Electrical resistivity response due to variation in embankment shape and reservoir levels

The safety of the center-core type of fill dam structure was assessed by examining the effects of the distortion of electrical response verified in terms of two-dimensional (2D) apparent resistivity and its inverted sections from three-dimensional (3D) modeling for the embankment. The distortion effect is due to 2D interpretation of the 3D structure of the embankment. From the analysis, it was found that water level was correctly described by the resistivity section around the middle part rather than each side at the end of the embankment. This is due to the 3D terrain effect when the material of the embankment is assumed to be horizontally uniform. In addition, when the slope of the outer rock-fill section is set as uniform, the resistivity section is more similar for sharper center-core slopes. On the other hand, when the rock-fill slope is steep, the resistivity section shows the water level at a lower position than the real one, and the 3D distortion effect at the end of the embankment is enhanced.     

井地-高密度电法联合数值模拟研究

从点源场的边值问题出发,利用有限元推导了井地电法和高密度电法2.5维的详细解法,并编制了相应的程序。利用均匀半空间的地电模型进行了正演计算,并与地下真实电阻率进行了对比分析,验证了算法的正确性。在此基础上对均匀介质中存在对称高阻体和边缘高阻体的模型,进行正演模拟,分别得到其井地电法的视电阻率断面图和高密度电法的视电阻率断面图。结果表明,结合两种方法可以对地下存在的高阻异常体进行有效精确地探测。     

剔除高密度电阻率法三极装置浅部不均匀体效应影响的方法及效果

基于总结出的高密度电阻率法三极测量装置下的二维正演拟断面中浅部不均匀体畸变规律和特征,设计了一套能有效剔除三极测量装置浅部不均匀体效应影响的数据处理方法,成功应用于野外实测数据的处理。     

On thermobarometry

Thermobarometry, the estimation of the pressure–temperature ( P – T ) conditions of metamorphism, can be undertaken by using pseudosection calculations as well as by conventional methods. Conventional thermobarometry uses only the equilibrium thermodynamics of balanced reactions between end-members of minerals, combined with the observed compositions of minerals. In contrast, pseudosections involve a forward calculation of mineral equilibria for a given rock composition. When related to observed rock data such as mineral assemblages, mineral proportions and mineral compositions, pseudosections have the power to provide valuable additional thermobarometric information. This is because the rock composition provides added constraints on P – T , unavailable in conventional thermobarometry, such as when minerals in the mineral assemblage are no longer stable, or when additional minerals join the mineral assemblage. Considering both conventional and pseudosection thermobarometry, a minimum requirement is that they use the same thermodynamic data and activity–composition models for the minerals involved. A new thermocalc facility is introduced that allows pseudosection datafile coding to be used for conventional thermobarometry. Guidelines are given and pitfalls discussed relating to pseudosection modelling and conventional thermobarometry. We argue that, commonly, pseudosection modelling provides the most powerful thermobarometric tools.     

激发极化法在准噶尔盆地含水区测定中的应用

采用激发极化法对准噶尔盆地盆五井区进行了含水区定位研究,通过分析视电阻率单点曲线和视极化率单点曲线,对研究区范围内的地质及地层的含水情况进行初步判断,识别了含水层及水层的顶界埋深;在这一基础上用二维直流电法反演软件对视电阻率资料进行了二维反演,使之更接近地层真电阻率。视极化率剖面图的分析表明盆五井区存在一定的地下水资源,其中P200201、P200202、P200207线所经过的区域的含水性较好。第三系上新统上独山子组（N₂d₂）地层为强含水层,第三系上新统下独山子组（N₂d₁）为弱含水层。盆五井区含水层的特点是高阻含水、低阻不含水,这反映出该区的砂岩沉积较为疏松,地下水的矿化度偏低。推测该研究区地下水的水质较好。     

Challenges in high-pressure granulite metamorphism in the era of pseudosections: reaction textures, compositional zoning and tectonic interpretation with examples from the Bohemian Massif

High‐pressure granulites are an important record of geodynamic processes in overthickened or subducted continental crust. Orthopyroxene‐free assemblages in granitic (ternary feldspar(s) + quartz + garnet + kyanite + rutile), intermediate (ternary feldspar(s) + quartz + garnet + clinopyroxene ± kyanite ±rutile ± titanite) and basic (garnet + clinopyroxene + plagioclase ± quartz + rutile) compositions indicate formation conditions at mantle depths. Clinopyroxene compositions in Variscan high‐pressure granulites are unusual in that they include omphacite (in plagioclase‐bearing rocks thus not eclogite) and Al‐rich diopside (i.e. indicating high Ca‐Tschermak content), with both yielding temperatures above 900 °C. Problems such as compositional zoning, multiple generations of key phases in reaction domains and unmixing of high‐temperature solid‐solution phases during cooling (ternary feldspars, omphacite) clearly indicate disequilibrium and require very careful interpretation as to which phases and compositions possibly represent a former equilibrium association. Pressure–temperature (P–T) determination by the pseudosection method, although allowing prediction of mineral assemblages, compositions and molar proportions for a fixed bulk composition for modelled P–T conditions, still requires reliable activity–composition information for the key phases feldspar and clinopyroxene as well as an interpretation of former equilibrium compositions in the investigated samples, i.e. the same restrictions applying to conventional thermobarometry. The interpretations of some recently determined pseudosections for the composition of Variscan clinopyroxene‐bearing high‐pressure granulites contradict numerous published P–T paths. However, quantitative information from thermobarometry or pseudosections must be integrated with key petrographic observations. In the case of the Variscan example, it is argued that petrographic observations and published P–T paths are consistent with mineral assemblages predicted in pseudosections and support existing tectonometamorphic models.     

珠江口盆地低阻油层饱和度评价方法

珠江口盆地的低阻油层具有高泥质含量、低电阻率以及低产能的特点, 其真实含油饱和度的确定存在很大的困难。首先分析了区域低阻油层的地质成因, 指出地层束缚水含量高, 以及由此形成的发达的导电网络是导致油层低阻的主要原因;在此基础上, 利用多种适合于泥质砂岩地层的饱和度模型进行计算, 并根据密闭取心资料, 重点分析了在不同地层条件下, 对岩样含油、水饱和度进行脱气校正、压实校正、体积系数校正以及漏失校正的方法;并结合核磁共振测井, 对测井计算的含水饱和度进行标定。结果显示, 低阻油层的含水饱和度平均在70%以上, 且印度尼西亚公式计算的含水饱和度与岩心校正后的饱和度以及核磁束缚水饱和度吻合均很好, 因此印度尼西亚公式是最适合本区域低阻油层的饱和度评价模型。     

Geothermobarometry on anatectic melts – a high-pressure Variscan migmatite from northeast Sardinia

We studied a high-pressure amphibole-bearing migmatite cropping out along the northeastern coast of Sardinia, a few kilometres northeast of Olbia, in order to improve our knowledge about its evolution using pressure–temperature (P–T) pseudosections. Thermodynamic calculations with PERPLE_X were undertaken in the system Na₂O–K₂O–CaO–FeO–MnO–MgO–Al₂O₃–TiO₂–SiO₂–H₂O using a haplogranitic melt model. Calculations were conducted for average compositions of the protolith and the tonalitic leucosome with different contents of H₂O to construct various pseudosections in the P–T range 2–20 kbar and 550–900°C. We demonstrate that the molar ratios Na/K and Si/Al of the anatectic melt are of high geothermobarometric value. Particularly the melt field in the P–T pseudosection for the protolith, contoured by Si/Al isolines, shows a significant decrease in this ratio with rising pressure. This ratio is only weakly dependent on variable H₂O contents of the amphibole-bearing migmatite as demonstrated by contoured T–H₂O pseudosections. The application of the Na/K and Si/Al ratios in melt yielded P–T conditions close to 13 kbar and 700°C, assigned as the conditions of partial melting. Evaluation of contoured P–T pseudosections for the leucosome composition resulted in conditions of about 10.5 kbar and 700°C for the crystallization of amphibole in the leucosome melt, and 9 kbar and 680°C for complete crystallization of this melt.     

大地电磁静态效应模拟研究

通过数值模拟分析模型宽度、厚度、埋深、电阻率等参数对视电阻率、视相位的影响,发现TM模式受静态效应影响严重;异常体产生的静态效应畸变随埋深变浅而增大,地表异常造成的畸变尤为严重;异常体与围岩的电阻率差异越大造成的静态效应越明显;相比厚度,同比改变异常体的宽度对视电阻率的影响更大.通过曲线拟合各参数与视电阻率的相关函数,估算参数计算异常体对视电阻率的影响程度,以达到改正的目的.     

大地电磁法中的RHOPLUS理论及其应用研究

简述了大地电磁法中的Rhoplus理论.结合实测资料,详细介绍了Rhoplus理论在检查视电阻率和相位资料间的一致性、估算任意频点的视电阻率和相位值及其上下限、合理拼接不同仪器系统采集的资料、发现或校正由仪器或处理软件的问题引起的曲线畸变、了解受干扰噪声影响造成曲线畸变的程度等方面的应用.     

High-frequency induction logging in deviated and horizontal wells: Geosteering and inversion

The study focuses on processing methods for high-frequency induction data from deviated and horizontal wells. Simulation of electromagnetic fields with analytical algorithms is used to study how the resistivity of rocks above and below the well influences the acquired data. The simulations show that the contribution of under- and overlying layers into the recorded responses is considerable and depends on resistivity contrasts between the layers and on the position of logging arrays relative to the layer boundaries. This fact has to be taken into account when estimating true resistivity of reservoirs and when inverting induction logs from horizontal wells for reservoir characterization. The method is applied to oil- and water-saturated reservoirs in West Siberia, which contain high-resistivity impermeable layers leading to overestimation of apparent resistivity. Due regard for the effect of these layers in inversion of induction logs provides high-quality resistivity estimates.     

A numerical comparison of enhancing horizontal resolution (EHR) technique utilizing 2D resistivity imaging

The effectiveness of inversion apparent resistivity data to determine accurately the true resistivity distribution over 2D structures has been investigated using a common inversion scheme based on smoothness-constrained nonlinear least-squares optimization with enhancing horizontal resolution (EHR) technique by numerical simulation. The theoretical model generates in RES2DMOD software at specific distance and depth using Wenner, Wenner–Schlumberger, and pole–dipole arrays were inverted. The inversion model was compared with the original 2D model in RES2DINV software. The study model includes horizontal layering, vertical resolution, and horizontal two layers with different resistivity. Also, the response to variations in data density of these arrays was investigated. The study shows the best array suitable to be used in the survey was chosen for real data acquisition at the actual site. Subsequently, the results from borehole were used to verify the results of 2D resistivity imaging method with and without EHR technique. Saturated zone (0–40 Ω-m) was found scattered at the depth of 10–20 m. The borehole is located at 63 m at 2D resistivity imaging survey which shows at depth 10–20 m is sandy silt. Highly weathered sandstone was found at 6 m depth with resistivity value of 800 Ω-m and SPT N value of 20. The bedrock was found at 27 m depth with resistivity value of 3,000 Ω-m and SPT N value of 50. The application of 2D resistivity imaging with EHR technique indicate the ability of the proposed approach in terms of density, depth, and resistivity value of anomalous and layer in a computationally and numerically efficient manner and to exhibit good performance in the data inversion.     

利用高密度电阻率法监测地下介质污染问题的数值模拟

对地下介质不同污染模式的电性分布特征,给出了不同的地电模型。针对不同的地电模型,利用2.5维有限元正演模拟的方法计算出了不同监测装置形式下的异常分布特征,并给出了针对不同污染模式的最优监测装置形式和异常分析方法。同时模拟分析了对于不同污染程度监测的可行性。     

Application of integrated geoelectrical methods in Khenadarreh (Arak,Iran) graphite deposit exploration

This paper is devoted to application of SP, resistivity and mise-a-la-masse method in Khenadarreh (Arak, Iran) graphite deposit exploration. Mise-a-la-masse method is one of the main geophysical exploration methods used to investigate conductive bodies with outcrops on the surface or in exploratory pits. By integrating mise-a-la-masse results with other geoelectrical methods, such as self-potential and resistivity surveys, useful results can be acquired. Powerful survey design, data processing and interpretation methods can make geoelectrical surveying results more reliable. Here, we applied integrated geoelectrical methods to explore the Khenadarreh-bala graphite deposit in the Sarband region of the Markazi province, Iran. We present a new and simple method for processing and interpreting mise-a-la-masse data, which corrects for the effect of distance from the current electrode in the conductive body. After correction, the residual potential values corresponded with the SP results and the apparent resistivity pseudosections. The geophysical results were confirmed by drill holes in the study area.     

低孔渗储层井周油藏侵入模拟及阵列感应电阻率校正方法

泥浆滤液侵入渗透性地层会改变井周地层流体的原始分布状态,使井周地层电阻率发生变化、储集层电测井响应失真。本文基于油水两相渗流理论和水对流扩散理论,应用数值模拟方法构建地层模型,研究低渗储层泥浆侵入过程中井周地层矿化度、含水饱和度、电阻率变化特征及井周油藏分布特征。对不同渗透率地层泥浆侵入的数值模拟结果表明:在低渗透储层,相同侵入时间内,随着地层渗透率的增加,泥浆侵入深度增加,且同一侵入深度的含油饱和度减小;随着侵入时间的增加,井周地层水矿化度、含水饱和度和地层电阻率的侵入剖面前缘均发生明显的变化。结合3口井的泥浆侵入数值模拟分析,发现阵列感应测井响应特征及其电阻率的幅度差异受泥浆侵入时间和侵入深度影响。不同流体性质层段的井周油藏动态演化规律模拟结果表明,井周油藏的变化情况与储层流体性质有明显关系。利用实例井侵入模拟结果与阵列感应测井响应的相关性,构建了侵入深度的计算公式;根据侵入阶跃剖面,利用几何因子理论构建了原始地层电阻率计算公式,用以辅助储层电阻率校正。正确认识井周地层泥浆侵入特征及井周油藏的分布情况有助于提高阵列感应测井评价储层的准确性。     

常规直流电法在黄河水库南屯地质灾害评价中的应用

黄河水库南屯进行勘查的主要目的就是通过物探方法初步查明地下溶洞存在的情况以及区内的地质概况,为该区地质灾害的评价提供必要的参考依据。通过多种物探方法实验,发现激电中梯、激电联剖及视电阻率测深效果相对较好。由于区内地下水较发育,而且已知塌陷部位都不同程度的含水,水导电性能好.。因此,在地表观测时呈现视电阻率低阻趋势。当溶洞中没有地下水存在时,视电阻率呈现高阻异常。这样就可以通过视电阻率的反映来发现地下溶洞。     

P–T evolution of Precambrian eclogite in the Sveconorwegian orogen,SW Sweden

Conditions of the prograde, peak‐pressure and part of the decompressional P–T path of two Precambrian eclogites in the eastern Sveconorwegian orogen have been determined using the pseudosection approach. Cores of garnet from a Fe–Ti‐rich eclogite record a first prograde and syn‐deformational stage along a Barrovian gradient from ~670 °C and 7 kbar to 710 °C and 8.5 kbar. Garnet rims grew during further burial to 16.5–19 kbar at ~850–900 °C, along a steep dP/dT gradient. The pseudosection model of a kyanite‐bearing eclogite sample of more magnesian bulk composition confirms the peak conditions. Matrix reequilibration associated with subsequent near‐isothermal decompression and partial exhumation produced plagioclase‐bearing symplectites replacing kyanite and clinopyroxene at an estimated 850–870 °C and 10–11 kbar. The validity of the pseudosections is discussed in detail. It is shown that in pseudosection modelling the fractionation of FeO in accessory sulphides may cause a significant shift of field boundaries (here displaced by up to 1.5 kbar and 70 °C) and must not be neglected. Fast burial, exhumation and subsequent cooling are supported by the steepness of both the prograde and the decompressional P–T paths as well as the preservation of garnet growth zoning and the symplectitic reaction textures. These features are compatible with deep tectonic burial of the eclogite‐bearing continental crust as part of the underthrusting plate (Eastern Segment, continent Baltica) in a collisional setting that led to an effectively doubled crustal thickness and subsequent exhumation of the eclogites through tectonic extrusion. Our results are in accordance with regional structural and petrological relationships, which demonstrate foreland‐vergent partial exhumation of the eclogite‐bearing nappe along a basal thrust zone and support a major collisional stage at c. 1 Ga. We argue that the similarities between Sveconorwegian and Himalayan eclogite occurrences emphasize the modern style of Grenvillian‐aged tectonics.     

Phase equilibria modelling of kyanite‐bearing anatectic paragneisses from the central Grenville Province

Kyanite‐bearing paragneisses from the Manicouagan Imbricate Zone and its footwall (high‐P belt of the central Grenville Province) preserve evidence of partial melting with development of metamorphic textures involving biotite–garnet ± kyanite ± plagioclase ± K‐feldspar–quartz. Garnet in these rocks displays a variety of zoning patterns with respect to Ca. Pseudosection modelling in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (NCKFMASHTO) system using measured bulk rock compositions accounts for the textural evolution of two aluminous and two sub‐aluminous samples from the presumed thermal peak to conditions at which retained melt solidified. The prograde features are best explained by pseudosections calculated with compositions to account for melt loss. The intersection of isopleths of grossular content and Fe/(Fe + Mg) relating to large porphyroblasts of garnet provide constraints on the P–T conditions of the metamorphic peak. These P–T estimates are considered to be minima because of the potential for diffusional modification of the composition of garnet at high‐T and during the early stages of cooling. However, they are consistent with textural observations and pseudosection topology, with peak assemblages best preserved in rocks for which the calculated pseudosections predict only small changes in mineral proportions in the P–T interval, in which retrograde reactions are inferred to have occurred between the thermal peak and the solidus. Maximum P–T conditions (14.5–15.5 kbar and 840–890 °C) and steep retrograde P–T paths inferred for rocks from the Manicouagan Imbricate Zone are comparable with those determined for mafic rocks from the same area. In contrast, maximum P–T conditions of 12.5–13 kbar and 815–830 °C and flatter P–T paths are inferred for the rocks of the footwall to the Manicouagan Imbricate Zone. The general consistency between textures, mineral compositions and the topologies of the calculated pseudosections suggests that the pseudosection approach is an appropriate tool for inferring the P–T evolution of high‐P anatectic quartzo‐feldspathic rocks.     

点电源下复杂角域地形影响及校正

角域是描绘起伏地形的基本单元,通过角域叠加可以获得复杂纯地形产生的视电阻率异常,进而可以进行地形校正。将简单角域视电阻率畸变进行组合叠加得到复杂角域地形的视电阻率畸变,分析了组合叠加误差,并与ANSYS软件数值模拟结果进行了对比。结果表明,在地形起伏不大的情况下,可以通过组合叠加获得复杂角域地形的视电阻率异常。最后给出了利用组合叠加法获得纯地形影响后应用比较法进行地形改正的模型算例;与水平地形时的结果对比表明,比较法可以削弱地形影响,突出局部地质体的异常。