含小断层煤层Rayleigh型槽波波场和频散分析

煤层隐伏小断层在煤田地质勘探期间能否查明,是影响煤矿安全生产的主要地质因素.煤层中传播的地震槽波,非常适用于探测煤田异常构造.本文利用谱元法模拟计算了含断层模型Rayleigh型地震槽波的产生、传播过程,对于直达槽波、反射槽波、透射槽波进行了波场特征分析,频散特征分析,频谱特征分析.根据不同小落差断层模型（垂直断距分别为1/4、1/2、3/4、1个煤厚;断层倾角分别为90°、60°、45°）,分析了Rayleigh型槽波传播过程中断层对反射槽波、透射槽波能量、频谱的影响效应.基于以上分析,对于小断层模型中的Rayleigh型地震槽波取得了一些规律性认识,并对Rayleigh型地震槽波的勘探应用做了一些探讨.     

THE INFLUENCE OF AN ASYMMETRY IN THE SEQUENCE ROCK/COAL/ROCK ON THE PROPAGATION OF RAYLEIGH SEAM WAVES*

Small offsets in hard coal seams can be detected with the aid of seam (channel) waves. Transmission and reflection of seam waves depend, among other parameters, upon the symmetry properties of the sequence rock/coal/rock. Two typical unsymmetrical sequences are found in European coal deposits: (i) coal seams with roof and floor of differing acoustic impedance and (ii) coal seams interlayered with rock and soil. Two-dimensional analog models with appropriate impedance contrasts are used to study the effect of the unsymmetrical layers upon the propagation of Rayleigh seam waves. Data analysis is based upon amplitude measurements both parallel and perpendicular to the layers and dispersion curves. The effect of unsymmetrical roof (rock 1) and floor (rock 2) was studied with models containing homogeneous coal seams. Leaky mode wave groups with phase velocities (c_R) in the range between the SV-wave velocities (β_{r1, 2}) of the two rock materials, i.e. β_r1≥c_R > β_r2, form a characteristic part of the Rayleigh seam wave signal. Using Knott's energy coefficient calculations it is shown that in that range energy leakage into the surrounding rock by refracted SV-waves is restricted to only one of the two interfaces, namely coal/rock 2. At the other interface, coal/rock 1, all waves are totally reflected. Thus, the high amplitudes of these leaky mode wave groups are explained by “quasi-normal mode” features. The influence of a dirt bed on wave propagation was studied in models where the roof and the floor have the same elastic properties. The maximum thickness of the dirt bed did not exceed 20% of the total seam thickness. The effect of the bed's location within the seam was also investigated. For all recorded normal-mode wave groups either the total seam or the coal layers could be regarded as wave guides. This was shown by the fact that the phases could be associated with the phase velocity dispersion curves calculated for the symmetrical sequence rock/coal/rock. These curves are relevant under the condition that the thickness of the coal layer assumed under the calculation coincides with the thickness of the effective wave guide of the respective wave groups. Wave groups guided in the total seam are not influenced by either the thickness or the position of the dirt bed. On the other hand, for wave groups guided in the coal layers, the quotient of signal amplitudes in the coal layers is influenced by the position of the dirt bed.     

USE OF ELECTRICAL RESISTIVITY METHODS FOR THE STUDY OF COAL SEAMS IN PARTS OF THE RANIGANJ COALFIELD (INDIA)*

The surface resistivity method has been used to study a few exposed coal seams located in the northwestern part of the Raniganj Coalfield (belonging to the Damodar Valley Gondwana basin), India. Different electrode configurations, viz., Wenner, two-electrode and half-Schlumberger, have been used with different electrode spacings in horizontal profiling. The vertical distribution of resistivity has been studied using Schlumberger configuration at different locations along the profile. Laboratory studies of resistivity (at partial to full water saturation conditions) and porosity of different coal, shaly sandstone and sandstone samples of the survey area had already been carried out before the field survey. The results indicated the presence of a good resistivity contrast between the coal seams and the surrounding formations. The field results did not bear this out: the resistivity responses of the coal formations on the profiles and sounding curves are not as clear as one would expect for such high contents. The Wenner profiles show a broad resistivity anomaly over the coal seams. Two-electrode profiles are less noisy than Wenner profiles. Sharp peaks have been observed over the coal seams. The half-Schlumberger configuration seems the best: all the coal seams and their edges can be accurately outlined on the resistivity curves. Borehole data close to the profile have been used for correlating the field results. There is good agreement with vertical electrical soundings.     

垂向裂隙各向异性煤层地震波响应

煤层中存在的裂隙会导致介质表现为各向异性,本文以HTI型煤层为例,结合各向异性介质弹性矩阵和各向异性裂隙理论,推导出不同充填物的垂直裂隙中各向异性参数表达式,将其应用于地震波响应分析;通过改进的交错网格差分法和各向异性Christoffel方程波场分解法,得到地震波合成记录和分解后的P波和SV波记录;将Thomsen群速度与相速度公式,经过坐标轴旋转变换,得到HTI型煤层中不同各向异性参数的地震波速度响应表达式;建立不同类型煤层地质模型,分析了裂隙密度、裂隙充填物以及煤层厚度等参数变化时的地震波响应特征.研究结果为分析垂向裂隙各向异性薄煤层地震波传播规律提供工具,为选用相应地震数据进行地震波各向异性参数反演提供依据.     

带地形的可控源电磁法2.5维各向异性介质正演研究

目前,对于可控源电磁法各向异性介质2.5维问题,主要采用一次场、二次场分离的方法消除场源奇异性并降低截断边界对计算区域的影响.该方法数值计算精度高,但是很难适用于复杂地形条件下的数值模拟.针对复杂地形问题,基于总场的有限元方法表现出一定的优越性,然而,这种方法存在场源奇异性问题和截断边界问题.本文采用基于总场计算的方法对带地形的可控源电磁法2.5维各向异性介质进行模拟研究,推导了考虑电导率和介电常数各向异性的2.5维控制方程;引入网格加密-收缩算法降低场源奇异性的影响范围,提升数值计算效率;引入行波分解边界条件降低截断边界的影响;提出任意采样反傅里叶变换方法,快速、高精度地计算出空间域电磁场分量.理论模型数值算例中:首先,验证了本文算法的有效性;其次,对任意各向异性倾角产生的可控源电磁响应规律进行研究;最后,采用山丘模型对各向异性介质电磁场的响应规律进行了模拟和分析.     

基于PML边界条件的二阶电磁波动方程GPR时域有限元模拟

完全匹配层(PML)作为一种稳定高效的吸收边界条件,广泛应用于基于一阶电磁波动方程的探地雷达(GPR)数值模拟中.为解决基于二阶电磁波动方程的GPR数值模拟的吸收边界问题,本文借鉴二阶弹性波动方程的PML边界条件构建思想,提出了一种适合二阶电磁波动方程GPR时域有限元模拟的PML边界条件.从二阶电磁波动方程出发,基于复拉伸坐标变换,推导了PML算法的频域表达式;通过合理构造辅助微分方程,得到了PML算法的时域表达式,并以变分形式(弱形式)加载到GPR时域有限元方程中,实现了PML边界条件在二阶电磁波动方程GPR时域有限元模拟中的应用.在此基础上,对比了无边界条件、Sarma边界条件和PML边界条件下均匀模型的波场快照、单道波形、时域反射误差和能量衰减曲线,结果表明:PML边界条件的吸收效果要远优于Sarma边界条件,具有近似零反射系数.一个复杂介质模型的正演模拟验证了PML边界条件在非均匀地电结构中电磁波传播模拟的良好吸收效果.     

Modified image algorithm to simulate seismic channel waves in 3D tunnel model with rugged free surfaces

The existence of rugged free‐surface three‐dimensional tunnel conditions in the coal seams, caused either by geological or mining processes, will inevitably influence wave propagation characteristics when the seismic waves go through the coal mines. Thus, a modified image algorithm has been developed to account for seismic channel waves propagating through this complicated topography with irregular free surfaces. Moreover, the seismic channel waves commonly exhibit damped and dispersive signatures, which is not only because of their own unique sandwich geometry of rock–coal–rock but also because of the viscoelastic behavior of coal. Considering the complexity of programming in three‐dimensional tunnel models with rugged free surfaces, an optimized vacuum grid search algorithm, enabling to model highly irregular topography and to compute efficiently, is also proposed when using high‐order staggered finite‐difference scheme to simulate seismic channel wave propagations in viscoelastic media. The numerical simulations are implemented to investigate the accuracy and stability of the method and the impact of coal's viscoelastic behavior on seismic channel wave propagation characteristics. The results indicate that the automatic vacuum grid search algorithm can be easily merged into high‐order staggered finite‐difference scheme, which can efficiently be applied to calculate three‐dimensional tunnel models with rugged free surfaces in the viscoelastic media. The simulation also suggests that the occurrence of a three‐dimensional tunnel with free surfaces has a remarkable influence on the seismic channel wave propagation characteristics and elastic energy distribution.     

Wave propagation through a medium containing anisotropic permeability

Summary Reflection and refraction for an incident plane polarised electromagnetic wave have been studied for a gyrotropic medium of tensorial permeability as a boundary value problem. The evaluated field variables reveal the electromagnetic phenomena for such a medium.     

0.9 m薄煤层SH型槽波频散特征及波形模式

在0.9 m薄煤层中使用放炮方法做微震震源的条件,通过对所采集到的薄煤层槽波信号进行分析,发现薄煤层槽波在频域中存在高频和低频两个独立并且不连续的波段.其中高频区大约以2000 Hz为中心,低频域的中心频率约为490 Hz,并且高频域和低频域的能量差异不显著.通过时频分析,可以清晰地看到高、低两个频域几乎在同一时刻触发,并且其小波相关系数在这两个域中的分布规律表现出一定的相似性.通过对0.9 m薄煤层槽波频散曲线的理论分析可知,现场观测到的Airy震相的频率及速度和其理论值较为接近.震源置于煤层中心,且炸药能量对顶底板的扰动,对第二阶对称波形模式下槽波高频部分的形成起着关键作用.在这一对称高阶波形模式下槽波的波速基本上和煤层顶底板中S波的波速一致.由于这两个触发的波形模式在时间域中具有相似的特征,建议在高频域和低频域同时发育较好的薄煤层槽波勘探中,可以采用在同一时间域中高低频相结合的方法提高利用槽波勘探分析的效果.     

VERTICAL SEISMIC PROFILING IN COAL*

The intellection of seismic wave propagation in coal measures demands direct observation of the wavefield progression. Two vertical seismic profiles with high spatial and temporal sampling, were recently recorded in the Sydney Basin coalfields as part of an experimental coal seismic program. Static corrections and interval velocities were obtained by an automated system to determine first kicks and pulse rise times. Upgoing and downgoing waves were separated in the f—k-plane using a novel technique of contour slice filtering. The isolated upgoing waves clearly display reflections from the major coal seams within the stratigraphic sequence. The downgoing wave spectra were subjected to attenuation analysis. The deduced specific quality factor Q for Permian coal measure rocks lies in the range 20–70. Similar estimates were obtained in the time domain from measurements of pulse broadening. Synthetic VSP seismograms, computed using an exact recursive formulation, are an indispensable aid to interpretation. They illustrate the filtering effects of coal seams and sequences, and the effects of the contribution of internal and free-surface multiple reflections in the recorded wavetrains.     

Microcosmic analysis of ductile shearing zones of coal seams of brittle deformation domain in superficial lithosphere

For many years, the study on ductile shearing zones mainly focuses on the strain analysis and the characteristics of microcosmic structure of felsic duc-tile shearing zones in deep layers (8—10 km). Discus-sions on the process of the development of small-type and mini-size ductile shearing zones have been re-ported in recent years[1,2]. 揇uctile deformations in brittle deformation擺3] and 揵rittle deformations in ductile deformation擺4] are rather a familiar phenome-non in nature. Researchin…     

Analysis of quality factors for Rayleigh channel waves

To facilitate investigation of the effect of imperfect elastic dissipation on thepropagation of Rayleigh-type channel waves and use of their quality factors in investigationsof the properties of coal seams, a simple method for calculating the quality factor QR isproposed in this paper. Introduction of complex velocities into the dispersion function allowscalculation of the dispersion function of Rayleigh-type channel waves in coal seams. By thecontrol variable method, we analyzed changes in QR with changes in coal seam thickness andP- and S-wave Q-factors within the coal seam and adjacent rock layers. The numerical resultsshow that the trend of the QR curve is consistent with the group velocity curve. The minimumQR value occurs at the Airy phase frequency; the Airy phase frequency decreases as coal seamthickness increases. The value of QR increases with increasing Qs2 （quality factor for S wavein coal seam）. We can compensate for the absorption of Rayleigh-type channel waves usingthe computed QR curve. Inversion of the QR curve can also be used to predict the thicknessesand litholoeies of coal seams.     

露天煤矿地下水电磁探测与类型判别

为准确确定露天煤矿地下水分布与类型,根据不同赋存条件电磁差异,基于磁场强度与电阻率参数进行异常划分与类型判别。以新疆某露天煤矿为例,采用高精度磁法通过磁异常强度确定烧变岩边界,并采用高密度电法和瞬变电磁法通过电阻率圈定低阻异常区,结合异常响应差异分别推断地下水分布范围与赋存类型。地面钻探验证探测成果的准确性。结果表明:露天煤矿烧变岩水与砂岩裂隙水电磁响应差异明显,基于磁场强度与电阻率的双参数综合电磁法能准确圈定其分布范围,并判别其赋存类型。     

Coal top detection by conductively guided borehole radar wave imaging for open cut blast-hole drilling

Damage to the top of coal seams, caused by incorrect blast stand-off distances, results in coal losses of up to 10–15% to the Australian open cut coal mining operations. This is a serious issue to be addressed. Here we propose to use a new forward-looking imaging technique based on the borehole radar technology to predict the coal seam top in real time while drilling blast holes. This is achieved by coupling the conventional borehole radar waves on to a steel drill rod to induce a guided wave along the axial drill rod. The drill rod ahead of the borehole radar behaves as a forward-looking antenna for the guided waves. Both numerical modelling and field trials simulating a drill rod as an antenna are used to investigate the feasibility of the proposed technique for prediction of the coal top under typical open cut environments. Numerical modelling demonstrated that conductivity of the overburden is the most important factor affecting our ability to see coal seams ahead of the drill bit, the guided borehole radar waves could be used for top coal prediction and a theoretical prediction error less than 10 cm and a forward-looking capability of 4–6 m can be achieved. Field trials at Australian open cut coal mines also demonstrated that guided borehole radar waves can be observed and used for prediction of coal top ahead of drill bit during blast-hole drilling in resistive, open cut environments (the average resistivity should be higher than 75 Ωm).     

Application of the nearly perfectly matched layer for seismic wave propagation in 2D homogeneous isotropic media

Numerical modelling plays an important role in helping us understand the characteristics of seismic wave propagation. The presence of spurious reflections from the boundaries of the truncated computational domain is a prominent problem in finite difference computations. The nearly perfectly matched layer has been proven to be a very effective boundary condition to absorb outgoing waves in both electromagnetic and acoustic media. In this paper, the nearly perfectly matched layer technique is applied to elastic isotropic media to further test the method's absorbing ability. The staggered‐grid finite‐difference method (fourth‐order accuracy in space and second‐order accuracy in time) is used in the numerical simulation of seismic wave propagation in 2D Cartesian coordinates. In the numerical tests, numerical comparisons between the nearly perfectly matched layer and the convolutional perfectly matched layer, which is considered the best absorbing layer boundary condition, is also provided. Three numerical experiments demonstrate that the nearly perfectly matched layer has a similar performance to the convolutional perfectly matched layer and can be a valuable alternative to other absorbing layer boundary conditions.     

CALCULATION OF DISPERSION CURVES AND AMPLITUDE-DEPTH DISTRIBUTIONS OF LOVE CHANNEL WAVES IN HORIZONTALLY-LAYERED MEDIA*

We present dispersion curves, and amplitude-depth distributions of the fundamental and first higher mode of Love seam waves for two characteristic seam models. The first model consists of four layers, representing a coal seam underlain by a root clay of variable thickness. The second model consists of five layers, representing coal seams containing a dirt band with variable position and thickness. The simple three-layer model is used for reference. It is shown that at higher frequencies, depending on the thickness of the root clay and the dirt band, the coal layers alone act as a wave guide, whereas at low frequencies all layers act together as a channel. Depending on the thickness, and position of the dirt band and the root clay, in the dispersion curves of the group velocity, secondary minima grow in addition to the absolute minima. Furthermore, the dispersion curves of the group velocity of the two modes can overlap. In all these cases, wave groups in addition to the Airy phase of the fundamental mode (propagating with minimum group velocity) occur on the seismograms recorded in in-seam seismic surveys, thus impeding their interpretation. Hence, we suggest the estimation of the dispersion characteristics of Love seam waves in coal seams under investigation preceding actual field surveys. All numerical calculations were performed using a fast and stable phase recursion algorithm.     

The history and dynamics of a welded pyroclastic dam and its failure

Igneous intrusions in coal seams are found in 80 % of coal mines in the Huaibei coalfield, China, and coal and gas outburst accidents have occurred 11 times under a 120-m-thick sill in the Haizi mining field. The magma’s heat had a significant controlling effect on coal seam gas occurrence. Based on theoretical analysis, experimental tests and site validation, we analyzed the temperature distribution following magma intrusion into coal measure strata and the variations in multiple physical parameters and adsorption/desorption characteristics between the underlying coal seams beneath the sill in the Haizi mining field and coal seams uninfluenced by magma intrusion in the adjacent Linhuan mining field. The research results show that the main factors controlling the temperature distribution of the magma and surrounding rocks in the cooling process include the cooling time and the thickness and initial temperature of the magmatic rock. As the distance from sill increases, the critical effective temperature and the duration of sustained high temperatures decrease. The sill in the Haizi mining field significantly promoted coal seam secondary hydrocarbon generation in the thermally affected area, which generated approximately 340 m³/t of hydrocarbon. In the magma-affected area, the metamorphic grade, micropore volume, amount of gas adsorption, initial speed of gas desorption, and amount of desorption all increase. Fluid entrapment by sills usually causes the gas pressure and gas content of the underlying coal seams to increase. As a result, the outburst risks from coal seams increases as well.     

Characteristics of Seismoelectric Wave Fields Associated with Natural Microcracks

Properties of seismoelectric waves in relation to natural earthquakes have been investigated. The electromagnetic disturbances were analyzed to test the hypothesis that pulse-like electric variations are directly related to microcracks as source. Because variation is very difficult to detect, there have been few quantitative field investigations. We used selected events with clear S and P phases from the data catalog obtained before the Tohoku earthquake in 2011. The electric strength of the fast P wave (P_f), S wave (S), and electromagnetic wave (EM) associated with formation of cracks of tensile mode were estimated. The co-seismic electric signal accompanied by the S wave has the largest strength, well above the noise level, and the EM wave has the lowest strength. Analytical estimation of the ratio of the strengths of the P_f and EM phases to that of the S phase by use of Pride’s equations gave results partially in agreement with observation (the order was Ap_f > A_s > A_em). The strength of the observed electromagnetic mode is approximately two orders of magnitude larger than that estimated from the theory. We suggest this greater strength can be attributed to the converted modes at layer contracts or to the effect of the boundary between free atmosphere and crust. Overall agreement between observations and theoretical estimates suggests that electromagnetic anomalies, crustal deformation, and groundwater changes can be investigated on the basis of the unified equations for the coupled electromagnetics, acoustics, and hydrodynamics of porous media.     

常温压条件下六种变质程度煤的超声弹性特征

煤弹性是反映煤的物质组分和结构的重要力学特征之一.在钻孔与测井的约束下,运用地震勘探获取煤层的弹性特征以反映其物性等,对于煤炭井工开采和煤层气储层评价及开发具有重要的工程意义,而其中,煤样的超声实验是实现地震反演煤层物性的基础.鉴于此,本文针对中国义马、阜康、淮南、平顶山、鹤壁和焦作6个矿区采集的6种不同变质程度的原煤样30块,在常温常压条件下分别进行了沿煤层走向、倾向及垂直层理三个方向煤样的实验室超声波测量.测试结果显示:煤样纵横波速度在走向、倾向、垂向三个方向上依次减小,存在各向异性,且P波速度的平均各向异性强于S波;品质因子与弹性模量在三个方向上也存在较大差异,且S波的品质因子大于P波的品质因子;弹性模量除泊松比外,均小于一般的沉积岩.通过本实验与分析进一步证明了:Gardener与Castagna公式不适用于中国煤田的煤岩弹性描述,并给出了精度更高的经验公式.     

The Influence of a Local Fault Zone on High Energy Tremor Occurrence During Longwall Mining of a Coal Seam

Underground mining of coal seams in the Upper Silesian Coal Basin in Poland is accompanied by seismic activity of varying magnitude. The investigations which have been performed for several years distinguished high energy mine tremors connected directly with mining or coupled with geological structures, such as large faults. In mined seams, local fault zones occur. Faults in these zones are usually small, with throws comparable with coal seams thicknesses. Local fault zone may be responsible for the occurrence of high energy tremors as well as large faults, as presented in this article. An analysis of source mechanism of high energy tremors generated during longwall mining of the coal seam No. 510, with presence of a local fault zone, in one of the Polish hard coal mines in the Upper Silesian Coal Basin was performed. For this purpose, the seismic moment tensor inversion method was used. In most of foci, the process of shear predominated. Determined nodal plane parameters were correlated with parameters of faults forming the local fault zone. High energy tremors were generated mostly by dislocations on faults of the local fault zone. Weakening of roof rocks in the neighborhood of local fault zone takes an important role too, and was responsible for share of implosion in the focal mechanism.