Seismicity is often observed to occur during underground mining activities. This happens because the creation of mining voids (tunnels, stopes and excavations) changes the stress conditions in the rock. These stress changes may be large enough to fracture the rock, resulting in seismic evens. Deeper mines generally experience a greater seismic hazard because of higher stresses.
Olaf Goldbach, business development manager at the Institute of Mine Seismology, explains that in order to manage the seismic risk, the relationship between mining and seismic activity needs to be understood.
Seismic monitoring objectives
Routine seismic monitoring has become an integral part of assessing the seismic hazard in mines. The monitoring objectives should be formulated up-front in a seismic hazard management plan (SHMP) and include:
Rescue: To detect and locate potentially damaging seismic events, to alert management and to monitor seismic activity during rescue operations.
Prevention: To confirm the rock mass stability related assumptions made during the mine design process and to adapt the design while mining so that seismicity can be prevented or better managed. For example, the seismic information can help to delineate geological structures, resolve the direction of principal stresses, determine the expected ground motion produced by large seismic events using a ground motion prediction equation, monitor the propagation of a cave front or measure the levels of ground motion on the surface of excavations for support design.
Hazard assessment: To quantify the expected exposure to seismicity in terms of the probability of occurrence, within a specific period of time in a given area, of a potentially damaging event. Short term hazard assessment helps determine the re-entry period into the working areas after an increase in seismic activity caused by larger seismic events or after production blasts. Long-term seismic hazard assessment estimates the size of the next record-breaking large seismic event and the probabilities of occurrence in time and in space.
Alerts: To detect sudden and unexpected changes in the spatial and/or temporal behaviour of using short-term activity tracking that may indicate rock mass instability and affect working places in the short-term. Such unexpected changes in seismic patterns should be accompanied by an appropriate trigger action response plan (TARP).
Back analysis: To improve both the mine design and seismic monitoring processes. All seismic events regardless of size that resulted in a fatality, injury or damage should be back analysed thoroughly. It is also advisable to back analyse all near misses, i.e. seismic events that did not result in consequences, but had the potential to do so. Results of such forensic back analyses should form the basis for a regular critical review of the applied seismic risk management strategy, guidelines and procedures, included the SHMP and corresponding TARPs.
A seismic system needs to be sensitive enough to record small seismic events (or cracks in the rock). This information is required to assess the seismic rock mass response to the changing stress conditions caused by mining so that preventative measures can be initiated.
On the other hand, larger seismic events also need to be recorded in order to assess the seismic hazard, i.e. the probability of occurrence, within a given time period in a given area, of potentially damaging events, and to design rock support. Today’s digital data acquisition equipment has a very high dynamic range, allowing it to record a wide range of ground motions resulting from small fractures to large rockbursts.
The general principle is that the denser the seismic array (i.e. greater number of sensors in a given volume of rock), the more sensitive the seismic system, (allowing smaller events to be recorded), and the greater the location accuracy of the recorded seismicity.
Different sensors are used depending on the levels of ground motion to be recorded, e.g. geophones or accelerometers, or a combination of both. In practice, the number and choice of sensors in a seismic system is determined by the size of the mine and its monitoring needs. A tailored solution is offered to each client.
Knowledge is power
Most mines start by installing a seismic monitoring system to detect large seismic events, such as rockbursts, as these occurrences impact on the safety of mine workers and can damage mining infrastructure and equipment. At its most basic level, a seismic system provides the following information:
- When did the seismic event occur?
- Where was it located?
- How big was it?
The two quantities that describe the size of a seismic event in a physically meaningful way are seismic energy and seismic potency (or moment); the commonly quoted ‘magnitude’ scale is arbitrary and non-physical. However, modern digital seismic recording equipment, along with the latest seismological analysis methods, allow one to deduce much more information about the nature of the rock mass.
In addition to its role as a mine safety tool (detecting large seismic events), a seismic monitoring system is often also used as a mine design tool. Careful examination of the recorded vibrations, the tracking in space and time of seismicity and sophisticated inversion methods such as moment tensor analysis reveal important details about the changing stress states of the mined rock mass, the levels of ground shaking at various points around a mine and the failure processes leading to rockbursts. This information is important for mine design, the choice of mining method and layout planning, as well as support design considerations.
These days, in order to achieve the seismic monitoring objectives defined in the SHMP, the recorded seismic data is combined with numerical stress modelling, to obtain a more holistic understanding of the changing stress conditions resulting from mining activities.
The Institute of Mine Seismology (IMS) has been a leader in mine seismic monitoring for 29 years and with more than 260 seismic systems that are currently operational in 33 countries around the world. In Africa approximately 70 IMS seismic monitoring systems are installed in mines in South Africa, Zambia, DRC, Tanzania, Namibia, Mali, Lesotho and Mozambique. www.imseismology.org