ABSTRACT

Blötberget, in the Ludvika Mines of Bergslagen mineral district of central Sweden, is well-known for its iron-oxide, sometimes apatite-rich, deposits. There is also a renewed interest in exploring and mining the deposits due to accessibility to the market and recent advancements made in the mining and metallurgical technologies. During two field campaigns (2015 and 2016), high-resolution reflection seismic data were acquired using both cabled- and wireless-recorders as well as a landstreamer system.

In this study, we have merged the two datasets and process them together to provide deeper information on the extension of the mineralization and potential unknown resources at depth. We show how the merged dataset images the mineralization much better and deeper than known also potential reflections under the known ones that can be targeted through a drilling program. This study demonstrates reflection seismic method is a powerful tool for imaging iron-oxide mineralization at depth. We argue that they should be acquired more routinely at the site for mineral exploration purposes. It also paves the way for justifying a 3D seismic survey in the area.

The paper is available here.

ABSTRACT

Within the H2020-funded Smart Exploration project existing legacy seismic data acquired in the Ludvika Mines are analysed in order to delineate the deposits in depth. Here we present a velocity model derived using first-break traveltime tomography, which represent the near-surface materials at high resolution and can be directly used for refraction static calculations or incorporation and for depth imaging algorithm. Our results are consistent with derived velocities from downhole logging data and show a strong vertical velocity gradient in the upper first hundred meters. In mineral exploration clear images of the subsurface and an improved characterization of mineral deposits are required to reduce the risk before drilling. Especially in prestack depth imaging workflows, which are successfully applied to hardrock seismic data, a reliable velocity model is required that represents the lateral and vertical variations in lithology and assures the robustness of the velocity model within the application of depth migration routines at the same time. A special focus of this work lies on the derivation of a detailed near-surface velocity model, which accounts for strong scattering effects due to lateral inhomogeneities as well as for topographic effects on the reflections.

The paper is available here.

ABSTRACT

Over the last few years, there has been a proliferation of seismic solutions, which employ specific combinations of equipment, acquisition, and processing techniques that can be applied in hard rock situations to improve the imaging resolution. The latest developments in seismic processing, in particular, make it worthwhile to re-process the legacy data to enhance the resolution of the data. This is particularly important in the mining regions where no new data are available or the acquisition of new data is expensive or not allowed due to new environmental regulations. In this work we demonstrate, through a case study from one of the world’s deepest gold mines in South Africa, how revisiting, recovering and re-processing of the seismic data acquired decades ago can significantly improve the quality of the interpretations. The information can benefit future mine planning operations by providing a better estimation of the resources and inform in the siting of the sinking of future shafts. Thus, any future mineral exploration plans could take the information obtained from the re-processed legacy seismic data into account when planning either 2D or 3D seismic surveys.

The paper is available here.

ABSTRACT

Volcanogenic-hosted massive sulphide (VMS) deposits show a strong seismic contrast (mainly due to their density) against almost all lithological host rocks therefore justifying their direct targeting using seismic methods (Salisbury et al., 2000; Malehmir et al., 2012 and references therein; Malehmir et al., 2013) when there is adequate signal-to-noise (S/N) ratio and suitable geometry. While there are earlier published accounts illustrating the use of seismic methods for direct targeting of deep-seated VMS deposits elsewhere (Matthew, 2002; Malehmir and Bellefleur, 2009), a number of attempts were done in Europe during the early 90s for this purpose and these have been overlooked for unknown reasons.

The paper is available here.

ABSTRACT

Europe is in need of fresh aluminum for its vast variety of developments and Greece has the potential to deliver. Delphi Distomon S.A is one of the largest bauxite producers in Greece and is interested to explore new deposits in new unexploited areas. Logistics, accessibility, environmental issues and high cost are key obstacles in the application of a high-definition 3D active seismic survey. Hence, an alternative integrated method of exploration will be carried out based on gravity, magnetotelluric and passive seismic methods. As a preliminary step for an optimized acquisition scheme, a dynamical approach is followed that utilizes a lithology model created by available drilling data, its transformation to a density one, the forward modelling and the comparison of the synthetic data with a previous gravity study in the area. The preliminary results of the analysis gave the chance to identify the vulnerabilities of the lithology and the equivalent geophysical model. As it was observed, they should be enriched during the survey with additional geological information and in situ observations. The emerged models can contribute remarkable to the stage of the processing of the different geophysical methods as well as the final stage of the integrated interpretation.

The paper is available here.

ABSTRACT

Europe has a favourable geology for a wide variety of commodities necessary for improving our modern life sustainably and environmentally friendly. The Smart Exploration initiative answered one of the seven societal challenges offered by the European Innovation Partnership on Raw Materials on new solutions for sustainable production of raw materials – new sensitive exploration technologies. The project involves 27 partners from nine European countries comprising of 11 research institutions, 11 small and medium enterprises and 5 stakeholders. It primarily focuses on developing cost-effective, environmentally friendly tools and methods for geophysical exploration in highly challenging brownfield areas to meet the ever-increasing community (social acceptance) and environmental issues, as well as reduce the return time (from exploration to production). The aim is to not only generate new technological and methodological markets, but also to create results that will also allow for improved exploration in the EU countries and beyond. Planned prototypes and their potential to impact the market as well as methodological developments will be introduced. Furthermore, the value of legacy data through the use of both traditional and innovative approaches, reviving these datasets and illustrating their potential for deep (+500 m) targeting of mineralized bodies will be discussed.

The paper is available here.

ABSTRACT

We present a feasibility study for surface wave tomography for mineral exploration. We apply a typical seismological approach, the Two Station Method to a hard-rock site, at exploration scale. Even with this method, we are able to separate the two propagation modes typical of these sites. After windowing the traces by picking one propagation mode in the group velocity matrix, we pick the phase velocity dispersion curve in the cross-multiplication matrix. We finally propose a plot consisting of slices of tomographic pseudo volumes, which allows us to understand the penetration depth we can have. Furthermore, it gives us a first indication of the velocity anomalies in the area.

The paper is available here.

ABSTRACT

The word “seismics” in the geoscience community is often used synonymously with “oil and gas”, despite its successes in other applications, for example, in mineral exploration, engineering application, mine planning and safety.Over the past few decades, the method has been developed and successfully used for mineral exploration, mine planning, and safety in “hard rock” metallogenic provinces worldwide (e.g., Australia, Europe, Canada, and South Africa), leading to the discovery of giant minerals and metal deposits.However, despite these successes, the method’s capabilities in mining still remains less-known to many geoscientists and some mining companies are still reluctant to use it for “hard rock” exploration and mining.The purpose of this paper is to demonstrate how the reflection seismic method has been successfully used to explore and discover some of the world’s largest mineral and metal deposits that are located deep underground – where exploration drilling is more costly and risky. A wide range of case studies from hard rock environments are covered, for example, from South Africa and Canada.

The paper is available here.

ABSTRACT

Without doubt for a successful and long-lasting career, it is important to understand how to balance and promote personal and professional teamwork achievements. The modern age has brought many useful communication tools, making it easier to network and reach beyond conventional academic journals. This study, through a survey and statistical analysis of the results, shows how the young professionals pursuing their temporary position within the H2020-funded Smart Exploration project predict their future career, communicate their work with the outside world and envisage a positive work/research environment for their success. Smart Exploration has provided an opportunity to bridge the gap among universities, mineral exploration industry and small and medium enterprises by employing over 15 young professionals. It provides hands-on field experience using modern equipment and methods, as well as showing them how a collaborative and integrated team is tackling mineral exploration challenges within the EU. The project focuses on research and innovation, thereby tasks and activities have been designed with a great emphasis on young professionals, diversifying their activities across engineering and geosciences. This multidisciplinary environment has led to a much more secure feeling to promote their research and development, and a probability for a sustainable career.

The paper is available here.

Bergslagen is one of the richest mineral districts in Sweden for base and precious/critical minerals and metals. In this work a mineral system approach for targeting of iron-oxide apatite-bearing mineralisation has been developed. GIS-based mapping of prospectivity for this type of mineralisation has been carried out with a focus on Ludvika mining area from Blötberget to Håksberg with known and high-quality iron-oxide deposits. According to spatial analysis on mappable criteria’s, strong positive airborne magnetic anomalies, density of structures and the contact between felsic volcanic rocks and granites are crucial for this type of mineralisation in the study area This GIS-based model will also be used in targeting of iron-oxide deposits at depth in the Blötberget area. However, the mineral systems approach considers the origin of deposits in the framework of lithospheric-scale processes from the time-honored aspects of the source, fluids, transport and physical and/or thermo-dynamical traps. Applied to exploration strategy, this approach allows for more predictive models. Rather than matching patterns, knowledge of the underlying geological processes and tectonic-structural setting can be used for identifying areas with higher probability of finding deposits of interest. Furthermore, this method can broaden the scope of prospectivity indicators and allows for earlier and more efficient fertility assessments.

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ABSTRACT

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 full paper is available here.

ABSTRACT

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 hardrock 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. Secondly, 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 mineralisation zone, are enhanced and better visualised 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 hardrock 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.

This article is protected by copyright. All rights reserved

The paper is available here.

ABSTRACT

Seismic detection of faults, dykes, potholes, and iron‐rich ultramafic pegmatitic (IRUPs) bodies is of great importance to the platinum mining industry, as these structures affect safety and efficiency. The application of conventional seismic attributes (such as instantaneous amplitude, phase, and frequency) in the hard‐rock environment is more challenging than in soft‐rock settings because the geology is often complex, reflectors disrupted, and the seismic energy strongly scattered. We have developed new seismic attributes that sharpen seismic reflections, enabling additional structural information to be extracted from the hard‐rock seismic data. The symmetry attribute is based on the invariance of an object with respect to transformations such as rotation and reflection; it is independent of the trace reflection amplitude, and hence a better indicator of the lateral continuity of thin and weak reflections. The reflection‐continuity detector attribute is based on the Hilbert transform; it enhances the visibility of the peaks and troughs of the seismic traces, and hence the continuity of weak reflections. We demonstrate the effectiveness of these new seismic attributes by applying them to a legacy 3D seismic dataset from the Bushveld Complex in South Africa. These seismic attributes show good detection of deep‐seated thin (∼ 1.5 m thick) platinum ore bodies and their associated complex geological structures (faults, dykes, potholes, and iron‐rich ultramafic pegmatites). They provide a fast, cost‐effective, and efficient interpretation tool that, when coupled with horizon‐based seismic attributes, can reveal structures not seen in conventional interpretations.

The paper is available here.

ABSTRACT

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 hard‐rock 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 (V_Sz) and time‐average P‐wave velocity (V_Pz) profile are obtained directly from the dispersion curves, after inverting only a reference curve. The V_Pz 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.

ABSTRACT

Near-surface velocity models are important for deep imaging of mineral deposits with seismic exploration. The near-surface can be quite complex from loose, highly heterogeneous materials to stiff, fractured rocks. Surfacewave analysis can be an effective method to image the shallow subsurface of such challenging environments.

Here, we propose a workflow that includes several processing and inversion steps. Initially, for the optimization of the processing parameters, we assess the presence of sharp lateral variations with a method based on the measured energy of Rayleigh waves. Then, using a moving window of receivers, we extract Rayleigh-wave dispersion curves along the acquisition line as the maxima of the f-k spectrum. Finally, the dispersion curves are inverted using a laterally constrained inversion scheme. The proposed methodology has been tested on legacy data from a mining field.

The paper is available here.

Manzi, M., Malehmir, A., Durrheim, r.J.

ABSTRACT

Seismic methods have become, and will continue to be, an important tool to help unravel structures hosting mineral deposits at great depth. This work demonstrates how the reflection seismic method (mainly 3D seismics) has been successfully used to explore some of the world’s largest platinum deposits and map the gross structural architecture that controlled the formation of these ore bodies, support mine planning and contribute to safety. A case study from the Bushveld Complex in South Africa is presented, demonstrating the importance of advanced 3D seismic attributes for high-resolution structural mapping.

The paper is available here.

Sadegi, M., Bastani, M., Malehmir, A., Luth, S., Mardsen, P.

The paper is available here.

Brodic, B., M., Malehmir, A., Juhlin, C., Dynesius, L., Bastani, M., Palm, H.

ABSTRACT

During the last few decades there has been an increased demand for infrastructure, along with a greater awareness of environmental issues in the construction industry. These factors have contributed to an increased interest in using seismic methods for near surface characterization, particularly in urban environments. Seismic sensors not affected by anthropogenic electromagnetic noise are therefore important, as well as an acquisition system that is easy to deploy, move and non-invasive. To address some of these challenges, a multicomponent broadband MEMS (micro-electro mechanical system) based landstreamer system was developed. The landstreamer system is fully digital, therefore it is less sensitive to electrical or electromagnetic noise. Crosstalk, leakage and tilting tests show that the system is superior to its predecessors. The broadband nature of the sensors (theoretically 0–800 Hz), 3C (three-component) recording and the close spacing of the sensors enable high-resolution imaging. The current streamer configuration consists of 20 sensors 4 m apart and 80 sensors 2 m apart. The streamer can easily be combined with wireless recorders for simultaneous data acquisition. In this study, we present results from testing of the streamer with various sources, such as a shear wave vibrator and different types of impact sources. MEMS-sensors and their high sensitivity allowed recording clear reflections that were not observed with coil-based sensors. A complementary test was also carried out at a planned access ramp for an urban tunnel where potential poor quality rocks had been identified by drilling. First-break traveltime tomography showed that these poor quality rocks correlate with low velocity zones. The presented landstreamer system has great potential for characterizing the subsurface in noisy environments.

The paper is available here.

Balestrini, F., Draganov, D., Malehmir, A., Mardsen, P., Ghose, R.

ABSTRACT

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.