Publications – On Journals

Smart Exploration: from legacy data to state-of-the-art data acquisition and imaging

Malehmir, A.,  Donoso, G., Markovic, M., Maries, G., Dynesius, L., Brodic, B.,Pacheco, N., Marsden, P., Bäckström, E., Penney M., and Araujo, V.

First Break, Vol. 37, N. 8, August 2019

ABSTRACT

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

Manzi, M., Malehmir, A., and Durrheim, R.

First Break, Vol. 37, N. 8, August 2019

ABSTRACT

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

Marques, F., Matos, J.X.,Sousa, P.,Represas, P., Araújo, V., Carvalho, J., Morais, I., Pacheco, N., Albardeiro L. and Gonçalves P.

First Break, Vol. 37, N. 8, August 2019

ABSTRACT

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.

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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