Combined analysis of seismicity and numerical modelling

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Geological setting and initial stress state

Garpenberg ore is a volcanogenic hydrothermal deposit formed mainly by stratabound replacement below the seafloor, within the caldera vent of a large shallow marine rhyolite-dacite volcano (Allen et al., 2003). The volcanogenic hydrothermal process can be explained by seepage of cold seawater through fractures into a shallow submarine volcano. Here the fluid undergoes a high-temperature reaction due to the presence of hot rocks and it thermally expands, becomes lighter and flows up again towards the ocean. The mineralization process starts during seawater rise, when metals present in volcanic rocks are dissolved and concentrated as polymetallic sulphides deposits. For a complete description of the volcanogenic hydrothermal process and the consequent formation of massive sulphide deposits, the reader is referred to Tornos et al. (2015).
At Garpenberg, an extensive limestone reef was present just below the seafloor, and it formed both a barrier and a chemically reactive trap that focused precipitation of metals along the base of the limestone (Allen et al., 2003). Moreover, just after, or immediately prior to the mineralization, Garpenberg volcano formed a caldera, with the emptying of the magma chamber and the subsequent collapse of the volcano edifice. This complex process leads to the formation of the Garpenberg limestone-skarn hosted deposit, that is the largest known concentration of polymetallic sulphide orebodies in the region, containing mainly galena (lead – Pb), sphalerite (zinc – Zn), chalcopyrite (copper – Cu), together with silver (Ag) and gold (Au) (Jansson, 2011). After the formation, the ore deposit has been significantly modified by subsequent tectonic deformations and metamorphism (Allen et al., 2003).
Fig. 2.3 – Geological map of Garpenberg area (on the left) and geological cross section of Lappberget orebody (on the right) (after Ahmadi et al., 2013). The cross section is taken along the NW-SE profile shown in the geological map (on the left). In the cross section, depths and lengths are given in meters and in the mine coordinate system. Note that X and Y directions are inverted between mine coordinate system and Ineris one, such that the X reported in the figure corresponds to the Y in Ineris coordinates.
Lappberget is one of the biggest orebodies of the entire deposit, presenting a steeply inclined, almost vertical structure (Fig. 2.3 on the right). It is situated in a syncline and hosted in a limestone unit, which is the only non-volcanic rock of the formation, below a superficial stratum of volcanic and sedimentary rocks (in yellow in Fig. 2.3). The limestone presents different grades of metamorphism, showing gradual alteration into dolomite and skarn as the mineralization is approached, and with zones of transformation of the dolomite into talc that are very common at the contact with the ore. The orebody itself, which will be described in more details in the next section, is mainly constituted by sulphides in the form of massive volumes as well as in small veins. The lowest unit of the formation, characterized by different layers of metamorphic volcanogenic rocks, is mainly constituted by quartzite and schist (van Koppen, 2008). No faults are observed in the proximity of Garpenberg mine area. A trust fault is encountered at more than 1 km distance, toward the south-east direction (Fig. 2.3 on the left).
The here described geology is extremely simplified as each unit presents many different rock types. Indeed, mine geologists have classified more than 190 rocks which can be encountered underground in the mine. The exhaustive list of rock types is presented in Tab. A.1. For the scopes of this thesis, such a detailed geological description is not needed, thus, rock types will be significantly simplified, as it will be discussed in Chapter 5.

Lappberget orebody and weakness zones

Lappberget orebody presents a whole vertical extension ranging between -435 m and -1600 m, as modelled by mine geologists. Focusing on the study area of this thesis (the block 1250), between – 1108 m and -1257 m, the orebody (light red in Fig. 2.4) has a maximum length of about 300 m (along the X direction), while its width can be extremely variable, being thinner in the eastern area and wider toward the west. The maximum width, which is encountered in the central area of 1250 block is of about 140 m.
Fig. 2.4 – Orebody, weak and very weak geological zones in block 1250. Weak and very weak zones configuration is updated at October 2017. Gray profiles indicate mine galleries.
The ore appears to be heterogeneous: its eastern area (type A ore) is characterized by massive sulphide deposits mainly hosted in the limestone/dolomite unit, while, moving toward the west, the ore presents an irregular and heterogeneous mineralization (type C ore) within a host rock of mica-quartzite (Olsson 2018, personal communication, 2 May). Normally, this latter type of mineralization is referred to as “stringer” or “impregnated”, as the minerals occur in veins networks inside the host rock. The veins can be very diffuse as well as very dispersed (Olsson 2016, personal communication). Aside from the stringer mineralization, C ore presents as well massive sulphide bands of a few centimeters in thickness. These bands are called “remobilized ore” and they follow the tectonic structures in the rock mass, containing bigger amount of silver and lead compared to zinc (van Koppen, 2008).
All along the orebody the presence of mechanically weak zones is observed (in green Fig. 2.4). These areas occur as intrusions of large to small size, with a complex geometry inside the orebody and its host rock. The geological definition of these weak zones is quite complex and cannot be based only on one lithology type or one specific mineral type. Weak zones of the studied area are composed by lenses of schists in the metal deposit, as well as by rock blocks containing massive amounts of soft minerals, like talc and chlorite. Moreover, zones of contacts between different rock types are also regarded as weak zones, especially when rock types of highly varying stiffness are mixed together (Olsson 2017, personal communication, 1 September). In addition to weak zones, mine geologists identified also the so-called very weak zones (in red in Fig. 2.4). These areas are characterized by the massive presence of schists extremely rich in talc. As visible in Fig. 2.4, weak and very weak zones are more intensely deployed along the eastern side of the mine, in coincidence with the A ore zone. Their intensity is gradually reduced toward the C ore zone in the western side of the mine, where very weak zones are not encountered.
Weak and very weak zones raise significant questions in term of mine sequencing and induced seismicity, as it will be later discussed. However, it is difficult to get a complete picture of weakness zones profiles. Indeed, their geometry is known only on the horizontal plane, per each level of the mine. This means that, their vertical extension between two consecutive levels (around 25 meters) is not precisely known, neither modelled by mine geologists. Moreover, the level-by-level geometry of weakness zones is in constant update as a new drift is excavated and geological surveys are made possible. In this thesis, we will always refer to the geometry update of October 2017 which is shown in Fig. 2.4, further changes to this configuration will not be considered.
The described setting, in terms of orebody and weak zone features, results in a marked variability of geological characteristics moving from the east toward the west side of the mine, i.e. from the A to the C ore. Firstly, the orebody cannot be considered as a homogeneous rock mass with constant properties. Indeed, the mineralization and the bedrocks change significantly. Similarly, weak zones distribution differs in the horizontal direction. Based on these evidences, the eastern side of the mine can be considered extremely heterogeneous. The limestone/dolomite is abundantly replaced by massive volumes of sulfides and skarn, which are cut by large weak zones of schist. This means that very stiff rock masses are interbedded with extremely soft materials, making the peculiarity of this area. Indeed, schist can also be found in the central and the western area and the mica quartzite can be schistose in itself, but schist within the eastern side stands out due to the fairly difference to the dolomite and the massive sulfides (Olsson 2018, personal communication, 3 May). On the contrary, the central and the western area of the mine can be considered more homogeneous, with the mica quartzite interrupted by smaller sulphide volumes and the presence of less diffuse weak zones.
Small fractures due to geology or induced by mining are both observed in Lappberget, even if no major fractures are reported. However, geological surveys routinely done by mine geologist do not include the analysis of fractures or joints. Therefore, their location, intensity and orientation are not known, which constitutes an important lack of information for the scope of this thesis.

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Initial stress state and elastic rock mass properties

In underground mines, the stress state can be split in two components: (i) the virgin or initial state of stress prior to any underground excavation and (ii) the induced stress, i.e. the stress variation due to mine openings. Initial stresses are dependent on many parameters such as depth, rock mass properties and geological (tectonic) settings, which define the local stress state within rock masses. When openings are excavated, local stresses need to be redistributed in surrounding rocks, resulting in the alteration of the initial stress pattern. This change in stress intensity and/or orientation due to mine excavations is referred to as mine-induced stress. Knowledge about stress state before and after excavations is extremely important as it strongly influences the local rock mass response to mining, both in terms of seismic and aseismic strains.
Stress measurements in Garpenberg were firstly performed in 2004 by SINTEF company with the overcoring method, at levels 883 and 967 below ground surface, in close proximity to Lappberget orebody (precise coordinates are not available). Later, in December 2014, new stress measurements were carried out by Ineris within block 1250. These latter measurements were performed by the overcoring method with CSIRO Hi strain cells, in two boreholes drilled from Stope 15 toward Stope 13 at 1155 m depth (Fig. 2.5a and b). During the measurement campaign, permanent CSIRO Hi cells were as well installed (PD and PH in Fig. 2.5b) for monitoring quasi-static stress changes during mining.

Geophysical and geotechnical monitoring in Lappberget

As already mentioned in Chapter 1, block 1250 of Lappberget has been instrumented with a monitoring network in the end of 2014. The network, installed by Ineris, consists of, both, strain cells and seismic sensors for monitoring quasi-static strain changes and microseismic activity as excavations progress.
Permanent CSIRO strain cells, PD and PH, have been installed close to Stope 13 at level 1157. More precisely, PH cell has been set up in a horizontal borehole within the secondary Stope 14, while, PD cell has been set up in a downward borehole in Stope 13 (Fig. 2.5 and Fig. 2.11). With this configuration, strain monitoring aims at determining strain, and in turn stress variations in Stope 13 between levels 1157 and 1182, which constitutes the rock volume planned to be mined last in the entire column 13. At the same time, PH cell ensures the comparison of stress state between primary and secondary stopes until the end of the exploitation in Stope 13 (Tonnellier et al., 2016). Strain cells perform one measurement per hour of 12 strains in different directions, with accuracy about ± 5 µm/m. This allows determining stress changes by inversion, assuming an elastic behavior of the rock.

Microseismic data

During the two-years period analyzed the seismic monitoring network recorded more than 800 events within the studied area. After back-analysis of 2015 data and localization improvements, as it will be described in the next Chapter, retained events are more than 760. Indeed, events localized in the upper levels of the mine, outside block 1250, were discarded from further analysis. This choice is motivated by monitoring strategy, but also because of a lack of information about production data outside block 1250, which are fundamental for the objectives of this thesis.
If compared with other deep hard rock mines, 1250 block of Lappberget orebody is not particularly seismically active. Indeed, microseismic activity rate is on average in the order of 2 events per day, which is a low value compared with other deep underground mines (e.g. Fritschen, 2010; Kgarume et al., 2010; Kubacki et al., 2014; Vallejos and McKinnon, 2011).
Fig. 2.14 reports spatiotemporal characteristics of mine blasts and microseismic events localized around mine galleries. We observe a temporal correlation between microseismic activity and PB (Fig. 2.14c). Indeed, the cumulative curve of MSE shows a stepwise trend with important increases of seismicity rate immediately after some PB. This tendency, which appears intensified in the second half of 2016, highlights that seismic activity is not homogeneous during time but punctually enhanced by the occurrence of PB. However, seismic rock mass response appears to be very variable from a blast to another, as, in some cases, seismicity rate remains almost constant despite the considerable number of performed blasts.
Spatial distribution of MSE (Fig. 2.14a and b) points out two main seismically active zones deployed in the central area and in the eastern side of the mine. The correlation between seismic activity and PB positions is not straightforward. Indeed, the intense production of the western side did not result in an intense seismic activity, while most of the events are observed in the eastern side where few PB were performed.
These preliminary observations on the reciprocal spatiotemporal distribution of MSE and mine blasts, highlight some correlations between mining and seismic activity, even if the nature of this correlation is not clear and needs deeper investigations. It is the aim of this thesis to understand if mining-induced stresses are solely responsible for seismic activity in Lappberget orebody, or if other factors, such as for example the geological setting and the mining sequence, may play a significant role.

Table of contents :

Chapter 1 – Introduction
1.1 Mining-induced seismicity mechanisms
1.2 Monitoring of mining-induced seismicity and hazard assessment
1.3 Motivation, strategy and structure of this thesis
Chapter 2 – Study area: Garpenberg mine and Lappberget orebody
2.1 An introduction to Garpenberg mine
2.2 Geological setting and initial stress state
2.2.1 Lappberget orebody and weakness zones
2.2.2 Initial stress state and elastic rock mass properties
2.3 Mining method and sequencing
2.4 Geophysical and geotechnical monitoring in Lappberget
2.4.1 Extensometer data
2.4.2 Strain measurements
2.4.3 Microseismic data
2.5 Seismic activity and observed damage
Chapter 3 – Seismic data processing
3.1 Routines of seismic data acquisition and processing
3.1.1 Type of recorded seismic signals
3.1.2 Challenges and common errors in daily data processing
3.2 Picking consistency evaluation – The Wadati analysis
3.3 Evaluation of microseismic network performances
3.3.1 EMAP algorithm methodology
3.3.2 EMAP application to Lappberget microseismic network
3.4 Considerations about the extension of the analyzed area
3.5 Seismic source parameters estimation
3.5.1 Considerations on source parameters uncertainties
Chapter 4 – Rock mass response to mining
4.1 Spatiotemporal behavior of microseismic activity and mine blasts
4.1.1 Seismic sequences and clusters
4.2 Analysis of seismic source parameters
4.2.1 Temporal variation in b-value
4.3 What drives seismicity?
4.4 Analysis of geotechnical observations
4.5 Summary and discussion
Chapter 5 – Numerical modelling
5.1 Numerical modelling techniques
5.2 Model choice and strategy
5.3 Description of the model
5.3.1 Model geometry and boundaries
5.3.2 Model meshing
5.3.3 Initial and boundary conditions
5.3.4 Modelled elements and mechanical effect of paste fill
5.3.5 Constitutive laws and mechanical properties
5.3.6 Simulated mining sequence
5.4 Comparison with in situ geotechnical measurements
5.5 Model results and interpretations
5.5.1 Analysis of stress distribution
5.5.2 Analysis of strain distribution
5.5.3 Analysis of plastic zones and influence of weak geological materials
5.5.4 Temporal evolution of model parameters
5.6 Discussion and conclusion
Chapter 6 – Combined analysis of seismicity and numerical modelling
6.1 Relating induced seismicity with geomechanical modelling
6.2 Strategy of comparison in our work
6.3 Qualitative comparison at large-scale
6.3.1 Plastic zone and seismic activity
6.3.2 Instability criteria and seismic activity
6.4 Quantitative comparison at small-scale
6.4.1 Model and seismic parameters at punctual locations
6.4.2 Model and seismic parameters at spheres location
6.5 Summary and conclusion
Chapter 7 – Summary, conclusions and perspectives
7.1 Microseismic and geotechnical data analysis and interpretation
7.2 Numerical modelling and mining-induced seismicity
7.3 General perspectives


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