SPECIAL TOPIC: LEADING GEOSCIENCES IN A NEW ERA Cover mapping with passive seismics at the Boulia Prospect, Mount Isa Province, Queensland, Australia Charles D. Beard1,2*, Nicholas Arndt1,2, Richard Lynch1 and Jamin Cristall3 demonstrate that the ambient-noise surface-wave tomographic method can accurately and inexpensively map the thickness of sedimentary cover that obscures potentially mineralised regions globally. Abstract To determine the thickness of sedimentary cover overlying Proterozoic basement, an ambient-noise surface-wave tomographic (ANSWT) survey was conducted in the Boulia region of the Mt Isa Province in Queensland, Australia. An array of 100 three-component seismic nodes was deployed along a 30 km section and ambient seismic noise was recorded for 19 days. In the resultant seismic model, the top-basement contact is resolved as a sharp, subhorizontal interface at a depth of about 700 m, where seismic velocity (Vs) increases downward from about 2500 to 3500 m/s. The accuracy of the retrieved top-basement contact was confirmed by comparison with drill hole intersections and with results from active seismic and magnetotelluric surveys. This study demonstrates that the ANSWT method can accurately and inexpensively map the thickness of sedimentary cover that obscures potentially mineralised regions globally. Introduction Near-surface mineral deposits are becoming exhausted and mining companies are now required to explore deeper (e.g., Schodde 2017), commonly below surficial layers of younger sedimentary rocks, alluvium, glacial deposits or laterites in tropical regions (e.g., Blewett 2013; Arndt et al. 2017; Gonzalez-Alvarez et al. 2020). Superficial sediments cover a very large fraction of potentially mineralised zones, obscuring an estimated 80% of the Australian continent. To open up these areas for mineral exploration, the Australian government has supported the UNCOVER programme which supports the development of new technologies for exploring beneath covered areas in Australia and similar programmes are underway in Canada and other countries. While geochemical methods can provide some indications of the presence of ore bodies buried beneath pre-mineral cover by tracing the aureoles that overlie certain types of mineralization (e.g., Guffey et al. 2018), geophysical methods provide the only reliable approach to determine the thickness and architecture of the cover and to image mineralised zones beneath it. Inductive-source electromagnetic and magnetotelluric soundings have limited ability to resolve the base of conductive cover while 1 Sisprobe SAS | * Corresponding author, E-mail: cdb53@cam.ac.uk 2 depth estimates from gravity and magnetic data are ambiguous. Active-source reflection seismic surveys have a long and successful history in hydrocarbon exploration (e.g., Hanssen 2011; Polychronopoulou 2018) and have been used in several regions for the exploration for mineral deposits (e.g., Salisbury and Snyder 2007; Malehmir et al. 2012) but they are not optimised for determination of absolute cover thickness and are currently too expensive to be applied widely in this industry. Ambient-noise surface-wave tomography Passive seismic methods, which use ambient sources of seismic vibrations, including ocean swells, wind in trees or traffic, were developed as a research tool to investigate the structure of the crust and mantle. (e.g., Shapiro and Campillo 2004; Sabra et al. 2005; Campillo 2006; Brenguier et al. 2007). These methods do not require the deployment of sources for seismic waves, such as explosives or vibroseis trucks and therefore have a lower environmental impact, are easier to deploy, and cost much less than active seismic surveys (Ramm et al., 2019; Dales et al., 2020). Microseismic events can be used as sources for body wave tomography (Nakata and Nishida, 2019), a technique that is widely used in monitoring volcanos and faults (Vanorio et al., 2005; Mitchell et al., 2013; Brenguier et al., 2019; Hillers et al., 2019), and in civil engineering applications. This approach is rarely suitable for mineral exploration, however, because image quality depends on the magnitude and distribution of earthquakes, and many exploration sites are in regions with little seismic activity. The passive seismic approach has also been used in the oil and gas industry, commonly as a complement to active seismic surveys (e.g., Chmiel et al., 2019), but it has been taken up by mineral exploration companies only recently and tentatively. Some notable examples in which passive seismic methods have been used in mining or mineral exploration are Olivier et al. (2015); Chamarczuk et al. (2018); Ramm et al. (2019); Dales et al. (2020); Afonin et al. (2021); Gil et al. (2021). Ambient-noise surface-wave tomography (ANSWT) is a passive seismic method that is relatively new to mineral exploration. In this method, correlations of seismic ambient noise recorded Université Grenoble Alpes, Université Savoie Mont Blanc | 3 Anglo American, Australia DOI: xxx FIRST BREAK I VOLUME 40 I JUNE 2022 1 SPECIAL TOPIC: LEADING GEOSCIENCES IN A NEW ERA at two stations (sensors) are used to reconstruct the response of the subsurface, as if one of the receivers were acting as a source and the other one as a receiver. Surface waves are retrieved from seismic noise, and they are used to compute tomography using a two-step method described by Mordret et al. (2013) for travel time tomography, and by Sambridge (1999) and Mordret et al. (2014) for depth inversion. Technical details of the methods, with specific reference to the mapping of cover thickness, will be given in Lavoué et al. (in prep). Velocity models prepared using surface wave ambient noise methods have a relatively poor spatial resolution that limits their ability to directly image ore bodies (Chamarczuk et al., 2018; Ramm et al., 2019; Afonin et al., 2021; Xu et al., 2021). With the exception of large massive sulphide deposits or iron ore bodies, the dimensions of the mineralised zone are usually too small or the contrast between the seismic velocity of the mineralisation and surrounding rocks is usually too low. The method can, however, be used to provide valuable information about the geological setting and the structures that control the mineralisation (e.g., Xu et al., 2021), and, as is demonstrated in this paper, it is a valuable tool for mapping the thickness and characteristics of post-mineralization sedimentary cover. Here we describe how ANSWT has been used to map sedimentary cover at the Boulia exploration site in Queensland, Australia. The Mount Isa Province and Boulia Prospect The Mount Isa Province in Queensland, Australia, contains several world-class, sediment-hosted sulphide deposits including Mount Isa (Forrestal, 1990; Lilly et al., 2017). This enormous, high-grade deposit has been a major producer of Cu as well as Pb, Zn and Ag for over a century. Other more recently discovered deposits, including Century and Hilton, are major sources of Pb, Zn and Ag (Large et al., 2005). The mineralisation is hosted in variably metamorphosed Proterozoic shales and is believed to have formed during the interaction of deep-sourced basinal fluids with near-surface sedimentary rocks. The Boulia prospect, located in the south western part of the Province, is the site of active exploration by several companies (Figure 1). In this region, the mineralised Proterozoic rocks are covered by thick sequences of Cambrian to Cretaceous sedimentary rocks of the Georgina and Eromanga Basins. The Eromanga Formation consists of Jurassic–Cretaceous sandstone, mudstone, and siltstone (thickness ∼ 100 m) and crops out mainly in the eastern part of the study area. Contact relationships indicate that the base of the Eromanga Formation dips at a shallow angle to the east. Limestones of the underlying Georgina Formation (thickness ~ 600 m) crop out mainly to the west. Geological mapping provides little information about the base of the Georgina Formation nor about the depth and geometry of the contact with the Proterozoic basement. The poorly consolidated and weathered Quaternary and Tertiary units can be expected to have the slowest seismic velocities, followed by intermediate velocities in most of the Cretaceous to Cambrian units. A sharp downward increase in velocity should mark the contact between the sedimentary units and the seismically faster metamorphosed rocks of the basement; this change in velocity will be used to map the depth of sedimentary cover. Existing data sets for the Boulia prospect Several types of geological and geophysical data for the Boulia region have been collected by private companies and government geological surveys, and are available for download from the Geological Survey of Queensland Open Data Portal (https:// geoscience.data.qld.gov.au/). Knowledge of the geology of the Figure 1 a) A geological map of the southern portion of the Mount Isa metallogenic province showing geophysical surveys at the Boulia prospect, the Proterozoic Mount Isa block, and the sedimentary cover sequence. b) A simplified stratigraphic section. The Proterozoic metasedimentary rocks that may contain mineralization are covered by Cambrian–Ordovician Georgina limestone (thickness ~600 m) and Jurassic–Cretaceous sedimentary rocks of the Eromanga basin (thickness ~100 m map is after Mond et al. (1977) and uses the Lambert conformal conic projection). The stratigraphic section is after Casey et al. (1960). 2 FIRST BREAK I VOLUME 40 I JUNE 2022 SPECIAL TOPIC: LEADING GEOSCIENCES IN A NEW ERA Figure 2 Airphoto of the Boulia Prospect showing the location of passive seismic nodes, the 3D passive seismic Vs model, 2D active-source reflection seismic profiles, magnetotelluric profiles and three exploration drill holes. Boulia region is built on mapping and interpretation of drill-hole data by Green et al. (1963); Kress and Simeone (1993); Withnall et al. (2013); Cook et al. (2013). Figure 1, a geological map of the Boulia region, is extracted from the Detailed Digital Surface Geology dataset provided by the Geological Survey of Queensland https://www.data.qld.gov.au/dataset/queensland-geology-series. Two of three exploration drillholes in the area penetrate the sedimentary cover and intersected basement. Geoscience Australia conducted a deep crustal active-source seismic survey over the Boulia region in 2014 and 2015. Data were recorded along three traverses, 14GA-CF2 (369 km), 14GA-CF3 (339 km) and 15GA-CF3 (140 km) with the aim of determining the architecture of Proterozoic basement and the depth of sediment cover. Links to the raw data and interpretations are provided by Fomin and Costelloe (2015). The 2D active seismic profiles were reprocessed and reinterpreted by HiSeis (2018). That study focused mainly on structures within the Proterozoic basement and identified complex folding and faulting and a possible mafic intrusion. The survey also identified a strong, subhorizontal reflector, shown in Figure 3, that is interpreted to mark the base of sedimentary cover. Prestack time-migrated sections from the active seismic surveys are shown in Figure 3, with the depth conversion having been computed using the cover–basement contact from a single exploration drillhole (MJRD0001). For this reason, the depth of cover in the pre-stack time-migrated sections might be biased toward shallower or greater depths, depending on lateral variations in seismic velocity within the cover sequence. Additional information on cover thickness is provided by Frogtech Geoscience (2018), which used data from the Geoscience Australia Australian National Gravity Database and satellite data from Sandwell et al. (2014) to interpret basement structure and the thickness of sedimentary cover. They also summarise data and interpretations from Milligan et al. (2009, 2010) who compiled and interpreted data from airborne magnetic surveys. An airborne EM survey (approximately 1200 line km) was also conducted in the survey area. Simpson and Heinson (2020) describe a magnetotelluric (MT) survey used to model the depth of the Eromanga and Georgina basins. The survey collected data from 809 broadband MT and 855 audiomagnetotelluric stations, which were distributed along E-W transects to the north of, and overlapping with the active seismic lines, as shown in Figure 1. Downhole resistivity logs from three petroleum exploration wells near the study area were used to generate 1D resistivity models which provided a basis for the 2D interpretation shown in Figure 4. The passive seismic survey The aim of this survey was to determine whether the ANSWT passive seismic technique could resolve the thickness and internal characteristics of the sedimentary units that overly the potentially mineralised basement rocks of the Mount Isa Province (Figure 1). An array of 100 three-component seismic nodes was deployed along a ∼30 km section of road, the same as that followed by the active seismic survey line 14GA-CF3 (Figures 1 and 2). This road runs subparallel to the E-W striking magnetotelluric profiles. The nominal inter-station distance was 300 m with a minimum of 250 m. Ambient seismic noise was recorded by 99 receivers for 19 days from 27/11/2019 to 13/12/2019 (one node was not operational during the acquisition). Frequencies from 0.4–2.5 Hz were used to compute a 3D shear wave velocity model following procedures described by Mordret et al. (2013). The spectral content of the seismic records indicate variations in the quality of data between stations. Overall the receivers recorded low-quality seismic noise, as is to be expected for a location in the middle of a continent and far from centres of human activity. Even though the quality of ambient seismic noise in this isolated region is relatively poor, the results described FIRST BREAK I VOLUME 40 I JUNE 2022 3 SPECIAL TOPIC: LEADING GEOSCIENCES IN A NEW ERA below demonstrate that this noise was adequate to achieve the aims of the project. The survey area was overlain with a 2D raster onto which depth profiles were fitted (Figure 2). At each point, a dispersion curve was determined from the seismic records via a frequency-dependent travel-time tomography analysis. Modelled 1D velocity depth profiles were then generated by fitting depth and velocity parameters to describe the observed dispersion curves following the methodology of Sambridge (1999); Mordret et al. (2014). Three fitted parameters are depths of interfaces separating layers of constant shear-wave velocity. Four additional parameters are the shear-wave seismic velocities of these layers (Table 1). A total of 24,000 models were generated. The final model presented is the average of the 300 models with the lowest misfits, with misfit being defined as the difference in velocity between the modelled 1D depth profile and the observed local dispersion curve. This Monte-Carlo technique also allows estimation of model uncertainties with depth, via the standard deviation of the distribution of the 300 selected velocity models. Velocity was interpolated horizontally between the 1D depth profiles to obtain the final 3D velocity model. 4.1. Results The passive seismic model covers a wedge-shaped area 32 × 4 km to a depth of 2 km below present ground surface (Figure 1, 2). In this survey, the top 200 m was not effectively resolved due Parameters Units Min Max V0 m/s 1500 4000 V1 m/s 1500 4000 V2 m/s 2000 4000 V3 m/s 2000 4500 D0 m 0 400 D1 m 400 1000 D2 m 1000 2500 Table 1 Parameterisation of the local 1D Vs models. to a coarse inter-station spacing (nominally 300 m) and a lack of available high-frequency ambient noise during the survey period. From 200 m to 600 m depth, the shear-wave velocity is between 2200 and 3000 m/s with strong lateral and vertical variations. Below 600–700 m velocities are faster, between 3000 and 3900 m/s. The model is well-resolved from about 200 to 1400 m depth, and with a lateral resolution ranging from 500 to 1000 m. Uncertainties on velocities are less than 10%. Misfits increase toward the eastern part of the survey area where the 1D depth-profile models fit the observed dispersion curves less well. Figure 3 shows a vertical cross-section through the ANSWT model in the western part of the survey area where it coincides Figure 3 Vertical cross-sections showing the ANSWT passive seismic model, the 2D active seismic pre-stack time migration model (greyscale, HiSeis 2018) and in the middle panel, the two techniques superimposed. Inset shows top-basement position in drillhole MSD-003, compared with the ANSWT model and the active seismic profile. 4 FIRST BREAK I VOLUME 40 I JUNE 2022 SPECIAL TOPIC: LEADING GEOSCIENCES IN A NEW ERA Figure 4 A vertical cross-section through the Boulia Prospect showing (a) our ANSWT passive seismic velocity model, and (b) 2D magnetotelluric profile MT-04 from Simpson and Heinson (2020), with isovelocity surfaces from our ANSWT model overlain. Bm-CP is the top basement contact interpreted from the inversion of the MT data. with 2D active-source seismic survey line 14GA-CF3 and magnetotelluric line MT-04 (see locations in Figure 1). In this area, S-wave velocities in the sedimentary cover are relatively constant, varying from about 2000 to 3000 m/s. At a depth of around 700 m, the model velocities increase sharply with increasing depth to 3200 to 3600 m/s. Discussion Interpretation of 3D velocity model Shear-wave seismic velocities for the sedimentary rocks of the Eromanga and Georgina Formations are expected to be about 1500–2500 m/s for sandstone and 2200–3000 m/s for limestone, based a global compilation of petrophysical studies (Ji et al., 2002). These velocities are significantly slower than the 3200–3800 m/s expected for the metamorphosed basement rocks. Across most of the survey area, the S-wave velocity models resolve a sharp downward increase in shear wave velocity at a depth of 700 m. Based on the expected petrophysical parameters and the sharp downward increases in velocity, we interpret the top-basement contact as a Vs isovelocity value of 3000 m/s. In the section shown in Figure 3, the 3000 m/s contact is at a depth of about 700 m and is relatively flat lying. Uncertainty on our model velocity determinations is less than 10 %, corresponding to a basement depth precision of about ± 70 m. The contact between the Eromanga Formation and underlying Georgina Formation within the cover is not resolved in this ANSWT model (Figures 3 and 4). This could be due to the proximity of this contact to the ground surface, and the relative insensitivity of this particular array configuration at depths of less than 200 m. Alternatively, there may be insufficient contrast in the S-wave velocity between the two cover formations, limiting the ability of ANSWT to image the interface between them. Comparison with existing exploration data sets The collar of drill hole MSD-003 is 500 m to the north of the magnetotelluric profile shown in Figure 4, and is located in the NW of our survey area (Figure 2). Considering the shallow dip of the undeformed sedimentary formations at the Boulia Prospect, it provides an independent test for the depth for the top-basement contact resolved by our passive seismic model (Figure 3). The top-basement contact in this drill hole is at a true vertical depth of 740 m (measured depth = 789 m) and is in close agreement with the 3000 m/s isovelocity surface in our passive seismic Vs model. The 2D reflection active seismic profile 14GA-CF3 was processed using velocity information from drill hole MJRD0001, which is in the NE of our survey area (Figure 2), and its collar is FIRST BREAK I VOLUME 40 I JUNE 2022 5 SPECIAL TOPIC: LEADING GEOSCIENCES IN A NEW ERA located 900 m south of the active seismic profile. HiSeis (2018) converted reflection time to depth and the generated a pre-stack time-migrated reflection profile from the active survey data (Figure 3). As a result, their top-basement contact is fixed to the depth of a prominent subhorizontal reflector at the site of the drill hole. Lateral variations in the velocity structure of the Eromanga and Georgina Formations (both on and off-profile) have potential to introduce undetectable bias to these profiles, deepening or shallowing the resolved ‘top basement’ prominent reflector relative to reality. At the position of drill hole MSD-003, the active seismic profile resolves a prominent subhorizontal reflector 100 m deeper than the 3000 m/s isovelocity surface in our ANSWT model (Figure 3). The generally good agreement in position and shape between this active-seismic reflector and the 3000 m/s passive seismic isosurface suggests that lateral variations in cover velocity are small, with faster cover in the W, close to hole MSD-003 relative to near hole MJRD0001 in the E. Active-source reflection seismic sursurveys have several disadvantages. They have the potential to include undetectable bias in the depth of reflected interfaces, may be influenced by off-profile geology and are associated with a high environmental and economic cost. They are therefore poorly suited for the determination of absolute cover thickness during province-scale exploration campaigns. The magnetotelluric survey was conducted along a series of E-W profiles that partially overlap with the area covered by the active and passive seismic surveys (Simpson and Heinson, 2020, Figures 1 & 2). Profile MT-04 resolves the Eromanga sediments as a relatively thin layer (≤ 100 m) near the ground surface (Figure 4). It shows that the contact with the underlying Georgina Formation dips to the East, and that the Eromanga Formation appears to become thinner toward the west, in agreement with the geological map which shows that the younger Eromanga rocks crop out mainly in the eastern part of the survey area (Figure 1). The basement top is less well-defined, interpretation being complicated by a conductive stratum in the Georgina Formation (Simpson and Heinson, 2020). In contrast to the Eromanga–Georgina contact, the basement top resolved by MT dips at a shallow angle (~5°) to the west. This is broadly consistent with regional geological mapping, which shows that the Georgina Basin becomes deeper to the northwest (Figure 1, Casey et al. 1960; Mond et al. 1977). The presence of highly-conductive strata in the cover sequence, highlights the need to incorporate local petrophysical information when using magnetotelluric data to determine cover thickness. Much like the active-source seismic profiles, 2D magnetotelluric surveys are additionally subject to bias introduced by off-profile variations in geology. The close agreement between the results from our ANSWT test survey, drill logs and the existing geophysical data confirms the value of passive seismic surveys as a means of mapping the depth and geometry of cover--basement contacts during province-scale exploration. The precision of our ANSWT model at ~700 m depth is better than that of the MT technique (±70 m cf. ±250 m, Simpson and Heinson 2020). Unlike active-seismic profiles, the ANSWT model fitting procedure accounts for lateral changes in the velocity of the cover sequence. Our method 6 FIRST BREAK I VOLUME 40 I JUNE 2022 produces 3D velocity models that are not sensitive to off-axis geological variations, as is the case with all geophysical surveys measured along 2D profiles. Prior knowledge of the petrophysics of local rocks is generally not critical for mapping cover thickness because strong seismic velocity contrasts usually exist between fast crystalline basement rocks and slow sedimentary rocks or unconsolidated sediments. The resolution of ANSWT models can be greatly improved by the use of 2D grids of seismic nodes, rather than line profiles. This is especially the case in survey areas with stronger and higher-frequency sources of ambient seismic vibrations than were present at the Boulia Prospect. Where to apply ANSWT in mineral exploration? The main advantages of ANSWT derive from its relatively low cost and minimal environmental impact. The seismic nodes are simple to deploy and do not require the preparation of tracks nor the cutting of grid lines. Although their location must be recorded accurately, there is considerable flexibility as to where they are placed, which means that local unfavourable locations (outcrops, rivers, swamps, buildings) can be avoided. Because of the low environmental impact, ANSWT can often be carried out in sensitive areas where other surveys are prohibited. The method also produces 3D models and basement depth results that are not biased by complications such as the presence of conductive sedimentary units. The main disadvantage is the relatively low spatial resolution, both vertical and horizontal, especially when compared to active-source geophysical methods. Due to its resolution and cost, ANSWT is best applied during exploration at the province to district scale (100 km to 10 km lateral) to map in 3D geological features on the order of kilometres to hundreds of meters in size. In special circumstances it is possible to design surveys for finer-scale investigation, though this requires the presence of higher-frequency ambient noise, and application to features relatively close to the surface. ANSWT is sensitive only where there are significant variations in the S-wave velocity of adjacent rocks. As with all geophysical surveys, local petrophysical measurements can be helpful during both survey design and for the fitting and interpretation of the seismic models. Summary Ambient noise surface wave tomography is a promising technique for mapping the thickness and internal characteristics of sedimentary cover sequences. This survey demonstrates that the method can accurately and inexpensively map sedimentary cover overlying prospective regions. The precision with which the basal contact can be mapped is better than that obtained using magnetotelluric profiles and far less expensive than activesource seismic surveys. As with other techniques, interpretation of the generated 3D geophysical models must be done with an understanding of the geological context and the petrophysical properties of the local lithologies. Acknowledgements The authors wish to thank Anglo American for funding the passive seismic survey, for field logistical support at the Boulia site and for permission to publish. We thank members of Sisprobe, especially Anaïs Lavoué for conducting the field work and SPECIAL TOPIC: LEADING GEOSCIENCES IN A NEW ERA generating the 3D passive seismic models and Sophie Beauprêtre for comments on an earlier version of this manuscript. Lindskog, L., Spicer, B., Carbonell, R., Orlowsky, D., Carriedo, J. and Hagerud, A. [2021]. Reflection seismic imaging to unravel subsurface geological structures of the Zinkgruvan mining area, References Afonin, N., Kozlovskaya, E., Heinonen, S. and Buske, S. [2021]. Nearsur- central Sweden. Ore Geology Reviews 137, 104306, doi:https://doi. org/10.1016/j. oregeorev.2021.104306. face structure of the Sodankylä area in Finland, obtained by passive Gonzalez-Alvarez, I., Goncalves, M.A. and Carranza, E.J.M. [2020]. seismic interferometry. 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