Microseismic Energy Research Articles

Mechanism and Control of Rockburst Induced by Draining Spatial Islands and Squaring

A small-pillar gob-side roadway showed rockburst appearance during the mining of a gob-side working face located in the Shaanxi-Inner Mongolia mining area. This study examines the 2202 gob-side working face of a coal mine in Inner Mongolia as a case study. A stress evolution model was built for the static-stress spatial islands formed by drainage regions and goafs based on the spatial relationships between drainage regions and goafs. The average microseismic frequency and energy of the high-stress zone of spatial islands were at least 1.37 times of those of other zones, validating the presence of spatial islands. The dynamic and static load effects of working face squaring were obtained based on the evolution of the stope roof as well as changes in microseismic data. Microseismically active zones were advanced to 200 m–300 m on working faces. The rockbursts induced by high static loads and dynamic and static loads formed by spatial islands and squaring were calculated. According to calculation results, the critical stress concentration value under high static loads was 3.27; the critical static stress concentration value under dynamic and static loads was 1.81. The superposition of drainage boundary stress, goaf lateral stress, and lead stress might reach the critical stress concentration under dynamic load disturbances, causing rockbursts. A stress adjustment scheme was established, including overall hydraulic fracturing of the external roof of the drainage region, reduction of stoping speed, and pressure relief of large-diameter boreholes. The stress adjustment scheme was implemented on-site and supplemented by monitoring and early warning methods to safely advance by the first squaring region.

Global characteristics of ambient seismic noise

Ambient seismic noise becomes more and more important and helpful on assisting velocity model inversion, earthquake detection, and ground motion prediction. Based on the analysis of continuous seismic data and ocean wave height, we find that the ocean wave height and winter storms have a controlling factor on the DF microseismic energy level and its frequency extent in time scale. It shows that high and low DF microseismic energy accompanied with wide and narrow frequency range consistent with the high wave height period (when the ocean is stormier) and low wave height period, respectively. Since DF microseism is dominated by Rayleigh waves, its energy attenuates very quickly when it travels through shoreline to the continent crust. Our observations give a quality factor Q of about 83 for DF energy traveling from the middle of the Atlantic to the central of Europe. We observe a lower energy level of SPDF (short period DF) than that of LPDF (long period DF) for the continent stations, however a reversed situation for the island stations. It suggests that short period DF energy decays faster than the long period one. High-frequency ambient noise is called microtremor. The microtremor for the island station with low elevation has a semidiurnal modulation in phase with ocean tide. The microtremor for the station at other locations are from the anthropogenic activities which have diurnal, weekly, and annually variations.

Passive-seismic image-domain elastic wavefield tomography

SUMMARY Elastic time-reverse imaging offers a robust wavefield-based approach for locating microseismic events; however, event location accuracy greatly depends on the veracity of the elastic velocity models (i.e. VP and VS) used for wave propagation. In this study, we propose a methodology for microseismic image-domain wavefield tomography using the elastic wave equation and zero-lag and extended source images, the focusing of which is used as a quality control metric for velocity models. The objective function is designed to measure the focusing of time-reversed microseismic energy in zero-lag and extended event images. The function applies penalty operators to source images to highlight poorly focused residual energy caused by backpropagation through erroneous velocity models. Minimizing the objective function leads to a model optimization problem aimed at improving the image-focusing quality. P- and S-wave velocity model updates are computed using the adjoint-state method and build on the zero-lag and extended image residuals that satisfy the differential semblance optimization criterion. Synthetic experiments demonstrate that one can construct accurate elastic velocity models using the proposed method, which can significantly improve the focusing of imaged events leading to, for example, enhanced fluid-injection programs.

Application of Distributed Optical Fiber Technology for Coal Bump Prevention

The 8939 working face in Xinzhouyao coal mine is a high coal bump proneness panel. For coal bump prevention, rib holes are drilled for pressure relief purpose. The deformational behaviour of the pressure relief borehole is studied using distributed optical fiber sensing technology. The strain of the surrounding coal and the pressure relief range were measured from 0 hrs to 402 hrs after hole drilling. Based on the analysis of pressure relief procedure, combining with borehole observation, the crack development, limited equilibrium, collapse, and compaction stages of the borehole were estimated as 0∼72 h, 72∼190 h, 190∼402 h, and greater than 402 h, respectively. Consequently, the hole drilling is modified to 110 m ahead of the working face to achieve better pressure relief effect. Microseismic monitoring shows that, after hole drilling optimisation, the high-energy microseismic events and average energy of microseismic events are reduced significantly.

A novel geophysical method for fractures mapping and risk zones identification in a coalmine, Northeast, China

A novel approach for mapping induced fractures and associated lineaments has been introduced based on microseismic data, and thus their contribution towards identification, and prediction of the risk zones which may lead to a collapse or rockburst in a coal mine. By taking Tashan Coal Mine (TCM) in Datong coalfield of China as an example, the real-time microseismic data was acquired for a period of 7 months. The microseismic data was processed using standard processing schemes followed by source location and parameters estimation. The proposed approach utilizes microseismic events, trace them to map the induced fractures, and based on their network to interpret associated lineaments. Based on the configuration, geometry, and network, thousands of fractures were identified which vary in length. Regional lineaments have been interpreted based on the orientation of these fractures. The fractures and lineaments distribution were further correlated with computed microseismic energy distribution. The spatial distribution of microseismic energy helped in identification of a geological discontinuity (fault) that clearly separates the study area into northern and southern parts. The reactivation of this fault may pose significant threats to mass stability, life, and property. The northern half is characterized by more and short length fractures while southern by prolonged and fewer fractures. The upper northern half is dominated by high energy (≥50, 000 J), while the lower part is characterized by variable energy pattern (22,000 to 40, 000 J). It is inferred from the energy-lineaments correlation figure, the northern half describes relatively stronger rock type in comparison to the southern part suggesting different rock deformational behavior. The fractures and lineaments converging towards a single point define risk zones where future collapse or rockburst can be expected. This identification of risk zones provides a strong basis for the real-time prediction of future catastrophic hazards in the coal mines. This study may benefit global underground engineering researchers for hazards prediction and establishing of the early warning systems.

Prediction of rockbursts in a typical island working face of a coal mine through microseismic monitoring technology

Coal has dominated the energy structure in China for a long time. In recent years, dynamic disasters such as rockbursts have affected the safety of coal mine in production with the increase of coal mining depth. Microseismic monitoring is a non-intrusive monitoring technique that can be used to collect much key information: it has been widely applied in coal mine safety monitoring. In this paper, the failure mechanism and energy conversion in a coal-rock mass during mining were investigated, revealing the controlling effect of the surrounding rock mass on the coal seam. microseismic events during mining were monitored by using a self-built high-precision microseismic monitoring system, a variety of analytical methods have been used to quantify the monitoring results, and two rockburst accidents have been identified. The responses of different parameters before the rockburst were analyzed, and the response characteristics of different parameters to rockburst were summarized. The functional relationship between microseismic energy and rock damage was established by classical Benioff strain theory. Combined with the microseismic energy, strain, the degree of damage and the damage, the model of coal-rock mass was verified and modified through physical experiments. A new parameter for rockburst prediction (the energy rockburst warning (ERW) index) was defined and the numerical relationship between ERW and microseismic energy was deduced. The monitoring data were analyzed using this model and results showed that compared with other analysis, the new model can effectively assess the stability of coal-rock mass to a certain extent.

Locating microseismic events using multiplicative time reversal imaging based on decoupled wavefields in 2D VTI media: Theoretical and synthetic cases studies

Estimation of microseismic event location is of great significance in unconventional resources development and exploitation with its advantage of imaging the fractures induced by hydraulic fracturing stimulation. Time reversal imaging technique (TRI), which reversely propagates the recorded microseismic energy to refocus to its real location, has been verified as a satisfying method in noisy environment. However, the location images obtained from TRI techniques are frequently smeared with strong imaging noises and artifacts resulting from the cross talk of coupled wave modes coinciding in space and time with sparse measurements especially in heterogeneous anisotropic media, resulting in erroneous location estimations. To mitigate these disturbances, a multiplicative time reversal imaging method based on decoupled wavefields is proposed (DC-MTRI). Wave-mode separators constructed based on Christoffel equation are first utilized to fully separate quasi P wave (qP) and quasi vertically pozlarized shear wave (qSV) wavefields at each receiver in 2D vertical transverse isotropy (VTI) media. Multiplicative time reversal imaging condition is then applied to the auto and cross-correlations of decoupled wavefields of each receiver to construct the final location image. The synthetic examples show that the wave-mode separators applied in this paper can fully separate qP and qSV potentials in 2D VTI media compared to Helmholtz decomposition. As a consequence, the proposed method substantially improves the quality of the location image with a much sharper focus and less noisy imaging energy at shallow depth of the sophisticated Sigesbee 2A model. In addition, the numerical experiments show that DC-MTRI can give robust source location images with distinct radiation pattern of high resolution for sparse acquisition plan. Furthermore, DC-MTRI has same sensitivity on velocity perturbations compared with conventional TRI techniques. Therefore, the proposed technique can potentially be employed as the constraints to estimate source mechanism and update the velocity structure in the future.

Multifield Environmental Analysis and Hazards Prevention of Steeply Inclined Deep Coal Mining

Due to the different in‐situ stress level, mining stress state, and surrounding rock properties of steeply inclined deep coal mining, the mutation mechanism of underground engineering rock mass is complex. This paper studies the cause and control of mining disaster of steeply inclined deep coal resources in Wudong coal mine. The results show that the structural field is the key of multifield analysis, and particularly a large area of suspended roof is easy to expand energy and induce dynamic hazards. By means of borehole television‐ (BT‐) transient electromagnetic (TEM) detection, it is found that there are hidden dangers of roof safety and suspected water hazards in Wudong coal mine, and the roof above the detection area (+575 m south roadway to 2250–2600 m) is in a suspended state; there is a suspected water‐rich area in the range of 2320–2340 m and 2390–2400 m, and the lowest vertical height is +613.8–+615.5 m. Exploring and releasing the water in the aquifer effectively reduced the water pressure; in +575 m south roadway, +587 m measure roadway, and blasting chamber, the suspended roof blasting holes are constructed. Microseism‐ (MS‐) TEM monitoring shows that the apparent resistivity fluctuates significantly, the microseismic energy and events have been significantly reduced, and it is maintained at a low level for two consecutive weeks, confirming the effectiveness of the stable release of the high‐stress roof in the +575 m near stope area; at the same time, the safeguard measures for long‐term roof dynamic monitoring are constructed.

Study on Mechanism of Influence of Mining Speed on Roof Movement Based on Microseismic Monitoring

Based on the study of the movement rule of the rock strata under the influence of the mining speed in the typical working face of Dongjiahe coal mine, the distribution of microseismic events and the variation characteristics of microseismic parameters in the slow and fast advancing stage are compared and analyzed, and the mechanism of the rock strata activity under the influence of the mining advancing speed is revealed from the perspective of the microfracture. The results show that the movement of the roof strata and the stress adjustment of surrounding rock have certain timeliness. The maximum advanced distance of microseismic events in the slow and fast stages is 185 m and 130 m, respectively, and the maximum lag distance of microseismic events in the goaf is 120 m and 180 m, respectively. The time of stress adjustment of surrounding rock is short, and the load transfer of the roof is insufficient. The advanced distance of microseismic events is increased, and the lag distance decreases. The percentage of microseismic events in the total number of events is 47% and 38%, respectively, in the slow and fast stages of advancing. With the increase of mining speed, the intensity of roof strata activity in the goaf is weakened. The rock failure decreases and the volume of broken block increases, and roof collapse and rotary subsidence are insufficient. During the nonpressure period, the maximum development elevation of microseismic events is +350 m and +300 m, respectively, in the slow and fast stages, while with the development elevation of microseismic events in the roof pressure near +390 m, increasing the mining speed cannot change the final failure height of the overburden. During the analysis period of roof pressure, the concentrated release of microseismic energy in the faster stage is 183% of that in the slower stage. The increase of large moment magnitude event frequency leads to the decrease of b value. The risk of roof instability and strata behavior increases.

Experimental Study on the Effect of Advancing Speed and Stoping Time on the Energy Release of Overburden in an Upward Mining Coal Working Face with a Hard Roof

In order to study the influence of advancing speed and stoping time of a coal face on the scale and frequency of rock burst, the energy release characteristics of an overburden fracture under six advancing speeds and four stoping times are studied by theoretical analysis and similar simulation experiments. The distribution characteristics of microseismic events before and after stoppage are compared, and the load/unload response ratio is introduced to analyze the relationship between the synergistic effect of advancing speed and stoping time and the characteristics of microseismic events in coal and rock mass. The mechanism of rock burst induced by the advancing speed and stoping time effect in the working face is studied, and the coordinated regulation and mitigation of advancing speed and stoping time are analyzed and completed. The results show that the effect of advancement speed and stoping time is very important to the energy release of overburden. The energy released by microseismic events during stoping is exponentially related to the advancing speed. The change of advancing speed causes the change of microseismic event characteristics, reflecting the evolution process of overburden structure and its energy. During stoping, the secondary microseismic events disturbed by mining occur frequently, leading to the significant difference of energy released by microseismic events during stoping. After stoping, the microseismic energy is more than four times higher than that during the stop period, and the risk of coal seam impact is high during the stope period. The synergetic change of advancement speed and stoping time changes the cycle of energy accumulation and release. The response ratio of loading and unloading considering the effect of advancement speed and stoping time is established by using the corresponding ratio of loading and unloading, and the impact risk of the coal seam is quantitatively analyzed. Based on the monitoring and analysis of microseismic events, the safety mining index of coordinated control with the energy of a single microseismic event of 180 J is established, and the best advancing speed of the working face is determined to be 4 m/d. According to the corresponding ratio of loading and unloading, the reasonable stoping time of different advancing speeds and the corresponding advancing speed of different stoping times after the resumption of mining are determined, so as to provide a reference for the safe and efficient mining of similar rock burst mines.

Study on Deformation and Energy Release Characteristics of Overlying Strata under Different Mining Sequence in Close Coal Seam Group Based on Similar Material Simulation

For the characteristics of overburden deformation and energy release under different mining sequences of close-distance coal seam groups, physical material similar simulation experiments were carried out, and comprehensive monitoring methods such as dial gauge, total station, micro-seismic monitor, and pressure sensor were used to test the Guangou Coal Mine. The comparative analysis of the initial mining and the upward recovery of the B4-1 coal seam is carried out to study the migration law, mine pressure distribution, and energy release characteristics of the overlying strata during W1145 mining face mining and residual coal mining. The results show that the maximum subsidence of surface and rock formation caused by re-mining of B4-1 residual coal is 0.96 m and 2.57 m respectively, which is 0.42 m and 0.47 m lower than that of W1145 working face. The boundary angle, moving angle, and rock stratum formed by the upward recovery of the remaining coal seam are 79.3°, 81.1°, and 67.5° respectively, which are smaller than the 80.9°, 82.3°, and 75.8° formed by the first mining. The cumulative development height of the fracture zone caused by upward mining is 115.7 m, which is 8.0% smaller than the cumulative development height of the downstream fracture zone of 125.8 m. When the up-level mining is carried out, the fragmentation effect of the rock layer below the key layer is strong, which makes the loosely broken rock block have a better supporting effect. Therefore, the residual coal mining time is longer than that of the first mining. The initial pressure step of the residual coal recovery is 139.2 m, and the average step of the cycle is 34.2 m, which is significantly larger than the 128.0 m and 26.0 m of the first mining. The loose rock strata that are disturbed by the upward recovery are more likely to be broken. Therefore, there are more micro-seismic events during the re-mining of the remaining coal. The B4-1 residual coals have a total of 945 incidents of re-seismic micro-seismic events, which is 292 more than the W1145 working face. After the B2 coal seam mining disturbance, the energy of some rock layers above the B4-1 coal seam is released, so that the micro-seismic energy caused by re-mining of the remaining coal seam is small. Through microseismic monitoring, it can be concluded that the accumulated energy in the process of upward re-mining of remaining coal seam is less than that in the process of downward mining of W1145 working face. Upward recovery is more likely to cause damage in the disturbed loose rock formation. Therefore, the frequency of micro-seismic events during the upward recovery is higher, and the partial energy release of the rock after the disturbance is caused, so that the source energy generated in the unit length of the upward recovery is smaller than the initial one.

Mechanism by Which Backfill Body Reduces Amount of Energy Released in Deep Coal Mining

Deep coal mining is unavoidable, and the complex mining environments and the increasing dangers associated with ultrahigh energy accumulation and release from mining disturbances renders it extremely difficult to maintain a safe and stable stope. Solid backfilling technology directly uses coal gangue and other solid wastes in the mining area to fill the gob after mining. Support from the backfill body can inhibit the movement of overlying rock strata and significantly alleviate the influence of mining. In this study, the correlations between the deformation of gangue filling material and the characteristics of energy dissipation were examined under lateral uniaxial compression. The strain energy density distributions of backfilling and caving mining methods were simulated using numerical modeling. The results showed that the strain energy density distribution of backfilling mining was less concentrated, and its peak value was lower than that of caving mining by 51.0%, indicating that backfilling could effectively reduce the amount of energy released from mining rocks. The dense backfill mining area of the No. 9301 face in Tangkou Coal Mine was used as a case study. Measures for controlling the backfill body compaction for reducing the amount of energy released from mining rocks were proposed. These measures include optimizing the support structure and filling material formula, controlling the preroof subsidence, and ensuring an appropriate number of tamping strokes. The monitoring results of the backfilling quality, surface subsidence, and microseismic energy of No. 9301 working face in Tangkou Coal Mine showed that when the backfill body filling ratio control value was 82.28%, the total number of microseisms and the amount of energy released from the mining working face were significantly lower compared to those of the caving method. This study demonstrated that the backfill body could effectively reduce the amount of energy released from mining rocks, thereby realizing management of mine earthquake and sustainable deep coal mining.

Research on the Law of Large-Scale Deformation and Failure of Soft Rock Based on Microseismic Monitoring

Based on the existing Canadian ESG microseismic monitoring system, a mobile microseismic monitoring system for a soft rock tunnel has been successfully constructed through continuous exploration and improvement to study the large-scale nucleation and development of microfractures in the soft rock of the Yangshan Tunnel. All-weather, continuous real-time monitoring is conducted while the tunnel is excavated through drilling and blasting, and the waveform characteristics of microseismic events are analysed. Through the recorded microseismic monitoring data, the variation characteristics of various parameters (e.g., the temporal, spatial, and magnitude distributions of the microseismic events, the frequency of microseismic events, and the microseismic event density and energy) are separately studied during the process of large-scale deformation instability and failure of the soft rock tunnel. The relationship between the deterioration of the rock mass and the microseismic activity during this failure process is consequently discussed. The research results show that a microseismic monitoring system can be used to detect precursors; namely, the microseismic event frequency and energy both will appear “lull” and “active” periods during the whole failure process of soft rock tunnel. Two peaks are observed during the evolution of failure. When the second peak occurs, it is accompanied by the destruction of the surrounding rock. The extent and strength of the damage within the surrounding rock can be delineated by the spatial, temporal, and magnitude distributions of the microseismic events and a microseismic event density nephogram. The results of microseismic analysis confirm that a microseismic monitoring system can be used to monitor the large-scale deformation and failure processes of a soft rock tunnel and provide early warning for on-site construction workers to ensure the smooth development of the project.

Regional hazard prediction of rock bursts using microseismic energy attenuation tomography in deep mining

Rock burst prediction is a worldwide challenge that we have long tried to overcome. This study tentatively proposed a method to regionally predict rock burst hazards using microseismic energy attenuation. To verify the feasibility of the proposal, first, the mechanism of microseismic energy propagation and attenuation in rock medium was explored, and dominant attenuation characteristics of microseismic waves were analyzed. Second, a spatial attenuation model of microseismic energy was established, and the average energy attenuation coefficient for each wave path was defined. A 3D seismic energy attenuation inversion algorithm was put forward, and the corresponding computation matrix was developed. Third, a continuous microseismic field investigation was carried out in a deep coal mine. Seismic energy attenuation coefficient was confirmed using the calibrated focus position and energy determination. Based on data discretization processing, energy attenuation inversion and tomography, potential rock burst hazard regions were strictly zoned in mining areas. Finally, regional prediction results obtained from the microseismic energy attenuation were compared with the direct measurement results obtained from the classical drilling dust method to verify the reliability of proposed approach. It turns out that rock burst hazard regions predicted by the microseismic energy attenuation agreed well with the objective hazardous situations. Seismic energy attenuation coefficient is a significant evaluation factor that directly mirrors the inelastic performance of rock medium. Energy attenuation coefficient threshold used for determining the rock burst hazard regions was 3.0 km−1. Reliability of the seismic energy attenuation inversion and tomography was closely related to the spatial distribution of microseisms in a localized region. The optimum spatial density of microseisms was 0.2 m−3. Regional rock burst prediction using microseismic energy attenuation is an effective approach for revealing potential hazardous regions in deep mining conditions. This approach improves the pertinence of geological hazard prevention and provides a beforehand reference for targeted hazard management.

Focus Energy Determination of Mining Microseisms Using Residual Seismic Wave Attenuation in Deep Coal Mining

Based on the energy attenuation characteristics of residual wave in deep rock, a method was developed to determine the microseismic focus energy. Differential energy loss in infinitesimal spreading distance is logically deduced, upon which energy attenuation equation was established. With a logarithmic transformation, a linear relation of the residual seismic energy with distance is formulated. Its intercept was used to determine the microseismic focus energy. The result is compared with that determined by the energy density method. The reliability of the determined focus energy and the impact of the built‐in velocity threshold on the residual wave energy computation are discussed. Meanwhile, the energy absorption coefficient used for representing the absorption characteristics of the rock medium in the mining region under study is also clarified. Key findings show that the microseismic focus energy confirmed by the residual wave attenuation is reliable. The result’s accuracy is quite high, especially for the events in deep rock with great homogeneity. The developed focus energy computation method is closely dependent on the integrity of waveform, accuracy of repositioning, and reliability of effective components extraction. The new method has been shown to be effective and practical.

Research of Rock Burst Risk Induced by Mining and Field Case in Anticlinal Control Area

Stress concentration caused by tectonic stress and mining disturbance in coal mines induces a unique type of rock burst. No. 3201 working face controlled by an anticline structure in the Shandong mining area is used as the research background. The formation mechanism for anticlines is analyzed. Theoretical research shows that the bigger the tectonic couple is, the smaller the foundation stiffness, and the greater the bending degree and elastic strain energy of the coal will be. The distribution characteristics of abutment pressure and maximum principle stress in anticlinal control areas are analyzed using UDEC numerical software. The results show that rock bursts result from interactions between abutment pressure and residual tectonic stress. The “connection‐overlay‐separation” phenomenon of abutment pressure presents with working face advancement. Furthermore, the energy criterion for rock burst initiation is established based on the energy principle. Residual energy “E0 − EC” and rock burst danger characteristics during mining are discussed. Based on the simulation results, microseismic monitoring data for No. 3201 working face are analyzed, and the law of microseismic energy is consistent with the variation law for the residual energy “E0 − EC” at the peak of the simulated abutment pressure. The microseismic energy and frequency are higher during mining, increasing the risk of rock burst events. It can provide scientific basis for prevention and control of rock burst.

Full wavefield decomposition of high‐frequency secondary microseisms reveals distinct arrival azimuths for Rayleigh and Love waves

AbstractIn the secondary microseism band (0.1–1.0 Hz) the theoretical excitation of Rayleigh waves (Rg/LR), through oceanic wave‐wave interaction, is well understood. For Love waves (LQ), the excitation mechanism in the secondary microseism band is less clear. We explore high‐frequency secondary microseism excitation between 0.35 and 1 Hz by analyzing a full year (2013) of records from a three‐component seismic array in Pilbara (PSAR), Australia. Our recently developed three‐component waveform decomposition algorithm (CLEAN‐3C) fully decomposes the beam power in slowness space into multiple point sources. This method allows for a directionally dependent power estimation for all separable wave phases. In this contribution, we compare quantitatively microseismic energy recorded on vertical and transverse components. We find the mean power representation of Rayleigh and Love waves to have differing azimuthal distributions, which are likely a result of their respective generation mechanisms. Rayleigh waves show correlation with convex coastlines, while Love waves correlate with seafloor sedimentary basins. The observations are compared to the WAVEWATCH III ocean model, implemented at the Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), which describes the spatial and temporal characteristics of microseismic source excitation. We find Love wave energy to originate from raypaths coinciding with seafloor sedimentary basins where strong Rayleigh wave excitation is predicted by the ocean model. The total power of Rg waves is found to dominate at 0.35–0.6 Hz, and the Rayleigh/Love wave power ratio strongly varies with direction and frequency.

Stability analysis and reinforcement evaluation of high-steep rock slope by microseismic monitoring

The stability of excavated high-steep rock slopes is most significant in the long-term safety of large-scale hydropower projects. Dagangshan rock slope had an unloading fractured zone (XL-316) and a fault (f231) during excavation, which resulted in a high risk of slope instability. Anti-shear tunnels were constructed for enhancing the overall shear resistance and reinforcing the slope at six different elevations. The microseismic technique was employed to investigate microseismic activities during excavation and after reinforcement. An integrated method of microseismic energy density and the magnitude-frequency relation based on microseismic monitoring was proposed to analyse the stability and reinforcement effect of the slope. During excavation, a large number of microseismic events took place with few low-magnitude events and numerous high-magnitude events. The aggravated damage occurred in the major discontinuities during excavation. The number of low-magnitude microseismic events increased after reinforcement, and the b value obviously increased, which means the slope stability was substantially enhanced. Microseismic activities were evidently reduced, and the accumulated microseismic energy increased slowly. The stresses were transferred towards the edge of the anti-shear tunnel, and a small number of low-energy events occurred. The targeted reinforcement has served well for its purpose and improves the mechanical performance of the discontinuities. The application of the proposed method was verified by 3D RFPA-Centrifuge software. The proposed method can be considered as an effective way to evaluate the reinforcement effect and stability of high-steep rock slopes.

Analysis for Microseismic Energy of Immediate Rockbursts in Deep Tunnels with Different Excavation Methods

AbstractThis study integrates microseismic data and data from hundreds of rockbursts of different intensities that occurred in a water drainage tunnel and four deep headrace tunnels at Jinping II Hydropower Station in Sichuan Province, China. The tunnels with overburden depths between 1,900 and 2,525 m, the maximum principal stress of which reaches 63 MPa in a rock mass, is composed mainly of marble. The tunnels have a total length of 12.4 km. The microseismic energy produced during the development of immediate rockbursts induced by the excavation by the drill-and-blast method (DBM) and a tunnel-boring machine (TBM) were studied. The results indicate that the daily maximum microseismic energy can be used as a basis for estimating rockburst intensity. The daily maximum microseismic energy (logarithm) corresponds to intense rockbursts (>6), moderate rockbursts (between 5 and 6), weak rockbursts (between 4 and 5), and no rockbursts (<4). Before the occurrence of intense rockbursts, there were numerous weak a...

Anthropogenic sources stimulate resonance of a natural rock bridge

AbstractThe natural modes of vibration of bedrock landforms, as well as the sources and effects of stimulated resonance remain poorly understood. Here we show that seismic energy created by an induced earthquake and an artificial reservoir has spectral content coincident with the natural modes of vibration of a prominent rock bridge. We measured the resonant frequencies of Rainbow Bridge, Utah using data from two broadband seismometers placed on the span, and identified eight distinct vibrational modes between 1 and 6 Hz. A distant, induced earthquake produced local ground motion rich in 1 Hz energy, stimulating a 20 dB increase in measured power at the bridge's fundamental mode. Moreover, we establish that wave action on Lake Powell, an artificial reservoir, generates microseismic energy with peak power ~1 Hz, also exciting resonance of Rainbow Bridge. These anthropogenic sources represent relatively new energy input for the bridge with unknown consequences for structural fatigue.