Seismic Monitoring in Mines

Objectives in Seismic Monitoring

• The most common objective of seismic monitoring in underground mines is to locate areas in which rock mass failure is causing seismic events. Temporal and spatial understanding of seismic events helps to minimise seismicity related mine production delays, while facilitating the identification of potential safety hazards due to changing ground conditions.
  
  

  Understanding mine seismicity can be achieved through developing insight into two factors, seismic hazard and seismic source mechanism. Seismic hazard is generally defined as the likelihood of occurrence of a seismic event of a given size. Understanding seismic hazard is important as it gives indications of the potential size of future seismic events. Analysis of seismic hazard aims to identify the parts of a mine more likely to have large seismic events.
  
  

  Seismic source mechanism is the rock mass failure mode that causes a seismic event. Understanding seismic source mechanism is important for two main reasons. Firstly, seismic source mechanism often dictates when seismic energy will be released. For stress fracturing related seismic source mechanisms, seismic energy release usually occurs as a result of stress change due to mine blasting. For seismic source mechanisms related to geological features such as faults, the release of seismic energy may be quite poorly related to discrete blasts.
  
  

  The second important implication of seismic source mechanism is related to event size. Various seismic source mechanisms have different characteristic maximum event sizes. For instance, stress change seismic events are rarely larger than Richter magnitude 0 to +0.5. Pillar bursting seismic events can be up to about Richter magnitude +2 to +3. Fault-slip seismic events can be up to magnitude +5, however in Australian mines, these events are rarely larger than Richter +3. By understanding the seismic source mechanism, an indication of the worst-case seismic event magnitude is suggested.
  
  

  Seismic hazard and seismic source mechanism are dominated by the factors controlling the local rock mass failure, namely: stress, geology and mining influences. Consequently, hazard and mechanism in mines vary in time and space.
  
  

Definitions

• A seismic event is a transient dynamic stress wave caused by failure or fracturing in a rockmass. Effectively, a seismic event is the sound (vibration) of rock slipping or rock breaking. Seismic events are a normal response of a rockmass to the stress changes caused by the creation of mining excavations. Most underground mines have seismic events that can be heard and felt.
  
  

  The seismic source mechanism is the mode of deformation or failure of a rockmass that causes the seismic stress wave to be created. Typical seismic source mechanisms include:

  • Slip on existing geological features,

  • Creation of new fractures in a rockmass due to high stress or crushing, and

  • Tensile failure of intact rock or a rockmass.
    
    

  Seismic hazard is generally defined as the likelihood of occurrence of events of certain magnitude. There is no widely accepted measure used to quantify seismic hazard. If the likelihood of large seismic events is high, seismic hazard is considered high. If the likelihood of large seismic events is low, seismic hazard is considered low.
  
  

  Magnitude is a relative measure of the strength of a seismic event based on measurements of maximum displacement at a given frequency. Numerous magnitude scales have been employed in the past. Richter Magnitude is the most widely recognised magnitude scale. It was originally developed based on earthquakes in Southern California. It is widely used to describe the strength of natural earthquakes.
  
  

  The following table contains a qualitative relation between the magnitude of a seismic event and how it is felt in a mine, based on experience with mine seismicity in several Canadian and Australian mines.
  
  

  Approximate Richter Magnitude	Description
  -3	* Small bangs or bumps heard nearby. Typically these events are only heard relatively close to the source of the event. * This level of seismic noise is normal following development blasts in stressed ground. * Events are audible but the vibration is too small to be felt. * Not detectable by most microseismic monitoring systems.
  -2	* Significant local ground shaking. * Felt as good thumps or rumbles. * The event may be felt more remote from the location (more than 100 metres away). * Often detectable by a microseismic monitoring system.
  -1	* Major local ground shaking. * Often felt by many workers throughout the mine. * Should be detectable by a seismic monitoring system. * Similar vibration to a distant underground secondary blast.
  0	* Vibration felt and heard throughout the mine. * Bump may be felt on surface (hundreds of metres away), but may not be audible on surface. * Vibration felt on surface similar to those generated by a development face blast.
  +1	* Felt and heard clearly on surface. * Vibrations felt on surface similar to a major production blast.
  +2	* Vibration felt on surface is greater than very large production blasts. * Geological survey earthquake sensors can usually detect events of this size.
  +3	* Event is detected by earthquake monitors throughout Australia.

  • Richter magnitude of a seismic event and how it is felt in a mine.