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
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
ISSI's core business is "the development and marketing of technologies, methodologies and services in monitoring and modelling the seismic response of rock mass and engineering structures to natural and/or induced forces, as well as safety and environmental policing". Its field of expertise covers seismic monitoring and
modeling of, for example, mines, dams, municipalities and cities (for example, San Francisco which lies on the San Andreas fault line) and has now extended to assisting volcanologists in the study of volcanic activity.
The company has four divisions - a technology division based in Stellenbosch in the Cape; mining and customer divisions at AngloGold Ashanti's West Wits operations; and ISS Pacific based in Perth, Australia. Each division has a specific focus:
- technology: research & development, software engineering, and system engineering;
- mining: seismological processing, analysis & interpretation (SPAI) and modelling with data;
- customer services: hardware & software maintenance, customer support and technical services; and
- ISS Pacific: system engineering, SPAI, and modelling with data.
Seismology-related services applicable to the mining industry include:
- support: technical and operational assistance of system design, installation, training and maintenance;
- seismological services: routine seismological processing, analysis, interpretation and reporting;
- modelling: mine layout design audit for rockmass stability and integration for controlled mining; and
- engineering seismology: measurements and characteristics of ground motion and structure vibration.