.1). As part of this, new precompetitive datasets have been acquired within the southern Thomson region including surface geochemistry (Main and Caritat, 2016), airborne electromagnetic data (Roach, 2015), and gravity and magnetotelluric data for new geological interpretations. These datasets will support an improved understanding of the: thickness and nature of cover; geology obscured by sedimentary cover or regolith; and, mineral systems potential of the region. Magnetotellurics is a passive geophysical technique where the Earth s natural time-varying electrical and magnetic fields are recorded to provide a measure of subsurface conductivity/resistivity (note that these two physical properties are related, as resistivity is the inverse of conductivity). The frequencies of magnetic and electrical sources recorded provide different depth information. Audio magnetotellurics (AMT) records higher frequency data and images shallower electrical structure, and broadband MT (BBMT) records lower frequencies that image relatively deeper electrical structure. Although the depth of investigation (DOI) of the technique varies according to the local resistivity structure, in general AMT images the shallowest upper crust only (to approximately 4 km depth) and BBMT images the crust (up to 60 km depth). For the southern Thomson region, BBMT was acquired along two long and one shorter transect (Figure 1.2) to provide detail of the entire crust of the region, while AMT was acquired along shorter and higher-resolution lines to provide knowledge of the coverbasement relationships of the region. Within this document we detail the acquisition, processing and analysis of the MT data, and present preliminary resistivity models. These data, their analyses and models are released as underpinning datasets to support future geophysical analyses and geological interpretation. Accordingly, this report focuses primarily on the core geophysical aspects from acquisition to preliminary modelling. Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 5

7 Figure 1.1: Location of the Thomson Orogen and Southern Thomson Project area in the border region of northwestern NSW and southwestern QLD. 1.2 Data acquisition parameters and site/traverse locations Zonge Engineering and Research Organisation (Zonge) were contracted by GA to conduct a BBMT survey in the project area in northern New South Wales and southern Queensland between October and December The contractor also obtained a test line of AMT data during this period to determine whether the method would provide useable data in this terrain. Two orthogonal horizontal components and the vertical component of the magnetic field and two orthogonal horizontal components of the electric field data were recorded at 199 sites on three lines: GA1, GA2 and GA3. (Figures ; Table 1.1). Broadband magnetotelluric data were recorded over periods of hours per site, providing data in a frequency range of 400 Hz to Hz. Site spacing between BBMT stations varied from 3.5 and 5 km. Audio magnetotelluric data were also recorded at 27 sites, at approximately 1 km station spacing, on a section of line GA3 from Hungerford south into New South Wales to test the suitability of the AMT technique for use in the project region (along a portion of line GA3: Figures ; also Table 1.1). Based on these results, further AMT data were then acquired by the contractor during January and February 2015 at a further 259 sites on five lines (GA5, GA6, GA7a, GA7b and GA8; Figures ; also Table 1.1) within the project area to investigate the resistivity structure of the shallow crust and basin cover. Station spacing on the AMT lines was approximately 1 km. The majority of AMT data were acquired during a 2 hour period during the day, with a small number of sites acquired overnight. All sites yielded data within a frequency range of 1000 Hz to 0.1 Hz. 6 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

8 Table 1.1: Details of survey lines from the Southern Thomson Project MT acquisition. Line ID Type Number of Measurement Sites Site location diagram Notes GA1 BBMT 99 Figure 1.4 GA2 BBMT 15 Figure 1.4 GA3 BBMT & AMT 85 Figure 1.5 (BBMT) Figure 1.6 (AMT) Includes 27 AMT sites for testing GA5 AMT 38 Figure 1.7 Intersects BBMT line GA3 GA6 AMT 67 Figure 1.7 Higher resolution AMT inset on line GA1 GA7a AMT 61 Figure 1.7 Provides higher resolution inset on BBMT line GA1 GA7b AMT 37 Figure 1.8 Intersects BBMT line GA3 GA8 AMT 56 Figure 1.7 Intersects BBMT line GA3 Stations are numbered according to the following scheme: yy-station_maximum period. This is applied per line, so station 14-42_2350 on line GA3 was acquired in 2014, is station 42 along the line, and has data to a maximum period of 2350 s. To differentiate later with the presentation of lines that intersect, station numbers may be prefixed with the line and the year and maximum period omitted for simplicity: in this case, the above station would be GA3-42. Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 7

9 Figure 1.2: Locations of BBMT and AMT acquisition lines in the Southern Thomson Project area overlain on 1:1M scale Surface Geology of Australia (Raymond et al., 2012). Station locations for each individual line are shown in Figures Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

10 Figure 1.3: Locations of BBMT and AMT acquisition lines in the Southern Thomson Project area overlain on the total magnetic intensity anomaly map of Australia, 6 th edition (Geoscience Australia, 2015). Station locations for each individual line are shown in Figures Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 9

11 Figure 1.4: Locations of BBMT sites along traverses GA1 and GA2 overlain on the total magnetic intensity anomaly map of Australia, 6 th edition (Geoscience Australia, 2015). Other traverses are depicted by grey lines. Note: not all sites are depicted due to point density. 10 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

12 Figure 1.5: Locations of BBMT sites along traverse GA3, overlain on the total magnetic intensity anomaly map of Australia, 6th edition (Geoscience Australia, 2015). Other traverses depicted by grey lines. Note: not all sites are depicted due to point density. Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 11

13 Figure 1.6: Locations of AMT sites along traverse GA3, overlain on the total magnetic intensity anomaly map of Australia, 6th edition (Geoscience Australia, 2015). Other traverses and also GA3 BBMT sites in the vicinity of the AMT sites on GA3 are depicted in grey. Note: not all sites are depicted due to point density. 12 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

14 Figure 1.7: Locations of AMT sites along traverses GA5, GA6, GA7a and GA8, overlain on the total magnetic intensity anomaly map of Australia, 6th edition (Geoscience Australia, 2015). Other traverses are depicted by grey lines. Note: not all sites are depicted due to point density. Figure 1.8: Locations of AMT sites along traverse GA7b, overlain on the total magnetic intensity anomaly map of Australia, 6th edition (Geoscience Australia, 2015). Other traverses are depicted by grey lines, with AMT traverses depicted as thicker lines. Note: not all sites are depicted due to point density. Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 13

15 1.3 Data processing and quality assurance Field acquisition for the MT data for this project occurred in two campaigns (see Section 1.2), with each campaign reported on separately by Zonge (Zonge, 2014; 2015). Zonge performed preliminary processing of the MT data using the following procedure: 1. Phoenix SSMT2000 software was used to process raw time series data. Processing included remote referencing, primarily to reduce noise in the magnetic data. 2. A magnetic field declination of 9 (from the 2005 IGRF) was used to rotate the time-series data to true north-east coordinates. 3. Frequency domain data including cross-spectra for all recorded channels were produced using SSMT2000 software. Review, editing and conversion to impedance tensors were conducted utilising Nord-West Ltd MT Corrector software. At this point polarity errors in acquisition were corrected and outlying impedance estimates on all recorded components were removed before producing electronic data interchange (.edi) data files for further review. Spline functions were also fitted to multiple sections of the impedance estimates were also calculated and exported at this stage. On a weekly basis, during acquisition, Zonge provided the following data and information to GA for data quality assurance and quality control (QA/QC): Field notes including line, station, date, receiver reference, receiver files, Ex and Ey ground contact resistivity, and station duration information for each site. Processing notes including impedance tensor data quality, likely causes of noise, repeat site requirements and crew comments. Data plots containing screen shots of off-diagonal components of impedance tensor estimates. A clean.edi file containing 20 or more separate impedance estimates for each site. A spline.edi file containing the best fit impedance estimates for each site based on the calculation of the spline functions to smooth the impedance estimates. These files are included in the accompanying electronic data package in the data folder. Clean files contain _clean in the filename, and splined files contain _spl in the filename. Geoscience Australia staff checked the field notes, processing notes, data plots and data files once they were downloaded as part of GA s Quality Assurance/Quality Control process. As part of the QA/QC process, time-series data were plotted as multi-sections of impedance estimates to assess data quality. A clean.edi file usually contains 20 or more multi-sections of impedance estimates derived from a segment of the time-series data. A segment of good quality time-series data produces consistent multi-sections of impedance estimates (Figure 1.9), while poor quality time-series data will produce scattered impedance estimates (Figure 1.10). Repeat acquisition was undertaken at sites exhibiting poor quality time-series data. Repeat measurements were not performed where lower data quality was unavoidable due to environmental conditions or weak natural source signals as assessed by GA geophysicists. Spline files were imported into Schlumberger WinGLink software for further review. Off-diagonal components of impedance tensors (XY and YX) were stacked together along the profile to check the consistency of data at neighbouring sites (Figure 1.11). Static shifts between XY and YX at each site were also recorded for reference during further analysis. 14 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

16 Figure 1.9: An example of high quality time-series data as indicated by consistent multi-sections of impedance estimates. The limited scatter of the points highlights the consistency within the data for this site. Figure 1.10: An example of poor quality time-series data as indicated by highly scattered multi-sections of impedance estimates. In contrast to Figure 1.9, these data show considerable scatter indicating significant noise. Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 15

17 Figure 1.11: Stacked XY and YX of sounding curves on BBMT line GA Depth of investigation Electrical resistivity varies over many orders of magnitude across a region due to crustal and mantle conductivity variations, and thus the effective depth of investigation (DOI) can also vary significantly across the same region (Jones, 1983). In general, DOI is deeper in resistive material and shallower in conductive material. Depth of investigation also increases for lower frequencies or higher periods. Below we will discuss the various measures of the depths to which these data can be regarded as reliable. To understand the possible maximum DOIs of these MT data, we analysed for DOI using the Bostick transformation (Jones, 1983). We examined the DOI along the BBMT traverses GA1-3 (Figure 1.13), and also for the AMT traverses in Queensland (lines GA5-8: Figure 1.14). The effective investigation depth is about 1/2 DOI, due to the nature of electromagnetic surveying. The intensity of the electromagnetic field deceases exponentially with increasing the depth h into the Earth. The reason behind that is a time-varying magnetic field is accompanied by a time-varying induced electric field, which in turn creates secondary time-varying currents and secondary magnetic field. The induced currents produce a magnetic flux that is opposite to the external source field, thus the total magnetic flux is decreased. At a depth of δ into the Earth, the amplitude of the electromagnetic field decreases to 1/e of its value at the Earth surface, this distance is called the skin depth, defined by Simpson and Bahr (2005) as δ = 2/μ 0 σω (Equation 1) where μ 0 is the permeability of free space, and ω = 2πf, f is the frequency in Hz, and σ is a half-space of a given conductivity (S/m). 16 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

18 The skin depth δ is also commonly referred to as depth of investigation or penetration depth. In the MT community, a penetration depth is generally accepted that in a half space medium of resistivity equal to apparent resistivity at that particular period T defined by the Bostick transform (Bostick, 1977): h = ρ a (T)T = 1 δ (Equation 2) 2πμ 0 2 from Equation 2, Bostick penetration depth is approximately 0.7 times the skin depth (Figure 1.12). There is another concept closely related to the penetration depth, which is the sensitivity of the impedance tensor data to the earth resistivity (ρ: the inverse of conductivity i.e. ρ = 1/σ). For 1-D model, an analytical solution for the sensitivity function (Oldenburg, 1979) describes that a small perturbation to the resistivity ρ to the Earth resistivity model ( ) produces a small change g to the impedance tensor data g: S(z, σ, ω) = kze 2kz (Equation 3) where k = iωμ 0 σ and Z is the depth normalised by the skin depth δ. As an example to illustrate the relationship of the sensitivity function and skin depth, the sensitivity for electromagnetic variation with a frequency 10 Hz in a half-space of conductivity 0.01 S/m (100 ohms) is calculated. Note that maximum of the sensitivity occurs close to a depth of half the skin depth (Figure 12). At the depth δ, the sensitivity value is 0.19, and at the depth 0.2δ, sensitivity value is 0.189, therefore the investigate depth range is between 0.2δ to δ with the maximum sensitivity at depth of 0.5δ. Note that the relationship between the normalised sensitivity and the normalised depth is not dependent on the given frequency and resistivity used in the calculations. The effective depth of investigation (DOI) should therefore be regarded as 0.5δ. Figure 1.12: Graph of depth (normalised to skin depth) versus sensitivity. Note that the sensitivity peaks at half of the skin depth. This gives the effective depth of investigation. As it varies by local resistivity structure, in this report we refer to depth of investigation (DOI) that is derived from the Bostick transform (see Equation 2). The DOI as reported in the figures below should be regarded as a maximum and the effective depth to which the data can be regarded as reliable would be approximately half of the reported DOI. Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 17

19 From the analytical solution, we present depth of investigation data which are based on the Bostick transform (Equation 2). These should be regarded as a maximum depth and effectively the data will image reliably to an effective investigation depth of half these values. This will depend on the local electrical structure and the relationship is more complicated for an earth that consists of multiple layers of differing conductivities. The relative differences between the DOI for sites as reported below do reflect where sites are capable of imaging to greater or shallower depths, however. The DOI for lines GA1 and GA2 are in the order of 100 km (effective investigation depth is 50 km), implying the presence of comparatively resistive material (Figure 1.13). In contrast, line GA3 shows more variability in the DOI, with values varying from km (effective investigation depth is from 30 km to 100 km). The DOI in the central to southern part (30 30 S to S) of this line is relatively shallow, in the order of 60 km (effective investigation depth is about 30 km), potentially as a result of the presence of relatively conductive elements in the crust and mantle. The greater DOI in the northern part of the profile, in the order of 200 km (effective investigation depth is 100 km), suggests the presence of much more resistive elements in the lower crust and uppermost mantle. The DOI of AMT data, as measured at periods of 1 second (s) across 5 AMT survey lines, is relatively uniform at between 1.2 km and 1.6 km (effective investigation depth is 0.6 km to 0.8 km) for all data for this study (Figure 1.14). Figure 1.13: Calculated DOI at a period of ~1000 s for BBMT lines GA1 through GA3. 18 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

20 Figure 1.14: Calculated DOI at a period of ~1 s for AMT lines GA5 through GA8. Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 19

21 2 Data analysis 2.1 Dimensionality Magnetotelluric data are sensitive to the dimensionality of the electrical structure beneath and surrounding the MT site. Data can be: 1-dimensional (1D), where the same resistivity layering extends in effectively infinite directions all around the site; 2-dimensional (2D), where the resistivity varies along a line away from the station but is effectively infinite in a direction perpendicular to this direction; or, 3-dimensional (3D), where the resistivity structures vary in all directions. To determine dimensionality and directionality of the MT data acquired in the Southern Thomson Project area, we use the phase tensor analysis of Caldwell et al. (2004), parameterised by Booker s (2014) scheme, to estimate the dimensionality and directionality of the resistivity structures beneath the study area. According to this approach, a phase tensor Φ in measurement coordinates can be parameterised by: Φ=R 1 (θ ellipse ) [ Φ a 0 0 Φ b ] R(ѱ)R(θ ellipse ) where θ ellipse is the angle between the x-axis of the measurement coordinates and one of the phase tensor ellipse axes. Angle ѱ is a normalised skew angle which is rotationally invariant. [ Φ a 0 0 Φ b ] is a 2D case phase tensor, and R is a unitary coordinate rotation operator. A graphical representation of the phase tensor parameters can be presented as a phase ellipse generated by applying a phase tensor rotated around a unit circle (Figure 2.1). 20 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

22 Figure 2.1: Graphical representation of phase tensor ellipse parameters (from Booker, 2014). The phase tensor provides a measure of dimensionality for the MT data: In the 1D case, the phase ellipse is a circle. In the 2D case, the angle ѱ is zero. A point on the unit circle crosses each axis of the ellipse exactly when the point on the ellipse crosses the same axis. Although strictly 2D structure requires 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 (Booker, 2014). In the 3D case, the angle ѱ is not zero. A point on the unit circle crosses each ellipse axis when the point on the ellipse is not on one of the semi-axes. The angle (θ ellipse ) is the 2D regional strike angle if ѱ is zero. Note that the regional strike has an ambiguity of 90 so interpretation requires integration with other data available, such as structural geological knowledge. To illustrate the results for the Southern Thomson Project region, the phase tensor ellipses for BBMT survey lines GA1 (Figure 2.2) and GA3 (Figure 2.3) are plotted for periods from s to 2380 s. Ellipses are plotted away from the sites as period increases. The ellipses are coloured by their normalised skew angle ѱ. At short periods the phase ellipses at all stations are close to circular, indicating that shallow resistivity structures beneath the study area are ~1D. The resistivity structure becomes 2D or quasi-2d as the period increases (i.e. for greater depths) and normalised skew angle ѱ is less than 6 (red ellipses). At periods over 1000 s, and therefore at greater depths, normalised skew angle ѱ is over 6 (yellow or green), indicating a 3D resistivity structure. Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 21

23 Figure 2.2: Phase tensor ellipses at periods from s to 2380 s for BBMT survey line GA1. 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 is painted by normalised skew angle (ѱ), where red indicates a skew angle of less than 6 degrees. Thirteen periods are used to generate the plot: 0.004, 0.017, 0.073, 0.294, 1.1, 18, 74, 149, 436, 877, 1190, 1754 and 2380 s, with increasing periods plotted further from the station points. These periods relate to the upper few hundred metres down to the lower crust, although the specific depth depends upon the conductivity beneath and surrounding the station. 22 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

24 Figure 2.3: Phase tensor ellipses at periods from s to 2380 s for BBMT survey line GA3. 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 is painted by normalised skew angle (ѱ), where red indicates a skew angle of less than 6 degrees. Thirteen periods are used to generate the plot: 0.004, 0.017, 0.073, 0.294, 1.1, 18, 74, 149, 436, 877, 1190, 1754 and 2380 s, with increasing periods plotted further from the station points. These periods relate to the upper few hundred metres down to the lower crust, although the specific depth depends upon the conductivity beneath and surrounding the station. 2.2 Directionality As magnetotelluric surveying measures the orthogonal horizontal components of both the electrical and magnetic fields, magnetotellurics is sensitive to the direction of conductivity features (such as faults) in the subsurface. In this section, we examine the directionality for the two longest BBMT lines, GA1 and GA3. Figure 2.4 presents angle θ ellipse, which is the angle between the x-axis of the measurement coordinates and the phase tensor major axis at all sites on lines GA1 and GA3. Angles of east-west orientation dominate for periods of greater than 1000 seconds (~ the mid to lower crust), which may suggest an east-west orientation to deeper structures within the basement of the Thomson Orogen. Figure 2.4 also presents histograms of phase tensor major axis directions at periods of 1191, 2016 and 2383 s for lines GA1 and GA 3. Sixty two percent of data fall in the range 90 to 130 (-90 to - 50 ), and 47% of the data in this range fall between 100 and 110 (or between -80 and -70 ) with a dominant azimuth of 105. Similarly on line GA3, 67% of phase tensor major axis directions fall within a range of 90 to 130 (-90 to -50 ), and 47% of values within this range are between 100 and 110, with a dominant azimuth of 105. Accordingly, a value of 105 is adopted as the regional strike for the BBMT data. The regional strike of 105 means we define the transverse electric (TE) mode impedance Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling 23

25 as corresponding to electric current flowing parallel to an approximately east-west regional strike, while the transverse magnetic (TM) mode data correspond to electrical current flowing across the north-south regional strike. Figure 2.4: A) The phase tensor major axis of periods from s to 2380 s with 90 ambiguity at all sites on BBMT survey lines GA1 and GA3. Periods are 0.004, 0.017, 0.073, 0.294, 1.1, 18, 74, 149, 436, 877, 1190, 1754 and 2380s. B) Histogram of phase tensor major axis for data of periods 1191, 2016 and 2383 s for sites on line GA1. C) Histogram of phase tensor major axis for data of periods 1191, 2016 and 2383 s for sites on line GA Apparent resistivity structure Qualitative insights into variations in the subsurface structure along the model profiles can also be obtained from pseudo-sections of the apparent resistivity and phase data of the TM and TE modes; these two modes are complementary and represent different components of the impedance tensor. To analyse the apparent resistivity structure, the lines were first projected onto straight 2D sections, with data rotated to match the strike of these 2D lines. To illustrate the padding used for the modelling (Section 3), the padding at the start and ends of lines is included. Note that note all stations are labelled on some lines due to station density, in these cases start and end stations are included and every 5 th station is also included. To simplify the figures, stations are labelled only by station number and maximum period and referred to below by their line number and station only (e.g. site 14-42_2350 for line GA3 is labelled on the pseudosections as 42_2350 and is referred to by only its station number and line so GA3-42). The station naming convention is discussed in section GA1 Figures 2.5 and 2.6 are apparent resistivity pseudo-sections for BBMT survey line GA1, showing TE and TM modes respectively. The line comprises 99 measurement sites; the northwestern-most site (site _spl) was excluded due to poor data quality. In the middle of the pseudo-sections of both the TE and TM modes, phase value is >45 at short periods and deceases with increasing period. This suggests the presence of a conductive surface layer. The left part of the profile near site GA1-85 is characterised by high apparent resistivity at short periods which may indicate a relatively shallow resistive feature. 24 Southern Thomson Magnetotelluric (MT) Survey Report and Data Modelling

26 Figure 2.5: Pseudo-section of apparent resistivity and phase data of the TE mode for line GA1. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 25

27 Figure 2.6: Pseudo-section of apparent resistivity and phase data of the TM mode for line GA1. 26 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

28 2.3.2 GA2 Figures 2.7 and 2.8 are apparent resistivity pseudo-sections for BBMT survey line GA2, showing TE and TM modes respectively. The line comprises 15 measurement sites, and the southern end of this line (GA2-01 intersects GA1 at station GA1-54. Both the TE (Figure 2.7) and TM (Figure 2.8) modes show a two layer resistivity structure, with a conductive upper layer over a lower resistive layer. Figure 2.7: Pseudo-section of the apparent resistivity and phase data of the TE mode on line GA2. Figure 2.8: Pseudo-section of the apparent resistivity and phase data of the TM mode on line GA2. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 27

29 2.3.3 GA3 (BBMT) Figures 2.9 and 2.10 present apparent resistivity pseudo-sections for BBMT survey line GA3, showing TE and TM modes respectively. The line comprises 85 measurement sites, and it intersects line GA1 at site GA3-68 (site GA1-79 on GA1). Site GA3-9 site was excluded due to poor data quality. Both TE and TM mode data exhibit a similar pattern in apparent resistivity and phase data up to a period of 10 s, suggesting that the shallow resistivity structure is 1D. This shallow feature is characterised by low apparent resistivity and a phase value > 45, indicating the occurrence of a near-surface conductive zone. At periods greater than 10 s, TM mode data suggest the presence of a nearly continuous deep resistive layer along much of the profile. Finally, between sites GA3-23 to GA3-39 the near-surface conductive zone appears thicker (100 km to 160 km from the west side of the profile) GA5 Figures 2.11 and 2.12 are apparent resistivity pseudo-sections for AMT survey line GA5, showing TE and TM modes respectively. Line GA5 intersects (at site GA5-36) BBMT line GA3 (site GA3-62). Site GA5-25 was excluded due to poor data quality. The pseudo-sections of both modes show that, for frequency ranges of 1000 Hz to 1 Hz, a conductive zone dominates along the profile. At frequencies less than 1 Hz, a resistive zone is represented at depth (especially in the TM data), suggesting that the AMT data is reliable beneath the overlying conductive zone GA6 & GA7a Figures 2.13 and 2.14 are combined apparent resistivity pseudo-sections for survey lines GA 6 and GA7a, which are two separate segments of a larger continuous survey line. Line GA7a has 61 measurement sites running in parallel with BBMT line GA1 between sites GA1-95 and GA1-84). Line GA6 is located further to the east, and has 67 measurement sites running in parallel with BBMT line GA1 between sites GA1-84 and GA1-70. In both TM (Figure 2.13) and TE (Figure 2.14) modes, pseudo-sections for the combined survey lines show a similar structure, indicating that these data can be treated as 1D. The dominant feature is a pronounced shallowing of the underlying resistive structure, separating a conductive structure from a frequency band of 1000 Hz to 1 Hz. This feature is consistent with a similar observation on the pseudo-section for BBMT line GA1 (Figures 2.5 and 2.6) GA7b Figures 2.15 and 2.16 are apparent resistivity pseudo-sections for survey line GA7b, showing TE and TM modes respectively. Line GA7b sits north of line GA6, and comprises 37 measurement sites. The easternmost site on line GA7b (GA7b-25) intersects BBMT line GA3 at site GA3-70. Pseudo-sections for both modes show a two layer resistivity structure with a conductive layer over a resistive layer GA8 Figures 2.17 and 2.18 are apparent resistivity pseudo-sections for AMT survey line GA8, showing TE and TM modes respectively. The line comprises 56 measurement sites. The westernmost site (GA8-10) intersects BBMT line GA3 at site GA3-72. The resistivity structure along the line in both TE (Figure 2.17) and TM (Figure 2.18) modes is very similar to AMT line GA7b (Figures ), with a conductive upper layer over a resistive lower layer. 28 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

30 Figure 2.9: Pseudo-section of the apparent resistivity and phase data of the TE mode on line GA3 (BBMT). Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 29

31 Figure 2.10: Pseudo-section of the apparent resistivity and phase data of the TM mode on line GA3 (BBMT). 30 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

32 Figure 2.11: Pseudo-section of the apparent resistivity and phase data of the TE mode on line GA5. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 31

33 Figure 2.12: Pseudo-section of the apparent resistivity and phase data of the TM mode on line GA5. 32 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

34 Figure 2.13: Pseudo-section of the apparent resistivity and phase data of the TE mode on line GA6 and 7a. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 33

35 Figure 2.14: Pseudo-section of the apparent resistivity and phase data of the TM mode on line GA6 and 7a. 34 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

36 Figure 2.15: Pseudo-section of the apparent resistivity and phase data of the TE mode on line GA7b. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 35

37 Figure 2.16: Pseudo-section of the apparent resistivity and phase data of the TM mode on line GA7b. 36 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

38 Figure 2.17: Pseudo-section of the apparent resistivity and phase data of the TE mode on line GA8. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 37

39 Figure 2.18: Pseudo-section of the apparent resistivity and phase data of the TM mode on line G8. 38 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

40 3 Two dimensional (2D) modelling 3.1 Introduction The apparent resistivity and phase pseudo-sections for the data acquired in the southern Thomson Orogen region depict the data, but they do not contain depth information (the DOI of MT data depends upon local resistivity structure). In order to obtain measures of true resistivity with respect to depth along the profiles, these data must be modelled. The dimensionality analysis conducted in Section 2 indicates that the data can be sufficiently modelled as 2D. This also naturally fits with the data that were acquired along 2D traverses. To conduct 2D modelling of the profiles, all BBMT and AMT measurement sites are projected to straight lines close to the observed profiles (for example, the projected profile for BBMT line GA1 is depicted in Figure 3.1). This procedure is analogous to generating a common depth point line used for processing of non-linear seismic reflection survey lines. Data are 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 TE and TM modes are inverted using the WinGLink inversion code, in which static shift is also a model parameter. The initial model was a uniform half space of 100 Ωm. Standard deviation is used as the default data error, which is otherwise set to 10% for apparent resistivity and phase if data errors are not recorded. An error floor of 20% in apparent resistivity and an equivalent value for phase (5.8 ) were applied in the inversion. Figure 3.1: BBMT Line GA1 and its projected profiles for 2D modelling. This is representative of the approach taken for projection of MT sites onto 2D profiles, potentially in segments where large changes in line azimuth are required by data acquisition locations. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 39

41 Inversions were run in two stages. Firstly a uniform half space with a resistivity of 100 Ωm was chosen as an initial model and TE mode data were inverted to produce a smoothed resistivity structure model. The output model of TE mode inversion was then set as an initial model for both TE and TM mode inversions. A coarse mesh and a fine mesh model were produced for BBMT survey lines GA1 and GA3 (a coarse mesh was not required due to the significantly fewer number of stations along the GA2 line). The coarse mesh model presents a simple model which has the minimum structures necessary for fitting the data. The fine mesh model provides insight into the structures in the coarse model and gives a better estimate of their depths and shapes. It should be noted that structures in the fine mesh models may be exaggerated due to the model space being over-parameterised. Broadband magnetotelluric data inversion parameter settings used for WinGLink are: solve for the smoothest model regularization Laplacian: Uniform grid Laplacian operator regularization Order: Minimize integral of Laplacian (m) **2 weighting Function: Weighting enabled: Y, Horizontal alpha factor: 1.2, Beta factor in weighting: 3 for BBMT data weighting Function: Weighting enabled: Y, Horizontal alpha factor: 1.5, Beta factor in weighting: 2 for AMT data minimum block dimension (H): 500, Min. block dimension (V): 500 tau for smoothing factor= 15 static Shift: Compute Static Shift= Y, Static Shift Variance for constraint= 5, Static Shift Damping Factor= The maximum number of iterations is set to 300; inversion stopped when the normalised RMS no longer decreases. The observed lines and their corresponding projected profiles for 2D modelling for all BBMT and AMT survey lines are presented in the following sections. In the 2D model, the vertical axis shows depth in metres and the horizontal axis shows distance in metres. We have produced these models as a preliminary set to be used for future geological interpretation. More targeted inversions, using more detailed starting models, would be recommended for more reliable interpretation. As the data are projected onto a straight line (or straight line segments), we show only the locations of the start and end stations for each line to simplify the figures. Finally, all models include elevation, though it is not apparent for the BBMT inversions due to their much larger scale. For this reason, BBMT models are depicted as models with respect to depth, and AMT models are represented with respect to elevation (from the Australian Height Datum) D models GA1 Line GA1 was modelled as two profiles: a NW to SE profile, and an approximately N to S profile. The NW to SE profile (long profile) starts at site GA1-100 and ends at GA1-16, while the N to S profile 40 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

42 (short profile) starts at site GA1-21, and ends at GA1-02. Changes in modelled resistivity structure across the shared point between the two stitched lines as a result of applying a coarse mesh is shown clearly in Figure 3.2. The application of a fine mesh model (Figure 3.3) significantly reduces this boundary effect. Figure 3.2: A 2D coarse mesh model for BBMT line GA1.The long and short profiles are stitched at site GA1-16. The change in resistivity structure is obvious across the common site GA1-16 due to a coarse mesh. Figure 3.3: A 2D fine mesh model for BBMT line GA1. The long and short profiles are stitched at GA1-16. The change in resistivity structure across the common site GA1-16 is relatively smooth compared to the coarse mesh model shown in Figure GA2 Line GA2 was modelled as a single profile (Figure 3.4), running approximately SW-NE. A coarse model was not required due to the fewer number of sites acquired along this line (when compared with BBMT lines GA1 and GA3). The model mesh was slightly shallower than a total depth of 60 km, also due to there being fewer data along this line. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 41

43 Figure 3.4: A 2D fine mesh model from inversion of TE and TM data on BBMT line GA2. Line GA3 was modelled as a single traverse, running approximately north-south. A coarse model (Figure 3.5) was first produced in order to provide a good starting model for the fine detail inversion (Figure 3.6). Figure 3.5: A 2D coarse mesh model for BBMT line GA3. 42 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

44 Figure 3.6: A 2D fine mesh model for BBMT line GA3. The maximum depth for the model is metres GA5 Modelling for line GA5 was conducted on a WNW-SSE projection line; only a fine model was required because of the high station density along the line (Figure 3.7). The model results are generally two layers, with a conductive layer at the surface of variable depth and a resistive basement. A more conductive feature cuts the basement in the centre of the line between 24 km and 28 km. Figure 3.7: A 2D fine mesh model for AMT line GA5. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 43

45 3.2.4 GA6 & GA7a Lines GA6 and GA7a are modelled as both a single profile, due to multiple azimuth changes along the line, which is a multiple-section profile (Figures 3.8 and 3.9). The appearance of the resistivity structures in Figure 3.7 are compressed in the horizontal direction compared to the resistivity structures imaged in Figure 3.8 from the same data. This difference highlights the ambiguities that can be introduced when modelling data as single straight lines, as opposed to shorter multiple line segments, and highlights why modelling should be conducted along multiple 2D transects in regions where the acquisition line may deviate significantly from an ideal straight line. As lines GA6 and GA7a were acquired coincident with part of BBMT line GA1, we also compared the AMT (single line model) and the BBMT results (Figure 3.10). The AMT data are higher resolution than the BBMT data, and this closer station spacing better defines the basement-basin interface, which is interpreted to be the boundary between the upper red conductive layer and the lower yellow-green moderately conductive layer). The BBMT data, however, can penetrate deeper and better map the basement features underneath the moderately conductive basement. The AMT data can be considered reliable to around 1 km depth, as shown in Figure GA7b Line GA7b was modelled as a single west-east line and shows a consistent resistivity structure with cover imaged as low resistivity and the basement as higher resistivities (Figure 3.11). The depth of cover varies only slightly along this line GA8 Line GA8 was modelled as a single west-east profile, and shows a two-layer resistivity structure (Figure 3.12). There is more variation in the depth of cover imaged along this line when compared to line GA7b. 44 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

46 Figure 3.8: A 2D fine mesh model using a single west-east model for lines GA6 and GA7a. Figure 3.9: A 2D fine mesh model using multiple straight-line segment models for lines GA6 and GA7a. Note that the resistivity structure is less compressed horizontally when compared with the single line model (Figure 3.8), however, there are some artefacts from the segment joins along the line. Note also that the position of anomalies along the line is different compared to the single straight line due to the differing projection of stations onto the line. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 45

47 Figure 3.10: Comparison, to a depth of 1 km, of single line model for AMT data along GA6 and GA7a (upper) and BBMT data along line GA1 (lower). 46 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

48 Figure 3.11: A 2D fine mesh model for line GA7b. Figure 3.12: A 2D fine mesh model for line GA8. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 47

49 4 Conclusions Broadband magnetotelluric and AMT data were acquired as part of the Southern Thomson Project in two field campaigns in 2014 and 2015, with data acquired at approximately 600 sites. Analysis and modelling of these data indicates that: a) The BBMT was able to map the resistivity structure of the upper and lower crust successfully, including the gross architecture of the covering Eromanga Basin b) The BBMT data also defined a deeper regional east-west electrical strike which requires further integration with other geological data in the region c) The AMT data mapped the cover sequences in higher spatial resolution, and was able to penetrate at least 1 km beneath the surface of the Earth to map near-surface crustal features. These data will be integrated with other datasets including gravity data and airborne electromagnetic data (see Roach 2015) as part of the Southern Thomson Project, leading to understanding of the basin and crustal architecture of the southern Thomson Orogen (Folkes, 2016) and the nature of the boundary between the Thomson and Lachlan orogens. Future work on these data will concentrate on geologically constrained inversions of the MT data to better understand the detailed resistivity structure of the cover and the basement within the Southern Thomson Project. 4.1 Acknowledgements The authors acknowledge the traditional owners and landholders within the Southern Thomson Project region, without whose cooperation these data could not have been collected. The survey was planned with the help of colleagues from the State geological surveys including Rosemary Hegarty (GSNSW), and Janelle Simpson (GSQ). Ian Roach and Andrew McPherson are thanked for their thorough reviews of this Record. Liejun Wang and Jingming Duan were responsible for data QA/QC at Geoscience Australia. Data were analysed and modelled by Liejun Wang. This Record was written by Richard Chopping and Liejun Wang. This Record is published with the permission of the CEO, Geoscience Australia. 48 Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling

50 References Booker, J.R The magnetotelluric phase tensor: a critical review. Surveys in Geophysics 35(1), Bostick, F.X A simple almost exact method of MT analysis. Workshop on Electrical Methods in Geothermal Exploration. United States Geological Survey, Contract number Caldwell, T.G., Bibby, H.M. and Caldwell, C The magnetotelluric phase tensor. Geophysical Journal International 158, Geoscience Australia, Magnetic Map of Australia grid sixth edition 80m cell size. Digital dataset, Geoscience Australia, Canberra, Australia. Folkes, C Integrating gravity, seismic, AEM and MT data to investigate crustal architecture and cover thickness: modelling new geophysical data from the Southern Thomson region. ASEG-PESA 2016 Abstracts, Adelaide. Jones, A.G On the equivalence of the Niblett and Bostick transformations in the magnetotelluric method. Journal of Geophysics 53, Main, P.T. and Caritat, P. de, (2016). Southern Thomson Region Geochemical Survey, Southwestern Queensland and Northwestern New South Wales - The Chemical Composition of Surface and Near-Surface Catchment Outlet Sediments. Geoscience Australia Record, 2016/11, 136 pp. DOI: /Record Oldenburg, D. W., 1979, One-dimensional inversion of natural source magnetotelluric observations, Geophysics, 44, pp Raymond, O.L., Liu, S., Gallagher, R., Zhang, W. and Highet, L.M. (2012). Surface Geology of Australia 1:1 million scale dataset 2012 edition. Digital dataset, Geoscience Australia, Canberra, Australia. Roach, I. C. (editor) The Southern Thomson Orogen VTEMplus AEM survey: Using airborne electromagnetics as an UNCOVER application. Geoscience Australia, Canberra. Record 2015/29. Simpson, F. and Bahar, K Practical magnetotellurics. Cambridge University Press, Cambridge. 272p. Zonge, Thomson Prospect MT Survey Summary, October December Zonge Engineering and Research Organisation Pty Ltd. Unpublished report produced for Geoscience Australia. Zonge, Thomson Prospect AMT Survey Summary, January February 2015, Zonge Engineering and Research Organisation Pty Ltd. Unpublished report produced for Geoscience Australia. Southern Thomson Magnetotelluric (MT) Survey Report and Preliminary Data Modelling 49