INTERMEDIATE AND LONG-TERM HAZARD
Assessment of intermediate and long-term seismic hazard
The assessment attempts to evaluate the probability of a large seismic event occurring in a specific area of the mine within a certain interval .
The following techniques have been routinely used in South African gold and platinum mines since 2000.
Types of seismic hazard assessment
Long-term assessment relates to the planning of future mining, and generally relies on a numerical modelling (e.g. calculation of ERR and ESS). 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.
Medium-term assessment is implemented on a monthly basis. The areas of interest are identified using seismic data (clusters of seismic events), geological settings (structures) and mining layout (active stopes). The following types of analyses are applied for each area:
- Time history of seismic parameters (apparent stress and 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 to 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 recognise an anomalous response to production.
- The seismic proxies of stress (energy index and apparent stress) are compared to corresponding characteristics derived from numerical modelling (average pillar/abutment stress).
Finally, a hazard rating of the areas of interest is provided.
Short-term assessment is conducted daily or as often as a per-shift basis. This includes monitoring the behaviour of a set of seismic parameters for each area of interest. Examples of these seismic parameters include seismic activity rate, energy index, cumulative seismic displacement and seismic Schmidt number. The hazard rating is then assigned according to variations in these parameters.
I think ‘time’ should be in the sentence. Interval could refer to something else.