Seismicity risk management – increasing output in a tight economy

By Ricardo Ferreira and Yi Huang, Golder Associates

As the economy tightens and the availability of extractable ore decreases, mining companies are naturally looking for ways to extract more value from their existing deposits. One way of doing this is by “high-grading” their deposits, and increasingly, by recovering ore from high grade pillars in previously mined areas of their operations, or mines that are reaching maturity.

But while there can be good financial returns, remnant pillar mining isn’t the immediate golden egg it first appears to be. Serious consideration must be given to the technical challenges involved. This means that allowance has to be made for seismicity induced within the ground during normal operations, and all risks - including safety, technical and financial - must be considered before a decision is made to commence with remnant pillar mining.

It’s standard practice in countries such as South Africa, where mines are deeper and mining usually takes the form of tabular excavations instead of the more massive-style underground mining that is common in Australia. But seismic monitoring is often considered as a “nice-to-have” in Australia, rather than a necessity, with mining engineers generally waiting until they are on the verge of getting ground deformation or until conditions start to look dangerous before considering bringing in a seismologist.

A seismologist would argue, as in South Africa, that seismic monitoring is vital from the first day of operations in order to guide the mining process and provide valuable baseline data against which to make later decisions. It is easy to begin mining with preconceived ideas about how the process will go, which don’t always correspond to reality. The seismic “rock talk” is a universal language which provides a window to see the way the rock is moving and provide an accurate location of events with comparable and consistent analysis and interpretation.

An ongoing seismic monitoring program throughout the life of the mine also allows for strategies to be quickly devised to control each seismic risk as it is encountered, according to the geotechnical setting. This saves both time and money, and can contribute to a safer working environment. And in a mine with long-term seismic monitoring, historical data allows for simpler decision-making about “value-added” mining methods such as remnant pillar mining.

An in-depth seismic monitoring program can also greatly increase ore recovery rates. By monitoring the rock mass response to mining, a deep understanding of how much of the ore is being extracted compared to the amount originally planned, can be gained. Most seismicity is highly localised around extraction areas, so the seismic progression can give a very good indication of where rock is breaking, and how much is caving.

It’s true that seismic monitoring isn’t cheap; but it can be excellent value for money. By integrating a seismic system with remotely monitored ground movement monitoring techniques (such as extensometers, stress cells and co-axial cables) or geophysical imaging techniques (such as resistivity and microgravity), the additional ore that may be extracted due to the detailed knowledge of the ore position and ground deformation characteristics may well pay for the cost of the seismic system many times over. Furthermore, by adopting early implementation of a comprehensive and robust seismic monitoring regime, mining engineers will generally have a much clearer understanding of the mining process, with better risk management inevitably leading to increased peace of mind.

And for those who say that seismic monitoring is too complicated or complex for the average mine operator to understand, recent advances in data processing and software systems for management and analysis of seismic monitoring data have resulted in the analysis of seismic data becoming much more user friendly.

For further information, contact Ricardo Ferreira on tel: +61 8 9213 7600 or email: riferreira@golder.com.au

Caption for image:

A three-dimensional representation of a cluster of seismic events below an open-pit mine (the same theory applies to underground mines). Seismic events are displayed as coloured spheres; warmer colours denote higher event magnitudes. The coloured grids represent the expected location error of the seismic system, with warmer colours representing higher location error. Note that high location errors are only observed on the outskirts of the mine area.