Resolution of freshwater and saline water aquifers by composite geophysical data analysis methods

Abstract

The resolution of the freshwater and saline water aquifers in a coastal terrain (Mahanadi Basin, India) is updated. We analysed electrical borehole log data at four sites and compared the water resistivity regime of the freshwater and saline water zones obtained from electrical borehole logging, with the resistivity regime obtained by interpreting vertical electrical sounding (VES) data. The multilayer VES data interpretation is modified to a simple model, containing only the freshwater zone and the saline water zone. The composite geophysical parameters of the freshwater and saline water zones, in particular the resistivity and longitudinal unit conductance regime, are identified. The resolution obtained from the composite geophysical data analyses is very clear and convincing. The composite longitudinal unit conductance regime of the saline water zones is very high compared to that of the freshwater zones. This makes the identification of the two aquifers easy and increases its reliability. A technique which enables analysis of composite geophysical data of freshwater and saline water zones at VES sites in the vicinity of the borehole log sites is proposed. The significance of longitudinal unit conductance in resolving the freshwater and saline water aquifers is illustrated graphically. The proposed technique is validated by correlating the longitudinal unit conductance and resistivity with the total dissolved solids. The efficiency of the technique is validated by carrying out discriminant function analysis.

Citation Hodlur, G. K., Dhakate, R., Sirisha, T. & Panaskar, D. B. (2010) Resolution of freshwater and saline water aquifers by composite geophysical data analysis methods. Hydrol. Sci. J. 55(3), 414–434.     

STATISTICAL TECHNIQUES APPLIED TO BOREHOLE GEOPHYSICAL DATA IN GOLD EXPLORATION1

Gold, as a trace element, is not directly detectable by geophysical borehole logging techniques. Geophysical logging methods have therefore been used to infer the presence of structural features and alteration processes associated with gold. Since these features and processes differ with the style of gold mineralization, a unique relation between geophysical anomalies and gold cannot be established. However, in a particular environment, such an association can be established. We examined geophysical log data (self potential, induced polarization, resistivity, gamma, temperature, and temperature gradient) and drill core from the Barber-Larder property in NE Ontario. The geophysically detectable alteration processes associated with gold were identified as sericitization and pyritization, by means of factor analysis. A linear discriminant function was constructed which allowed the zones of economic mineralization (nominally 0.05 oz/ton or 1.5 ppm) to be identified with a 75% success rate based on geophysical log data alone. This rate was achieved without sophisticated non-linear data transformations.     

Numerical sensitivity test of three-electrode laterolog borehole tool

Finite element numerical simulation has been carried out to investigate quantitatively the response of the three-electrode laterolog borehole tool (LL3) on radial and vertical heterogeneity of the rock. In order to calculate the apparent resistivity from the electric potential and the current discharge of the measurement electrode the probe coefficient of the LL3 tool with finite electrode extent was determined. Two independent methods, a finite element modeling and a semi-analytical solution, resulted in the probe coefficient of approx. 0.15 m with a relative deviation of 2.4% due to the different geometry, resolution and electronics of the models. It was established that LL3 is only slightly sensitive to the presence of mud when the borehole diameter is d ≤ 30 cm and the ratio of the resistivity of rock and the borehole mud is 1 ≤ R _t/R _m ≤ 1000. Vertical heterogeneity test pointed out that the layer boundaries can be localized exactly even for thin bedded layer (with a thickness of 1 m) and the presence of low-resistive borehole mud. Correction factors were suggested to decrease the biasing effect of the low-resistive borehole mud and the shoulder beds on the apparent resistivity observed by LL3. Finally, it was verified that the probe has large penetration depth with excellent vertical resolution, what explains the enduring popularity of the LL3 tool in well logging.     

庐枞盆地科学钻探地球物理测井及深部铀异常的发现

庐枞盆地砖桥科学钻探ZK01孔为深部探测技术与实验研究专项在庐枞盆地施工的钻探验证孔,全井段实施了连续取心和地球物理测井工作.测井工作分三次完成,测井总深度1994.02m.测井项目包括视电阻率、极化率、磁化率、纵波速度、超声成像、自然伽马、密度、井斜、井径、井温、泥浆电阻率、井中三分量磁测等10多种方法,获得了钻孔剖面原位物性参数、钻孔几何形态及井壁超声图像.通过对地球物理测井和钻孔岩心编录等资料的研究,完成了岩性的人工识别与支持向量机判别,建立了钻孔测井解释岩性剖面;通过对矿化地层的测井响应分析,将电阻率和磁化率作为粗安岩矿化的识别标识;根据超声成像测井资料推断本地区深部地层最大水平主应力方向为南北走向.在ZK01孔1500~1900m发现放射性异常,对铀当量大于万分之一的21处异常进行了定量解释,铀矿化段累积厚度93.02m,为庐枞地区深部找铀矿提供了重大线索.     

Geoelectrical structure and hydrogeological investigations of the southern Rif Cordillera (Morocco)

An electrical prospecting survey is conducted in the Rharb basin, a semi‐arid region in the southern part of the Rifean Cordillera (Morocco) to delineate characteristics of the aquifer and the groundwater affected by the marine intrusion related to Atlantic Ocean. Analysis and interpretations of electrical soundings, bi‐logarithmic diagrams and the geoelectrical sections highlight a monolayer aquifer in the southern part, a multilayer system in the northern part of the Rharb basin and lenticular semi‐permeable formations. Several electrical layers have been deduced from the analysis of bi‐logarithmic diagrams: resistant superficial level (R₀), conducting superficial level (C₀), resistant level (R), intermediary resistant level (R′), conducting level (Cp) and intermediary layer of resistivity (AT). Spatial distribution of the resistivity deduced from the interpretation of apparent resistivity maps (AB = 400 and 1000 m) and the decreasing of resistivity values (35–10 Ωm), in particular in the coastal zone show that this heterogeneity is related to several anomalies identified in the coastal area, which result from hydraulic and geological processes: (i) heterogeneous hydraulic conductivity in particular in the southern part of the Rharb; (ii) lateral facies and synsedimentary faulting and (iii) the relationship between the electrical conductivity and chloride concentration of groundwater shows that salinity is the most important factor controlling resistivity. The distribution of fresh/salt‐water zones and their variations in space along geoelectrical sections are established through converting subsurface depth‐resistivity models. Copyright © 2010 John Wiley & Sons, Ltd.     

On the 3-D inversion of vertical electrical soundings: Application to the South Ismailia area—Cairo desert road, Cairo, Egypt

A code for 3-D resistivity modelling and inversion of vertical electrical soundings has been developed based on the finite-element technique and regularisation method. Synthetic data were used to test the effectiveness of the code and to examine the resolving capability of the Schlumberger array in investigating 3-D resistivity distributions. The code was applied to experimental data set constituted by 35 Schlumberger soundings collected near the Cairo city in order to study the subsurface resistivity distribution. The results have shown that valuable imaging of the subsurface resistivity distribution can be constructed even when the vertical electrical soundings are acquired in a sparse field data set.     

Evaluation of electric parameters of anisotropic sandy-shaly Miocene formations on the basis of resistivity logs

In this paper, electrical properties of an anisotropic rock formation are discussed. Electrical anisotropy parameters, i.e., horizontal resistivity, R _H , and vertical resistivity, R _V , and anisotropy coefficient in the sandyshaly Miocene formation, are determined on the basis of induction logs and laterologs. Graphs of R _V = f(R _H ) calculated for different resistivities of sandstones and shales and variable volumes of those two components allow us to conclude about the correctness of the applied method for determining the resistivity parameters of autochthonous Miocene formations.     

一种评价致密砂岩储层孔隙结构的新方法及其应用(英文)

致密砂岩储层的孔隙结构对其渗透性和电性影响显著,是此类复杂储层岩石物理研究的关键。针对仅从连通喉道半径评价渗透率的多解性以及储层孔隙结构与电性关系研究欠缺等不足,综合影响物性的主要因素,提出了一种同时考虑孔隙度、最大连通喉道半径及分选性三种因素的新型孔隙结构参数δ的计算公式。利用岩心及实测数据对比分析表明,δ值能够较连通喉道半径等传统方法更精确地刻画致密砂岩储层渗透性,同时它与储层电性具有密切关系,可用于估算地层因素F和胶结指数m。据此提出将孔隙结构对电阻率的影响进行归一化校正以及基于核磁共振测井预测储层完全含水电阻率R0的评价方法,从而突出储层流体性质变化引起的电性变化,并提供了一种新的致密砂岩储层流体识别思路,研究结果得到了实验资料和实际测井试油资料的验证。     

Effect of high-pressures on the electrical resistivity of natural zeolites from Deccan Trap, Maharashtra, India

We report here the electrical resistivity measurements on two natural zeolites–natrolite and scolecite (from the Killari borehole, Maharashtra, India) as a function of pressure up to 8 GPa at room temperature. High-pressure electrical resistivity studies on hydrous alumino-silicate minerals are very helpful in understanding the role of water in deep crustal conductivities obtained from geophysical models. The results obtained by magneto-telluric (MT) soundings and direct current resistivity surveys, along with the laboratory data on the electrical resistivity of minerals and rocks at high-pressure–temperature are used to determine the electrical conductivity distribution in continental lithosphere. The electrical resistivity of natural natrolite decreases continuously from 2.9 × 10⁹ Ω cm at ambient condition to 7.64 × 10² Ω cm at 8 GPa, at room temperature. There is no pressure-induced first order structural phase transitions in natrolite, when it is compressed in non-penetrating pressure transmitting medium up to 8 GPa. On the other hand scolecite exhibits a pressure-induced transition, with a discontinuous decrease of the electrical resistivity from 2.6 × 10⁶ to 4.79 × 10⁵ Ω cm at 4.2 to 4.3 GPa. The observed phase transition in scolecite is found to be irreversible. Vibrational spectroscopic and X-ray diffraction studies confirm the amorphous nature of the high-pressure phase. The results of the present high-pressure studies on scolecite are in good agreement with the high-pressure Raman spectroscopic data on scolecite. The thermo gravimetric studies on the pressure-quenched samples show that the samples underwent a pressure-induced partial dehydration. Such a pressure-induced partial dehydration, which has been observed in natural scolecite could explain the presence of high conductive layers in the earth's deep-crust.     

The Influence of Topology on Hydraulic Conductivity in a Sand-and-Gravel Aquifer

A field experiment consisting of geophysical logging and tracer testing was conducted in a single well that penetrated a sand-and-gravel aquifer at the U.S. Geological Survey Toxic Substances Hydrology research site on Cape Cod, Massachusetts. Geophysical logs and flowmeter/pumping measurements were obtained to estimate vertical profiles of porosity ϕ, hydraulic conductivity K, temperature, and bulk electrical conductivity under background, freshwater conditions. Saline-tracer fluid was then injected into the well for 2 h and its radial migration into the surrounding deposits was monitored by recording an electromagnetic-induction log every 10 min. The field data are analyzed and interpreted primarily through the use of Archie's (1942) law to investigate the role of topological factors such as pore geometry and connectivity, and grain size and packing configuration in regulating fluid flow through these coarse-grained materials. The logs reveal no significant correlation between K and ϕ, and imply that groundwater models that link these two properties may not be useful at this site. Rather, it is the distribution and connectivity of the fluid phase as defined by formation factor F, cementation index m, and tortuosity α that primarily control the hydraulic conductivity. Results show that F correlates well with K, thereby indicating that induction logs provide qualitative information on the distribution of hydraulic conductivity. A comparison of α, which incorporates porosity data, with K produces only a slightly better correlation and further emphasizes the weak influence of the bulk value of ϕ on K.     

Efficiency of joint use of MRS and VES to characterize coastal aquifer in Myanmar

The productivity and the water quality of coastal aquifers can be highly heterogeneous in a complex environment. The characterization of these aquifers can be improved by hydrogeological and complementary geophysical surveys. Such an integrated approach is developed in a non-consolidated coastal aquifer in Myanmar (previously named Burma).A preliminary hydrogeological survey is conducted to know better the targeted aquifers. Then, 25 sites are selected to characterize aquifers through borehole drillings and pumping tests implementation. In the same sites, magnetic resonance soundings (MRS) and vertical electrical soundings (VES) are carried out. Geophysical results are compared to hydrogeological data, and geophysical parameters are used to characterize aquifers using conversion equations. Finally, combining the analysis of technical and economical impacts of geophysics, a methodology is proposed to characterize non-consolidated coastal aquifers.Depth and thickness of saturated zone is determined by means of MRS in 68% of the sites (evaluated with 34 soundings). The average accuracy of confined storativity estimated with MRS is ± 6% (evaluated over 7 pumping tests) whereas the average accuracy of transmissivity estimation with MRS is ± 45% (evaluated using 15 pumping tests). To reduce uncertainty in VES interpretation, the aquifer geometry estimated with MRS is used as a fixed parameter in VES inversion. The accuracy of groundwater electrical conductivity evaluation from 15 VES is enough to estimate the risk of water salinity. In addition, the maximum depth of penetration of the MRS depends on the rocks' electrical resistivity and is between 20 and 80 m at the study area.     

Simple relative space–time scaling of electrical and electromagnetic depth sounding arrays: implications for electrical static shift removal and joint DC-TEM data inversion with the most-squares criterion

A simple scaling relationship is shown to facilitate comparison, correlation and integration of data recorded using the common experimental configurations in electrical and electromagnetic depth sounding. Applications of the scheme to field data from typical geological and landfill environments show that it is robust and, where transient electromagnetic (TEM) data are available, enables easy identification and quantification of electrical static shift (galvanic distortion) in magnetotelluric and direct current (DC) sounding curves. TEM‐based procedures are suggested for both the direct removal of static shift in DC sounding curves and effective joint data inversion with the most‐squares criterion in the presence of static shift. A case study of aquifer characterization using sounding data from borehole sites in the Vale of York in England shows that static shift is a common problem in this glacial‐covered terrain and demonstrates the effectiveness of the proposed joint DC‐TEM inversion strategy in handling distorted soundings.     

Theoretical eduction and numerical simulation researches on the relationship between resistivity and water saturation of waterflood oil zone

In the process of water displacing oil, the relationship between resistivity and water saturation is the fundament of the quantitative research on the waterflooded grade and the remaining oil saturation with well logging data. A large number of core analysis data and production data are cumulated in the process of oil field exploitation, which offers the basis for the above research. This paper educed two methods from the Archie equation and material balance theory to calculate the quantitative relationships between R _z and S _w, and between R _t and S _w. The relationships set up by the two methods are similar to those set up by the real core measurements. The results can be used to analyze influencing factors and determine saturation quantitatively.

     

5频等参数阵列相位差感应电阻率

随着复杂油气田的广泛勘探,对测井技术提出了新的要求,阵列感应电阻率的出现可得到较准确的地层真电阻率,但我国的阵列感应电阻率测井仪主要依赖进口,限制了我测井不能在与仪器供给商有利益冲突的国家或地区进行同类作业服务.本文介绍了5频等参数阵列相位感应电阻率测井仪器原理、设计流程,与常规感应电阻率、俄罗斯VIKIZ测井资料的对比.阵列相位差感应电阻率测井仪较好的解决了常规感应测井仪器中存在的纵向分辨率差、径向探测深度浅、井眼影响大、不能解决复杂侵入剖面等问题,具有较好的分辨能力,适合于水淹层测井、薄层测井、剩余油测井.     

Identification of coal seam strata from geophysical logs of borehole using Adaptive Neuro-Fuzzy Inference System

Different parameters obtained through well-logging geophysical sensors such as SP, resistivity, gamma–gamma, neutron, natural gamma and acoustic, help in identification of strata and estimation of the physical, electrical and acoustical properties of the subsurface lithology. Strong and conspicuous changes in some of the log parameters associated with any particular stratigraphy formation, are function of its composition, physical properties and help in classification. However some substrata show moderate values in respective log parameters and make difficult to identify or assess the type of strata, if we go by the standard variability ranges of any log parameters and visual inspection. The complexity increases further with more number of sensors involved.An attempt is made to identify the type of stratigraphy from borehole geophysical log data using a combined approach of neural networks and fuzzy logic, known as Adaptive Neuro-Fuzzy Inference System. A model is built based on a few data sets (geophysical logs) of known stratigraphy of in coal areas of Kothagudem, Godavari basin and further the network model is used as test model to infer the lithology of a borehole from their geophysical logs, not used in simulation. The results are very encouraging and the model is able to decipher even thin cola seams and other strata from borehole geophysical logs. The model can be further modified to assess the physical properties of the strata, if the corresponding ground truth is made available for simulation.     

Atmospheric anomalies and anomalies of electricity in the near-surface atmosphere before the Kamchatka earthquake of January 30, 2016, based on the data from the Paratunka Observatory

The 15-min data of vertical ionosphere sounding and 10-min data from measurements of the vertical component (Ez) of the near-surface quasistatic atmospheric electrical field and the respective values of electrical conductance of near-surface air at the Paratunka complex geophysical observatory in the period from January 28 to January 30, 2016 have been analyzed to reveal the possible anomalies preceding the M = 7.2 earthquake that occurred on January 30, 2016, at 0325 UT. The distance between the observatory and epicenter was 117 km. These anomalies have been revealed, and the majority of them, in our opinion, may be related to the processes of earthquake preparation.     

FREQUENCY ELECTROMAGNETIC SOUNDING USING A VERTICAL MAGNETIC DIPOLE*

A horizontal transmitter loop (vertical magnetic dipole) is used for frequency electromagnetic (FEM) soundings. The frequency ranges from approximately 6 Hz to about 4000 Hz. The vertical and radial magnetic field components are measured for 20 frequencies per decade several hundred meters from the transmitter loop. A very small bandwidth is selected for amplification using a reference signal. An Apple computer is used for data acquisition. A computer program for Marquardt inversion optimizes the parameters for the n-layer case: the resistivities and thicknesses of individual beds and a correction factor for the primary magnetic field. Interpretation of each component individually yields practically the same parameters. Examples from the field are given with interpretation; comparison with dc resistivity measurements is provided. The ratio of vertical/radial magnetic field components vs. frequency can be transformed simply into apparent resistivity vs. apparent depth. This can be done in the field to obtain an estimation of the depth of the layer boundaries. FEM results are compared with Schlumberger d.c. sounding obtained at the same site.     

地球物理测井在多年冻土厚度和地下冰调查中的应用

基于青藏高原多年冻土区三个钻孔的地球物理测井数据和钻孔编录资料,我们对多年冻土厚度和多年冻土层内地下冰与地球物理测井数据之间的关系进行了相关的分析研究.研究表明,当地层为土壤类型时,可以使用井径和侧向测井曲线来判断多年冻土层厚度;而当地层为致密的基岩时,不能使用上述两种测井曲线来判断多年冻土层厚度.此外,还可以使用长源距伽马-伽马曲线和侧向测井曲线来识别多年冻土层内部分地下冰层的位置,其前提条件是地下冰层具有一定的厚度,或即使厚度较薄,但连续出现.这一研究结果对于利用地球物理测井曲线来调查多年冻土情况具有一定的应用价值.     

基于小波变换图像分割技术的电成像测井资料裂缝、孔洞面孔率提取方法

对于裂缝、溶蚀孔洞发育的碳酸盐岩缝洞储层,如何从测井资料中提取裂缝、溶蚀孔洞信息是评价储层有效性的关键问题.为了从电成像测井静态图像上准确地分割出清晰的裂缝、溶蚀孔洞子图像进而提取其参数信息,本文提出了一种基于小波变换模极大值图像分割技术的电成像测井资料缝洞面孔率提取方法.以钮扣电极电导率曲线为对象,先消除井壁凹凸不平导致的地层背景噪声的影响,利用小波变换模极大值图像分割方法得到包含裂缝和溶蚀孔洞目标的子图像,根据子图像提取裂缝-孔洞总面孔率、裂缝面孔率、孔洞面孔率等信息.本文利用塔里木盆地奥陶系碳酸盐岩地层的电成像测井数据提取了缝洞面孔率参数,还利用同井岩心CT扫描图像计算的平均缝洞面孔率、双侧向电阻率、常规测井资料三孔隙度模型计算的相对连通缝洞孔隙度进行了对比,并进行了试油验证.对比表明,电成像测井裂缝-孔洞总面孔率、裂缝面孔率、孔洞面孔率与岩心CT扫描图像平均缝洞面孔率、双侧向电阻率、相对连通缝洞孔隙度、试油结果均有较好的一致性.这一方面验证了采用本文方法提取的裂缝-孔洞总面孔率、裂缝面孔率、孔洞面孔率的合理性,另一方面表明所提取参数可用于指示缝洞型碳酸盐岩储层的渗透性和有效性.     

基于电阻率敏感性函数的随钻电磁波测井探测特性研究

为研究地层电性变化时不同区域对电磁波测井响应的贡献分布,从响应信号对地层参数求偏导的角度,给出一种新的电阻率敏感性函数定义,引入模式匹配法对纵向成层、径向非均匀介质敏感性分布进行快速模拟;通过对敏感性函数纵、横向积分给出了单发双收线圈系纵、横向探测范围,研究了井眼、频率、背景地层电阻率等对探测特性的影响.结果表明:敏感性函数能够定量表征响应对地层纵向与径向微观与宏观敏感性,幅度比与相位差敏感性分布形态类似,幅度比较广较深,而相位差分辨率高,敏感范围小;敏感性函数进行径向积分后可表征仪器的探测深度,与伪几何因子对比达到了同样的效果;背景地层电阻率在1~100Ωm变化时,工作频率2 MHz下幅度比探测深度约为0.6~2.3m,相位差为0.3~0.8m,幅度比50%纵向积分敏感性层厚约为0.3~1.6m,相位差约为0.2~0.6m;异常体与背景地层电阻率对比度在1~50变化时,引起的探测深度与敏感性层厚差异约为0.1~0.2m,远小于地层电阻率的影响.