Publications – On Journals
Smart Exploration: from legacy data to state-of-the-art data acquisition and imaging
First Break, Vol. 37, N. 8, August 2019
During the last decade, and possibly in years to come, miner-al exploration geophysics has strongly pushed itself towards developing new instruments and hardware solutions capable of addressing the ever challenging near-mine and brownfield explo-ration issues. New data will be acquired but higher noise levels and restricted access due to mining activities and infrastructure increase the challenge of acquiring data of sufficient quality to answer key geologic questions and define additional resources. Companies that value their existing data and reassess them rigorously and continuously are likely to benefit. However, new generations of geophysicists and mineral explorationists tend to prefer modern data from new instruments than the so-called ‘legacy data’. Legacy data by definition are those that have been acquired in the past, but their revival requires a significant amount of time and money, and still it may not always be possible to yield rewarding results. These data often suffer from bad documentation, inaccurate co-ordinates, and are stored on devices (e.g., tapes or hard copies) that makes it difficult to access or they may have partly been corrupted. However, legacy data are still valuable, especially if they are from brownfield or near-mine exploration sites and can be revived and reworked. Legacy data have the advantage of: • being less contaminated by noise from mining activities and infrastructures, • being acquired in places that now might be inaccessible (i.e., logistical challenges), • being cheaper to reprocess and reinterpret than to collect new data as data acquisition often makes up the bulk of a survey’s cost. Legacy data can also provide a first assessment if new and advanced data acquisition would lead to new knowledge and discoveries. Smart Exploration, an H2020-funded project involving 27 partners from nine European countries and six exploration sites was launched in late 2017. Its goal is to address some of these challenges concerning legacy data, to develop new geophysical instruments for a wide range of applications and to generate new targets at the explo-ration sites while training new generations of young professionals for these purposes. Here, we focus on re-evaluating the potential of some of the legacy data available from the exploration sites and together with new instruments developed during the project present how in-mine and brownfield mineral exploration can advance utilizing existing mining infrastructure. For example, we show the importance of legacy data from a 1996 dataset enabling imaging of a world-class +150 Mt massive sulphide deposit at depth as well as another case study showing that additional iron-oxide resources may be present downdip and under the known mineralized bodies. The development of a GPS-time system and an E-vibrator helped to then acquire a semi-3D tunnel-surface seismic dataset utilizing exploration tunnels in the Neves-Corvo mine.
The paper is available here.
Giving the legacy seismic data the attention they deserve
First Break, Vol. 37, N. 8, August 2019
Key minerals may soon be in short supply as shallow mineral deposits are mined-out; therefore exploration for economically feasible deep-seated deposits to sustain a long-term global growth is a great challenge. New deposits are likely to be found using reflection seismic surveys in combination with drilling, field geological mapping and other geophysical methods. Seismic methods have already have contributed significantly to the discovery of some of the world’s major mineral deposits (Milkereit et al., 1996; Pretorius et al., 2000; Trickett et al., 2004; Malehmir and Bellefleur, 2009; Malehmir et al., 2012). However, use of the method is not widespread because it is deemed to be expensive. Although improvements in computing capabilities have led to cost reductions, the costs are still beyond exploration budgets of many companies. Thus, mining companies have had little financial ability to acquire new reflection seismic data, and very little governmental support has been available to acquire research seismic surveys for mineral exploration. Over the last few years, there has been a proliferation of seismic solutions that employ various combinations of equip-ment, acquisition, and processing techniques, which can be applied in hard rock situations to improve the imaging resolution (Denis et al., 2013). The best acquisition solutions to date have come from the deployment of high-density receiver and source arrays which the extension of the seismic bandwidth to six octaves using broadband sources (Duval, 2012). Another area of seismic research has focused on surface seismic acquisition using three-component (3C) microelectro-mechanical (MEMS-based) seismic landstreamers (Brodic et al., 2015), coupled with wireless seismic recorders, and surface-tunnel-seismic surveys (Brodic et al., 2017). However, numerous difficulties have been encountered, even with these innovative acquisition seismic approaches. Seismic surveys acquired in the mining regions suffer from noise produced by the drilling, blasting and transport of rock and the crushing of ore. Furthermore, in some mining regions the acquisition of new data is not permitted due to new environmental regulations. In such a fast evolving seismic technological era, legacy reflection seismic data are often regarded by mining companies and geoscientists as inferior compared with the newly acquired data. This paper demonstrates that if the legacy data are properly retrieved, reprocessed, and interpreted using today’s standard techniques, they can be of significant value, particularly in the mining regions where no other data are available or the acqui-sition of new data is difficult and expensive. The development of multitudes of processing algorithms and seismic attributes, in particular, make it worthwhile to reprocess and interpret legacy data to enhance the detection of steeply dipping structures and geological features below the conventional seismic resolution limits (i.e., a quarter of the dominant wavelength), which was not possible with the tools that were available when the data were originally acquired and processed. The new information obtained from the legacy data may benefit future mine planning operations by discovering new ore deposits, providing a better estimation of the resources and information that will help to site and sink future shafts. Thus, any future mineral exploration project could also take the geological information obtained from the reprocessed and interpreted legacy seismic data into account when planning new advanced seismic surveys (Manzi et al., 2018). The latest seismic algorithms are particularly interesting to South Africa’s deep mining industry because South Africa has the world’s largest hard rock seismic database, which could benefit from new processing techniques and attributes analyses. These techniques could be applied to legacy seismic data to identify areas of interest, improve structural resolution and to locate deeper ore deposits. Seismic attributes, in particular, could be used to identify any subtle geological structures crosscutting these deposits ahead of the mining face that could affect mine planning and safety.
The full paper is available for reading and download here.
The role of land gravity data in the Neves- Corvo mine discovery and its use in present-day exploration and new target generation
First Break, Vol. 37, N. 8, August 2019
Several blind massive sulphide deposits associated with the Iberian Pyrite Belt (IPB) Volcano-Sedimentary Complex (VSC) (Figure 1) were discovered in SW Iberia using joint interpretation of geo-logical and geophysical models, such as Neves-Corvo (Albouy et al., 1981; Leca et al., 1983) and Lagoa Salgada (Oliveira et al., 1998) in Portugal, and Valverde and Las Cruces in Spain. In the IPB Portuguese sector, the former government agencies Serviço de Fomento Mineiro (SFM) and Instituto Geológico e Mineiro (IGM), as well as LNEG, fostered the acquisition of systematic geophysical surveys, in particular gravimetry, in the region during the second half of the 20th century. Since the 1960s, the former SFM carried out detailed ground surveys over N-S for E-W grids with distances between survey stations of 200, 100 and 50 m grid size (Oliveira et al., 1998). This enabled the identification of several potential targets which attracted the interest of important international investors and led to a continuous investment in geophysical research, based on ground and airborne surveys (Matos et al., 2019). The discovery of several massive sulphide deposits, includ-ing the world-classNeves-Corvo Cu-Zn-Sn deposit in 1977 (see location in Figure 1, Albouy et al., 1981; Carvalho et al., 1996; Carvalho et al., 1999; Oliveira et al., 2013) was a direct result of joint efforts of mining companies (a consortium formed by the Soc. Mineira e Metalúrgica de Peñarroya Portuguesa, Soc. Mineira de Santiago/Emp. Mineira e Metalúrgica do Alentejo and Societé d’Études de Recherches et d’Exploitations Minières), and former SFM exploration surveys (Albouy et al., 1981; Leca et al., 1983; Carvalho et al., 1999; Matos et al., 2019). The consortium invested significantly in exploration (Albouy, et al., 1981), namely: processing of the SFM-acquired gravity data (covering an area of 300 km2), collection of new gravity surveys (190 km2), more than 200 km of electric resistivity and magnetic profiles, as well as very-low-frequency (VLF) studies on several drill holes. The Neves-Corvo deposits occur along a NW trend, with seven deposits dispersed in a large complex antiform structure (Carvalho et al., 1996; Araújo and Castelo Branco, 2010; Oliveira et al., 2013). Understanding the geometry of subsurface orebodies requires accurate geological mapping based on surface surveys and/or borehole logging (Matos et al., 2019). The accuracy of the conceived model for geology and ore deposit at Neves Corvo, however, is limited by the sparse geologic outcrops and a biased distribution of drill holes. This has driven an impetus towards acquisition of geophysical data which can be more uniformly sampled and is typically less expensive to collect. Among the different geophysical methods employed, a strong response is expressed within the gravity data. This is the result of the large physical property contrast between high density, massive sulphide deposits and the volcano-sedimentary host rock lithologies (Neves, Corvo, Graça and Zambujal). The uppermost lenses are located in the NE flank of a gently dipping structure (10º-40º NE), at depths between 230 m (Corvo) and 350 m (Neves) (Albouy et al., 1981; Carvalho et al. 1996; Matos et al., 2019). These were therefore the first four orebodies to be discovered, while the gravitational response of the deeper Neves-Corvo mas-sive sulphide lenses were weaker and more difficult to recognize. With less obvious gravitational anomalies to guide exploration at such depths, rock density studies become a key issue in the geophysical characterization of Neves Corvo. In the case of the Semblana deposit (2010, ~800 m depth), ground electromagnetic surveys and extrapolation of favourable geology down dip from the Zambujal area were utilized for exploration in addition to the gravity data (Araujo and Castelo Branco, 2010). The use of gravity for direct detection of massive sulphides in the IPB has limitations. In areas covered by thick Flysch sediments (locally >1 km) where the VSC occurs at greater depths, the gravitational response is weak and more so a function of regional geologic elements. Localized variations in density, such as those caused by high density basic rocks or black shales with dissem-inated pyrite, are common within the VSC sequences. Intense rock weathering, low density siliceous shales or volcanogenic sandstones also contribute to a complex and multi-layered gravity profile. The elevated copper grades of the Neves-Corvo deposits justified more investment in exploration. The possibility of new discoveries with high metal content warranted an extension of exploration research, to explore deeper structures in the area (>>500 m depth). Considerable efforts were exerted in areas such as the Neves-Corvo-Corte Gafo, a 600 km2 polygon located NE of the mine site (Carvalho et al., 1996; Matos et al., 2019). With technical support of the former SFM gravity team, Somincor/Lundin implemented a multidisciplinary programme of gravity and magnetic surveys (9015 points covering an area of 314.5 km2, as well several profiles of transient electromagnetics (TEM, 215.5 km), magne-totellurics (27.0 km) and reflection seismic data (24.0 km). At a regional scale, a multitude of geophysical methods were deployed to characterize specific exploration targets throughout the IPB. These included: deep seismic reflection, electrical resistivity induced polarization, electromagnetic EM 37, pulse electromagnetic, transient electromagnetic, vertical transient elec-tromagnetic, vertical electrical soundings and magnetotellurics. In structurally complex zones, such as the Semblana area (Araujo and Castelo Branco, 2010), down-hole electromagnetic surveys were essential in the identification and delineation of the primary mineralized trends. In the Neves-Corvo region, seismic profiles were used by Lundin/Somincor to define key tectonic structures (Araújo and Castelo Branco, 2010; Inverno et al., 2015; Matos et al., 2019).
The paper is available for reading and download here.
Introduction to the special issue on “Cost-effective and innovative mineral exploration solutions"
Malehmir, A., Manzi, M., Draganov, D., Weckmann, U., Auken, E.
Geophysical Prospecting, Vol. 68, No 1, January 2020, pp. 3 – 6
The mineral exploration industry is again in a boom to provide new resources of critical raw materials as well as base and precious metals. This is evident from the globally increased expenditure reported for 2018 (International Mining 2018). Global demand towards green technologies requires sustainable flow of primary raw materials into the so-called whole value chain. Without exploring in new areas (greenfields), the chance of finding giant deposits or providing fresh resources to the market and sustaining the value-chain loop would be minimal. However, it is convenient in the short term to explore in brownfield areas and near mines to make use of existing infrastructures and to avoid new environmental footprints. In such circumstances, the mineral exploration industry is further challenged to not only provide high-quality and highresolution deep-targeting solutions but also to do it in a costand environmentally effective way. This is particularly significant in the European Union (EU) and in regions with thick cover, requiring new and much more sensitive exploration technologies for direct deposit targeting and geological characterization. In addressing these, there are several research and innovation initiatives worldwide trying to tackle some of these challenges like, to list a few, Smart Exploration in the EU and Metal Earth in Canada. There are also a number of cooperative research centre (CRC) projects in Australia as well as in Africa where favourable geology exists but exploration is challenged to find new “tier 1” deposits at depth (+500 m).
The full paper is available here.
Deep reflection seismic imaging of iron-oxide deposits in the Ludvika mining area of central Sweden
Markovic, M., Maries, G., Malehmir, A., von Ketelhodt, J., Bäckström, E., Schön, M., Mardsen, P.
Geophysical Prospecting, Vol. 68, No.1, January 2020, pp. 7 – 33
Reflection seismic data were acquired within two field campaigns in the Bl ¨ otberget, Ludvikamining area of central Sweden, for deep imaging of iron-oxide mineralization that were known to extend down to 800–850 m depth. The two surveys conducted in years 2015 and 2016, one employing a seismic landstreamer and geophones connected to wireless recorders, and another one using cabled geophones and wireless recorders, aimed to delineate the geometry and depth extent of the iron-oxide mineralization for when mining commences in the area. Even with minimal and conventional processing approaches, the merged datasets provide encouraging information about the depth continuation of the mineralized horizons and the geological setting of the study area. Multiple sets of strong reflections represent a possible continuation of the known deposits that extend approximately 300 m further down-dip than the known 850 m depth obtained from historical drilling. They show excellent correlation in shape and strength with those of the Bl ¨ otberget deposits. Furthermore, several reflections in the footwall of the known mineralization can potentially be additional resources underlying the known ones. The results from these seismic surveys are encouraging for mineral exploration purposes given the good quality of the final section and fast seismic surveys employing a simple cost-effective and easily available impact-type seismic source.
The paper is available here.
Seismic depth imaging of iron-oxide deposits and their host rocks in the Ludvika mining area of central Sweden
Bräunig, L., Buske, S., Malehmir, A., Bäckström, E., Schön, Mardsen, P.
Geophysical Prospecting, Vol. 68, No 1, January 2020, pp. 24 – 43
The development of cost-effective and environmentally acceptable geophysical methods for the exploration of mineral resources is a challenging task. Seismic methods have the potential to delineate the mineral deposits at greater depths with sufficiently high resolution. In hardrock environments, which typically host the majority of metallic mineral deposits, seismic depth-imaging workflows are challenged by steeply dipping structures, strong heterogeneity and the related wavefield scattering in the overburden as well as the often limited signal-to-noise ratio of the acquired data. In this study, we have developed a workflow for imaging a major iron-oxide deposit at its accurate position in depth domain while simultaneously characterizing the near-surface glacial overburden including surrounding structures like crossing faults at high resolution. Our workflow has successfully been showcased on a 2D surface seismic legacy data set from the Ludvika mining area in central Sweden acquired in 2016. We applied focusing prestack depth-imaging techniques to obtain a clear and well-resolved image of the mineralization down to over 1000 m depth. In order to account for the shallow low-velocity layer within the depth-imaging algorithm, we carefully derived a migration velocity model through an integrative approach. This comprised the incorporation of the tomographic near-surface model, the extension of the velocities down to the main reflectors based on borehole information and conventional semblance analysis. In the final step, the evaluation and update of the velocities by investigation of common image gathers for the main target reflectors were used. Although for our data set the reflections from the mineralization show a strong coherency and continuity in the seismic section, reflective structures in a hardrock environment are typically less continuous. In order to image the internal structure of the mineralization and decipher the surrounding structures, we applied the concept of reflection image spectroscopy to the data, which allows the imaging of wavelength-specific characteristics within the reflective body. As a result, conjugate crossing faults around the mineralization can directly be imaged in a low-frequency band while the internal structure was obtained within the high-frequency bands.
The paper is available here.
Potential of legacy 2D seismic data for deep targeting and structural imaging at the Neves–Corvo massive sulphide-bearing deposit, Portugal
Donoso, J.A., Malehmir, A., Pacheco, N., Araujo, V.,Penney, M., Carvalho, Spicer, B., J., Beach, S.
Geophysical Prospecting, Vol. 68, No 1, January 2020, pp. 44 – 61
Seismic methods are becoming an established choice for deep mineral exploration after being extensively tested and employed for the past two decades. To investigate whether the early European mineral-exploration datasets had potential for seismic imaging that was overlooked, we recovered a low-fold legacy seismic dataset from the Neves–Corvo mine site in the Iberian Pyrite Belt in southern Portugal. This dataset comprises six 4–6 km long profiles acquired in 1996 for deep targeting. Using today’s industry-scale processing algorithms, the world-class, ca. 150 Mt, Lombador massive sulphide and other smaller deposits were better imaged. Additionally, we also reveal a number of shallow but steeply dipping reflections that were absent in the original processing results. This study highlights that legacy seismic data are valuable and should be revisited regularly to take advantage of new processing algorithms and the experiences gained from processing such data in hard-rock environments elsewhere. Remembering that an initial processing job in hard rock should always aim to first obtain an overall image of the subsurface and make reflections visible, and then subsequent goals of the workflow could be set to, for example understanding relative amplitude ratios. The imaging of the known mineralization implies that this survey could likely have been among one of the pioneer studies in the world that demonstrated the capability of directly imaging massive sulphide deposits using the seismic method.
The paper is available here.
Improved target illumination at Ludvika mines of Sweden through seismic-interferometric surface-wave suppression
Balestrini, F., Draganov, D., Malehmir, A., Mardsen, P., Ghose, R.
In mineral exploration, new methods to improve the delineation of ore deposits at depth are in demand. For this purpose, increasing the signal-to-noise ratio through suitable data processing is an important requirement. Seismic reflection methods have proven to be useful to image mineral deposits. However, in most hard rock environments, surface waves constitute the most undesirable source-generated or ambient noise in the data that, especially given their typical broadband nature, often mask the events of interest like body-wave reflections and diffractions. In this study, we show the efficacy of a two-step procedure to suppress surface waves in an active-source reflection seismic dataset acquired in the Ludvika mining area of Sweden. First, we use seismic interferometry to estimate the surface-wave energy between receivers, given that they are the most energetic arrivals in the dataset. Second, we adaptively subtract the retrieved surface waves from the original shot gathers, checking the quality of the unveiled reflections. We see that several reflections, judged to be from the mineralization zone, are enhanced and better visualized after this two-step procedure. Our comparison with results from frequency-wavenumber filtering verifies the effectiveness of our scheme, since the presence of linear artefacts is reduced. The results are encouraging, as they open up new possibilities for denoising hard rock seismic data and, in particular, for imaging of deep mineral deposits using seismic reflections. This approach is purely data driven and does not require significant judgment on the dip and frequency content of present surface waves, which often vary from place to place.
The paper is available here.
Surface-wave analysis for static corrections in mineral exploration: A case study from central Sweden
Papadopoulou, M., Da Col, F., Mi, B., Bäckström, E., Schön, M., Marsden, P., Brodic, B., Malehmir, A., Socco, L.V.,
Geophysical Prospecting, Vol. 68, No 1, January 2020, pp. 214 – 231
In mineral exploration, increased interest towards deeper mineralizations makes seismic methods attractive. One of the critical steps in seismic processing workflows is the static correction, which is applied to correct the effect of the shallow, highly heterogeneous subsurface layers, and improve the imaging of deeper targets. We showed an effective approach to estimate the statics, based on the analysis of surface waves (groundroll) contained in the seismic reflection data, and we applied it to a legacy seismic line acquired at the iron-oxide mining site of Ludvika in Sweden. We applied surface-wave methods that were originally developed for hydrocarbon exploration, modified as a step-by-step workflow to suit the different geologic context of hardrock sites. The workflow starts with the detection of sharp lateral variations in the subsurface, the existence of which is common at hard-rock sites. Their location is subsequently used, to ensure that the dispersion curves extracted from the data are not affected by strong lateral variations of the subsurface properties. The dispersion curves are picked automatically, windowing the data and applying a wavefield transform. A pseudo-2D time-average S-wave velocity and time-average P-wave velocity profile are obtained directly from the dispersion curves, after inverting only a reference curve. The time-average P-wave velocity profile is then used for the direct estimation of the one-way traveltime, which provides the static corrections. The resulting P-wave statics from the field data were compared with statics computed through conventional P-wave tomography. Their difference was mostly negligible with more than 91% of the estimations being in agreement with the conventional statics, proving the effectiveness of the proposed workflow. The application of the statics obtained from surface waves provided a stacked section comparable with that obtained by applying tomostatics.
The paper is available here.
Body-wave passive seismic interferometry revisited: mining exploration using the body waves of local microearthquakes
Polychronopoulou, K., Lois, A., Draganov, D.
Geophysical Prospecting, Vol. 68, No 1, January 2020, pp. 232 – 253
As the global need for mineral resources is constantly rising and the exploitable concentrations of these resources tend to become increasingly complex to explore and exploit, the mining industry is in a constant quest for innovative and cost-effective exploration solutions. In this context, and in the framework of the Smart Exploration action, an integrated passive seismic survey was launched in the Gerolekas bauxite mining site in Central Greece. A passive seismic network, consisting of 129 three-component short-period stations was installed and operated continuously for 4 months. The acquired data permitted detection of approximately 1000 microearthquakes of very small magnitude (duration magnitude ranging between –1.5 and 2.0), located within or at a very close distance from the study area. We use this microseismicity as input for the application of passive seismic interferometry for reflection retrieval, using the body waves (P- and S-wave coda) of the located microearthquakes. We retrieve by autocorrelation zero-offset virtual reflection responses, per component, below each of the recording stations. We process the acquired results using reflection processing techniques to obtain zero-offset time and depth sections, both for P- and for S-waves. In the context of the present work, we evaluate one of the acquired depth sections, using an existing seismic line passing through the Gerolekas passive seismic network, and we perform forward modelling to assess the quality and value of the acquired results.We confirm that passive seismic reflected-wave interferometry could constitute a cost-effective and environmentally friendly innovative exploration alternative, especially in cases of difficult exploration settings.
The paper is available here.
Application of surface‐wave tomography to mineral exploration: a case study from Siilinjärvi, Finland
Da Col, F., Papadopoulou, M., Koivisto, E., Sito, Ł., Savolainen. M., Socco, L.V.,
Geophysical Prospecting, Vol. 68, No 1, January 2020, pp. 254 – 269
In order to assess the feasibility and validity of surface-wave tomography as a tool for mineral exploration, we present an active seismic three-dimensional case study from the Siilinj ¨ arvi mine in Eastern Finland. The aim of the survey is to identify the formation carrying the mineralization in an area south of the main pit, which will be mined in the future. Before acquiring the data, we performed an accurate survey design to maximize data coverage and minimize the time for deployment and recollection of the equipment.We extract path-averaged Rayleigh-wave phase-velocity dispersion curves by means of a two-station method. We invert them using a computationally efficient tomographic code which does not require the computation of phase-velocity maps and inverts directly for one-dimensional S-wave velocity models. The retrieved velocities are in good agreement with the data from a borehole in the vicinity, and the pseudo three–dimensional S-wave velocity volume allows us to identify the geological contact between the formation hosting most of the mineralization and the surrounding rock. We conclude that the proposed method is a valid tool, given the small amount of equipment used and the acceptable amount of time required to process the data.
The paper is available here.
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Smart Exploration has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No.775971