Geoelectrical investigations by means of resistivity methods in karst areas in Romania

The applicability of resistivity methods to ground water investigations is well recognized. As water-saturated rock formations have a lower electrical resistivity than dry ones, an electrical resistivity survey should result in low resistivity anomalies. Normally, such anomalies are interpreted to indicate areas of potentially significant ground water flows. In karst areas, however, interpretation may not be as straightforward: for example, large electrically conductive domains can represent water bearing zones, whose fluid-permeability may be poor; alternatively, fast flow conduits, which may be unsaturated, occur as slender objects, and not as clear anomalous features. In order to deal with such extreme heterogeneities, resistivity investigations require some specific adjustments. One example is the so-called “mise à la masse” method. In Romania, it was used to trace the cold karst water inflows that detrimentally affected the commercial exploitation of a thermal spring, Hercules at Baile Herculane. Conventional geoelectrical approaches––such as using resistivity highs to detect air-filled cave passages, are proved to be less efficient in the considered karst investigations.     

Geostatistical integration using 2-D electrical resistivity and 3-D gravity methods for detecting cavities in a karst area

Two-dimensional (2-D) electrical resistivity and three-dimensional (3-D) gravity explorations were undertaken to estimate the 3-D distribution of karst cavities at the area of Yongweol-ri in Korea where ground subsidence has occurred. Although the gravity method is a low-cost way of analyzing a 3-D continuous structure, its vertical resolving power is poor. In contrast, the electrical resistivity method can provide a 2-D and/or 3-D subsurface structure with a much higher lateral and vertical resolution than the gravity method. Accordingly, geostatistical methods and density information were used to enhance the 2-D resistivity structure revealed by the electrical resistivity method into a 3-D structure. The assumptions are, first, that each data set senses the same underlying geological structure in terms of different material properties and, secondly, that two different material properties are correlated locally or globally throughout the entire target area. As a result, the distribution of limestone cavities can be estimated under the assumption that they are mostly filled with groundwater and clayey soils and have abnormally low levels of resistivity and density. The estimated distribution corresponds with the grouting, borehole imaging and monitoring data. In this example, it can be seen that the integration analysis of 2-D electrical resistivity and 3-D gravity methods is a very powerful tool for 3-D subsurface imaging and that the method can provide enhanced imaging capabilities for 3-D cavities.     

使用MAPGIS中的DTM功能处理化探数据

利用MIPGIS中的DTM功能,结合Microsoft Excel处理水系沉积物测量、土壤测量化探数据的方法及成图时注意的一些问题,处理化探数据方法便捷准确。     

Application of electrical resistivity tomography (ERT) technique to detect underground cavities in a karst area of South Korea

Recently, the electrical resistivity tomography (ERT) technique has been increasingly applied to underground cavity detection filled with groundwater and/or clay. In this study, an ERT survey was conducted to examine the spatial distribution and shape of underground cavities in a karst area in Korea. Numerical ERT modeling and inversion were carried out to select appropriate field survey parameters. Two geological structures, similar to those that exist in the field survey area, were modeled: a sinkhole model and a vein-type structure. Electrode configurations using pole–pole, pole–dipole, dipole–pole, and dipole–dipole arrays were tested with the two geologic models and compared. The numerical modeling showed that the resistivity distribution of the ERT tomogram using the pole–dipole array produced the best representation of the two models. The pole–dipole and pole–pole array configurations were applied in an ERT field survey using six boreholes. The field ERT results show that cavity areas filled with clay or groundwater appeared as low-resistivity anomalies in the limestone formation. This was particularly evident in the inclined vein-type structures attributed to fracture zones, where the field data were similar to the modeling results. Therefore, the ERT technique could be effectively used for detecting underground limestone cavities under similar geological conditions as those at our study area.     

薄层灰岩浅部岩溶发育特征及分布模型

碳酸盐岩往往含有较多泥质夹层,多发育浅部岩溶裂隙,影响工程基础稳定性。文章依托广西合山市溯河矿区某道路浅部岩溶调查成果,分析了薄层灰岩岩溶宏观形态和垂直分布特征,按照溶蚀程度将其分为碎屑残留带、表层岩溶带和下部包气带;通过室内溶蚀试验,分析了矿物组成、CO2及水动力条件影响下的溶蚀规律;对比分析了薄层和厚层岩溶发育特征。结果表明:（1）薄层灰岩浅部岩溶发育程度与微地貌有关,通常在山谷和地下水强径流带溶蚀作用更为强烈;（2）薄层灰岩溶蚀优先开始于方解石,在岩石表面形成溶孔、粗糙程度增加,提供了侵蚀性溶液流动通道;（3）薄层灰岩岩溶较厚层灰岩岩溶规模小,分布较为均匀,由地表土体渗入地下岩体的地下水含有较多侵蚀性CO2水溶液,会促进岩溶作用的进行;（4）薄层灰岩浅部岩溶发育分布模型为:发育均匀的微小溶蚀裂隙在溶蚀下渗过程中遇到软弱夹层时会沿水平向扩展并随进一步溶蚀作用被充填,垂向深部发展时溶蚀作用逐步消散。     

南川地区溶洞及采空区地震资料针对性处理方法

为解决地震勘探中地下溶洞及采空区位置地震资料成像效果差的难题,利用正演模拟结合实际资料分析,总结该类区域地震资料的频率、能量和信噪比特征,在此基础上,提出一套针对性的处理方法,包括微测井约束层析静校正、弱信号提取与补偿、叠前五维数据规则化和基于构造约束的网格层析速度建模。通过在南川地区的应用,地震资料信噪比和波组连续性得到明显改善。证明了此方法可以有效提高地震勘探中溶洞及采空区位置地震资料品质。      

Magnetic measurements in electrical prospecting by resistivity methods

The electrical resistivity and induced polarization (IP) methods are widely used in geological mapping, prospecting and exploration of mineral deposits, engineering geology, hydrogeology, archaeology, and geotechnical and environmental applications. Historically, these methods have formed the basis of the electrical prospecting technique. In these methods, a DC or low-frequency AC electrical current is introduced into the earth through a grounded transmitter line. The measured quantity is the electric field. However, if the earth’s resistivity or chargeability changes horizontally, this change gives rise to an anomalous magnetic field, which is studied by the magnetometric resistivity (MMR) and magnetic induced polarization (MIP) methods, respectively. Along with advantages, some shortcomings are inherent in the MMR and MIP techniques. Apparently, the main drawback of these methods is that the magnetic fields of both the transmitter line wire and ground electrodes on the surface are several orders of magnitude greater than the anomalous magnetic field response. This introduces a significant “noise” to magnetic-resistivity data. We investigate the potential of using a circular electric dipole (CED) in magnetometric resistivity techniques. It has been found that the application of a CED, instead of a conventional transmitter line, dramatically enhances the signal-to-noise ratio.     

随机岩溶介质的电阻率正演模拟

受岩溶发育的影响,岩溶地区电阻率分布具有很大的不确定性。采用传统的正演方法在一定程度上不能满足岩溶探测的需要。为了更真实地反映岩溶介质的电性特点,本文提出了一种根据随机过程的谱分解理论来构造随机介质模型的方法,并以此建立了岩溶地区随机地电模型。对理论和实测地电模型的正演计算表明,随机介质能更有效模拟实际地电结构,正演结果更接近实测的异常形态。因此,岩溶地区的电阻率正演模拟计算采用随机介质模型是十分有必要的,一方面有助于对异常的认识,另一方面,随机介质模型可以更好地模拟岩溶地区的地电模型。      

高分辨地电阻率法探查小窑采空区的应用效果

介绍了利用高分辨地电阻率法探查大柳塔煤矿小窑采空区的作业方法和效果.钻探验证表明,这种探查方法可靠,所推断的采空区位置准确.这一探查结果为地面注浆灭火措施提供了科学依据.     

高密度电法岩溶探测模型及其在变电工程中的应用

在我国西南部,由于地质条件复杂,许多变电工程不可避免地建于岩溶发育地区,因此探明站址底部隐伏岩溶的位置、规模、埋深及充填物是工程勘察的重点。采用高密度电法进行探测,通过数值模拟计算,总结分析不同填充类型岩溶的地电响应特征,为现场探测、分析、解释溶洞特征提供科学依据;结合站址底部隐伏岩溶探测工程案例,说明高密度电法能有效地查明地下隐伏岩溶,为变电工程合理选址及设计基础深度提供可靠依据。     

岩溶探测中的弹性波CT方法

弹性波CT方法,又称地震波层析成像技术.这种技术利用大量的地震波速度信息进行专门的反演计算,得到测区内岩土体弹性波速度的分布规律.利用这种方法探查岩溶的分布、形态及连通性与其它测试方法比较,分辨率高、效率高、空间位置准确.在岩溶发育地区进行工程勘察,采用常规钻探与弹性波CT成像相结合的勘察方法,可消除工程安全隐患.     

超高密度电法在上覆型岩溶勘察中的应用

南京地铁4号线桦墅站—仙林东站区间现场钻探时在部分钻孔中发现规模较大的溶洞,为探明区内溶洞的大小和分布情况,结合区域工程地质与水文地质条件,分析岩土介质的地球物理特征,采用超高密度电法对研究区进行地电测量,经反演分析获取地电阻率图像,结合岩溶的电阻率异常响应特征,在隧道纵横方向上分别采用地面方式和井井方式相互验证,判别岩溶的分布范围、走向、大小及埋藏深度。研究结果表明,超高密度电法在上覆型岩溶勘察中具高精度、高效率、直观性等特点,解释成果与钻探揭示验证一致,为土-岩-空洞三元结构的探测提供了一种新方法。     

Linear regression models for estimating true subsurface resistivity from apparent resistivity data

Simple linear regression (SLR) models for rapid estimation of true subsurface resistivity from apparent resistivity measurements are developed and assessed in this study. The objective is to minimize the processing time and computer memory required to carry out inversion with conventional algorithms. The arrays considered are Wenner, Wenner–Schlumberger and dipole–dipole. The parameters investigated are apparent resistivity (\(\rho _a \)) and true resistivity (\(\rho _t\)) as independent and dependent variables, respectively. For the fact that subsurface resistivity is nonlinear, the datasets were first transformed into logarithmic scale to satisfy the basic regression assumptions. Three models, one each for the three array types, are thus developed based on simple linear relationships between the dependent and independent variables. The generated SLR coefficients were used to estimate \(\rho _t\) for different \(\rho _a\) datasets for validation. Accuracy of the models was assessed using coefficient of determination (\(R^{2})\), F-test, standard error (SE) and weighted mean absolute percentage error (wMAPE). The model calibration \(R^{2}\) and F-value are obtained as 0.75 and 2286, 0.63 and 1097, and 0.47 and 446 for the Wenner, Wenner–Schlumberger and dipole–dipole array models, respectively. The SE for calibration and validation are obtained as 0.12 and 0.13, 0.16 and 0.25, and 0.21 and 0.24 for the Wenner, Wenner–Schlumberger and dipole–dipole array models, respectively. Similarly, the wMAPE for calibration and validation are estimated as 3.27 and 3.49%, 3.88 and 5.72%, and 5.35 and 6.07% for the three array models, respectively. When compared with standard constraint least-squares (SCLS) inversion and Incomplete Gauss–Newton (IGN) algorithms, the SLR models were found to reduce about 80–96.5% of the processing time and memory space required to carry out the inversion with the SCLS algorithm. It is concluded that the SLR models can rapidly estimate \(\rho _t\) for the various arrays accurately.     

在金矿勘探中应用STING-SWIFT高密度电法

STING-SWIFT高密度电法系统可以应用于多金属、金属硫化物矿,地下水、浅部煤矿等资源勘探,也可以从事工程质量、无损检测,隐患勘探等.在金矿勘探中主要采用偶极-偶极装置,电极数42个,可以探测150 m以内的电阻率和充电率的精细结构,以推断精细的地质和电子性导电矿物结构.通过对黑龙江金厂、河北峪耳崖、青海门源多拉隆洼等金矿区的工作表明,该系统不同的参数设置往往会给结果带来很大差异;尤其是对于不同的矿区,灵活调整工作参数、极距、最大偶极距、测量时间、误差限等,将能收到事半功倍的效果.     

电阻率法和激发极化法在地下水勘查中的应用

依据提供的物性参数的不同,广泛应用于找水工作的电法手段可分为电阻率法和激发极化法两大类,其中电阻率法提供电阻率参数,主要解决与赋水有关的构造问题(如赋水层位或断裂构造)。方法上有常规电法、电磁法,可根据地区赋水部位的不同选择适当的方法。激发极化法则利用含水层的激发极化效应进一步确定目标层位或构造的赋水性。电阻率法和激发极化法相互配合,在找水工作中相得益彰。     

时移电阻率法归一化数据反演分辨电阻率结构微小变化

时移电阻率法能够应用于监测地下水污染物运移、衡量坡面稳定性等工程和环境问题.本文利用归一化时移电阻率法数据反演结果识别地下微小电阻率结构变化.首先,将初次采集数据作为背景数据,利用其归一化其他时刻时移电阻率法数据;再次,实现背景数据、时移数据以及归一化后时移数据的非线性共轭梯度反演.在合成数据算例中,保持观测系统不变,用相同反演参数和均匀半空间参考模型参与反演.反演结果表明,相对背景电阻率而言,归一化数据反演结果能够直接地、有效地分辨较为微小的电阻率结构动态变化,而传统电阻率反演不能分辨.     

Reliability of dipole-dipole electrical resistivity tomography for defining depth to bedrock in covered karst terranes

 Sinkhole collapse is one of the main limitations on the development of karst areas, especially where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown that sinkholes are likely to develop in cutter (enlarged joint) zones as a result of subterranean erosion by flowing groundwater. Because of the irregular distribution of pinnacles and cutters on the bedrock surface, uncertainties arise when "hit-or-miss" borehole drilling is used to locate potential collapse sites. A high-resolution geophysical technique capable of depicting the details of the bedrock surface is essential for guiding the drilling program. Dipole-dipole electrical resistivity tomography (ERT) was used to map the bedrock surface at a site in southern Indiana where limestone is covered by about 9 m of clayey soils. Forty-nine transects were conducted over an area of approximately 42,037 m². The electrode spacing was 3 m. The length of the transects varied from 81 to 249 m. The tomographs were interpreted with the aid of soil borings. The repeatability of ERT was evaluated by comparing the rock surface elevations interpreted from pairs of transects where they crossed each other. The average difference was 2.4 m, with a maximum of 10 m. The discrepancy between interpreted bedrock-surface elevations for a transect intersection may be caused by variations in the subsurface geology normal to the transect. Averaging the elevation data interpreted from different transects improved the ERT results. A bedrock surface map was generated using only the averaged elevation data at the transect junctions. The accuracy of the map was further evaluated using data from four exploratory boreholes. The average difference between interpreted and actual bedrock surface-elevations was less than 0.4 m. The map shows two large troughs in the limestone surface: one coinciding with an existing sinkhole basin, while the other is in alignment with a small topographic valley. Because sinkholes were observed at the same elevation interval in similar valleys in the vicinity, the delineated trough may have implications for future land use at the site. Received: 4 January 1999 · Accepted: 8 March 1999     

充水溶洞特征的高密度电阻率法反演分析研究

在高密度电法勘探中,不同性质的溶洞其反演电阻率特征有差异,溶洞为空洞时反演电阻率表现为高阻,充满水时表现则为低阻,而充水溶洞的电阻率反演特征很难判断.文章通过构建不同充水程度的溶洞模型进行正演模拟,利用最小二乘法对模型的地电断面进行反演,对比不同反演成像模型的地电响应特征.结果表明,充水溶洞的电阻率随溶洞充水量的增加而减小,低阻的范围随着水量的增加而不断扩大,其可分辨性强.野外探查资料进一步验证了模拟结果,所获得的认识对地下溶洞的电法探测分析具有一定的指导意义.     

新疆戈壁地区大功率激电勘探接地电阻改善方法

在新疆戈壁地区年降雨量少,水分蒸发量大,地表长期处于干燥状态,易形成干燥薄层干质第四系堆积. 物探大功率激电勘探需要较高电流,在干燥的地表处埋设电极接地电阻往往较大,给供电造成较大困难. 为有效解决此类问题,实现大功率激电勘探的可行性,同时兼顾提高生产效率,在参考借鉴前人研究成果以及生产经验的基础上,总结并付诸应用了改善干燥薄层松散地表地区接地电阻的方法技术,即采用湿土掩埋多孔片状电极. 该方法的应用,不仅解决了该类型地区电极供电问题,同时极大提高了生产效率,野外勘探数据质量也大幅提高,取得了较好的应用效果.