An original method is proposed for the evaluation of moment tensor components using time-domain calculations of low-frequency displacement amplitudes with the first-wave polarities attached. The technique is suitable for applications involving large amounts of data that require a fast response time, such as those encountered in the analysis of induced seismicity. Simulation tests indicate an excellent agreement with inversions based on the far more labor intense spectral processing. The overall results of waveform simulation analysis allow the conclusion that this inversion approach is reliable in retrieving the geometrical aspects of the seismic source. Inversions for the pure-shear mechanism and a general mechanism, including a tensile component, are shown to be robust for various wave-type solutions, under reduced array coverage, hypocentral mislocation, and the addition of Gaussian normally distributed noise. The influence of errors in first-arrival polarities and low-frequency displacement amplitudes are subsequently studied for a better understanding of the applicability limits.

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.

No abstract

A microseismic monitoring system provides a vital window into a rock mass to see where stress induced fracturing is occurring in relation to mining operations. A main factor for the accuracy of the microseismic locations is the velocity model assumed for the rock mass. The majority of mines that use microseismic systems use a single velocity model for location purposes which assumes the same elastic modulus properties throughout the volume. This study shows examples of event locations that were calculated using a velocity model that accounts for multiple complex shaped geological units each with their own properties. The method allows multiple voids to be added that could be air filled, brine filled, or cement paste back filled, thus mimicking mining and geotechnical operations such as stope mining, cave mining, solution mining, or underground cavern storage. Going beyond the dots, microseismic systems provide an important way to understand the failure mechanics of the rock fracturing. With good data quality, each located event can be solved for the source mechanism (moment tensor) and interpreted in terms of whether the event is dominantly tensile opening, closing, or shear slip. The orientation of each event failed zone can be quantified providing useful information about the discrete fracture network (DFN). This paper provides examples of source mechanism solutions using a full 3D velocity model. It is shown that the ray path of each sensor does affect the source mechanism solution when comparing a single velocity model solution and a 3D velocity model solution. Microseismic systems offer important daily information for mine operation, safety and planning. Improvements to the accuracy of seismic results by using enhanced processing methods and regular calibration, allow a mine to more confidently integrate seismic results with numerical models and make decisions. This is especially important as mines move to different excavation methods such as block caving and extend to greater depths and stresses. Recently, microseismics are playing an essential role in applications such as the monitoring of shale gas hydraulic fracturing and underground storage caverns (Baig et al. 2012). Being able to monitor a 3D volume of rock deep under the ground in real time is very important for tracking any rock fracturing and coalescence that may be occurring. https://papers.acg.uwa.edu.au/p/1410_48_Collins/ 3D velocity model with complex geology and voids for microseismic location and mechanism DS Collins et al.

Ð A novel seismic moment tensor inversion approach is applied to microseismic events from the Kidd mine with moment magnitudes ranging between À1.2 and 0. Data consist of 35 events recorded on 8 triaxial accelerometers installed underground. Reliable solutions are obtained for 21 events, of which 14 represent pure shear mechanisms, whereas the remaining 7 exhibit a signi®cant positive volumetric component (72±76%), along with some pure shear failure (15±20%). Further analysis indicates that 6 of the events characterized by high volumetric components are located within a sill pillar on the 4,700 level and have subvertical P and subhorizontal T axes. This is in agreement with the presence of tensile cracks close to openings for incipient pillar bursting. The pure shear events are located outside the sill pillar, between the 4,600 and 4,800 levels, and on the 5,600 level within a highly fractured rockmass. For the latter events, the subvertical nodal planes are found to match closely the orientation of subvertical NW-SE fractures aligned parallel to the major faults in the area.