By Taylor Milne, Andrew Weir-Jones, and Alireza Taale
With mine- and drill-site safety becoming an increasingly crucial topic in many projects today, new technologies are arising to seismically monitor the structural integrity of any mine or drill site that may consist of rock structures or structural discontinuities.
Rock masses exhibit extremely complex behavior, and rock mechanics and ground control have been the subject of considerable fundamental and applied research throughout the world since the 1950s.
Ground control requires an understanding of structural geology, rock properties, ground water and ground stress regimes, and of how these factors interact. Tools include the methods of site investigation and rock testing, measures to minimize damage to the rock mass caused by blasting, the application of design techniques, monitoring and ground support.
Several important developments have taken place in rock mechanics and ground control in recent years, including the development of ground monitoring instruments, which record, digitize and process acoustic emissions produced by micro-earthquakes (pops and cracks) in order to provide an insight on initiation and evolution of fractures or rock mass failures which is often referred to as Passive Microseismic Monitoring (PMM).
PMM can be deployed relatively easily and can provide enough reliable data to make decisions about the probability of structural failure within a mine or drill site. A proper PMM installation will help monitor the structural integrity of a mine, and avoid induced seismicity events with the potential to put lives at risk and cause substantial damage to equipment and nearby structures.
When an applied load causes rock to fracture some of the released strain energy propagates through the surrounding rock mass as vibrations, which can be detected by the appropriate sensors. In some cases these are felt or heard by people; mineworkers often hear minor events as snapping or clicking noises; small earthquakes, Magnitude 3.0 on the Richter scale, can be felt near the epicenter by the general population.
The fracturing of the rock can be caused by tensile, compressive or shear forces. The location and energy released by the fracture can be determined with reasonable accuracy by analyzing the arrival times and characteristics of the vibrations.
On a global scale this is how seismologists locate the hypocenters and estimate the intensities of earthquakes. On a local scale, the procedure can be used to locate shear surfaces beneath a landslide or pillar failures in a mine, and on a still smaller scale, this is how the extent and location of the induced fractures created within a reservoir by a hydraulic-fracture operation can be derived.
States of Stress
Every natural or human activity on or in the Earth’s crust can cause changes in its state of stress. Under some circumstances these perturbations of the stress field can trigger events which release enormous amounts of elastic strain energy that was stored within the deformed rock mass. An example is the 1906 San Francisco earthquake, which was caused by the shear rupture of the San Andreas fault along a length of nearly 500 km, the actual relative movement across the fault reached nearly 9 m in some locations.
There has been a dramatic evolution of technology over the last 40 years. With state-of-the-art digital recorders, tri-axial down-hole sensor packages and real-time data rendering, engineers can monitor geomechanical phenomena at depths of greater than 10,000 ft. In the mining sector, subsurface stress changes and rock fractures can be monitored in near real-time so precisely operators can shut down immediately if a safety or operational situation arises.
The optimization of the sensor arrays used to acquire passive microseismic data is dependent upon many factors. These include aspects of the array in relation to the expected acoustic source, number of sensors being deployed, acoustic dampening properties of the surrounding rock, surrounding natural and anthropogenic noise that could be heard by the sensors, and of course, accessibility, environmental conditions and work crew safety.
In order to understand how the acoustic waves are travelling from the source to the receiver, we must know the physical properties of the rock the wave is propagating through, specifically its density, seismic velocity and anisotropy. Knowing these factors allows a geophysicist to accurately trace the micro-seismic signal back to its correct source location, with an acceptable degree of uncertainty.
Finally, once the locations of micro-seismic events are known, real-time interpretations can be made with regards to mine safety. This translates directly into savings because on-site equipment, manpower and production all benefit from reduced downtime associated with past practices, in particular the need to shut down operations while local geophysicists examined multiple lines of data.
There are also a couple of different strategies for deploying sensors: the first is a surface array, usually laid out in a grid pattern. These types of systems suffer from inherent noise problems; they tend to pick up acoustic emissions from everything including road traffic, work-over rigs and pumps. For this reason, the number of sensors deployed is substantially more than in a down-hole array where the inherent noise level is much less.
The second type of system is a down-hole system. This type of system is typically designed to be permanent or semi-permanent and it is deployed after an earth model has been developed for a specific job site that will go into production. Alternatively, there may be environmental concerns that need to be monitored over the Life Of Field (LOF). There is some capital cost associated with the deployment of LOF systems but the major benefits include security, no ambient noise to corrupt the data and reliability. These systems can be deployed and require virtually no maintenance over their useful service life, which can easily exceed 20 or more years.
Surface and buried arrays continue to evolve at an exponential rate. Reliability and clarity in the algorithms continue to provide solid data so that operators and concerned parties feel comfortable that mining operations are doing everything in their power to mitigate the potential of a catastrophic failure contaminating groundwater, watercourses or human occupied areas.
PMM provides a useful means of securing the integrity of a mining operation. Each mine is unique and will have different geo-mechanical properties, will require different surface or buried array specifications, and will have different fracture characteristics, however the hazards associated with fluid injection remain consistent.
Taylor Milne, GIT, is a junior geophysicist at The Weir-Jones Group of Companies in Vancouver, Canada. Andrew Weir-Jones is operations manager at The Weir-Jones Group and Alireza Taale is business development manager. The company supplies passive microseismic and acoustic emission monitoring solutions to the oil and gas, mining, and heavy construction industries in North America and overseas. Its website is www.weir-jones.com.