Seismic Hazard Assessment
What is seismic hazard assessment?
The assessment attempts to evaluate the probability of a large seismic event occurring in a specific area of the mine within a certain time interval.
Types of seismic hazard assessment
The Long-term assessment is related to planning of future mining and generally relies on a numerical modelling (i.e. calculation of ERR, ESS and others). Seismic data is mainly used to verify and calibrate the model. Historical seismic data is also extremely useful for mine planning in and near old mined out areas.
The Medium-term assessment is implemented on a monthly basis. The areas of interest are identified using both seismic data (clusters of seismic events), geological settings (structures) and mining layout (active stopes). The following types of analysis are applied for each area:
- time history of seismic parameters (Apparent Stress, Hazard Magnitude) is considered over a long-term period (years where data is available). The patterns of unstable behaviour of the rock mass are identified. Dynamics of these parameters are also compared with the history of the production;
- spatial distributions of seismic proxies of inelastic strain and stress are studied. Potential regions of instability are detected in terms of an asperity model;
- time-of-day distribution of seismicity is examined to recognize an anomalous response to production;
- the seismic proxies of stress (Energy Index, Apparent Stress) are compared with corresponding characteristics derived from numerical modelling (average pillar / abutment stress).
Finally a hazard rating of the areas of interest is provided.
The Short-term assessment is done either daily or even on a per-shift basis. This includes monitoring of the behaviour of a set of seismic parameters (for example seismic activity rate, Energy Index, cumulative Seismic Displacement, seismic Schmidt number) for each area of interest. The hazard rating is then assigned according to variations in these parameters.
The above techniques have been routinely used in South African gold and platinum mines since 2000.