Robust inversion of vertical electrical sounding data using a multiple reweighted least-squares method

The root cause of the instability problem of the least-squares (LS) solution of the resistivity inverse problem is the ill-conditioning of the sensitivity matrix. To circumvent this problem a new LS approach has been investigated in this paper. At each iteration, the sensitivity matrix is weighted in multiple ways generating a set of systems of linear equations. By solving each system, several candidate models are obtained. As a consequence, the space of models is explored in a more extensive and effective way resulting in a more robust and stable LS approach to solving the resistivity inverse problem. This new approach is called the multiple reweighted LS method (MRLS). The problems encountered when using the L ₁- or L ₂-norm are discussed and the advantages of working with the MRLS method are highlighted. A five-layer earth model which generates an ill-conditioned matrix due to equivalence is used to generate a synthetic data set for the Schlumberger configuration. The data are randomly corrupted by noise and then inverted by using L ₂, L ₁ and the MRLS algorithm. The stabilized solutions, even though blurred, could only be obtained by using a heavy ridge regression parameter in L ₂- and L ₁-norms. On the other hand, the MRLS solution is stable without regression factors and is superior and clearer. For a better appraisal the same initial model was used in all cases. The MRLS algorithm is also demonstrated for a field data set: a stable solution is obtained.     

垂直井眼中三维阵列感应测井资料自适应井眼校正

三维感应测井受井眼环境影响很大,如何消除这些不利影响是有效获取各向异性地层电导率真值并提高资料处理和解释质量的一项重要工作.本文以一维柱状各向异性地层模型为基础,通过最小平方反演技术从实际测量资料中快速反演井径、偏心距、泥浆电导率、地层水平电导率和各向异性系数等模型参数,设法减小井眼环境对测量结果的影响.首先,利用仪器偏心情况下不同旋转角电导率张量的关系方程,建立旋转角提取与校正方法,获取无旋转角三维感应测井数据.在此基础上,根据事先计算好的井眼校正库并结合多维非线性有限元逼近技术,建立快速计算三维感应测井响应以及Jacobi导数矩阵的插值公式,并利用自适应正则化迭代反演技术不断修改模型参数,实现理论合成资料与输入资料的最佳拟合.最后,通过反演出的模型参数计算三维感应测井资料所有分量的校正量,实现三维感应资料的井眼校正.理论模型和实际资料的处理结果均证实,在低阻井眼泥浆情况下,一维柱状模型中的反演方法仍然可以提取出较可靠的地层水平和垂直电导率,且井眼环境校正对于受井眼环境影响相对较小的测井曲线具有非常好的校正效果.     

二维阵列线圈核磁共振地下水探测理论研究

核磁共振法(Magnetic Resonance Sounding, MRS)是一种直接探测地下水的地球物理方法,目前只能对水平层状的含水层进行一维测深,对于尺寸小于线圈直径的二维或三维含水构造成像时,其灵敏度和横向分辨率很低.本文从研究二维阵列线圈核磁共振地下水探测方式的可行性出发,推导了地面发射线圈产生的椭圆极化激发场和阵列接收线圈的核磁共振响应核函数的表达式,数值计算了二维阵列线圈测量方式的MRS信号初始振幅图像和探测灵敏度,通过与二维发射/接收同一线圈测量方式对比,得到结论如下:二维阵列线圈测量方式可直接确定地下含水构造的水平位置,但需更大的激发电流强度和接收灵敏度;当中浅层(2,以上三种分辨率均由仪器噪声水平所决定.通过实施增加接收线圈的数量和匝数、提高激发脉冲矩和接收灵敏度等改进措施,现有的仪器系统可用二维阵列线圈测量方式.本文提出的理论和方法,将促使二维阵列线圈核磁共振方法在地下暗河、喀斯特溶洞和堤坝渗漏等二维或三维含水构造快速探测方面得到应用.     

Deep resistivity structure of the Dikili-Bergama region, west Anatolia, revealed by two- dimensional inversion of vertical electrical sounding data

Within the framework of the National Marine Geological and Geophysical Program, we re‐examined deep vertical electrical sounding (VES) data. The data, measured in 1968 by the General Directorate of Mineral Research and Exploration (MTA) of Turkey with the aim of exploring the deep resistivity structure of the Dikili–Bergama region, focus on the geothermal potential. The geoelectrical resistivity survey was conducted using a Schlumberger array with a maximum electrode half‐spacing of 4.5 km. The two‐dimensional (2D) inversion was utilized to interpret the VES data that were collected along 15‐ to 30‐km profiles. The 2D resistivity–depth cross‐sections obtained show very low resistivity values near the Dikili and Kaynarca hot springs. The 2D inversion results also indicate the presence of fault zones striking nearly N–S and E–W, and fault‐bounded graben‐horst structures that show promising potential for geothermal field resources. The 2D gravity model, which is in good agreement with the density variation of the region, supports the resistivity structure revealed by 2D inversion. The lithology information obtained from the borehole near Kaynarca also confirms the results of the resistivity interpretation and the density model.     

我国多参数激电测深找水应用综述

近40年来,地球物理技术在我国的地下水勘查中得到了比较多的应用,IP、TEM、EH4、CSAMT、SNMR等物探找水方法不断涌现.本文在介绍激电找水原理和技术方法的基础上,通过3个不同类型地下水勘查的成功实例,表明多参数直流激电测深这一传统的地球物理勘查方法在地下水勘查中仍具有独特的优势.提出了含水层的激电异常特征、含...     

Modeling of vertical sounding ionograms for nonmonotonous ionospheric layers

The problem of radiosignal reflection from a plane layered isotropic ionosphere has been considered. The vertical sounding ionograms for ionospheric layers with a complicated structure have been modeled. It has been demonstrated that the structure of reflected signals depends on the degree of ionospheric irregularities. Diffuse reflection of radiosignals, caused by the vertical irregularity of the ionosphere, has been detected.     

Models of deep electrical conductivity obtained from data on global magnetic variational sounding

An analysis of published papers containing magnetovariation sounding data (MVS) is carried out. A catalogue of the most trustworthy MVS parameters' values covering periods from 6 hours up to 11 years is compiled. With the help of averaged data of the magnetovariation sounding a planetary geoelectric profile is constructed meeting the requirements of up-to-date understanding of the physical processes in the Earth.     

Use of marine controlled-source electromagnetic sounding for sub-basalt exploration

The problem of imaging sedimentary structure obscured by high-velocity layers, such as carbonate, basalt or salt, using conventional seismic techniques is well known. When this problem is encountered in offshore areas, marine electromagnetic data can provide valuable, alternative and complementary constraints on the structure. We concentrate on the use of marine controlled-source electromagnetic (CSEM) sounding in the frequency domain. The feasibility of mapping sub-basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice. As a starting point the problem is addressed in terms of 1D resistivity structure. We show that sub-basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown. The presence of small-scale inhomogeneities in the basalt (which cause much of the scattering in seismic data) is shown to have no appreciable effect on the ability of the CSEM data to detect the sediments. The modelling is then extended to 2.5 dimensions. Again the presence of sub-basalt sediments can be readily detected and their properties and geometry quantified, especially if the electromagnetic data are combined with constraints from complementary geophysical techniques.     

Groundwater potential estimation using vertical electrical sounding measurements in the semi-arid Khanasser Valley region,Syria

Abstract

Traditional and polar vertical electrical sounding (VES) techniques were used for computing and characterizing the transmissivity of the Quaternary and Palaeogene aquifers in the Khanasser Valley, northern Syria. The VES technique with its different applied approaches is inexpensive, easy and gives rapid results with high spatial resolution. The Dar-Zarrouk parameters were determined to estimate the groundwater potential of the aquifers. The results show the mean transmissivity of the Quaternary aquifer to be 49 m²/d (minimum: 10 m²/d; maximum: 131 m²/d; standard deviation: 32 m²/d), and that of the Palaeogene aquifer is 0.94 m²/d (minimum: 0.31 m²/d; maximum: 1.54 m²/d; standard deviation: 0.33 m²/d).

Editor D. Koutsoyiannis

Citation Asfahani, J., 2013. Groundwater potential estimation using vertical electrical sounding measurements in the semi-arid Khanasser Valley region, Syria. Hydrological Sciences Journal, 58 (2), 468--482.     

Born近似快速三维反演井地电法数据

本篇研究了井中电偶极激发地面接收的井地电法的快速反演成像问题.我们采用了Born近似方法和重加权正则化共轭梯度法(RRCG)算法.数值计算的结果表明Born近似是一种有效的井地电法三维快速反演方法,同时也说明井地电法监测油水前驱和储层边界预测的观测数据可以用该方法进行快速三维反演成像.     

Quaternary aquifer transmissivity derived from vertical electrical sounding measurements in the semi-arid Khanasser Valley Region,Syria

Vertical electrical sounding technique (VES) is used as an alternative approach to pumping test for computing the Quaternary aquifer transmissivity in the Khanasser Valley, Northern Syria. The method is inexpensive, easy and gives faster results with higher special resolution than the traditional pumping technique. The hydraulic conductivity values obtained using VES agree with the pumping test results, which in the Khanasser Valley vary between the order of 0.864 and 8.64 m/day (10⁻⁵ and 10⁻⁴ m/s). The probable location of the Quaternary aquifer in the Khanasser Valley is obtained through the transmissivity map derived from VES. The knowledge of transmissivity is fundamental for modeling and management processes in the Khanasser Valley. Other similar semiarid regions can benefit from the approach successfully applied in the study area.     

基于双频磁共振的未知拉莫尔频率地下水探测方法

在利用地面磁共振方法进行地下水探测时,要求发射频率与拉莫尔（Larmor）频率相同.由于地磁场的不均匀性和噪声干扰,实际测量时无法获得准确的Larmor频率,导致发生偏共振现象.如果假设磁共振或频率偏量未知时,反演得到的含水量和弛豫时间（T₂^*）将存在较大误差.为了解决未知Larmor频率情况下的准确探测问题,本文在频率环测量方案的基础上提出了双频磁共振探测方法,即只需知道Larmor频率的范围区间,通过区间外的两个频率进行偏共振激发,利用频率差值的一半作为频率偏量计算核函数,即可消除未知Larmor频率的影响.通过仿真实验,证明了在电阻率大于100 Ωm时,双频磁共振和偏共振的核函数偏差较小.对于双层含水层模型,双频磁共振和偏共振的信号偏差小于3 nV,因此得到双频磁共振信号可以用偏共振核函数进行反演.通过对假设磁共振、假设偏共振和双频磁共振反演结果的对比,可以得到在未知频率偏量和设定频率偏量改变时,双频磁共振均能得到准确的反演结果.但是,当环境噪声增加和电阻率降低时,反演结果的准确度降低.最后,通过长春市烧锅镇采集数据的反演结果与已知钻探资料进行对比,验证了双频磁共振探测方法的有效性和准确性.     

Multiple scattering effects under vertical sounding of equatorial ionosphere

This paper presents a theoretical study of the influence of electron density small-scale irregularities on radio propagation under vertical sounding of the equatorial ionosphere. The sounder is assumed to radiate in the plane perpendicular to geomagnetic field lines. An approximate analytical solution of the equation of radiation energy balance in a plane layer of randomly inhomogeneous plasma has been obtained for this case. Analysis of the results shows that multiple scattering leads to attenuation of signal power and change of the signal arrival angles in the sounder vicinity.     

基于双频磁共振的未知拉莫尔频率地下水探测方法

在利用地面磁共振方法进行地下水探测时,要求发射频率与拉莫尔（Larmor）频率相同.由于地磁场的不均匀性和噪声干扰,实际测量时无法获得准确的Larmor频率,导致发生偏共振现象.如果假设磁共振或频率偏量未知时,反演得到的含水量和弛豫时间（T₂^*）将存在较大误差.为了解决未知Larmor频率情况下的准确探测问题,本文在频率环测量方案的基础上提出了双频磁共振探测方法,即只需知道Larmor频率的范围区间,通过区间外的两个频率进行偏共振激发,利用频率差值的一半作为频率偏量计算核函数,即可消除未知Larmor频率的影响.通过仿真实验,证明了在电阻率大于100 Ωm时,双频磁共振和偏共振的核函数偏差较小.对于双层含水层模型,双频磁共振和偏共振的信号偏差小于3 nV,因此得到双频磁共振信号可以用偏共振核函数进行反演.通过对假设磁共振、假设偏共振和双频磁共振反演结果的对比,可以得到在未知频率偏量和设定频率偏量改变时,双频磁共振均能得到准确的反演结果.但是,当环境噪声增加和电阻率降低时,反演结果的准确度降低.最后,通过长春市烧锅镇采集数据的反演结果与已知钻探资料进行对比,验证了双频磁共振探测方法的有效性和准确性.

     

井旁裂缝的双侧向测井物理模拟及特征分析

为深入认识裂缝性地层的双侧向测井响应特征,指导裂缝识别及裂缝参数表征,基于数值仿真开展了井旁裂缝的缩小比例物理模拟.在物理模拟平台构建的基础上,用不同浓度的氯化钠溶液模拟基岩,用不同宽度的镀锌板来模拟高导裂缝,实现测量过程的可视化.通过实验分析井旁裂缝的规模、发育位置和产状等因素对深、浅侧向电阻率的影响.研究表明:在仪器有效探测范围内,垂直缝延伸长度对深浅侧向电阻率的数值影响较小;裂缝离井壁越远,双侧向电阻率明显增大,但深浅侧向电阻率值的降低幅度不同;随着裂缝倾角增大,深侧向电阻率增大,浅侧向电阻率无明显变化,深浅侧向电阻率从负幅度差向正幅度差过渡,但临界角的观测十分困难,应用双侧向测井判断地层产状时,需特别注意其适用条件.     

Neogene aquifer properties specified through the interpretation of electrical sounding data,Salamiyeh region,central Syria

Twenty‐nine Schlumberger electrical soundings were carried out in the Salamiyeh region in Syria using a maximum current electrode separation of 1 km. Three soundings were made at existing boreholes for comparison. Aquifer parameters of hydraulic conductivity and transmissivity were obtained by analysing pumping test data from the existing boreholes. An empirical relationship between hydraulic conductivity determined from the pumping test and both resistivity and thickness of the Neogene aquifer has been established for these boreholes in order to calculate the geophysical hydraulic conductivity. A close agreement has been obtained between the computed hydraulic conductivity and that determined from the pumping test. The relationship established has, therefore, been generalized in the study area in order to evaluate hydraulic conductivity and transmissivity at all the points where geoelectrical measurements have been carried out. This generalization allows one to derive maps of the hydraulic conductivity and transmissivity in the study area based on geoelectrical measurements. These maps are important in future modelling processes oriented towards better exploitation of the aquifers. Copyright © 2006 John Wiley & Sons, Ltd.