As the global need for aluminum constantly rises, bauxite is considered to be a critical mineral, and the mining industry is in search of new and effective exploration solutions. In this context, we design and implement a purely earthquake-based passive seismic survey at the Gerolekas bauxite mining site in Greece. It is a very difficult exploration setting, characterized by rough topography, limited accessibility, and a very complex geotectonic regime. We gather a passive seismic data set consisting of four months of continuous recordings (May to August 2018) from 129 stand-alone 3C seismological stations. We then analyze this data set and extract 848 microearthquakes that will serve as sources for the application of local earthquake tomography (LET) and transient-source seismic interferometry (TSI) by autocorrelation. We apply LET to estimate the 3D P- and S-wave velocity models of the subsurface below the study area and TSI by autocorrelation to retrieve the zero-offset virtual reflection responses below each of the recording stations. The velocity models provide a relatively coarse image of a previously completely unexplored part of the mining concession, whereas the higher-resolution virtual reflection imaging illuminates in detail the different interfaces. We also reprocess three lines of legacy active seismic data that were shot in 2003, using the LET P-wave velocity model for depth migration, and confirm the improvement of seismic imaging. Finally, we evaluate the obtained results using well data and jointly interpret them, extracting useful information on the expected target depths and indicating that earthquake-based passive seismic techniques can be an innovative and environmentally friendly option for mineral exploration.

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Geothermal energy is a cleaner and more sustainable source of power, which plays a key role in the transition to a low-carbon economy. Sustainable and safe exploitation of geothermal resources, however, depends on our ability to understand and manage the associated seismic risks. In 2018, Nature's Heat geothermal project began operations in Kwintsheul, Netherlands, aiming to supply heat to 64 hectares of greenhouses. Between July and October 2019, a temporary seismic array was installed to monitor for possible seismic activity at the site. Microseismic moment-tensor inversion is a valuable tool for understanding the mechanics and structure of geothermal reservoirs, and for optimizing their exploitation. It can be challenging, though, to apply this technique when there are high levels of ambient seismic noise, as is often the case in geothermal operations in densely populated areas. In this study, we evaluate the feasibility of inverting the centroid moment tensor of microseismic events in Kwintsheul, using probabilistic moment-tensor inversions. We first test the probabilistic inversion using synthetic recordings of ambient seismic noise, after which we apply the technique to the low-magnitude (Md=0.16) event recorded on July 14, 2019. Our results give insight into the challenges and limitations of applying moment-tensor inversion to low-magnitude events in the context of geothermal operations in the Netherlands.@en

We present an overview of induced seismicity due to subsurface engineering in the Netherlands. Our overview includes events induced by gas extraction, underground gas storage, geothermal heat extraction, salt solution mining and post-mining water ingress. Compared to natural seismicity, induced events are usually small (magnitudes ≤ 4.0). However, due to the soft topsoils in combination with shallow hypocentres, in the Netherlands events exceeding magnitude 1.5–2.0 may be felt by the public. These events can potentially damage houses and infrastructure, and undermine public acceptance. Felt events were induced by gas production in the north of the Netherlands and by post-mining water ingress in the south-east. Notorious examples are the earthquakes induced by gas production from the large Groningen gas field with magnitudes up to 3.6. Here, extensive non-structural damage incurred and public support was revoked. As a consequence, production will be terminated in 2022 leaving approximately 800 billion cubic metres of gas unexploited. The magnitudes of the events observed at underground gas storage, geothermal heat production and salt solution mining projects have so far been very limited (magnitudes ≤ 1.7). However, in the future larger events cannot be excluded. Project- or industry-specific risk governance protocols, extensive gathering of subsurface data and adequate seismic monitoring are therefore essential to allow sustainable use of the Dutch subsurface now and over the decades to come.

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In 2018, the geothermal project Nature's Heat started its operations to supply heat to 64 hectares of greenhouses in Kwintsheul, Netherlands. The operation involves the extraction and reinjection of geothermal fluids at a depth of about 2.4km. Several studies suggested that geothermal operations in these parts of The Netherlands are unlikely to generate felt seismic events (M>2.0); nevertheless, adequate seismic monitoring techniques are essential to guarantee sustainable and safe use of the Dutch subsurface. Between July and October 2019, Delft University of Technology, Seismotech (Greece), and Gastreatment Services BV installed a passive seismic network to monitor the seismic activity over Nature's Heat geothermal reservoir. The seismic network consists of 30 three-component short-period seismic sensors placed at inter-station distances of approximately 150 m along two crossing lines. A challenge for seismic monitoring systems in urban areas is the high level of background noise. In Kwintsheul, anthropogenic noise dominates the spectrograms at frequencies higher than 2 Hz. Despite these high background-noise levels, a seismic event of ML = 0.0 (duration magnitude Md 0.16) was recorded by all seismometers of the array on July 14, 2019. To understand the relation of the event and improve the safety of the geothermal operation, we are developing a probabilistic monitoring and inversion scheme. This study aims to improve the seismic network's detection and hypocentre-determination capabilities and verifies via template matching if the detected seismic event is repeating over time (possibly at the background noise level). @en

As the quest for mineral resources becomes more and more challenging and exploration geophysics is actively evolving, the mining industry is in a constant pursuit of more efficient and cost-effective exploration methodologies that would provide a means of overcoming exploration limitations of the past. In this context, an integrated passive seismic survey was designed and launched at the Gerolekas bauxite mining site, in Central Greece, in the scope of checking the potential of passive seismic methodologies as a mining exploration alternative.@en

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.@en

Passive seismic surveys have long been applied in the context of exploration studies. Our research focuses on the exploitation of the natural seismicity that is present in passive seismic records and the extraction of useful information of the subsurface from different parts of the earthquake signal. In that scope, we initially install a dense seismological network in an area of interest that will permit the recording of local microseismicity and we detect and locate microearthquakes of very small magnitude that occur within a very close distance from this network. We then exploit the isolated earthquake signals, applying different passive seismic techniques, in order to extract the different pieces of information that they are carrying. More specifically, we use their P- and S-wave first arrivals to perform local earthquake tomography, acquiring a tomographic P- and S-wave velocity distribution of the subsurface below the area of interest. We also extract their P- and S-wave coda to perform reflected-wave seismic interferometry by autocorrelation, which provides zero-offset virtual reflection responses from virtual sources sending energy nearly vertically down below each station of the installed network. The acquired results, which are both individually interpreted and jointly evaluated, provide a valuable insight on the subsurface.@en