Microseismic Monitoring of a Controlled Collapse in Field II at Ocnele Mari, Romania

Abstract

Several decades of faulty exploitation of salt through solution mining led to the creation of an underground cavern containing several million cubic meters of brine. To eliminate the huge hazard near a densely inhabited area, a technical solution was implemented to resolve this instability concern through the controlled collapse of the roof while pumping the brine out and filling the cavern with sterile. To supervise this, an area of over 1 km 2 was monitored with a staggered array of 36 one-component, 15 Hz geophones installed in 12 boreholes about 160-360 m deep. A total of 2,392 seismic events with M w -2.6 to 0.2 occurred from July 2005 to March 2006, located within an average accuracy of 18 m. The b-value of the frequency-magnitude distribution exhibited a time variation from 0.5 to 1 and from there to 1.5, suggesting that the collapse initiated as a linear fracture pattern, followed by shear planar fragmentations and finally a 3-D failure process. The brunching ratio of seismicity is indicative of a super-critical process, except for a short period in mid-February when temporary stability existed. Event relocation through the use of a collapsing technique outlines that major clusters of seismicity were associated with the main cavern collapse, whereas smaller clusters were generated by the fracturing of smaller size nearby caverns. It is shown that one-component recordings allow for stable and reliable point source event mechanism solutions through automatic moment tensor inversion using time domain estimates of low frequency amplitudes with first polarities attached. Detailed analysis of failure mechanism components uses 912 solutions with conditional number CN \ 100 and a correlation coefficient r 2 [ 0.5. The largest pure shear (DC) components characterize the events surrounding the cavern ceiling, which exhibit normal and strike-slip failures. The majority of mechanism solutions include up to 30% explosional failure components, which correspond to roof caving under gravitational collapsing. The largest vertical deformation rate relates closely to the cavern roof and floor, as well as the rest of the salt formation, whereas the horizontal deformation rate is most prominent in areas of detected collapses.

Stability Assessment of the Excavated Rock Slope at the Dagangshan Hydropower Station in China Based on Microseismic Monitoring

“…e detailed specification can be found in [21]. An event can be located by the first P-wave arrivals at 4-24 sensors and S-wave arrivals at 1/4-1/2 based on the simplex algorithm [22]. Moreover, the system can evaluate seismic source parameters, such as the energy index, apparent stress, magnitude, and apparent volume.…”

Application of Microseismic Monitoring Technique in Hydroelectric Projects

“…Moreover, further analysis of the scale and characteristics of rock failure can be performed according to hypocenter parameters recorded by the system. Having recorded and processed a number of seismic events within a given volume of interest over time, one can then quantify the changes in the strain and stress regimes and in the rheological properties of the rock mass deformation associated with the microseismicity (Trifu and Shumila, 2010). This in turn allows estimation of quantities like rock mass stability over space and time, and thereby prediction of slope failure precursor or rockburst of deepburied tunnels.…”

“…The system then calculates in real time a preliminary hypocenter determination with a homogeneous velocity model and records the data on hard disk. This detailed specification can be found in (Trifu and Shumila, 2010;Xu et al, 2011b;Xu et al, 2010).…”

“…Therefore, it is the occurrence of microseismicity that characterizes if an unknown geological structure is active. Microseismicity related to these structures is also evidence of rock mass degradation from elevated stresses as each microseismic event indicates rock fracturing (Trifu and Shumila, 2010).…”

Subtask 2.2 - Creating A Numerical Technique for Microseismic Data Inversion

“…Being quite sophisticated, the techniques often provide data that are difficult to interpret. Continuous need in meaningful interpretation has recently drawn much attention to passive seismic monitoring (Liu et al, 2006;Trifu, 2008). Since its application requires proper interpretation of seismic data, some techniques to reach this goal have been developed and commercialized, i.e., mapping of a hydraulically induced fracture.…”