Evolution of stress fields and induced seismicity in operating mines

The authors propose a new procedure for establishment of the space-time relationships between the quantity and energies of seismic events and the stress-strain state parameters using deterministic information on variation of geomechanical fields and the statistical analysis of the induced seismicity data. The new procedure has been verified under conditions of Tashtagol iron ore mine.     

Laws of spreading and operational evaluation procedure for induced seismicity in mines and in mining areas

The scope of the article encompasses features of natural and induced seismicity and the change in frequency plots when natural seismicity turns into natural-and-induced seismicity in mining areas and when induced seismicity becomes natural-and-induced seismicity in mines. It is shown how induced seismicity is connected with the subsidence of overlying strata in mines—seismic process propagates together with the subsidence but seismic activity lowers in the time of maximum subsidence and intensifies when subsidence ceases or decelerates. The authors lay emphasis on estimates of seismic activity in mines and give details of an integrated index F procedure tested in mines and adjusted within the 10 year-long period of application.     

Natural and induced seismic activity in Kuzbass

Based on the plotted times of occurrence of seismic events, their timing within a work week and location of epicenters, it is illustrated that since the 1960s the seismicity in the Kuzbass territory has been a complicated natural and mining-induced phenomenon. Since the late 1980s, the natural and mining-induced seismicity has entered the second stage of development, with clustering of low-energy seismic events and strong shallow earthquakes in the areas under heavy mining and, in the first instance, at deep open pit mines. The representative event of this kind is the magnitude 5.2 earthquake at the Bachatsky Coal Open Pit Mine on June 19, 2013. It has been found that the natural and induced seismic activity is associated with deep faults that weakly show themselves in the upper layer of the earth crust but appear in the local relief, which is an evidence of their live emergence onto the daylight surface due to the mining impact. The article proposes the comprehensive research program for the natural and mining-induced seismic activity in the Kuzbass area and the development of early identification of seismically active zones.     

Passive tomography to image stress redistribution prior to failure on granite

Understanding stress redistribution within rock helps improve the safety and efficiency of mining projects. In order to better understand rock failure mechanism, a granite sample is prepared in a manner that facilitates shear failure along preconceived cracks. The testing process of this method employed several steps. First, the sample is loaded and then monitored in order to study the stress field associated with shear failure. Passive acoustic emissions systems are then used to monitor seismic events during the loading process. The data obtained from the test is processed by using the double-difference location algorithm. The velocity pattern of the sample is then calculated. The distribution of velocity is projected using double-difference tomography. It is shown that the sample failed along the plane where two parallel cracks were prepared. Most of the relocated events were located within or around the plane. The tomograms created from the recorded data accurately image the areas of interest within the rock samples. It is also shown that passive acoustic emissions monitoring systems coupled with double-difference tomography are capable of monitoring and forecasting the failure of rock.     

强矿震临界破裂阶段的岩体弹性波场

为探索井田范围较大尺度岩体破裂失稳的危险性预测,以抚顺老虎台井田为工程背景,反演连续12个月矿震弹性波的传播速度比（v_P/v_S）和质点振动振幅比（A _S/A _P）,分析强矿震临界发生阶段的弹性波场变异特征。结果显示,M_L≥1.5矿震的波场信息较敏感,在M L≥3.0（E≥1.8×10⁸J弹性能）矿震前,波速比和振幅比绝大多数存在显著的低值异常,预测M _L≥3.0矿震的信度分别为0.79和0.93,虚假异常和缺失异常较少,具有良好的应用前景。波速比和振幅比在强矿震临界破裂前存在的降低和恢复过程,可能指示能量核心区岩体的张性破裂扩容和裂隙闭合过程。     

矿山微震震源机制的初步研究

矿山微震震源机制是揭示和认识矿震发生机理,从事矿山微震监测预报、矿震灾害防治的基础和前提,也是从事矿震研究的理论基础之一。根据完整岩石破裂机理,从刚度理论的角度论证了岩石动力破坏的理论条件;在总结矿山常见的岩体破坏类型的基础上,对几种基本的矿震机制进行了全面的论述。     

Assessment of Horizontal External Stresses in the Vorkuta Mine Field Using Seismic Tomography Data

3D geomechanical model of the Vorkuta Mine, Pechora Coal Basin, has been developed and implemented numerically. By the data on dynamic events recorded by stationary seismic stations, tomography of the study object is performed, and velocity of elastic waves is recovered in the illuminated area of the object. The boundary value inverse problem on horizontal external stresses by stress distribution found using empirical relations between elastic wave velocities and stresses determined in the laboratoryscale experiments is formulated and solved.     

煤层气井水力压裂有效消突边界物理模型

针对低渗煤储层煤层气井水力压裂裂缝扩展范围和消突边界确定问题,以潞安矿区煤层气井为例,采用微地震裂缝实时监测数据、有限元地应力模拟技术及井下瓦斯抽采参数,对煤层气井水力压裂范围、裂缝几何形态、压裂前后地应力分布进行研究,建立了压裂裂缝扩展和消突边界物理模型,划分了菱形井网消突范围。结果表明:研究区裂缝类型属于PKN型,压裂区呈近似椭圆形,主裂缝沿最大水平主应力方向延伸,区内划分出铺砂区、最终解吸区、裂缝区、渗透区、气涌区等5边界;压裂区四周应力沿σ_H,σ_h分别升高19%和7%,区内下降15.5%和9.5%,地应力模拟结果与压裂边界物理模型相吻合;在排采达标情况下,有效消突边界小于压裂边界,与支撑剂铺置边界一致,有效消突边界之外存在突出危险区,菱形井网采用200 m×125 m布置方式更有利于井下对应区瓦斯防治。     

Microseismic monitoring in highly stressed granite: relation between shaft-wall cracking and in Situ stress

The excavation of the shaft at Atomic Energy of Canada Ltd's Underground Research Laboratory has been associated with wall failures and formation of overbreaks. Whole-waveform microseismic signals due to crack propagations induced by shaft sinking between 324 and 443 m of depth were recorded using a microcomputer-based acquisition system. An increased level of microseismic activity was observed for a period of about 2 hr after the blasts, the activity level decreased then to a rate of a few events an hour.A velocity survey was carried out mainly for source location calibration using seismic blasting caps detonated in 32 shallow horizontal boreholes drilled following a spiral pattern along the shaft. Both P- and S-wave velocities showed a gradual increase with depth elevation of the shots. A similar increase was observed in the dynamic Young's modulus. The location of the shots could be determined to an accuracy of less than 1 m using P- and S-wave arrival times of signals recorded after the shots and a least-squares source location algorithm.Processing of the excavation-induced data showed that cracking occurred around the bottom and the walls of the shaft near the newly-created faces. The damage area inside the wall is estimated at 1.5 m. Some correlation was observed between the horizontal distance of source locations from the shaft centre and the amount of overbreak observed along the shaft. Statistical methods were used to quantitatively analyse the distribution of source locations of blast-induced microseismic events in the horizontal plane. A clear clustering was observed in the NE-SW quadrant. This is compatible with the orientations of the minimum horizontal stress component and the overbreaks observed along the shaft.     

Mathematical Modeling of Kinematics of Central Asian Plates

A two-dimensional geomechanical model is developed for numerical modeling of the displacement, strain, and stress fields of the lithosphere of Central Asia. This model considers the basic structural elements, i.e., plates, microplates, and tectonic faults. For the fault edges, the relative displacements caused by the Indo-Eurasian collision at the velocity of Hindustan motion 55 mm/yr are calculated by means of finite-element method. It is shown that the velocity of opening of different sections of the Baikal rift ranges from 1.2 to 4.8 mm/yr.     

煤层气井多井压裂效果的数值分析

地面钻井抽采瓦斯已成为解决高瓦斯矿井安全问题的有效方法,但我国煤层的渗透率普遍较低,需要进行人工增透,其最有效的方法之一就是水力压裂。在对瓦斯抽采单井压裂效果分析的基础上,利用有限元分析软件RFPA对多井压裂效果进行了数值分析,结果表明:单井压裂时,裂缝在井周两侧沿最大水平主应力方向延伸;多井压裂设计时,采用对角线沿最大水平主应力方向的菱形布井方式,能在煤层中产生充分发育的裂隙和损伤区域,达到压裂煤层的目的;模型的边界条件对模拟结果产生较大影响,施加围压更符合实际情况。     

Seismic–Geomechanical Control of Water-Impervious Strata in Potassium Mines

The safety control system for water-soluble mineral mining in complicated ground conditions is presented. The system is based on geomechanical interpretation of surface and underground seismic monitoring data. Mathematical modeling of water-impervious strata takes into account the whole set of geological and geotechnical factors determined from the seismic observations. It is shown that the seismic–geomechanical monitoring allows stress state control in waterproof strata, which ensures prompt decisionmaking on extra protection and minimizes accident risks.     

Triggered Seismicity in a Flooded Former Coal Mining Basin (Gardanne Area,France)

We studied the correlation between seismicity and the water table level in an abandoned coal mine (Bouches-du-Rhône, France), closed in 2003, where groundwater has been pumped out since 2010 to prevent underground flooding. Microseismicity was first felt by the population in 2010 and a strongly felt seismic swarm occurred in November 2012. The origin of the seismicity was therefore questioned, in relation to both the potential instability of old, shallow galleries that might generate damage at the surface and a local seismic hazard assessment. A temporary dense seismic network in the area allowed us to analyse the spatial distribution of the seismicity in detail. Most of the seismicity was clearly located under the mine workings, highlighting that an existing fault system crossing the mining operation was being hydraulically activated, in accordance with the known tectonic extension regime. Our analysis clearly shows a spatiotemporal relationship between seismic migration and the level of the mine aquifer between 2013 and 2017. Thus, seismicity will persist with oscillations of the mining aquifer, depending on the pumping capacities and effective rainfall. Continuous hydraulic and seismic monitoring is necessary to better understand these phenomena and assess the associated risks.

     

Geomechanical support of geotechnical solutions in high stress mining

The authors emphasize advisability of the geomechanical laws and rules to be included in the geotechnical decision-making in terms of the Khibiny rock mass state assessment using numerical modeling. A procedure of finding seismic-hazardous sites in rocks by the seismicity parameters is described. The geomechanical control methods are presented for the surface and underground mining systems to achieve higher safety and efficiency mineral extraction.     

Three-dimensional geomechanical model of the Tashtagol iron-ore deposit

Conclusion The diverse information collected during fulfillment of the present study on the Tashtagol deposit as a geomechanical entity on the local level, and also the procedure developed for numerical modeling make it possible to perform a detailed analysis of the geomechanical situation that develops near mining operations and in the adjacent mass. The constructed model is “open” for the introduction of additional information on stress fields, the number, location, and state of faults, and other geomechanical information, and is also easily adapted for solution of dynamic (for example, on blast-induced breaking) problems. Similar models may be constructed for all iron-ore deposits of the Gornoi Shorii. The authors express appreciation to A. A. Eremenko, O. V. Kuchai, V. F. Khramtsov, and V. A. Shalaurov for data and information that they submitted on the structure and stress-state of the Tashtagol deposit. The study was performed with financial support of the Russian Fund for Fundamental Research (Grant No. 95-05-15604). Mining Institute, Siberian Division of the Russian Academy of Sciences, Novosibirsk. Translated from Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh. No. 3, pp. 28–37, May–June, 1998.     

Reconstruction of 3D stress field in coal–rock mass by solving inverse problem using tomography data

The theoretically evaluated multi-disciplinary approach enables determination of stress state of a coal–rock mass in the area of coal cutting using a package of geomechanical and geophysical information. The approach is based on successive solutions of two inverse problems in the framework of a geomechanical model: coal-bed tomography and assessment of horizontal components of external stress field. The numerical experiments demonstrate resolvability of the inverse problems given appropriate monitoring system ensures sufficient seismic coverage of a coal-bed in the domain of steep spatial gradients of elastic waves and the presence of regular composition in the frequency range of the order of hundreds of hertz in the sounding signal generated by a cutter–loader and/or other coal-face work machinery.     

Three dimensional stress determination by hydraulic fracturing for underground excavation design

The authors have developed a hydraulic fracturing system for three dimensional stress determination. The calculation procedure introduces new interpretations based on information obtained through laboratory hydraulic fracturing of intact rocks. Hydraulic fracturing data incorporated in the system must include at least both the fluid pressure at steady flow in a longitudinal fracture artificially produced around the borehole and the shut-in pressure in a transverse fracture which is apt to develop along a natural joint. Stress states in impermeable rock can then be evaluated by the system without assuming the direction of principal stresses.In this paper, the principle of the new method for three dimensional stress determination and an outline of the measuring system are described first, and then the results are reported of in situ measurements at several sites where the new system was applied.     

原位应力下裂缝性致密砂岩各向异性地震波速及其渗透率关联特征

超深层裂缝性非常规天然气藏的开发在能源领域所占比重越来越大,但目前关于该类型储层在原位应力下各向异性地震波速及渗透率的基础研究仍鲜有报道,一定程度上制约了其高效开发策略的制定进程。以塔里木油田克深2号裂缝性致密砂岩天然气储层为工程背景,利用先进的地球物理成像真三轴测试系统,开展了储层衰竭期间原位应力下完整砂岩与不同倾角裂缝岩样的各向异性地震波速以及各向异性渗透率演化规律实验研究,探讨了压缩波、剪切波、纵横波速比、3个主应力方向渗透率各向异性特性与内在机制,研究了地震波速与渗透率的内在关联特征。研究结果表明：首先,裂缝引起的结构非均质性以及三向不等应力作用下岩石各主应力方向微观结构的变形差异导致了地震波速的各向异性;储层衰竭期间地震波速度均随有效应力的增大近似线性增加,压缩波速度明显大于剪切波速度;纵横波速比随有效应力增大而单调减小,表明剪切波对外部荷载敏感性更强;通过压缩波速和纵横波波速比数据对比,发现剪切波S2速度对裂缝倾角更为敏感。其次,原位应力下各主方向渗透率均表现出显著的应力敏感性和结构（裂缝）敏感性,早期应力大小对储层衰竭中后期渗透率变化具有重要影响,渗透率各向异性随裂缝倾角...     

基于单纯形-最短路径射线追踪的微震震源混合定位算法

震源定位精度是评价微震监测效果的最重要指标。最短路径射线追踪法是一种求解复杂速度模型下地震波走时场及检源路径的算法,是一种重要的地震波走时正演方法。本文将该方法引入到矿山微震监测领域,与基于平均速度模型的常规单纯形定位方法相结合,建立了单纯形-最短路径射线追踪混合定位算法。首先采用最短路径射线追踪方法,计算观测系统中每个检波点到潜在震源点的反向射线路径及走时;然后基于均匀速度模型,采用单纯形算法计算得到一个初始解;最后在初始解的一个小区间内,计算各节点"走时-到时"差之和,取求和值最小值对应的节点作为震源点。层状速度模型和空区速度模型正演模拟实验表明,最短路径射线追踪方法计算得到的检源路径符合地震波传播基本规律。基于神东某矿31101工作面坚硬顶板压裂案例,以标定炮为参考,新方法定位误差4.35m,常规单纯形方法定位误差为19.63m;该方法可为层状及含空区等复杂地层微震监测等提供重要的理论支撑和技术支撑。     

Stress control of seismicity patterns observed during hydraulic fracturing experiments at the Fenton Hill hot dry rock geothermal energy site,New Mexico

Seismicity accompanying hydraulic injections into granitic rock is often diffuse rather than falling along a single plane. This diffuse zone of seismicity cannot be attributed to systematic errors in locations of the events. A new scheme for determining orientations and locations of planes along which the microearthquakes occurred was recently developed. The basic assumption of the method, called the three-point method, is that many of the events fall along well-defined planes; these planes are often difficult to identify visually in the data because planes of many orientations are present. The method has been applied to four hydraulic fracturing experiments conducted at Fenton Hill as part of a hot dry rock geothermal energy project. While multiple planes are found for each experiment, one plane is common to all experiments. The ratio of shear to normal stress along planes of all orientations is calculated using a best estimate of the current stress state at Fenton Hill. The plane common to all experiments has the highest ratio of shear to normal stress acting along it, so it is the plane most likely to slip. The other planes found by the three-point method all have orientations with respect to current principal stresses that are favourable for slip to occur along pre-existing planes of weakness. These results are consistent with the assertion that the rock contains pre-existing joints which slip when the effective stress is reduced by the increased pore fluid pressure accompanying the hydraulic injection. Microearthquakes occur along those planes that are favourably aligned with respect to the current stress field.