Occupational safety and health

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AngloGold Ashanti uses seismic systems developed by Integrated Seismic Systems International Ltd (ISSI) to assess 'ground behaviour', in an effort to reduce fall of ground incidents and accidents at its mines. Anglo American Corporation's Gold and Uranium Division had initially set up a research centre in 1985 to develop seismic systems at its Free State goldfields. By the time systems had been developed in 1988, a number of other South African gold mining companies, as well as other countries, were interested in the technology that had been developed. ISSI was then established as an independent company, whollyowned by Anglo American, catering for mining and non-mining environments. It then became a subsidiary of AngloGold when the company was formed in 1998, and more recently of AngloGold Ashanti following the business combination in 2004.

The seismic systems are a network of seismic stations that detect and record energy releases in the ground. This information is then transferred to the surface where it is processed, entered into a database, analysed and interpreted. AngloGold Ashanti has a computer aided design-based set of mine plans for all its underground operations and, when linked to the input from the seismic monitoring and analysis system, the performance of the mine plans can be determined. Essentially, the data is converted into management information which has a range of advantages, and allows AngloGold Ashanti to optimise its mine design.

The actual measurement of ground movement is with sophisticated measuring devices, called accelerometers and geophones. State-of-the-art in their design and sensitivity, these sensors, which monitor all underground movement, are able to distinguish between rock noise and mechanical noise, as made, for example, by locomotives. While all AngloGold Ashanti mines have sensors, the number of sensors installed depends on the mine's seismicity, the intensity of monitoring and size of the lease area. Tau Lekoa mine, for example has six sensors, while Great Noligwa has up to 60. Each network's seismic system has a defined area of accuracy, and seismic information is shared, not only with other mines, but also with industry and government on request.

Although the mining industry uses various magnitude scales to measure seismicity, the most commonly referred to is the Richter scale. To approximate the energy indicated by the Richter scale ranges, an event stated as magnitude -1.5 (the smallest event discernible by our seismic network) equates to energy released by detonating 3 kg of explosive whereas an event stated as a magnitude 10.0 (not noted in recorded history) approximates to the energy released from a one teraton explosion. Two types of seismic event are measured - earthquakes, where ground moves typically along faults or other geological-type structures; and rock bursts where mining-induced stress exceeds the rock's strength causing it to explode. In mining terms, a seismic event of magnitude 3.5 (73 metric tons) would be relatively large, but small in earthquake terms - earthquakes with magnitudes of 4.5 or greater are strong enough to be recorded by seismographs all over the world, including the South African government's nationwide seismic network.

ISSI's seismologists work closely with AngloGold Ashanti's rock engineers in linking seismic data with mine design. Mine design and layout is implemented in such a way that predicted large events are either mitigated or eliminated. Support systems are then installed to withstand these large events. (See case study: Fall of ground management in South Africa.) The success of the mine design layout and support has been proven at Great Noligwa mine where there have been several seismic events of 3.5 magnitude with zero injuries.

Another critical aspect in rating seismic events is not just the magnitude, or energy release, but the location or epicentre, which refers to the point on the earth's surface directly above where an earthquake has occurred, whereas the hypocentre measures the depth at which it occurred. The majority of major seismic events in the mining industry are related to geological anomalies and, if they occur at some distance from the workface, are fairly harmless. A seismic event which took place on 12 October 2005 at Great Noligwa mine at AngloGold Ashanti's Vaal River operations, measured 4.9 on the Richter scale, but took place 5.5 km below surface - which was about two kilometres below operations - and did not affect mining operations.

A common misconception about seismic monitoring is that actual seismic events can be predicted. However, one can only predict the likelihood of an event but not when and precisely where it will take place. Says Nigel Trevarthen, head of mining and mineral resource management at AngloGold Ashanti and recently appointed to the board of ISSI, "We use reality to verify our models and we use models in the design of our mines. Where monitoring shows that seismic activity is building up in a certain area, the mining methodology and strategy is revised. A case in point is seen at Great Noligwa when we decided to move six crews from an area of the mine as a precautionary measure in July 2005. This decision was reported to the market as one of the causes of a drop in the mine's production. The decision was vindicated when, following a 4.7 magnitude earthquake at the mine, no serious injuries were reported."

Seismic monitoring, an important tool in minimising fall of ground-related accidents and incidents, is constantly under review through ISSI's research projects. The secondment of a rock engineer to its Integrated Damage Rheology Model (IDRM) project coupled with the close relationship between ISSI and AngloGold Ashanti through dint of ownership, ensures that the latter is at the forefront of cutting-edge seismic system technology.