In the latest issue of its Digging Deeper newsletter, AMC’s Julian Watson, Senior Geotechnical Engineer, explains AMC began leasing seismic systems in Australia almost ten years ago in response to an increasing demand for installation of small-scale portable seismic systems in mines. The same seismic system leasing, event processing and analysis service will be available from AMC’s North American offices from August 2011. The service is based on reliable and relatively low cost smaller-scale portable or ‘stand-alone’ systems. Such factors make them attractive to operations that have started to experience minor seismicity, but which cannot justify the significant capital investment for a full mine-wide system. They are also cost effective for smaller operations with a relatively short mine life. Other applications where AMC has installed portable systems include: 

• Operations with rapid vertical advance requiring regular relocation of the sensors to keep up with the mining front
• Smaller satellite stoping areas that cannot easily be connected to an existing mine-wide system
• Pillars, including crown and ‘closure’ pillars subjected to increased loading.

The service provided by AMC includes the leasing of the seismic box and sensors on a monthly basis. This will normally include the design and installation of the system and processing of the seismic data. However, this can also be done by mine personnel with some training and assistance from AMC. If required, AMC can also provide detailed analysis and reporting of the recorded events. This type of study is often integrated with numerical modelling, allowing the seismic activity in the mine to be related to stress changes due to mining.

The Stand Alone GS (SAGS) unit used by AMC is a digital seismometer manufactured by the Institute of Mining Seismology Limited (formerly ISS), in South Africa. The SAGS unit is designed for use in applications where connection to central computer networks is not possible. The SAGS can determine its own timing (set from a GPS) and parameters without reference to a central computer. A maximum of six uniaxial sensors can be connected to a single unit. However, several SAGS units can be linked, allowing the addition of more sensors, which in turn provides greater flexibility and larger array coverage.

Once installed, AMC can provide ongoing support for the system over the internet. This allows AMC to connect remotely to review and maintain the SAGS unit. Once implemented, remote support can be provided by AMC from any location around the world. The seismic data collected by the SAGS unit can be used in a number of applications, including:

• Seismic hazard assessment
• Delineation of seismically active geological structures or pillars
• Monitoring of rock mass instability, including the initiation and propagation of caving in caving operations
• Identification of the transition from low stress to high stress conditions where increased level of support may be required
• Calibration of rock mass properties for use in numerical modeling studies.

AMC has many years of experience in the use of IMS seismic analysis software, which is capable of full waveform seismic source parameter analysis. All downloaded seismic events are manually processed to ensure accurate source locations and to remove any blasts or spurious events.

AMC can provide a detailed source parameter analysis of the recorded seismic event population. This includes the use of AMC’s in-house software to group or cluster spatially similar events or to group events that are closely related in time.

Types of analysis include:

• Temporal variations in seismic source parameters
• Energy, Moment, Energy Index, Apparent Volume
• Magnitude-frequency plots
• Event decay following blasts and re-entry times
• Source mechanisms analysis
• Analysis of ground motion parameters, including Peak Particle Velocity (PPV) and Peak Particle Acceleration (PPA).

These analyses are used in the management of seismic hazard. Management options usually include upgraded support, exclusion zones and periods, or changes to the mining method and extraction sequence. All of these options involve expenditure and disrupt normal mining operations to varying degrees. To develop an effective, properly focussed seismic hazard management plan requires an understanding of the timing, magnitude and location of seismic events in the mine and the main controls on this seismicity.

To aid in the assessment of seismic hazard, AMC routinely integrates numerical modelling with the interpretation of seismic data. The advantage of numerical modelling, assuming there is some model correlation between modelling and seismicity, is that it can be used to identify areas of elevated seismic hazard well in advance of mining, allowing sufficient time to take remedial measures or develop an alternate mine plan.