DETECTION OF CAVES IN A KARST FORMATION BY MEANS OF ELECTROMAGNETIC WAVES*

The application of classical geophysical methods in locating karst caves did not yield always unambiguous results. It was our task to verify a new method that has not yet been tested. In investigating caves in a karst formation, electromagnetic waves in the band 2–10 MHz were applied. The physical analysis of the problem is carried out, the measuring device used for the perradiation of the rocks between the boreholes is described, and the results of the measurements made in the karst area of East Slovakia (?SSR) are shown. The verification from mines is included. The results mentioned are demonstrated in a space model. The work performed in the mines verified unambiguously the interpretation of the indications measured. The results show the suitability of putting the electromagnetic method in question into the set of geophysical methods within the hydrogeological survey of karst areas.     

Geological environmental monitoring based on synchronous borehole geoacoustic and electromagnetic measurements: Use of natural electromagnetic radiation

The results of synchronous geoacoustic and electromagnetic measurements at three boreholes located in the Petropavlovsk-Kamchatsky geodynamic survey area with considerably different electromagnetic environments are studied. It is shown that reliable detection of geoacoustic emission responses to natural electromagnetic radiation in the range of 0.01–1.0 kHz is possible if geophones are placed in sufficiently deep boreholes. The results demonstrate the fundamental possibilty of using natural super-low-frequency electromagnetic radiation to monitoring of stress–strain states of the geological environment.     

井间电磁法综述

地面电磁法在实际工作中在很大程度上受到了探测深度和分辨率的限制.为了克服这些缺点,出现了井间电磁法.井间电磁法指的是在两个(或多个)钻孔中分别发射或接收电磁波信号,利用电磁波信号进行成像并探测井间物理性质的地球物理方法.由于发射机和接收机可以分别放置在很深钻孔中,其具有大透距、大探测深度的特点,因而广泛应用于工程环境物探、矿产勘查、石油勘探等中.针对不同的应用,产生了各具特点的一些特殊方法,包括井间无线电波成像、跨孔雷达、井间电磁成像.井间无线电波成像仪目前只测量电场强度数据,工作频率低,一般是单频的电磁波,频率范围通常在1 kHz至10 MHz.由于缺少走时数据修正射线路径,井间无线电波成像主要是进行基于直射线追踪的衰减层析成像.井间无线电波成像既可用于工程与环境地球物理也可用于找矿.跨孔雷达是钻孔雷达的一种探测方式,用高频电磁脉冲探测两个井孔间介电常数和电导率的变化.跨孔雷达层析成像也叫地质雷达CT,既可进行走时成像,还可进行衰减成像.一般来说,地质雷达CT的电磁波工作频率较高,中心频率通常在10 MHz和1 GHz之间,因此在分辨率指标上占有优势,跨孔雷达主要用于工程与环境地球物理.井间电磁成像采用更低的频率,测量复电磁信号,适合油气储集层监测,是一种地球物理前沿技术.经过在多个试验区初步试验表明,井间电磁成像是油藏研究的有效手段,可用于分析剩余油分布,寻找油气富集区,进而达到提高钻探高效井成功率和提高采收率的目的.本文详细介绍对比了这三种方法在理论和实践中的一些特点,并对未来的发展进行了展望.     

井间电磁波CT技术及其在岩溶勘察中的应用

井间电磁波层析成像技术是将医学CT应用于地球物理学中的一项新技术,它主要依据电磁波在不同介质中传播时的吸收差异,重建井间介质吸收系数的分布图像,形象及直观地展现研究区域的地质结构。本文详细地研究了井间电磁波层析成像技术的基本原理,对具有高低吸收电磁异常体的理论模型,根据一定的观测系统对电磁波的场强进行了数值模拟,并采用SIRT反演技术对吸收异常体的图像进行了重建;同时将该方法应用于广西某地拟建桥址的岩溶勘查之中,取得了良好的应用效果。     

Numerical modelling of resolution and sensitivity of ERT in horizontal boreholes

Resistivity in horizontal boreholes can give useful detailed information about the geological conditions for construction in rock, i.e. in front of a tunnel bore machine. This paper is an attempt to identify a suitable methodology for an effective measuring routine for this type of geophysical measurements under actual construction site conditions.Prior to any measurements numerical modelling was done in order to evaluate the resolution of different electrode arrays. Four different arrays were tested; dipole–pole, cross-hole dipole–dipole, cross-hole pole–tripole and multiple gradient array. Additionally the resolution of a combination of cross-hole dipole–dipole and multiple gradient was assessed. The 2D sensitivity patterns for various arrangements of the cross-hole dipole–dipole and multiple gradient array were examined. The sensitivity towards inaccurate borehole geometry and the influence of water in the boreholes were also investigated. Based on the model study the cross-hole dipole–dipole array, multiple gradient array and a combination of these were found to give the best result and therefore were used for test measurements in horizontal boreholes. The boreholes were 28.5 m long and drilled 6.5 m apart. Prototypes of semi-rigid borehole cables made it possible to insert multi electrode cables in an efficient way, allowing fast measurement routines. These measurements were then studied to determine their accuracy and applicability. The results showed a high resistivity rock mass at the site. A transition from high resistivity to slightly lower resistivity coincides well with a change in lithology from gneiss-granite to gneiss. It is likely that the shotcrete on the tunnel wall is seen as a low resistivity zone.The measurements are a valuable tool, but further development of the cables and streamlining of measuring routines have to be performed before the resistivity tomography can be used routinely in pilot holes during construction in rocks.     

Ray tomography: Errors and error functions

Tomography is the inversion of boundary projections to reconstruct the internal characteristics of the medium between the source and detector boreholes. Tomography is used to image the structure of geological formations and localized inhomogenieties. This imaging technique may be applied to either seismic or electromagnetic data, typically recorded as transmission measurements between two or more boreholes. Algebraic algorithms are error-driven solutions where the goal is to minimize the error between measured and predicted projections. The purpose of this study is to assess the effect of the ray propagation model, the measurement errors, and the error functions on the resolving ability of algebraic algorithms. The problem under consideration is the identification of a two-dimensional circular anomaly surveyed using crosshole measurements. The results show that: (1) convergence to the position of the circular anomaly in depth between vertical boreholes is significantly better than for convergence in the horizontal direction; (2) error surfaces may not be convex, even in the absence of measurement and model errors; (3) the distribution of information content significantly affects the convexity of averaging error functions; (4) measurement noise and model inaccuracy manifest in increased residuals and in reduced convergence gradients near optimum convergence; (5) the maximum ray error function increases convergence gradients compared with the average error function, and is unaffected by the distribution of information content; however, it has higher probability of local minima. Therefore, inversions based on the minimization of the maximum ray error may be advantageous in crosshole tomography but it requires smooth projections. These results are applicable to both electromagnetic and seismic data for wavelengths significantly smaller than the size of anomalies.     

井间电磁波CT在昆明地铁岩溶区地质勘察中的应用

井间电磁波CT技术是利用地层间介质的物性差异,不同性质的物质对电磁波的反射和吸收能力是不同的。井间电磁波CT技术在探测岩溶裂缝等方面有较好效果,本文利用井间电磁波CT技术,对昆明地铁四号线苏家塘站岩溶专项线路工程地下岩溶发育地段进行测试,通过对工区岩石电磁波吸收系数差异的研究,依据吸收系数特征,探测到地铁线路工程地下岩溶及裂缝发育,划分出4个层次的岩溶区,经与钻孔岩心资料对比,证实探测结果的正确性。且对工程设计和施工提供了必要的基础资料。      

Numerical study on the fracturing mechanism of shock wave interactions between two adjacent blast holes in deep rock blasting

With the application of electronic detonators, millisecond blasting is regarded as a significant promising approach to improve the rock fragmentation in deep rock blasting. Thus, it is necessary to investigate the fracturing mechanisms of short-delay blasting. In this work, a rectangle model with two circle boreholes is modeled as a particles assembly based on the discrete element method to simulate the shock wave interactions induced by millisecond blasting. The rectangle model has a size of 12 × 6 m (L × W) and two blast holes have the same diameter of 12 cm. The shock waves are simplified as time-varying forces applied at the particles of walls of the two boreholes. Among a series of numerical tests in this study, the spacing between two adjacent boreholes and delay time of millisecond blasting are considered as two primary variables, and the decoupling charge with a coefficient of 1.5 is taken into account in each case. The results show that stress superposition is not a key factor for improving rock fragmentation (tensile stress interactions rather than compressive stress superposition could affect the generation of cracks), whereas collision actions from isolated particles or particles with weakened constraints play a crucial role in creating the fracture network. The delay time has an influence on causing cracks in rock blasting, however, whether it works heavily depends on the distance between the two holes.

     

Electrical and electromagnetic borehole measurements: A review

Electrical and electromagnetic geophysical techniques have reached a high level of technological sophistication since they were first used in boreholes less than one hundred years ago. Borehole logging-the detailed determination of rock and fluid properties adjacent to the borehole, and borehole geophysics-extending the range of geophysical investigation large distances away from the borehole, are essential for exploration, assessment and production of earth resources, as well as for fundamental studies of the earth. Borehole electrical and electromagnetic methods incorporate 17 decades of the electromagnetic spectrum, from 1000-s geomagnetic studies, through resistivity and permittivity measurements, to high-resolution resistivity imaging, NMR and optical spectroscopy.     

冀东地区深孔地应力测量及其意义

为获得矿山深部开采和设计所需要的地应力基础资料,中国地质科学院地质力学研究所于2012—2013年在冀东地区滦县—乐亭大断裂附近开展了3个钻孔水压致裂的应力测量工作,钻孔孔深为500—1 000 m.地应力测量结果初步揭示了研究区地壳浅表层现今地应力的分布规律.实测结果表明:现今研究区域应力场优势方向为NW70°;3个测点的最大剪切应力与平均应力的比值um范围为0.24—0.48,平均值分别为0.30,0.40和0.30,均未达到地壳应力摩擦极限状态(um=0.6)的水平,而唐山大地震后不久研究区浅层获得的um则较大,个别点大于0.6,平均值为0.44,表明唐山大地震后研究区应力积累程度较低,应力释放较为充分;依据断层摩擦准则及拜尔利定律可知,研究区发生较大地震活动的危险性较低.      

Controlled‐source electromagnetic monitoring of reservoir oil saturation using a novel borehole‐to‐surface configuration

To advance and optimize secondary and tertiary oil recovery techniques, it is essential to know the areal propagation and distribution of the injected fluids in the subsurface. We investigate the applicability of controlled‐source electromagnetic methods to monitor fluid movements in a German oilfield (Bockstedt, onshore Northwest Germany) as injected brines (highly saline formation water) have much lower electrical resistivity than the oil within the reservoir. The main focus of this study is on controlled‐source electromagnetic simulations to test the sensitivity of various source–receiver configurations. The background model for the simulations is based on two‐dimensional inversion of magnetotelluric data gathered across the oil field and calibrated with resistivity logs. Three‐dimensional modelling results suggest that controlled‐source electromagnetic methods are sensitive to resistivity changes at reservoir depths, but the effect is difficult to resolve with surface measurements only. Resolution increases significantly if sensors or transmitters can be placed in observation wells closer to the reservoir. In particular, observation of the vertical electric field component in shallow boreholes and/or use of source configurations consisting of combinations of vertical and horizontal dipoles are promising. Preliminary results from a borehole‐to‐surface controlled‐source electromagnetic field survey carried out in spring 2014 are in good agreement with the modelling studies.     

Imaging a shallow salt diapir using ambient seismic vibrations beneath the densely built-up city area of Hamburg,Northern Germany

Salt diapirs are common features of sedimentary basins. If close to the surface, they can bear a significant hazard due to possible dissolution sinkholes, karst formation and collapse dolines or their influence on ground water chemistry. We investigate the potential of ambient vibration techniques to map the 3-D roof morphology of shallow salt diapirs. Horizontal-to-vertical (H/V) spectral peaks are derived at more than 900 positions above a shallow diapir beneath the city area of Hamburg, Germany, and are used to infer the depth of the first strong impedance contrast. In addition, 15 small-scale array measurements are conducted at different positions in order to compute frequency-dependent phase velocities of Rayleigh waves between 0.5 and 25 Hz. The dispersion curves are inverted together with the H/V peak frequency to obtain shear-wave velocity profiles. Additionally, we compare the morphology derived from H/V and array measurements to borehole lithology and a gravity-based 3-D model of the salt diapir. Both methods give consistent results in agreement with major features indicated by the independent data. An important result is that H/V and array measurements are better suited to identify weathered gypsum caprocks or gypsum floaters, while gravity-derived models better sample the interface between sediments and homogeneous salt. We further investigate qualitatively the influence of the 3-D subsurface topography of the salt diapir on the validity of local 1-D inversion results from ambient vibration dispersion curve inversion.     

Application of acoustic full wavetrains for the determination of lithology,reservoir and mechanical parameters of formation1

The significant development in acoustic full waveform logging during the last ten years has made it increasingly possible for log analysts to determine the physical properties of a rock formation in situ. Parallel to the methods applied to a single wavetrain during seismic processing, the new techniques, used for sets of wavetrains, have been successfully tested with acoustic full waveforms. Instantaneous characteristics analysis is included in this group of methods. This approach, leading to qualitative and quantitative interpretation, reveals the influence of small changes in physical properties on acoustic full wavetrains. Applications of complex acoustic waveform analysis for the determination of inhomogeneous zones are presented. Colour diagrams of instantaneous characteristics are used for the detection of fractured regions and slow formations with increased attenuation of acoustic waves. Results of the interpretation of individual acoustic full waveforms, based on cross-correlation and spectral analysis, using the IDNP and IDNS computer programs, e.g. velocities of compressional waves, shear and Stoneley waves, are presented. Since the bulk density of the rocks was known, it was possible, using the velocities of P- and S-waves obtained, to calculate the dynamic elastic moduli. We used the interpretation of acoustic full wavetrains to calculate porosity. The sonic porosity is compared to the porosity obtained from other logs and to that obtained from core sample analysis. The examples of acoustic full wavetrains were recorded in the Miocene sulphur-bearing limestones in central Poland. Field measurements were made using the domestic prototype equipment for well log recordings in shallow boreholes.     

Laboratory measurements of guided‐wave propagation within a fluid‐saturated fracture

A fluid‐saturated flat channel between solids, such as a fracture, is known to support guided waves—sometimes called Krauklis waves. At low frequencies, Krauklis waves can have very low velocity and large attenuation and are very dispersive. Because they propagate primarily within the fluid channel formed by a fracture, Krauklis waves can potentially be used for geological fracture characterization in the field. Using an analogue fracture consisting of a pair of flat slender plates with a mediating fluid layer—a trilayer model—we conducted laboratory measurements of the velocity and attenuation of Krauklis waves. Unlike previous experiments using ultrasonic waves, these experiments used frequencies well below 1 kHz, resulting in extremely low velocity and large attenuation of the waves. The mechanical compliance of the fracture was varied by modifying the stiffness of the fluid seal of the physical fracture model, and proppant (fracture‐filling high‐permeability sand) was also introduced into the fracture to examine its impact on wave propagation. A theoretical frequency equation for the trilayer model was derived using the poroelastic linear‐slip interface model, and its solutions were compared to the experimental results.     

微气压波与地震

地震之前长周期的地壳运动已为人们所关注,然而在零漂等问题尚未解决之前,长周期地震仪观测仍处于试验性阶段.另外,震前地气的喷溢和释放,只能在喷口处观测,距离稍远,就观测不到了,但由此而引起的大气振动和声学现象,却可在一定距离范围内观测到.因而可开展震前由长周期运动和地气等引起的大气声波、重力波等微气压波的观测研究.地震波所激发的次声[1],前人已有研究,但地震前的次声,中外文献还未见报道.1976年唐山地震后的1976年8月至1977年3月,笔者在北京首次开展微气压波的观测,发现了十余次地震前的微气压波,其特征为40—80秒的周期,1—5微巴的声压,波列很短,仅数分钟,出现于震前1—16小时.本文提出了识别震前微压波的分析方法,并讨论了其产生的可能机制和有关的临震现象.      

基于重构激发信号的电磁式可控震源相关检测参考信号方法研究

针对电磁式可控震源地震数据的相关检测,研究发现,在地下结构复杂、基板-大地耦合不佳时,常规方法——基于震源控制信号或基板附近信号作为参考信号检测得到的地震记录中,存在子波到时误差和虚假多次波问题.本文分析了上述问题的理论原因,并提出基于重构激发信号的相关检测参考信号方法(Correlation Detection Reference Signal Based on the Reconstructed Excitation Signal,CDRSBRES).首先,利用直达波与其他地震波到时不一致的特点,从震源基板附近信号中分离、提取直达波.然后,利用直达波重构震源激发信号并作为参考信号对地震数据进行相关检测.最后,应用谱白化技术提高检测结果质量.数值模拟研究表明,重构激发信号与理想激发信号的相关系数为0.9869,达到高度线性相关,CDRSBRES方法检测的地震记录在子波到时和波形特征上均与模型相符.随后,在某金属矿区开展了可控震源对比实验.与液压式可控震源MiniVib T15000检测结果相比,电磁式可控震源PHVS 500的检测结果中:基于震源控制信号的检测结果存在子波到时误差约0.012s,对应垂向精度误差约11.16m;基于基板附近信号的检测结果部分区域出现虚假多次波,信噪比降低;而CDRSBRES方法的检测结果子波到时误差约0.001s,对应垂向精度误差约0.93m,波形特征一致,相同区域无虚假多次波.综上,本方法适用于电磁式可控震源地震数据的高精度检测,尤其对于地下结构复杂区域的高分辨率地震勘探具有重要意义.     

Improved Resolution of Ambient Flow through Fractured Rock with Temperature Logs

In contaminant hydrogeology, investigations at fractured rock sites are typically undertaken to improve understanding of the fracture networks and associated groundwater flow that govern past and/or future contaminant transport. Conventional hydrogeologic, geophysical, and hydrophysical techniques used to develop a conceptual model are often implemented in open boreholes under conditions of cross-connected flow. A new approach using high-resolution temperature (±0.001°C) profiles measured within static water columns of boreholes sealed using continuous, water-inflated, flexible liners (FLUTe™) identifies hydraulically active fractures under ambient (natural) groundwater flow conditions. The value of this approach is assessed by comparisons of temperature profiles from holes (100 to 200 m deep) with and without liners at four contaminated sites with distinctly different hydrogeologic conditions. The results from the lined holes consistently show many more hydraulically active fractures than the open-hole profiles, in which the influence of vertical flow through the borehole between a few fractures masks important intermediary flow zones. Temperature measurements in temporarily sealed boreholes not only improve the sensitivity and accuracy of identifying hydraulically active fractures under ambient conditions but also offer new insights regarding previously unresolvable flow distributions in fractured rock systems, while leaving the borehole available for other forms of testing and monitoring device installation.     

Slopes of an airborne electromagnetic resistivity model interpolated jointly with borehole data for 3D geological modelling

We investigate a novel way to introduce resistivity models deriving from airborne electromagnetic surveys into regional geological modelling. Standard geometrical geological modelling can be strengthened using geophysical data. Here, we propose to extract information contained in a resistivity model in the form of local slopes that constrain the modelling of geological interfaces. The proposed method is illustrated on an airborne electromagnetic survey conducted in the region of Courtenay in France. First, a resistivity contrast corresponding to the clay/chalk interface was interpreted confronting the electromagnetic soundings to boreholes. Slopes were then sampled on this geophysical model and jointly interpolated with the clay/chalk interface documented in boreholes using an implicit 3D potential‐field method. In order to evaluate this new joint geophysical–geological model, its accuracy was compared with that of both pure geological and pure geophysical models for various borehole configurations. The proposed joint modelling yields the most accurate clay/chalk interface whatever the number and location of boreholes taken into account for modelling and validation. Compared with standard geological modelling, the approach introduces in between boreholes geometrical information derived from geophysical results. Compared with conventional resistivity interpretation of the geophysical model, it reduces drift effects and honours the boreholes. The method therefore improves what is commonly obtained with geological or geophysical data separately, making it very attractive for robust 3D geological modelling of the subsurface.     

Exact solutions of magnetohydrodynamics for describing different structural disturbances in solar wind

We use analytical methods of magnetohydrodynamics to describe the behavior of cosmic plasma. This approach makes it possible to describe different structural fields of disturbances in solar wind: shock waves, direction discontinuities, magnetic clouds and magnetic holes, and their interaction with each other and with the Earth’s magnetosphere. We note that the wave problems of solar–terrestrial physics can be efficiently solved by the methods designed for solving classical problems of mathematical physics. We find that the generalized Riemann solution particularly simplifies the consideration of secondary waves in the magnetosheath and makes it possible to describe in detail the classical solutions of boundary value problems. We consider the appearance of a fast compression wave in the Earth’s magnetosheath, which is reflected from the magnetosphere and can nonlinearly overturn to generate a back shock wave. We propose a new mechanism for the formation of a plateau with protons of increased density and a magnetic field trough in the magnetosheath due to slow secondary shock waves. Most of our findings are confirmed by direct observations conducted on spacecrafts (WIND, ACE, Geotail, Voyager-2, SDO and others).     

贴井壁声波测井仪的有限差分模拟研究

贴井壁声波测井仪在石油勘探开发中具有广泛的应用.它能够避开泥浆的影响而直接检测井壁附近的地层性质.但目前对此进行的数值模拟研究还仅局限于比较简单的模型.有限差分法可以模拟比较复杂的井眼模型,然而贴井壁声源的频率较高（80kHz左右）,使有限差分法计算量过大.本文应用局部加密交错网格有限差分方法解决了这类数值模拟中计算精度和计算量之间的矛盾.用这种算法模拟了井中有仪器时贴井壁声源激发的声场,并将其与无仪器时的声场作了比较.对模拟结果的验证和分析显示,该有限差分方法能够正确模拟贴井壁声源激发的声场.通过考察井中仪器本体对贴井壁声源激发声场的影响发现,测井仪器本体能够对直达波进行衰减而对环绕井壁传播的面波影响不大.