基于时延三维电阻率反演的岩溶地下河管道空间分布识别物理模拟研究

岩溶地下河管道空间分布的识别对岩溶区的各类地球科学工作意义重大,文章阐述了采用时延三维电阻率反演技术,开展对地下河管道空间分布识别的研究,在室内灰岩介质下的物理模拟实验结果表明：对雨季管道充水和枯季管道干涸时采集的电阻率数据进行时延反演后,地下河管道的模拟三维空间分布被很好地突显出来,时延反演效果大大地优于对单次采集数据的反演效果,管道充填水时的反演效果次之,管道充填空气时的反演结果很难有效识别地下河管道的空间分布情况。物理模型试验成果可指导野外实践中对岩溶地下河管道的探测研究。     

Geo-electrical delineations within Kubanni basin, north-central Nigeria

The research site is the whole landmass of the Federal College of Education, Zaria, seated on basement complex of north-central Nigeria. Direct current resistivity geophysical method was employed to characterise parameters such as the basement depth and topography, aquifer depth and thickness, weathered basement distribution as well as mapping of orientations of fractures and faults present in the premises using radial sounding technique. The conventional vertical electrical sounding (VES) Schlumberger array was carried out at 40 stations, and eight of which were radial stations. Radial sounding was used to establish resistivity anisotropy which gives clue for the choice of consistent VES profile direction used throughout the fieldwork. Results from the resistivity interpretation suggest three layers in most parts of the premises with some minor occurrence of two and four layers. The first layer (topsoil) has its thickness ranging between 3.5 and 14.0 m; second layer (weathered basement) thickness ranges between 9.0 and 36.5 m, while the third layer (fresh basement) is deepest (40.1 m) towards the eastern corner of the area. The aquifer depth ranges from 1.5 to 4.0 m with a thickness range of 5.0 to 14.0 m. The thickest aquifer occurs around the centre to the west in the area. Results from radial sounding show presence of resistivity anisotropy, an insight to fracturing and faulting; this is more pronounced around the west-central part of the premises.     

Use of vertical electrical soundings to delineate seawater intrusion in a coastal area of Byunsan,Korea

To delineate spatial extent of seawater intrusion in a small experimental watershed in the coastal area of Byunsan, Korea, electrical resistivity surveys with some evaluation core drillings and chemical analysis of groundwaters were conducted. The vertical electrical sounding (VES) method was applied, which is useful to identify variations in electrical characteristics of layered aquifers. The drilling logs identified a three-layered subsurface including reclamation soil, weathered layer and relatively fresh sedimentary bedrock. The upper two layers are the main water-bearing units in this area. A total of 30 electrical sounding curves corresponded mostly to the H type and they were further divided into three classes: highly conductive, intermediate, and low conductive, according to the observed resistivity values of the most conductive weathered layer. In addition, groundwater samples from 15 shallow monitoring wells were analyzed and thus grouped into two types based on HCO₃/Cl and Ca/Na molar ratios with TDS levels, which differentiated groundwaters affected by seawater intrusion from those not or less affected. According to relationships between the three classes of the sounding curves and groundwater chemistry, locations of the monitoring wells with low HCO₃/Cl and Ca/Na ionic ratios coincided with the area showing the highly conductive type curve, while those with the high ratios corresponded to the area showing low conductive curve type. Both the low electrical resistivity and the low ionic ratios indicated effects of seawater intrusion. From this study, it was demonstrated that the VES would be useful to delineate seawater intrusion in coastal areas.     

断层－充水溶洞上不同装置三维高密度电阻率法正演模拟响应特征分析

高阻碳酸盐岩中的低阻断层和充水溶洞是岩溶区地下水勘查的主要目标体。根据高阻中找低阻的原则,建立碳酸盐岩类裂隙溶洞水赋存模式的断层－溶洞地电模型,利用RES3D软件进行温纳、施伦贝格、偶极－偶极三种采集装置的正反演模拟计算,从三维反演结果、剖面、电测深曲线三个维度,对比分析不同装置下断层－溶洞目标体的地电响应特征和规律。结果显示：(1) 探测深度内,温纳、施伦贝格、偶极－偶极装置可有效识别断层及其上盘三倍于电极距规模的充水溶洞,无法分辨断层下盘二倍于电极距规模的充水溶洞;(2) 相同模型和观测条件下,偶极－偶极装置对目标体的识别能力最强,其三维反演结果可识别溶洞下边界,剖面中形成左凸低阻圈闭异常,不同测深点的曲线类型、拐点、极值点与模型设计最为贴近,且拐点对应岩性界面,极值点位于地质体的中心深度。该工作对野外观测方式的选取和地质解译有一定指导意义。     

Integrated geophysical studies to elicit the subsurface structures associated with Uranium mineralization around South Purulia Shear Zone,India: A review

South Purulia Shear Zone in India is an important zone that hosts uranium mineralization. Since detailed geophysical studies have not been carried out in this region, an integrated geophysical study using self-potential, resistivity, very low frequency electromagnetic and radiometric methods was performed to identify the subsurface structures that could host the hydrothermally altered uranium deposits in the area. The study reveals the wide and large magnitude of negative self-potential anomaly across the South Purulia Shear Zone. The peak negative self-potential anomalies are correlated with the low gravity and low resistivity anomalies measured along various profiles. The low self-potential, gravity and low resistivity anomaly zones are also correlated with conducting zones inferred from very low frequency electromagnetic measurements. Interpretation of self-potential data reveals multiple thick sheet-type vertical and/or inclined structures which might be associated with uranium mineralization. Schlumberger resistivity sounding data reveals an increasing trend of apparent resistivity with current electrode separations/depth. Apparent conductance measured simultaneously with resistivity measurement reveals an increase in current flow of current as depth increases. This exhibits the presence of thin conducting layers at these depths, which is not reflected in resistivity sounding data due to suppression problem. Also this conducting layer is consistent in various soundings and is connected from Raghunathpur to the South Purulia Shear Zone. Correlation of very low frequency and self-potential data shows that the structures are comparable and a radiometric profile also advocate that the conducting structure is associated with radioactive minerals. These structures are likely to be mineralized zones as hydro-uranium anomaly has also been reported from various locations in the area. Moreover, drilling results at a specific mine near the study area also confirms the presence of uranium mineralization. The hydrothermal activity associated with uranium mineralization seems to be still active in the area. Such combined geophysical studies are essential to understand this highly complex shear zone for the economic exploitation of its natural resources.     

鄂尔多斯盆地西北缘浩然柴达木地区下白垩统结构特征

鄂尔多斯盆地西北缘下白垩统为重要的找矿目的层,通过研究该套层位的结构特征,以期大致查明其深部发育情况,为盆地北部铀矿资源勘查与评价提供基础资料。文章通过对研究区可控源音频大地电磁(CSAMT)测深数据的反演处理,获得了沉积盖层的电性结构模型,结合区内地质、岩石物性及钻孔资料综合分析,认为下白垩统由浅至深主要发育两层地电结构,上部罗汉洞组纵向反映为明显的"上高下低"相对中高阻电性层,底板埋深一般在200 m左右;下部华池-环河组与洛河组整体反映为相对高阻电性层,底板埋深500~950 m,厚度400~700 m,走向北西,总体表现为北东部薄、南西部厚、中部更厚的沉积分布特征。     

A survey of the repository of groundwater potential and distribution using geoelectrical resistivity method in Itu Local Government Area (L.G.A), Akwa Ibom State, southern Nigeria

Vertical electrical sounding (VES), employing a Schlumberger electrode configuration, was used to investigate the sediments and aquifer repositories in Itu Local Government Area of Akwa Ibom state, southern Nigeria. This was done in sixteen (16) locations/communities with the maximum current electrode spread ranging between 800–1000m. The field data were interpreted using forward and iterative least square inversion modeling, which gives a resolution with 3–5 geoelectric layers. The observed frequencies in curve types include 31.25% of AKH, 18.8% of AAK and HK and 6.25% of K, QHK, AKH, KA and KHQ, respectively. These sets of curves show a wide range of variabilities in resistivities between and within the layers penetrated by current. The presence of K and H curve types in the study area indicates the alteration of the geomaterials with limited hydrologic significance to the prolific groundwater repository. A correlation of the constrained nearby borehole lithology logs with the VES results shows that the layers were all sandy formations (fine and well sorted sands to gravelly sands or medium to coarse-grained sands as described by nearby lithology logs) with some wide ranges of electrical resistivity values and thicknesses caused by electrostratigraphic inhomogeneity. The geologic topsoil (motley topsoil) is generally porous and permeable and as such the longitudinal conductance (S) values for the covering/protective layer is generally less than unity of Siemens (S < 1Ω^?1), the value considered for efficient protection of the underlying aquifers by the topmost and overlying layer. The spatial orientations and the leveling patterns of the most economically viable potential groundwater repository within the maximum current electrode separations has been delineated in 2-D and 3-D contoured maps. The estimated depth range for the desired groundwater repository is 32.6–113.1m and its average depth value is 74.30m. The thickness of this layer ranges from 27.9–103m while its average depth has been evaluated to be 63.02m. Also, its resistivity range and average value have been estimated to be 507–5612m and 3365.125Ωm     

论强化垂直聚焦测深法

本文首先讨论了强化垂直聚焦测深法（简称聚焦测深法）的视电阻宰表达式,提出了在实际工作中代换该法的简单装置及其观测方案。其次,详细分析了利用线性滤波法计算的聚焦测深法ρs理论曲线,指出了似增层性质及多种因素对ρs曲线的形状和特征点极距的影响作用。最后阐明了聚焦测深法在地质效能上不具有定勘探深度的原因。     

Hydrogeophysical investigation and estimation of groundwater potentials of the Lower Palaeozoic to Precambrian crystalline basement rocks in Keffi area, north-central Nigeria, using resistivity methods

The use of resistivity sounding and two-dimensional (2-D) resistivity imaging was investigated with the aim of delineating and estimating the groundwater potential in Keffi area. Rock types identified are mainly gneisses and granites. Twenty-five resistivity soundings employing the Schlumberger electrode array were conducted across the area. Resistivity sounding data obtained were interpreted using partial curve matching approach and 1-D inversion algorithm, RESIST version 1.0. The 2-D resistivity imaging was also carried out along two traverses using dipole–dipole array, and the data obtained were subjected to finite element method modeling using DIPRO inversion algorithm to produce a two-dimensional subsurface geological model. Interpretation of results showed three to four geoelectrical layers. Layer thickness values were generally less than 2 m for collapsed zone, and ranged from 5 to 30 m for weathered bedrock (saprolite). Two major aquifer units, namely weathered bedrock (saprolite) aquifer and fractured bedrock (saprock) aquifer, have been delineated with the latter usually occurring beneath the former in most areas. Aquifer potentials in the area were estimated using simple schemes that involved the use of three geoelectrical parameters, namely: depth to fresh bedrock, weathered bedrock (saprolite) resistivity and fractured bedrock (saprock) resistivity. The assessment delineated the area into prospective high, medium and low groundwater potential zones.     

Magnetotelluric Investigation of the Hydrothermal System and Heat Source in the Muine-Toyoha Geothermal Area, Hokkaido, Japan

Abstract. Magnetotelluric (MT) surveys were carried out around the Muine volcano, Hokkaido, Japan, where it is expected that the heat and metal source forming the polymetallic Ag-Pb-Zn-Cu-In Toyoha deposit is present at depth. Measurements were performed at 20 sites, 18 of which were located along a WSW-ENE profile traversing the north ridge of Mt. Muine. A resistivity model obtained from 2D inversion of the MT data shows subsurface specific conductive and resistive features. Conductive layers are present at the surface of Mt. Muine. The low resistivity is probably due to the clay-rich rocks associated with the hydrothermal alteration. A high resistivity layer, which corresponds to the pre-Tertiary Usubetsu Formation, crops out east of Mt. Muine and dips westward. At the west foot of Mt. Muine, relatively high resistive layers are widely exposed. The resistivity increases with depth and exceeds 1000 ohm-m. This fact indicates that this region is not influenced by the recent hydrothermal activity. An extremely conductive zone about 3–6 km wide and 6–9 km thick exists at a depth of 2 km below Mt. Muine. This zone mostly corresponds to an elastic wave attenuation zone detected by a seismic survey. It is interpreted as a large hydrothermal reservoir or melted magma, which is a heat source of the hydrothermal system in this area.     

Groundwater resources evaluation in calcareous limestone using geoelectrical and VLF-EM surveys (El Salloum Basin,Egypt)

Understanding and developing groundwater resources in arid regions such as El Salloum basin, along the northwestern coast of Egypt, remains a challenging issue. One-dimensional (1D) electrical sounding (ES), two-dimensional (2D) electrical resistivity imaging (ERI), and very low frequency electromagnetic (VLF-EM) measurements were used to investigate the hydrogeological framework of El Salloum basin with the aim of determining the potential for extraction of potable water. 1D resistivity sounding models were used to delineate geoelectric sections and water-bearing layers. 2D ERI highlighted decreases in resistivity with depth, attributed to clay-rich limestone combined with seawater intrusion towards the coast. A depth of investigation (DOI) index was used to constrain the information content of the images at depths up to 100 m. The VLF-EM survey identified likely faults/fractured zones across the study area. A combined analysis of the datasets of the 1D ES, 2D ERI, and VLF-EM methods identified potential zones of groundwater, the extent of seawater intrusion, and major hydrogeological structures (fracture zones) in El Salloum basin. The equivalent geologic layers suggest that the main aquifer in the basin is the fractured chalky limestone middle Miocene) south of the coastal plain of the study area. Sites likely to provide significant volumes of potable water were identified based on relatively high resistivity and thickness of laterally extensive layers. The most promising locations for drilling productive wells are in the south and southeastern parts of the region, where the potential for potable groundwater increases substantially.

     

三层倾斜层的电测深曲线正演计算与解析

本文是探讨三层倾斜层的直流电测深问题。文中通过三层倾斜层的数学模型,就各种参数(第一、二倾斜层的倾角及各倾斜层的电阻率值变化,测线平行、正交、斜交于倾斜层走向)进行了大量的电测深正演计算,并对正演计算结果加以探讨。计算结果表明,其值与三层水平层测深结果有较大的差别,如曲线的尾支渐近值差异,垂直倾斜层走向布极的测深曲线产生畸变,以及在某些地电参数下,测深曲线无法正确地反映地下的地电断面等。因此,在野外实际工作中,对于利用三层水平层的理论计算结果来解释三层倾斜层时,必须注意其解释误差。     

Mapping lithological variations in a river basin of West Bengal,India using electrical resistivity survey: implications for artificial recharge

Groundwater is a treasured earth’s resource and plays an important role in addressing water and environmental sustainability. However, its overexploitation and wide spatial variability within a basin and/or across regions are posing a serious challenge for groundwater sustainability. Some parts of southern West Bengal of India are problematic for groundwater occurrence despite of high rainfall in this region. Characterization of an aquifer in this area is very important for sustainable development of water supply and artificial recharge. Electrical resistivity surveys using 1-D and 2-D arrays were performed at a regular interval from Subarnarekha River at Bhasraghat (south) to Kharagpur (north) to map the lithological variations in this area. Resistivity sounding surveys were carried out at an interval of 2–3 km. Subsurface resistivity variation has been interpreted using very fast simulated annealing (VFSA) global optimization technique. The analysis of the field data indicated that the resistivity variation with depth is suitable in the southern part of the area and corresponds to clayey sand. Interpreted resistivity in the northern part of the area is relatively high and reveals impervious laterite layer. In the southern part of the area resistivity varies between 15 and 40 Ωm at a depth below 30 m. A 2-D resistivity imaging conducted at the most important location in the area is correlated well with the 1-D results. Based on the interpreted resistivity variation with depth at different locations different types of geologic units (laterite, clay, sand, etc.) are classified, and the zone of interests for aquifer has been demarcated. Study reveals that southern part of the area is better for artificial recharge than the northern part. The presence of laterite cover in the northern part of the area restricts the percolation of rainwater to recharge the aquifer at depth. To recharge the aquifer at depth in the northern part of the area, rainwater must be sent artificially at depth by puncturing laterite layers on the top. Such studies in challenging areas will help in understanding the problems and finding its solution.     

直流电阻率测深曲线解释中的直接反演法

本文根据在电导率随深度变化梯度不大的情况下，ＤａｒＺａｒｒｏｕｋ曲线与视电阻率曲线基本重合这一性质，通过各种变换，在电阻率测深中导出了一种可与大地电磁测深中Ｂｏ－ｓｔｉｃｋ反演方法相比美的直接反演方法，将视电阻率随极距变化曲线转换为电阻率随深度变化曲线。     

A direct inversion scheme for deep resistivity sounding data using artificial neural networks

Initialization of model parameters is crucial in the conventional 1D inversion of DC electrical data, since a poor guess may result in undesired parameter estimations. In the present work, we investigate the performance of neural networks in the direct inversion of DC sounding data, without the need ofa priori information. We introduce a two-step network approach where the first network identifies the curve type, followed by the model parameter estimation using the second network. This approach provides the flexibility to accommodate all the characteristic sounding curve types with a wide range of resistivity and thickness. Here we realize a three layer feed-forward neural network with fast back propagation learning algorithms performing well. The basic data sets for training and testing were simulated on the basis of available deep resistivity sounding (DRS) data from the crystalline terrains of south India. The optimum network parameters and performance were decided as a function of the testing error convergence with respect to the network training error. On adequate training, the final weights simulate faithfully to recover resistivity and thickness on new data. The small discrepancies noticed, however, are well within the resolvability of resistivity sounding curve interpretations.     

水下直流电阻率法数值模拟

利用有限单元法对水下电阻率测深进行数值模拟。与地面测量不同的是，水下电极所处的深度、水体本身和水底地形都会给电阻率数据造成较大影响，给数据的解释带来困难。若先通过计算，分析不同电阻率装置在水下测量时的视电阻率与水深的关系，再用有限单元法对水下的三维地电断面进行数值模拟，讨论水底地形对视电阻率的影响．即可用比较法消除地形影响。     

Deep Crustal Electrical Signatures of Eastern Dharwar Craton, India

Wide band magnetotelluric (MT) investigations were carried out along a profile from Kavali in the east to Anantapur towards west across the Eastern Ghat Granulite Terrain (EGGT), Eastern Dhanvar Craton (EDC) and a Proterozoic Cuddapah Basin. This 300 km long profile was covered with 20 stations at an interval of 12–18 km. The MT data is subjected to robust processing, decomposition and static shift correction before deriving a 2-D model. The model shows a resistive crust (−10,000–30,000 ohm-m) to a depth of 8–10 km towards west of the Cuddapah basin. The mid crust is less resistive (about 500 ohm-m) and the lower crust with a slight increase in resistivity (about 1,500 ohm-m) in the depth range of 20–22 km. The resistivity picture to the east of the Cuddapah basin also showed a different deep crustal structure. The resistivity of upper crust is about 5,000 ohm-m and about 200 ohm-m for mid and lower crust. The sediment resistivity of Cuddapah basin is of the order of 15–20 ohm-m. MT model has shown good correlation with results from other geophysical studies like deep seismic sounding (DSS), gravity and magnetics. The results indicate that the lower crustal layers are of intermediate type showing hydrous composition in Eastern Dhanvar Craton.     

Geoelectric section of the crust and upper mantle of the northern Sikhote-Alin from magnetotelluric sounding data

Magnetotelluric soundings (MTS) were conducted in a broad frequency range of 10 kHz to 0.001 Hz at a total of fifty-seven sounding sites of the profile spaced 5 km apart and intersecting the northern Sikhote-Alin across the strike. The analysis of the obtained magnetotelluric parameters has been made which shows three-dimensional geoelectric nonuniformities in the lower crust and upper mantle. The MTS curve interpretation was carried out in the framework of a three-dimensional model. As a result of the inverse problem solution, the geoelectric section has been constructed down to 150 km depth. The section distinguishes the crust with a resistivity higher than 1000 Ohm m and variable thickness between 30 and 40 km which is consistent with deep seismic sounding (DSS) data. The crust is subdivided into four blocks by deep faults, and each block is characterized by a set of parameters. The data support the existence of the Vostochny deep fault in the study area, whereas, on the contrary, the deep roots for the Central Sikhote-Alin fault have not been established. The upper mantle structure is nonuniform; three low-resistivity zones are identified that coincide with the boundaries of crustal blocks. In the revealed zones, an increase in the resistivity is noted from the continent to the Tatar Strait coast. A high-resistivity layer of 300–400 Ohm m was observed in the coastal area, which was steeply dipping from the crustal base down to 120 km depth and extended beneath the continent. Based on a set of geological and geophysical data, the ancient subducting plate is suggested in this area, and the evolutionary model of the region is proposed starting from the Late Cretaceous. The most probable mechanism of conductivity within the upper mantle is determined from petrological and petrophysical data. The low resistivity values are linked to dry peridotite mantle melting.     

The impact of subsurface geological structure on the groundwater occurrence using geophysical techniques in wadi El Kallabiyyah and wadi As Sabil,East Esna,Eastern Desert,Egypt

The present study aims to explain some hydrogeological problems that are related to geological setting in two wadis: El Kallabiyyah and As Sabil East Esna. For this purpose, land magnetic, vertical electrical soundings, and two-dimensional electrical resistivity imaging profiles were measured. The lithological succession comprises Upper Cretaceous to Quaternary deposits that rest on a Precambrian basement rock. The results of the interpretation of the land magnetic measurements in wadi El Kallabiyyah represent the depth to the basement rock which ranges from 260 to 660 m and increases towards the northeast trend, but in wadi As Sabil, there is a large basin at the center of the wadi with the recorded depth to the basement reaching 1,300 m due to the effect of two normal faults (F1 and F2). The results of the interpretation of the vertical electrical sounding and two-dimensional imaging confirmed the presence of these faults in wadi As Sabil. The results of the geoelectrical measurements in wadi El Kallabiyyah represented the water-bearing layer at the third unit that consists of sands, gravels, boulders, and clays. The presence of clay will affect the potentiality of a drilled well. For wadi As Sabil, the results of geoelectrical measurements led to the lower part of the second unit which acts as a water-bearing layer that consists of sands, boulders, gravels, and clays. It can be concluded that the best site for drilling wells in wadi El Kallabiyyah is at vertical electrical sounding no.5 with a total depth of not less than 230 m. In wadi As Sabil, the drilled wells must be concentrated at the center and the downstream.     

水平多层介质上水平电偶源频率电磁测深的阻抗实部等效电阻率

本文讨论了水平电偶源频率电磁测深中阻抗实部等效电阻率的具体定义和算法。用该方法计算的等效电阻率是电磁响应的单值函数,并极大地改善了视电阻率随频率变化的关系曲线,能直观地反映出地电断面随深度的客观变化,数值较接近地层真电阻率,且接近速度很快,假极值效应明显压低。数值计算和实际应用表明该方法是灵活、可行的,可很好地消除频率电磁测深中波区视电阻率曲线在非波区的畸变效应。