Multiridge Euler deconvolution

Potential field interpretation can be carried out using multiscale methods. This class of methods analyses a multiscale data set, which is built by upward continuation of the original data to a number of altitudes conveniently chosen. Euler deconvolution can be cast into this multiscale environment by analysing data along ridges of potential fields, e.g., at those points along lines across scales where the field or its horizontal or vertical derivative respectively is zero. Previous work has shown that Euler equations are notably simplified along any of these ridges. Since a given anomaly may generate one or more ridges we describe in this paper how Euler deconvolution may be used to jointly invert data along all of them, so performing a multiridge Euler deconvolution. The method enjoys the stable and high‐resolution properties of multiscale methods, due to the composite upward continuation/vertical differentiation filter used. Such a physically‐based field transformation can have a positive effect on reducing both high‐wavenumber noise and interference or regional field effects. Multiridge Euler deconvolution can also be applied to the modulus of an analytic signal, gravity/magnetic gradient tensor components or Hilbert transform components. The advantages of using multiridge Euler deconvolution compared to single ridge Euler deconvolution include improved solution clustering, increased number of solutions, improvement of accuracy of the results obtainable from some types of ridges and greater ease in the selection of ridges to invert. The multiscale approach is particularly well suited to deal with non‐ideal sources. In these cases, our strategy is to find the optimal combination of upward continuation altitude range and data differentiation order, such that the field could be sensed as approximately homogeneous and then characterized by a structural index close to an integer value. This allows us to estimate depths related to the top or the centre of the structure.     

Automatic Interpretation of Magnetic Data Using Euler Deconvolution with Nonlinear Background

The voluminous gravity and magnetic data sets demand automatic interpretation techniques like Naudy, Euler and Werner deconvolution. Of these techniques, the Euler deconvolution has become a popular choice because the method assumes no particular geological model. However, the conventional approach to solving Euler equation requires tentative values of the structural index preventing it from being fully automatic and assumes a constant background that can be easily violated if the singular points are close to each other. We propose a possible solution to these problems by simultaneously estimating the source location, depth and structural index assuming nonlinear background. The Euler equation is solved in a nonlinear fashion using the optimization technique like conjugate gradient. This technique is applied to a published synthetic data set where the magnetic anomalies were modeled for a complex assemblage of simple magnetic bodies. The results for close by singular points are superior to those obtained by assuming linear background. We also applied the technique to a magnetic data set collected along the western continental margin of India. The results are in agreement with the regional magnetic interpretation and the bathymetric expressions.     

重力全张量数据联合欧拉反褶积法研究及应用

全张量测量技术是在空中或海上用加载了多个加速度计的移动平台技术测量位场的五个独立分量.各张量分量包含不同方向的地下地质体信息,水平张量分量T_(xx)、T_(yy)、T_(xy)、T_(xz)、T_(yz)通常用于识别和映射与地质构造或地层变化有关的测量区域中的目标,垂直张量分量Tzz用于估计深度.然而,这些分量传统上是彼此分开解释,经常遇到错失关键信息的风险.本文所用全张量欧拉反褶积是在单独z方向的欧拉反演基础上发展而来的,它融合了重力异常垂直分量以及其三个方向导数、水平分量以及其三个方向导数.全张量数据信息得以有效应用的同时,欧拉反褶积结果也比常规欧拉反褶积结果更加收敛.最后,结合美国墨西哥湾地区实测航空FTG数据,用重力梯度张量数据进行联合欧拉三维反演研究,有效的识别岩盖的边界信息,划分岩盖范围,为进一步研究盖层底下深部复杂地质情况提供可靠的解释结果.     

Fast mapping of magnetic basement depth,structure and nature using aeromagnetic and gravity data: combined methods and their application in the Paris Basin

Assessment of deep buried basin/basement relationships using geophysical data is a challenge for the energy and mining industries as well as for geothermal or CO₂ storage purposes. In deep environments, few methods can provide geological information; magnetic and gravity data remain among the most informative and cost‐effective methods. Here, in order to derive fast first‐order information on the basement/basin interface, we propose a combination of existing and original approaches devoted to potential field data analysis. Namely, we investigate the geometry (i.e., depth and structure) and the nature of a deep buried basement through a case study SW of the Paris Basin. Joint processing of new high‐resolution magnetic data and up‐to‐date gravity data provides an updated overview of the deep basin. First, the main structures of the magnetic basement are highlighted using Euler deconvolution and are interpreted in a structural sketch map. The new high‐resolution aeromagnetic map actually offers a continuous view of regional basement structures and reveals poorly known and complex deformation at the junction between major domains of the Variscan collision belt. Second, Werner deconvolution and an ad hoc post‐processing analysis allow the extraction of a set of magnetic sources at (or close to) the basin/basement interface. Interpolation of these sources together with the magnetic structural sketch provides a Werner magnetic basement map displaying realistic 3D patterns and basement depths consistent with data available in deep petroleum boreholes. The last step of processing was designed as a way to quickly combine gravity and magnetic information and to simply visualize first‐order petrophysical patterns of the basement lithology. This is achieved through unsupervised classification of suitably selected gravity and magnetic maps and, as compared to previous work, provides a realistic and updated overview of the cartographic distribution of density/magnetization of basement rocks. Altogether, the three steps of processing proposed in this paper quickly provide relevant information on a deep buried basement in terms of structure, geometry and nature (through petrophysics). Notwithstanding, limitations of the proposed procedure are raised: in the case of the Paris Basin for instance, this study does not provide proper information on Pre‐Mesozoic basins, some of which have been sampled in deep boreholes.     

不同高度数据联合欧拉反褶积法研究

基于不同测量高度重力场及其梯度数据可同时对应同一场源并用于反演场源位置的分析原理,拓展不同高度场数据在欧拉反褶积法中的应用范围.首先,立足于对欧拉反褶积方法的理论研究基础,提出不同高度数据融合联合欧拉反演公式.其次,在理论模型上对多种高度数据联合反演做了测试分析计算,验证了不同高度场数据融合联合欧拉反褶积法能够改善位场解释中单一观测面数据计算带来的解的发散问题,收敛过程由此改善.最后,将本文方法应用于龙门山地区实际重力数据的解释,获得了研究区断裂分布特征.     

Euler deconvolution of the analytic signal and its application to magnetic interpretation

Euler deconvolution and the analytic signal are both used for semi‐automatic interpretation of magnetic data. They are used mostly to delineate contacts and obtain rapid source depth estimates. For Euler deconvolution, the quality of the depth estimation depends mainly on the choice of the proper structural index, which is a function of the geometry of the causative bodies. Euler deconvolution applies only to functions that are homogeneous. This is the case for the magnetic field due to contacts, thin dikes and poles. Fortunately, many complex geological structures can be approximated by these simple geometries. In practice, the Euler equation is also solved for a background regional field. For the analytic signal, the model used is generally a contact, although other models, such as a thin dike, can be considered. It can be shown that if a function is homogeneous, its analytic signal is also homogeneous. Deconvolution of the analytic signal is then equivalent to Euler deconvolution of the magnetic field with a background field. However, computation of the analytic signal effectively removes the background field from the data. Consequently, it is possible to solve for both the source location and structural index. Once these parameters are determined, the local dip and the susceptibility contrast can be determined from relationships between the analytic signal and the orthogonal gradients of the magnetic field. The major advantage of this technique is that it allows the automatic identification of the type of source. Implementation of this approach is demonstrated for recent high‐resolution survey data from an Archean granite‐greenstone terrane in northern Ontario, Canada.     

Imaging magnetic sources using Euler's equation

The conventional Euler deconvolution method has the advantage of being independent of magnetization parameters in locating magnetic sources and estimating their corresponding depths. However, this method has the disadvantage that a suitable structural index must be chosen, which may cause spatial diffusion of the Euler solutions and bias in the estimation of depths to the magnetic sources. This problem becomes more serious when interfering anomalies exist. The interpretation of the Euler depth solutions is effectively related to the model adopted, and different models may have different structural indices. Therefore, I suggest a combined inversion for the structural index and the source location from the Euler deconvolution, by using only the derivatives of the magnetic anomalies. This approach considerably reduces the diffusion problem of the location and depth solutions. Consequently, by averaging the clustered solutions satisfying a given criterion for the solutions, we can image the depths and attributes (or types) of the causative magnetic sources. Magnetic anomalies acquired offshore northern Taiwan are used to test the applicability of the proposed method.     

基于最小反演拟合差的重磁场源深度计算方法

以等效源及位场物性反演为基础,本文提出一种新的求取重磁场源深度的方法.该方法将一层等效源以一定的间隔从浅部向深部移动,并将等效源作为初始模型进行反演,当反演拟合差最小时,停止反演,此时的等效源底深即为所求场源的中心深度.由于仅需要反演一层等效源,比传统的物性反演计算时间大大减少,并且不需要进行深度加权约束.理论模型数据处理结果表明该方法能够获得较准确的场源深度:以长宽比为7.5的薄板模型为例,深度计算误差约为1个点距(25 m);以长宽比为0.5~1.5的厚板模型为例,深度计算误差小于1个点距(25m).将该方法应用于实测航磁梯度数据,计算的磁源中心深度在200~250m之间,钻井资料显示该异常由埋藏深度在200~300m的闪长岩引起,计算结果与钻井资料较吻合.     

基于垂向一阶导数与解析信号比值的欧拉反演方法

常规欧拉反褶积法中构造指数的选取以及分散解存在较多的问题,提出了基于联立垂向一阶导数与解析信号的欧拉齐次方程的RDAS-Euler反演方法。该方法可以更为精确的估计场源的范围及埋深,且不需考虑构造指数N的影响,避免了因构造指数不当而引起的反演误差。通过对单一地质体及组合地质体模型的实验证明本文方法能有效地完成目标体的反演工作,反演结果与理论值之间的误差小于10%,且相对于常规欧拉反褶积法更加稳定准确,能够更好的得到地质体边界及深度信息。将RDAS-Euler法应用于黑龙江省虎林盆地实测布格重力异常数据,获得了丰富的断裂信息,说明RDASEuler法增强了对断裂平面位置的识别能力。     

火山岩油气藏重磁电震综合预测方法及应用

通过准噶尔盆地陆东地区数十口钻井资料的对比分析、归纳总结,提出了火山岩油气藏重磁电震综合预测方法.将正则化下延与延拓回返垂直二次导数串联形成了一个新的滤波器,该滤波器相当于首先通过正则化下延将位场曲面延拓至地下目的层段,降低火山岩埋深对磁J异常幅值的影响,然后利用延拓回返垂直二次导数对弱信号进行增强,不仅提高了位场异常...     

鄂尔多斯南缘地区重力变化场源特征

以鄂尔多斯南缘地区布格重力异常数据及2014—2017年相对重力观测数据为基础,采用欧拉反褶积方法,对引起重力变化的场源深度进行反演,并对空间分布规律予以解释。通过构建理论模型,反演得到最优模型参数,并对实际数据进行计算和分析。为了减弱和消除欧拉解的发散性,利用水平梯度滤波法优化反演结果。结果表明:①构造指数为1时,适合对相对重力数据进行反演,当滑动窗口选择4—8倍测点间距时,可以获得较为可靠的场源参数;②在主要断裂附近,不同时间尺度的场源呈较好的一致性特征;③引起重力变化的场源深度集中在10—30 km,与鄂尔多斯南缘地区的震源深度基本一致;④不同时间尺度的场源位置相对分散,表明引起重力变化的物质流动具有随机性和波动性。     

辽宁地区时变重力场源变化特征分析

地震重力分析通过研究时变重力场变化获取地球内部介质物性变化信息.采用贝叶斯重力网平差方法对辽宁地区2011-2014年共计四年7期的流动重力观测资料进行处理,对研究区重力观测网总体监测能力做出分析,选用欧拉反褶积对研究区的重力变化场源深度及空间分布特征进行反演和解释.通过反演计算发现,在2013年灯塔Ms5.1地震前沈...     

Investigation of Crustal Thickness in Eastern Anatolia Using Gravity,Magnetic and Topographic Data

The tectonic regime of Eastern Anatolia is determined by the Arabia-Eurasia continent-continent collision. Several dynamic models have been proposed to characterize the collision zone and its geodynamic structure. In this study, change in crustal thickness has been investigated using gravity, magnetic and topographic data of the region. In the first stage, two-dimensional low-pass filter and upward analytical continuation techniques were applied to the Bouguer gravity data of the region to investigate the behavior of the regional gravity anomalies. Next the moving window power spectrum method was used, and changes in the probable structural depths from 38 to 52 km were determined. The changes in crustal thickness where free air gravity and magnetic data have inversely correlated and the type of the anomaly resources were investigated applying the Euler deconvolution method to Bouguer gravity data. The obtained depth values are consistent with the results obtained using the power spectrum method. It was determined that the types of anomaly resources are different in the west and east of the 40° E longitude. Finally, using the obtained findings from this study and seismic velocity models proposed for this region by previous studies, a probable two-dimensional crust model was constituted.     

Gravity and magnetic field interpretation based on transformations of horizontal gradients in the VECTOR system

A method of potential field processing based on the transformation of vectors of the total horizontal gradient in windows of various sizes is considered. The gradients are calculated at the centers of triangles, whose vertices are points of observations, as a rule, of gravity and magnetic fields. Averaging of horizontal gradients of the field rather than initial values of the field is the main distinction of this approach from the known methods. This procedure, referred to in this paper as vector scanning of the field, makes it possible to obtain layer distributions of field sources in a 3-D diagram that is a quasi-density model of the study medium within the framework of certain model concepts. The paper presents a model example demonstrating the possibility of separating the fields produced by two sources located on a vertical line and an example illustrating the application of this method to the interpretation of the gravity field in the zone of the geodynamic influence of the Urals.     

Magnetic basement: gravity‐guided magnetic source depth analysis and interpretation

We present a new integrated approach to the interpretation of magnetic basement that is based on recognition of characteristic patterns in distributions and alignments of magnetic source depth solutions above and below the surface of magnetic basement. This approach integrates a quantitative analysis of depth solutions, obtained by 2D Werner deconvolution of the magnetic data, with a qualitative evaluation of the Bouguer gravity anomalies. The crystalline/metamorphic basement and sedimentary cover have different origins, tectonic histories, lithologies and magnetic properties. These differences result in different geometries of magnetic sources associated with faults, fracture zones, igneous intrusions, erosional truncations, subcrop edges and other structural discontinuities. Properly tuned, 2D Werner deconvolution is able to resolve the intra‐sedimentary and intra‐basement magnetic source geometries into distinctly different distributions and alignments of calculated depth solutions. An empirical set of criteria, basement indicators, was developed for identification and correlation of the basement surface. The ambiguity of basement correlation with limited or non‐existent well control, which is common for onshore frontier and offshore explorations, can be reduced by incorporating the Bouguer gravity data into the process of correlation.     

Role of Lithology and Subsurface structures detected by potential field data in controlling the radioactive mineral accumulation at Natash area,Eastern Desert,Egypt

Wadi Natash area is located in the southern part of the Eastern desert of Egypt. It has a great importance for containing accumulations from the radioactive minerals of Uranium, Thorium and Potassium. An integrated potential study was carried out on the study area with the aim of locating depths to causative bodies with sufficient magnetic susceptibility that may represent magmatic intrusions with relation to the radioactivity location and delineate the subsurface structures affecting the area. Both magnetic and Bouguer data as well as radiometric data were interpreted rapidly for source positions and depths using Euler deconvolution, Werner deconvolution and 3D modeling techniques. The results deduced from the trend analyses show that the major fault trend affecting the area have NNW-SSE (Red Sea–Gulf of Suez trend) direction intersected by the less predominant NNE-SSW(The Gulf of Aqaba–Dead Sea trend) and WNW-ESE (Najd Fault System) fault trends. The causative bodies were imaged at depths ranging from 0.3 km to about 1.5 km. The depths along the interpreted profiles display discontinuities in potential field markers due to presence of the NNW-SSE fault trends act as pass channels for the hydrothermal solutions.It can be stated that the radioactive mineral accumulations were caused by the hydrothermal solutions rich with radioactive minerals as a result of intruding Natash volcanic to the granitic rocks. The Qouseir clastics and the Nudian sand stone were affected by these solutions and show a positive response for the radioactive minerals.     

九江-瑞昌矿集区的3D结构及对区域找矿的启示

本文分析了九瑞地区重、磁场的分布特征.运用最新的方法技术对已有的重磁资料重新进行处理:采用重磁多尺度边缘检测方法,对九瑞矿集区区域重力和航磁数据进行了边缘检测,并根据检测结果重新厘定了断裂系统的展布位置.在整理、分析九瑞地区地质、地层物性资料的基础上,对实测的1∶5万重磁数据进行较细致的准三维反演.同时将重磁三维物性反演应用到岩浆岩空间结构研究中,获得了矿集区地层结构及岩浆岩三维空间形态特征.根据反演所得磁化率强弱,分析了岩体的基性程度,为寻找与火山岩、侵入岩体有关的金属矿产提供了指示信息.最终建立的模型给出了地下地层的分布特征、控矿构造的展布规律、与成矿相关岩体的三维形态以及已知矿点的空间分布特征,为在九瑞矿集区的深部寻找隐伏矿体提供了新的信息.     

Some Insights into Topographic,Elastic and Self-gravitation Interaction in Modelling Ground Deformation and Gravity Changes in Active Volcanic Areas

Surface displacements and gravity changes due to volcanic sources are influenced by medium properties. We investigate topographic, elastic and self-gravitation interaction in order to outline the major factors that are significant in data modelling. While elastic-gravitational models can provide a suitable approximation to problems of volcanic loading in areas where topographic relief is negligible, for prominent volcanoes the rough topography could affect deformation and gravity changes to a greater extent than self-gravitation. This fact requires the selection, depending on local relief, of a suitable model for use in the interpretation of surface precursors of volcanic activity. We use the three-dimensional Indirect Boundary Element Method to examine the effects of topography on deformation and gravity changes in models of magma chamber inflation/deflation. Topography has a significant effect on predicted surface deformation and gravity changes. Both the magnitude and pattern of the geodetic signals are significantly different compared to half-space solutions. Thus, failure to account for topographic effects in areas of prominent relief can bias the estimate of volcanic source parameters, since the magnitude and pattern of deformation and gravity changes depend on such effects.     

有限长圆柱体磁异常场全空间正演方法

在经典位场理论中,许多简单形体位场异常难以通过积分得到全空间的解析式.圆柱体是一类很重要的理论模型体,常用于模拟圆柱状地质体或非地质体(如管线),但目前还不能用解析公式正演有限长圆柱体在三维空间里的磁异常,而多是采用近似简化为有限长磁偶极子或线模型代替.对于有限长圆柱体,特别是半径相对于上顶埋深较大时,这种近似的误差不可忽略.本文利用共轭复数变量替换法,推导出有限长圆柱体在全空间的引力位一阶、二阶导数,利用Poisson关系得到磁异常正演公式,进而利用有限长圆柱体磁异常正演公式求解管状体的磁异常,得到不同磁化方向、不同大小的管线产生的磁场的特征,并将其推广到截面为椭圆的情况.最后通过模拟计算定量给出了将圆柱体近似为线模型的条件.     

Can Euler deconvolution outline three-dimensional magnetic sources?

Severe limitations of the standard Euler deconvolution to outline source shapes have been pointed out. However, Euler deconvolution has been widely employed on field data to outline interfaces, as faults and thrust zones. We investigate the limitations of the 3D Euler deconvolution–derived estimates of source dip and volume with the use of reduced-to-the-pole synthetic and field anomalies. The synthetic anomalies are generated by two types of source bodies: (1) uniformly magnetized prisms, presenting either smooth or rough interfaces, and (2) bodies presenting smooth delimiting interfaces but strong internal variation of magnetization intensity. The dip of the first type of body might be estimated from the Euler deconvolution solution cluster if the ratio between the depth to the top and vertical extent is relatively high (>1/4). For the second type of body, besides dip, the source volume can be approximately delimited from the solution cluster envelope, regardless of the referred ratio. We apply Euler deconvolution to two field anomalies which are caused by a curved-shape thrust zone and by a banded iron formation. These anomalies are chosen because they share characteristics with the two types of synthetic bodies. For the thrust zone, the obtained Euler deconvolution solutions show spatial distribution allowing to estimate a source dip that is consistent with the surface geology data, even if the above-mentioned ratio is much less than 1/4. Thus, there are other factors, such as a heterogeneous magnetization, which might be controlling the vertical spreading of the Euler deconvolution solutions in the thrust zone. On the other hand, for the iron-ore formation, the solution cluster spreads out occupying a volume, in accordance with the results obtained with the synthetic sources having internal variation of magnetization intensity. As conclusion, although Euler deconvolution–derived solutions cannot offer accurate estimates of source shapes, they might provide a sufficient degree of reliability in the initial estimates of the source dip and volume, which may be useful in a later phase of more accurate modelling.