Mining induced seismicity
The scope of this article covers seismic sources, monitoring and analysis methods of seismic events and mine design in the context of seismicity management in underground mines. All Underground mines experience some degree of seismicity. Damage due to seismicity has been observed in various mines around the globe. Seismic events will occur as a result of blasting, fault slip, rockbursts or any other source of rock fracture and ground movement. Reducing seismicity in mines is a major challenge of deep underground mining. To minimize and mitigate damage from dynamic ground in an underground mine; understanding, assessing and designing with consideration of seismicity, and changes in seismicity, are essential.
- 1 Why Seismic Monitoring
- 2 Seismic Waveform Characteristics
- 3 Seismic Data Collection
- 4 Analysis of Induced Seismicity
- 5 Planning
- 6 References
Why Seismic Monitoring
Seismic monitoring in a micro scale provides useful information about the local state and stress conditions which can be used for identifying active structures and for better understanding of the rock mass behaviour (i.e. rock mass failure and its failure mechanism). This is done by analyzing the continuous four-dimensional seismic record from a rock mass as a result of progressive mining. On the other hand, in a macro scale, microseismic monitoring can be used to determine the current state of tectonic activity (Hudyma, 2003; McKinnon, 2013).
Seismic Waveform Characteristics
Seismic events are caused by the sudden release of energy as a result of stresses overcoming the strength of the rock, breaking it. This energy release is then propagated through the rock as pressure waves. Some waveform characteristics that are important to know for application to mines are as follows:
Compressional Waves and Shear Waves
There are three general waveforms produced from a single seismic event; P-waves, S-waves and surface waves. P-waves are compressional waves, and travel at the greatest velocity, making them the first-arrival waveforms. The S-Waves are shear waves, travel at a velocity slower than the P-waves making them the second arrival waveforms. Surface waves are not significant in mining applications. Figure 1 displays a seismic form with the first-arrival P-wave and second-arrival S-wave.
Magnitude is an assessment of the energy released during a seismic event, describing the event size . For mining applications, the Richter scale is used. Event magnitude is generally proportional to potential damage of a rockburst, as the magnitude increases the probability of a large rockburst increases . Table 1 displays the approximate relationship between the characteristic maximum event magnitudes with source mechanisms.
The hypocenter of a seismic event is the original location at which the seismic event has taken place. From the hypocenter, the waveform propagates outwards. The original sudden release of energy from fracturing rock, such as a fault slip or rockburst event, takes place at the hypocenter of the event. Commonly, the original seismic event will be the result of induced stresses from within the mine.
As a result of the sudden seismic event the stress, strain and particle velocity is propagated through the rock. This may alter the stress field, which may drive additional rock failure. For example, rockburst may occur in brittle rock when a sudden increase of stresses are present. Also, at the face of excavations, when the stress wave is reflected backwards, the tensile stresses of the waveform may be high enough to exceed the tensile strength of the rock, which is relatively low compared to compressive rock strength .
Seismic Data Collection
Collection of seismic data includes quantified results from monitoring in conjunction with qualitative observations in the mine. Results from monitoring must be compared to visual observations underground from routine inspections, in order to calibrate and accurately assess the monitor data .
Monitoring with Instrumentation
Monitoring in mines allows for observations to be assessed numerically. Seismic monitoring systems consist of a network of geophones, which measure the acoustic waveforms, generated from rock fracture, at the locations of the individual geophones. With the digitization of the geophone monitoring systems data is automatically uploaded for engineers to access, and patterns over large time-frames can be assessed .
Instrumentation used to assess seismicity, as well as strain and deformation as a result of seismicity, includes geophone networks, extensometers and strain gauge cells. Through strain gauge cells and extensometers, the ground stress and strain can be monitored in order to access damage due to seismicity. However, there is more inherent error associated with these two instruments. The effectiveness of using geophones is dependent on the accuracy of the assumed seismic velocities and the accuracy of the acoustic waveform reading. Also accuracy is commonly within a few meters when determining hypocenter location. However, by implementing a good seismic network of geophones, more accurate results can be achieved.
Implementation of a monitoring network must be done in order to minimize error by designing the spatial formation, volume and configuration of the network. Figure 2 displays a schematic diagram of a typical seismic monitoring network for an underground mine. The spatial formation of the geophones should take in account the distance to the event hypocenter as well as the structural geometry between the hypocenter and the geophones. The configuration of geophones around the hypocenter should be at a range of distances in order to detect several waveforms from a single event. The number of sensors used determines the volume of the monitoring network. The volume of the network will vary depending on the critical structures or stopes that are assessed. Seismic sensor configuration can be uniaxial or triaxial. Triaxial configurations are able to assess event magnitude, seismic energy and seismic moment, whereas the uniaxial geophones are more accurate in locating the source hypocenter. Uniaxial sensors are better for coverage in the mine, whereas triaxial sensors are better for post-processing seismic results.
In reality, the location of the seismic event will not be known, as the location of the seismic event will vary, as a result of dynamic stresses and deformation within the mine. However, the preliminary stability design of the mine should have a valuation of the at-risk structure and stopes of the mine, and the seismic monitoring systems should be configurable to the at-risk structures. Overall, geophone monitoring networks are recommended to monitor seismicity in a mine, as they are easy to install and the network can be configured to assess the particular failure mechanisms of the mine. If fault surfaces are exposed in the excavations, evidence of fault slip (movement along the fault surface) or strain should be observed.
Routine inspections of the excavation surfaces will provide information about the influence of seismic stress and seismic events. Seismicity in a mine will produce active ground, which will alter the observable surface of excavations. Overall, displacement of rock is indicative of changes in the stress field, and will provide information regarding the mine seismicity .
Proper inspection of the mine will include the excavation surfaces, exposed fault surfaces, failure of ground support, fresh loose rock and changes between these focuses before and after a large event. Stopes and drifts near to current development and production stopes should be inspected regularly. On the excavation surfaces, fresh cracks, buckling and loose rock are important to observe, as they may indicated changes in the stress field, as displayed in Figure 3. If there are exposed fault surfaces present in the mine, check if there is evidence of fault slip (movement along the fault) or strain. Joint surfaces are including in this description, as some joint surfaces may slip and become active faults due to a change of field stresses. Failure of ground support and additional loading of ground support are both indicators of changes to the stress field. Borehole breakouts, as displayed in Figure 4, can indicate the direction of the principle stress on a 2D plane, which are useful to quickly assess the change of the stress field in a particular drift.
Comparing inspection information with monitor data will allow for calibration of the data set and failure mechanisms present in the mine. Implementing regular, routine inspections are beneficial to determine a history of stability and excavation performance as development of the mine progresses.
Analysis of Induced Seismicity
Data analysis from information collected from a seismic monitoring network in a mine will determine the seismic parameters of the separate seismic event clusters in the mine. For each seismic cluster, seismic parameters are to be analyzed in order to design for mitigation of seismic damage. Common seismic parameters that will be discussed in this article are maximum expected magnitude, the Gutenberg-Richter relationship and source mechanism. Additional seismic parameters are discussed in. Understanding the seismicity hazard and failure mechanism of each event cluster indicates risks in the mine plan. The seismic hazard is assessed from the seismic likelihood and consequence which are determined form the Gutenberg-Richter relationship and the maximum expected magnitude, respectively.
To assess the seismicity in a mine, the single events must be clustered according to event location and time. There is high inherent variability in mines due to production and development blasting and equipment noise as well as from rock failure, therefore clustering of events is an important step. Seismic events will commonly cluster near in geologic discontinuances or at excavation boundaries, as displayed in Figure 5, where there are high concentrations of stresses and the difference in material properties will allow for higher changes in the stress field to occur. Commonly, individual event clusters will have different seismic sources.
Events are clustered by location of the event hypocenter as well as the time of the event. With digital monitoring, event timing is easily assessed. To determine the location of the seismic source, the differences between arrival times of the incoming waveforms to the geophone sensors are recorded, in order to find the distance of the hypocenter from at least three individual sensors with an assumed value for seismic travel velocity. Figure 6 displays the computation of a hypocenter from the arrival times of three individual sensors.
Seismic Source Mechanisms
The failure mechanism for each cluster of events may differ. To reduce complexity in the analysis, assessing the failure mechanism sources should be completed independently for each event cluster. The failure mechanism will vary between different clusters of events, due to the variation of failure mechanisms driving the seismicity, as displayed in Figure 7. Seismic source mechanisms in mines are either shear failures of geological structure, fault slip, or from tensile rockmass failure. From a variety of analysis techniques the source of the seismicity mechanism can be determined, whether it is from rockmass failure of a pillar or stope wall, or fault slip along a geological structure.
To differentiate fault slip failure mechanism from the rockmass failure mechanisms, P-waves and S-waves of events within the cluster are assessed. The ratio of seismic energy between the S-waves and P- waves indicate the type of failure mechanism for that event cluster. Dilatational, compressive or tensional, failure mechanisms, such as strain bursting (rock bursts), caving and pillar failure, produce more energy in the compression waves, P-wave. On the other hand, shear failure mechanisms, such as fault-slip on geological structure such as joints, faults and geological material boundaries. There is always more S-wave energy than P-wave energy, thus the S:P Ratio for dilatational failure is 3 -1 and the S:P Ratio for shear failure is >10. If the S:P ration of the event cluster is between these values, the failure mechanism will have both shear and dilatational failure mechanisms.
In the case of shear failure, the orientation of the fault plane, displayed as the moment tensor solution, can be determined through fault plane solutions (or also referred to as focal mechanism). The waveforms detected from a fault slip event will have either compressive or dilation polarity at the first arrival of the waveform, as displayed in Figure 8. Assessment of the waveform polarities at many seismic sensors surrounding the hypocenter will produce a moment tensor, as displayed in Figure 9. The moment tensor displays the orientation of the fault surface as well as an estimate of principal stresses present to activate the fault slip.
As previously discussed, there is more energy present in shear-waves that in pressure-waves. This is also displayed above in Table 1, where the seismic source mechanisms are generalized. Table 1 presents fault slip and shear failure mechanisms with greater energy. Ortlepp’s work also presents many other characteristics and patterns of rockbursts and seismic events with the seismic source mechanism.
Maximum Expected Magnitude
The maximum expected magnitude is a method to assess the consequence of a seismic event. The seismic moment and magnitude can only be assessed with triaxial sensors . Uniaxial sensors are unable to determine the seismic magnitude as the maximum displacement is unable to be assessed. The seismic magnitude is a measure of the ground displacement to assess the energy release and associated damage with an event, thus is a measure of consequence. The magnitude is calculated as follows:
Where A is the maximum displacement, T is duration of time and C is a correction factor for the path effects, site response and region of the seismic source. The maximum expected magnitude can be estimated by plotting the number of events of a specific magnitude to magnitude, as completed in the Gutenberg-Richter Relationship. Events of a larger magnitude occur less frequently than those of smaller magnitude, and when there is a large risk of a large seismic event there will be an increase in the number of smaller events . The intercept of the x-axis in the relationship, as displayed in the figure below, in the Gutenberg-Richter Relationship, is the estimated maximum expected magnitude.
This is related to the maximum expected magnitude estimated from the seismic source mechanism, as displayed in Table 1 .
The Gutenberg-Richter relationship is an assessment of the frequency at which seismic events of certain magnitude will occur. There is a power lase relationship between seismic event frequency and event magnitude. This relationship is as follows: .
Where M is event magnitude, N is the number of events equal or greater than the magnitude specified, and a and b are site constants, where a is data quality and b is the power law exponent. The b-factor is related to the likelihood of a seismic event occurring and is used to calculated the seismic hazard. The b-factor will vary greatly depending on the seismic source of the events, and will be individually determined for each separate cluster. Figure 10 displays the b-value analysis of two separate event clusters. Smaller b-values (<1) are indicative of fault-slip source mechanisms, and higher b-values (1.2 to 1.5) are indicative of a failure mechanism due to changes of the stress field. There is a larger probability of large magnitude events occurring when the b-value is smaller.
Seismic Hazard Magnitude
The seismic hazard is assessed for each event cluster, from the seismic likelihood and seismic consequence. The seismic likelihood is determined through the b-value of the Gutenberg-Richter relationship, and the seismic consequence is related to the maximum expected magnitude. Figure 11 displays the estimation of hazard magnitude, where the area under the Gutenberg-Richter relationship between the maximum expected magnitude and the minimum magnitude to cause damage. The minimum magnitude to cause damage is dependent on the condition of the mine excavations.
Seismicity is assessed in mines in order to reduce production delays and to improve mine safety. It is not possible to predict the location and timing of future rock bursts. However by understanding the common seismic failure mechanisms and high hazard clusters, damage can be successfully prevented and mitigated through strategic planning of the mine. Generally, damage from seismicity is driven by induced stresses from mining activity, geological structure and weakness of the rockmass . Methods to prevent and mitigate seismic damage include controlling stresses, continuous monitoring, implementing ground control and implementing re-entry time .
Controlling stresses in the rock effectively will minimize seismicity, by way of induced stresses from mining activity driving seismicity, as well as open voids. By controlling the progression of excavations, the evolution of stresses mine can be controlled, in order to reduce seismicity and mitigate damages. Changing the stope sequencing and mine layout will alter the seismicity. In areas of high risk, changing the stress field may reduce the risk for that region. This can be achieved by advancing the stopes upwards as a ‘chevron’ stope pattern (link), so that the high stresses will shed to the outside of the stopes and isolate the stresses from the excavations .
Another method to control stresses is to induce failure in specific high-risk regions. In particular, dykes and sills, which are commonly brittle rock, are prone to rockbursts and store more strain energy. By distressing and preconditioning the rock, the rock stiffness is decreased along with the ability to store strain energy, which decreases the probability of an uncontrolled brittle event. Distressing rock also allows for high stresses to be shifted elsewhere in the mines, decreasing the chances of a local rockbursting event. Blasting can also be used to trigger strain energy and strain relief. Rock burst events can be induced from blasting to induce failure in the surrounding rock, rendering it unable to retain high loads, thus decreasing the event magnitude .
As mining progresses, the stresses and seismic failure mechanisms present will evolve. In order for the seismicity changes to be observed, continuous monitoring through instrumentation and inspection should be implemented in all mine operations. Changing mitigation plans in mines is common and the monitoring network can be altered and improved as the seismicity progresses and evolves. Overall, damage prevention and mitigation is more application to mines, however smaller events will always precede a larger event in order to assist is estimating when and where a large event will occur .
Microseismics is used to assess small variation in rock from small seismic events. From observation of small events from excavation, potential failure planes can be determined. Through stress inversion, the stress field surrounding an excavation can be computed. A common issue in mines is repeated seismic loading, where some failure mechanisms will fail, but strains will continue to build causing more future failure. Microseismic monitoring can be utilized to identify failure mechanisms of repeated seismic loading .
In seismically active regions the deformation and damage that result from seismic activity effect the ground control utilized. From dynamic stress conditions of the rock and deformation, active ground support is needed. Frictional bolts, such as Split Set and Swellex, allow for movement of the excavation before the bolt reaches capacity. Support that will absorb energy should also be implemented to prevent collapse, such as Conebolts and yielding support. Pre-tensioned bolts are used in seismically active areas along with dynamic support in order to interlock the blocky rockmass and increase energy absorption. To contain falling rock, it is recommended to implement mesh or shotcrete to the excavation walls. Due to the high tensile stresses present in the seismic waveform, it is recommended to implement reinforced shotcrete with higher tensile strength than plain shotcrete .
Re-entry time is dependent on the seismicity present after blasts. If the mine or specific stopes are seismicity active after blasting, personnel and equipment are unable to enter the area of risk. Before re-entry, stabilization must be ensured after blasting. In some cases, it may take 5 hours for stresses to reach equilibrium after a blast . If seismic events are occurring at times that are unrelated to blast timing, such as fault-slip events, then the re-entry protocol cannot be dependent on managing blasting alone. Re-entry is timed to be when the seismic activity returns to the normal level. An estimate for timing can be determined from the Omori Power Law.
Where N is the number of events within a time period, t is the time period following the blast, and k and p are constants related to the total number of events and decay time period, respectively.Figure 12 displays an example of seismic event decay after blasting. The decay analysis provides an assessment of the decay of seismicity within a specific time period after blasting, and can be used to design the re-entry protocol of the mine .
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