Earthquakes with a local magnitude lower than 2.5 are called microearthquakes and are rarely felt. Human activities in hydrocarbon reservoirs, mining areas, large water reservoirs or geothermal sites induce microseismicity. Those activities involve changes in stress, pore pressure, volume and load, which can result in sudden shear failures in the subsurface usually along pre-existing weakness zones such as fault structures. Long-term microseismic monitoring in hydrocarbon reservoirs has the potential to reveal fracture geometry and to investigate the process of fluid fronts during production. With respect to safety and hazard issues, long-term monitoring can be valuable in evaluating processes resulting in well-casing failures and subsidence. Especially for the mining industry and for nuclear waste disposal sites microseismic monitoring provides valuable input for safety and hazard assessment.
Passive monitoring of microseismicity is nowadays a vital component of modern mining not only to minimize the risk for personnel and infrastructure, but also for the optimization of production. Although there is less experience with passive seismics in the hydrocarbon industry, it gained momentum in recent years, especially with the introduction of more advanced 3-component sensors.
NORSAR has developed software to analyze microseismic events, called MIMO, which is described in the next section. So far, we have applied this software to different scenarios worldwide e.g. in underground ore mines in Finland and Sweden (e.g. Pyhäsalmi ore mine), in a borehole across one of the most active fault zones in the world, the San Andreas Fault (San Andreas Fault Observatory at Depth, SAFOD), another international drilling project in Taiwan (Taiwan Chelungpu-fault Drilling Project), an unstable rock slope in a Norwegian fjord at Åknes. Several applications have also been in hydrocarbon reservoirs (e.g. Ekofisk, North Sea; Clinton County, Kentucky), geothermal reservoirs and shallow mining in Australia. The figures below show an example of microseismic events located in the Pyhäsalmi ore mine, Central Finland.
Microseismic events recorded in Pyhäsalmi ore mine, Central Finland, during 2003. Top left: view from North, top right: view from East, bottom left: view from South, bottom right: view from West. Grey shape represents ore body; pink pins are seismic stations; colored spheres are located events, size displays magnitude and coloring is according to time of year.
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