Inversion of deep-sea vertical electric soundings using the correlational similarity method

A method is proposed for the inversion of marine vertical electric sounding (VES) data in a tomographic approximation. A 2.5-D inversion is based on the correlational similarity method; this method is a natural stabilizer of the solution and provides the most compact image of an anomaly carrier. The algorithm possesses a high convergence rate. The inversion reduces to the expansion of anomalous stresses in orthogonal reference functions that are spatial focusing coefficients of a VES configuration. The coefficients were calculated with the help of a new method of contracting mappings allowing one to construct efficient algorithms for calculating electromagnetic fields on a dense spatial grid. Results of experimental in situ data inversion refined the morphology of deep-sea polymetallic sulfides of the Logachev-1 orefield in the Atlantic Ocean.     

Generalised Series Expansion (GSE) used in DC geoelectric–seismic joint inversion

Horizontally layered (1D) earth models are often assumed as a model estimate for the interpretation of geophysical data measured along 2D geological structures. In this process, the individual data sets are usually inverted independently, and it is considered only in a later phase of interpretation that these local (1D) models have common characteristic features. Taking account of these common attributes, instead of the successive independent interpretations, the lateral variations of geometrical and petrophysical parameters can be efficiently determined for the whole 2D structure by applying a series expansion. Using global basis functions, two advantages can be achieved: (i) choosing an appropriate number of basis functions helps us to restrict the complexity of the model; (ii) the integration of all the data sets measured along the profile gives rise to the application of simultaneous or joint inversion methods. This results in a decrease of the number of independent unknowns, a higher stability during the inversion and a more accurate and reliable parameter estimation.In this paper, a joint inversion algorithm is presented using DC geoelectric apparent resistivities and refraction seismic travel times measured along various layouts above a 2D geological model. To describe lateral variations series, expansions are used, and furthermore, to improve the often used approximation of a (locally) 1D forward modelling, the integral mean value of the horizontally changing model parameters (calculated along an appropriately defined interval) is introduced. We call the inversion procedure that combines series expansions and the concept of integral mean Generalised Series Expansion (GSE) inversion. The method was developed and tested for both the simultaneous (integrating data sets of one method or methods on the same physical basis) and the joint inversion (where data sets of methods on different physical bases are joined together), using synthetic and field data sets. It is also demonstrated that the equivalence problem inherent in the independent inversion of DC geoelectric data can efficiently be resolved by the use of the joint GSE inversion method in the cases of conductive and resistive equivalent geological models.     

Using Vertical Electrical Soundings for Characterizing Hydrogeological and Tectonic Settings in Deir El-Adas Area,Yarmouk Basin,Syria

The present study is aimed at characterizing the subsurface geological and tectonic structure in Deir El-Adas area, by using Vertical Electrical Sounding survey (VES) and hydrogeological investigations, in order to determine the causes of the failure for the majority of the wells drilled in the area. The survey data was treated in three different approaches including direct VES inversion, pseudo-2D method and horizontal profiling, in order to maximize the reliability of the data interpretation. The results revealed the presence of a local faulted anticline structure at the top of the Paleogene formation, underneath the basaltic outcrops where Deir El-Adas village is situated. The appearance of this subsurface anticline structure has complicated the local hydro-geological situation, and most likely led to limitation of the groundwater recharge in the area. Moreover, the performed piezometric and discharge maps indicated the presence of a notable groundwater watershed, in addition to feeble water productivity of the wells drilled adjacent to Deir El-Adas, mostly related to the subsurface geological and tectonic settings in the area.     

A multiscale electrical survey of a lateritic soil system in the rain forest of Cameroon

Resistivity investigations were carried out on an elementary watershed in SW Cameroon, firstly to assess the applicability of direct-current (DC) resistivity methods to solve various pedological problems in intertropical regions, and subsequently to determine the relationships between electrical resistivities and pedological properties of lateritic soil systems. The survey included measurements in pits with a small Wenner fixed-spacing array (SWA), vertical electrical soundings (VES) and vertical electrical “quick soundings” (VEQS) both using the Schlumberger configuration. The VES data were interpreted using a conventional multilayer inversion program to obtain best-fit models. Constraints to the interpretation of these data were provided by SWA and pedological information from existing observation pits. The results of the interpretation reveal five distinct geoelectrical layers overlying a resistive bedrock. The first is a thin organo-mineral upper layer with low resistivities in the range 250–450 Ωm. The second layer corresponds to micro-aggregated clayey materials and is more resistive (1300–1800 Ωm). The third represents the main part of ferruginous materials and is even more resistive (2000–4500 Ωm). The fourth corresponds to unsaturated saprolite and the last to saturated saprolite (ground water) with resistivities ranging from 800 to 1500 Ωm and from 150 to 250 Ωm, respectively. Estimates of soil volumes for the entire study area were obtained from VEQS interpretations. Most of the soil cover corresponds to saprolite (74%, being saturated by ground water), while topsoil and ferruginous materials represent 14 and 12%, respectively. Finally, geophysical results based upon 1-D inversion provide a satisfactory approximation of the various lateritic components' 3-D geometry over the watershed. The study provides original quantitative results concerning the behaviour of intertropical soil systems as well as some geomorphological keys for soil mapping at a regional scale.     

层状介质二维电阻率扰动反演方法

为解决考古、工程地质和地下水污染勘查中某些探测靶体与周围介质间电性差异不大的地质问题,本文提出了层状介质背景下二维电阻率扰动反演方法．根据观测得到的背景剖面视电阻率断面,确定出背景场的电性层结构,利用电阻率扰动方法计算出敏感矩阵,通过广义逆矩阵法求解出电阻率的增量．在商丘地下夯土城墙的应用表明,该方法有效地抑制了夯土城墙的旁侧效应和地下水噪声的影响,揭示出地下夯土城墙的埋深与几何形态．     

大地电磁一维连续介质反演的曲线对比法

根据电磁波的传播特性，把视电阻率随周期变化的曲线转化为电阻率随深度变化的曲线，并以此作为初始反演的地电模型．通过初始地电模型得到的视电阻率曲线与真实模型的视电阻率曲线的对比，对初始地电模型的电阻率值进行校正．校正后的反演模型的视电阻率曲线与真实模型的视电阻曲线的拟合程度有所提高．然后如此反复进行多次校正，获得与真实模型更为接近的反演模型，反演的拟合误差一般小于ｌ％．模型试验和实际例子表明，该方法的拟合程度优于Ｂｏｓｔｉｃｋ法．     

微动多阶瑞雷波SPAC系数反演方法及应用研究

为充分利用微动信号中基阶和高阶模式瑞雷波,本文研究了基于多阶瑞雷波SPAC系数直接反演的方法.该方法首先基于地层介质响应计算多阶瑞雷波的能量占比,考虑实际观测台阵有限台站个数对SPAC系数影响,正演计算多阶瑞雷波SPAC系数,再采用快速模拟退火算法对其反演以获得地下介质横波速度结构.在此基础上,本文通过数值模拟验证该方法的可靠性,分别选取三种典型地质模型,基于模式叠加算法合成理论微动信号,采用本文方法计算其理论多阶瑞雷波SPAC系数并反演,给出反演结果与真实模型对比.我们将该方法应用于上海中心城区的地质调查中,通过与钻探结果对比,进一步验证该方法的有效性.本文理论与实际应用研究表明,基于多阶瑞雷波SPAC系数直接反演的微动探测方法有助于提高反演结果的可靠性,尤其对含软硬夹层的复杂地层介质,可提高探测精度.     

SOME IMPROVEMENTS IN THE INTERPRETATION OF VERTICAL ELECTRIC SOUNDING CURVES*

The process of VES interpretation is discussed, including the following points. (a) Preliminary interpretation by means of master curves. It is shown that the positions of the auxiliary points K and Q depend on the resistivity of the substratum. The interpretation is improved if the auxiliary points are determined separately for each master curve. (b) The individual parts of the measured curves are shifted in overlapping MN electrode positions so that the total sum of squares of the shifts is minimal. (c) The ambiguity may be reduced by means of supplementary information or assumptions on the resistivities. Fixing the resistivities is not always possible because discrepancies may arise between the ground measurement and the well-logging data. The simultaneous interpretation of several VES curves is recommended assuming constant resistivities. This assumption may be subsequently verified by means of the F-test. (d) A nonlinear algorithm is proposed for the determination of confidence intervals. As the multi-dimensional confidence intervals are often very complicated, it is recommended to construct only one-dimensional confidence intervals for the estimable parametric functions. (e) A ‘double-least-squares’ optimization technique is presented. The optimization is performed on the estimable parametric functions, and the individual parameters are determined so that the solution remains near the initial guess. This technique is faster than the singular value decomposition.     

3-D seismic tomography for velocity and interface structure of the crust and upper mantle (theoretical part)

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Subsurface DC resistivity mapping: approximate 1-D interpretation

Resistivity prospecting is the main tool used to investigate the shallow structure of the ground. A series of new techniques for determining the 2-D and 3-D geometry of the ground is now finding increasing use, but the light and simple Wenner prospecting technique remains a practical and efficient tool for rapidly mapping lateral variations in resistivity. When the resistivity changes are smooth, 1-D modelling can be used to interpret the data, and the criteria governing this approximation can be defined from synthetic data generated by a 3-D slab-model. For a Wenner array, two quadripole configurations can be used, Normal and Dipole-Dipole. For these two configurations the width of the transition zone, the apparent anisotropy effect and the precision of the resistivity values recovered from 1-D inversion differ. However the simultaneous inversion of both sets of data gives better results than for either configuration by itself. Two examples illustrate that in geological contexts where the thickness of the weathered zone causes the changes in the apparent resistivity value, this parameter can be recovered from 1-D inversion.     

区域性的地震面波层析方法

本文提出了一个利用平面上长方形区域內的面波频散资料,求区域的三维速度结构的方法。将地震波慢度表示成二重Fourier级数,反演其系数为深度的函数,最后合成速度值,并以Love面波为例作了数值计算试验,绘制出了深度为150km的速度平面分布图。      

电阻率层析成像非线性反演

本文通过概率成像法、α中心法、遗传算法的有机结合, 实现了2-D电阻率层析成像非线性反演.首先,根据概率成像结果指示出的2-D地质异常体的相对电性和大致分布范围,合理地确定出α中心的位置与强度系数的变化范围.其次,以α中心法作为2-D地电结构的正演手段,采用遗传算法反演出各α中心的强度系数.在此基础上, 应用α中心法重新构建出地下介质电阻率的分布.通过对均匀半空间中赋存2-D地质异常体模型的反演试验, 获得了令人满意的结果.     

3-D crustal structure in the Agadir region (SW High Atlas,Morocco)

The 1960 Agadir earthquake (Mw 6.0) constitutes the most damaging earthquake event in Morocco. With the expansion of seismic networks during the last decade in Morocco, new seismic data have been collected in this region. The P and S arrivals at 19 stations located in Southern Morocco are used to investigate the lithosphere in the Agadir region. In this study, we use a linearized inversion procedure comprising two steps: (1) finding the minimal 1-D model and simultaneous relocation of hypocentres and (2) determination of local velocity structure using linearized inversion. The model parameterization in this method assumes a continuous velocity field. The resolution tests indicate that the calculated images give near true structure for the studied region from 0- to 45-km depth. The results show that the total crust thickness varies from 30 to 40 km in SW High Atlas and confirm the modest crustal tectonic shortening and thickening in the Atlas Mountains of Morocco. The inferred geological structure reconstructed from the calculated image illustrates the existence of fault-related folding. The evidence for coseismic ruptures in 1960 on the Kasbah anticline combined with the 1960 earthquake hypocentre located in the tomographic image determines the seismic potential of the active fault and related fold. The resulting tomographic image shows a high-velocity anomalies that could be associated with the location of deep active fault (10–30 km) associated with the fold structure. In the South Atlas, theses anomalies could be associated with the South atlas thrust front structure.     

COMPUTING THE KERNEL FUNCTION IN RESISTIVITY SOUNDING WITH AN ARBITRARY ELECTRODE CONFIGURATION*

A power series expansion can be used to obtain the kernel from apparent resistivities for an arbitrary electrode configuration. Three types of function are most appropriate for this purpose. The expansion coefficients can be by a least-squares method. In this case, ortho-normalization of the functions is of great advantage. An example with the Wenner configuration is given.     

庐枞矿集区矾山-将军庙地区AMT三维反演及地质结构解释

庐江-枞阳矿集区是长江中下游成矿带内的一个重要的多矿种成矿区.为揭示矾山-将军庙地区的地质结构以指导找矿,首先,在庐枞矿集区北部的矾山-将军庙地区开展了三维音频大地电磁数据采集工作,采用Rhoplus方法进行AMT数据死频带校正保证数据质量,利用相位张量分析地下介质的维性特征;其次,利用模块化三维反演并行代码ModEM开展了实测数据的三维反演,获得了一个典型的具有双层结构特征的火山岩盆地三维地下电性模型;最后通过电性模型及其他地质资料的综合解释,依据反演模型中近地表的低阻层和深部的高阻隆起区,分别勾画出了火山岩地层的厚度和深部侵入岩的分布范围,并在小岭地区发现地表浅部下方存在一个含矿次级火山机构的高导体.     

Geoelectric Characterization of Thermal Water Aquifers Using 2.5D Inversion of VES Measurements

This paper presents a short theoretical summary of the series expansion-based 2.5D combined geoelectric weighted inversion (CGWI) method and highlights the advantageous way with which the number of unknowns can be decreased due to the simultaneous characteristic of this inversion. 2.5D CGWI is an approximate inversion method for the determination of 3D structures, which uses the joint 2D forward modeling of dip and strike direction data. In the inversion procedure, the Steiner’s most frequent value method is applied to the automatic separation of dip and strike direction data and outliers. The workflow of inversion and its practical application are presented in the study. For conventional vertical electrical sounding (VES) measurements, this method can determine the parameters of complex structures more accurately than the single inversion method. Field data show that the 2.5D CGWI which was developed can determine the optimal location for drilling an exploratory thermal water prospecting well. The novelty of this research is that the measured VES data in dip and strike direction are jointly inverted by the 2.5D CGWI method.     

Effects of 1-D versus 3-D velocity models on moment tensor inversion in the Dobrá Voda area in the Little Carpathians region,Slovakia

Retrieving the parameters of a seismic source from seismograms involves deconvolving the response of the medium from seismic records. Thus, in general, source parameters are determined from both seismograms and the Green functions describing the properties of the medium in which the earthquake focus is buried. The quality of each of these two datasets is equally significant for the successful determination of source characteristics. As a rule, both sets are subject to contamination by effects that decrease the resolution of the source parameters. Seismic records are generally contaminated by noise that appears as a spurious signal unrelated to the source. Since an improper model of the medium is quite often employed, due to poor knowledge of the seismic velocity of the area under study, and since the hypocentre may be mislocated, the Green functions are not without fault. Thus, structures not modelled by Green functions are assigned to the source, distorting the source mechanism. To demonstrate these effects, we performed a synthetic case study by simulating seismic observations in the Dobrá Voda area of the Little Carpathians region of Slovakia. Simplified 1-D and 3-D laterally inhomogeneous structural models were constructed, and synthetic data were calculated using the 3-D model. Both models were employed during a moment tensor inversion. The synthetic data were contaminated by random noise up to 10 and 20 % of the maximum signal amplitude. We compared the influence of these two effects on retrieving moment tensors, and determined that a poor structural model can be compensated for by high-quality data; and that, in a similar manner, a lack of data can be compensated for by a detailed model of the medium. For examples, five local events from the Dobrá Voda area were processed.     

唐山滦县地区地壳速度结构和震源位置的非线性联合反演

本文描述了利用广义最小二乘非线性反演求解同时确定震源位置和速度结构的方法。给出了数值模拟的结果。并应用此方法对唐山滦县地区临时数字地震台网的资料进行了处理。获得了该区地壳地一维速度结构,并讨论了解的分辨率。     

大地电磁测深资料的联合反演研究

本文从大地电磁扬的色散关系出发,阐述了进行大地电磁资料一维联合反演的物理意义。论证了对于以色散关系相联系的一对大地电磁响应函数,特别是阻抗实部和虚部视电阻率进行联合反演的必要性和可能的应用前景。对比研究了大地电磁阻抗各单参量及其联合反演的效果。结果表明,在加快反演迭代速度,限定反演解的非唯一性、保证反演迭代的稳定进行等方面,联合反演比单参量反演具有明显的优越性;在大多数情况下,阻抗实部视电阻率和阻抗虚部视电阻率联合反演的效果要比通常所采用的常规视电阻率和阻抗相位的联合反演效果好。实际大地电磁资料的试验表明,各种不同平均阻抗的利用和综合反演解释将有助于获得更为可靠的结果。     

Adaptive reconstruction method of impedance model with absolute and relative constraints

Inversion for seismic impedance is an ill-posed and band-limited problem so that inversion results are non-unique and unstable and low and high frequency components of inversion results are missed. Combining regularization with constraints of sonic log data and geological structure information can help to alleviate these problems. To achieve this, we developed an inversion method by constructing a new objective function which includes edge-preserving regularization and soft constraint based on Markov random field (MRF). The method directly introduces absolute constraints with prior impedance and sonic log data in the objective function and indirectly achieves relative constraints with geologic structures of layer interfaces and faults by adjusting the regularization parameter which is the scaling parameter δ. Moreover, we improved the inversion result using anisotropic diffusion smoothing method. Optimization approach utilized in inversion is a fast simulated annealing (FSA). We test the method on both synthetic and field data examples. Tests on 2-D synthetic data indicate that aspects of the discontinuity in the inversion results are significantly improved by adding δ values in faults and layer interfaces. We obtained better results by combining the first-order neighborhood and the third-order neighborhood of MRF. The inversion results of the field data provide more detailed information of the layers. The results of nearby faults were improved by introducing the geological structure constraints.