ining operations, both open pit and underground, like the majority of large public and private enterprises, rely on the continuous operation of capital-intensive infrastructures to meet operational or production targets. However, unlike more conventional ventures, mining can be interrupted by unanticipated seismic shocks in seismically active areas.
Mineral processing plants, in particular, contain a multitude of large rotating machines that must run continuously to meet production targets. Mills and pumps, for example, are designed to operate within an envelope that takes into account variations in loading due to changes in feed and flow rates or work index, but their bearings are typically not designed to deal with the peak vertical and horizontal accelerations associated with seismic activity.
Similarly, large conveyors, skips and personnel hoists are typically not well suited to withstand significant horizontal accelerations when operating at full speed. Fortunately, we recognized these potential problems and have responded with an innovative and technologically sound answer.
In the event of an earthquake of a magnitude great enough to cause damage to processing or hoisting equipment, operators can now bring equipment to rest before the onset of any seismic event. By doing so, the forces due to accelerations associated with an earthquake do not combine with the normal operating loads to cause irreparable damage to equipment.
The technology we’ve developed is simply called the Earthquake Early Warning System (EEWS) and provides an advanced warning of an impending earthquake by recognizing that the destructive energy of an earthquake is related to large S-waves that are preceded by the smaller-amplitude P-waves arriving on site seconds earlier.
EEWS monitors ground motion in real time to detect the presence of P-wave signatures of a remote strong earthquake. The equipment consists of a set of triaxial vibration sensors mounted at various depths in two boreholes located at both ends of the critical facility.
Technically, mechanical vibration is converted into a fluctuating voltage signal that is digitized using 24-bit analog-to-digital converter. Multi-channel data streams are merged by the data acquisition equipment and sent to the central processing computer. Time synchronization is achieved using GPS technology. A central computer runs the seismic detection and proprietary classification software, which continuously analyzes data streams and triggers only when a set of multiple criteria is met.
Due to the multiple redundancies built into the system, a failure of individual components has no effect on the overall performance. For example, if a subset of the sensors fails, the software parameters will be adjusted to work with the remaining ones.
The output of the EEWS is a binary signal, i.e. yes/no, reporting the absence or presence of a precursor of the strong ground motion associated with the S-waves. If the early warning system reports a positive state, meaning a target P-wave is detected, the central computer will generate a visual and audio alarm, and will close the electric circuit connected to the external alarm sub-system. This autonomous decision-making takes less than a second.
This external system might be a control closure mechanism that turns off critical processes at the protected facility to minimize potential damage in terms of human lives and/or valuable equipment. Since shutting down the processes is usually associated with high costs, the system is required to be highly reliable and immune to false alarms.
Design and installation of an on-site seismic early warning system are more art than science, since local circumstances are unique and the system architect must take into account a series of parameters that will influence the effectiveness of the entire project.
In our experience, the cost of this technology is only a small fraction of the damages that may result should an earthquake strike a mine site.
*Special Report provided by Iain Weir-Jones, PhD, PEng, a mining engineer and president of Weir-Jones Engineering Consultants Ltd. Vancouver, and chairman of the Weir-Jones Group of Companies. Anton Zaicenco, PhD, is a seismologist and earthquake engineer at Weir Jones Engineering Consltants.