Magnetic data analysis at low latitudes using magnitude transforms

Magnitude transforms include magnitude magnetic anomalies (MMA), their gradients and Laplacians. They can be calculated from the total magnetic anomalies or other component anomalies. Magnitude magnetic anomalies have a space distribution different from that of the component anomalies. Their values are non‐negative and their respective patterns are similar to the positive gravity anomalies. Magnitude transforms are an effective tool for magnetic data analysis due to their simplified pattern and direct correlation with the space location of the source. They have advantages over the traditionally used reduction‐to‐the‐pole (RTP) transform, especially at low magnetic latitudes. The calculation of magnitudes of the anomalous field requires the total field data to be transformed into the component anomalies, while the reduction‐to‐the‐pole transform also includes a rotation of the magnetization vector, the orientation of which is usually assumed. For equal latitudes, the transfer functions of component‐component transforms in the frequency domain show better stability than the component‐component‐rotation transfer function. This is illustrated by a comparison of analytical expressions, and synthetic models of magnetic fields. The Dixon seamount case shows the possibilities for an improved data analysis and more confident source recognition at low latitudes using magnetic transforms.     

位场数据解释的Theta-Depth法

Theta图是利用位场(重磁)数据识别边界的常用方法,其表达式为重磁异常水平变化与垂直变化的比值函数.该方法计算浅源地质体边界的效果较好,而由于深源位场数据在换算过程中会产生趋同效应,在深源地质体识别应用中计算结果不准确,为此,本文提出Theta-Depth法并进行地质体埋深的计算.首先给出直接利用Theta图像进行场源体深度估算的方法,然后推导出基于Theta导数的线性方程来自动估算场源位置参数,本文方法可有效地利用Theta图像的特征为约束条件来提高反演结果的精度.理论模型试验证明本文提出的Theta-Depth法能有效地计算出场源体位置和深度.将该方法应用于满都拉地区实测磁数据的解释,帮助圈定了矿脉的分布.     

空间归一化边界识别方法用于判断地质体的水平位置及深度(英文)

边界识别是重磁数据解释中的常用方法之一,依据其结果可划分出地质体的水平范围。边界识别结果受地质体埋深及导数计算误差的影响所识别边界与真实边界之间存在一定的差距,且边界识别法无法直观地给出地质体的深度信息。为了获得异常体的水平位置和深度信息,本文提出空间归一化边界识别方法,其对不同深度的边界识别函数进行归一化计算,空间归一化边界识别法的最大值对应于异常体的水平位置和深度。常规边界识别结果的误差随理深的减小而减小,而空间归一化边界识别法是通过最大值来判断地质体的位置,最大值是在地质体处获得,因此归一化边界识别方法所获得的结果是准确的。通过理论模型试验证明归一化边界识别方法能有效地完成异常体的水平位置和深度的计算,所获得的水平位置和深度信息与理论值相一致,为下一步的勘探计划提供了更加可靠的依据。将其应用于实际航磁数据的解释,获得了断裂的具体分布形式。     

全张量磁梯度数据的斜导数特征值边界识别方法研究

本文提出了一种全张量磁梯度数据的斜导数边界识别方法,该方法利用全张量磁梯度数据定义了北向斜导数、东向斜导数和垂向斜导数.模型试验表明,北向和东向斜导数无法有效识别磁性异常体边界,异常形态复杂虚假异常较多,而垂向斜导数在高纬度斜磁化或者垂直磁化条件下可以清晰而准确得识别地质体的边界,具有一定的深度识别能力.与现有磁张量梯度的边界识别方法相比,识别效果好,可以有效均衡不同深度地质体的响应.将上述方法应用于大兴安岭地区实测航磁三分量数据转换得到的全张量数据,获得了研究区浅部磁性体的分布,并与三分量数据进行联合解释,取得良好的地质效果.     

基于Theta法改进的均衡滤波器在位场边界识别中的应用——以庐枞矿集区为例

利用观测数据确定地质体的边界位置是位场数据解译中的一项重要工作,传统的边界识别滤波器通常不能均衡深、浅部的地质体边界,近些年相关研究开始致力于发展均衡边界滤波器.本文基于Theta图法定义了新的边界识别滤波器,详述了滤波器波数域及空间域的主要计算公式,通过模型验证,该滤波器显著压制了Theta图法对深部地质体边界的放大作用,较好地平衡了深部和浅部边界.通过与传统的边界识别滤波器对比,本文定义的滤波器能够清晰且更加收敛地圈定出地质体的水平边界位置.以长江中下游成矿带庐枞矿集区为例,开展了1∶5万重磁数据的处理分析,并结合物性资料进行了讨论,结果表明:重磁数据的检测结果精确刻画了郯庐断裂带的位置;庐枞盆地的磁力数据检测边界整体与盆地的地质边缘一致,明确了边界断裂在深部倾向盆地内部;识别出庐枞盆地外围一系列环形边界,这些边界封闭区域与最新勘探发现的深部岩体及铁铜矿化体相对应,对于指导区域深部找矿工作有着重要的意义.     

Tilt梯度及其水平导数提取重磁源边界位置

探讨了用于探测异常源边界和形状的Tilt梯度及其水平导数的方法及其性质．通过模型试算,论证了Tilt梯度及其水平导数的有效性,并从理论上给予了证明．Tilt梯度及其水平导数的方法及其性质可广泛用于区域重磁异常的构造解释之中。     

Improved salt imaging in a basin context by high resolution potential field data: Nordkapp Basin,Barents Sea

The seismic imaging of salt diapirs in the Nordkapp Basin gave rise to considerable problems in defining their shape and volume. Independent information was added by integrating the interpretation with high resolution gravity and magnetic data. We developed a novel, iterative workflow, separated into sub‐categories: sediments, salt structures, basement and Moho. Distinctions between the sources of the anomalies from different depths was achieved by utilizing the different decay characteristics of gravity, gravity gradiometry and high resolution magnetic anomalies. The workflow was applied to the southern part of the Nordkapp Basin. It started with the sedimentary model derived from seismics, populated with measured densities and magnetic susceptibilities and a starting model for the base salt. The residual after the removal of this model was interpreted in terms of a crustal model, including flexural isostatic calculations for the Moho with the sedimentary load. The residual after the removal of crustal and early sedimentary model was used to tune the salt model. As these major and minor modelling steps depend on each other, an iterative process was applied to stepwise improve the density and magnetic susceptibility model. The first vertical gradient of gravity and the magnetic field were found to give most information about the cap rock of the diapirs. The improvement in salt imaging, integrated with results from controlled‐source electromagnetic and magneto‐telluric modelling is shown for the salt diapir Uranus, where a well, terminated in the salt, constrains the minimum of the depth to base salt.     

Interpretation of magnetic and gravity gradient tensor data using normalized source strength – A case study from McFaulds Lake,Northern Ontario,Canada

In this paper, we present a case study on the use of the normalized source strength (NSS) for interpretation of magnetic and gravity gradient tensors data. This application arises in exploration of nickel, copper and platinum group element (Ni‐Cu‐PGE) deposits in the McFaulds Lake area, Northern Ontario, Canada. In this study, we have used the normalized source strength function derived from recent high resolution aeromagnetic and gravity gradiometry data for locating geological bodies. In our algorithm, we use maxima of the normalized source strength for estimating the horizontal location of the causative body. Then we estimate depth to the source and structural index at that point using the ratio between the normalized source strength and its vertical derivative calculated at two levels; the measurement level and a height h above the measurement level. To discriminate more reliable solutions from spurious ones, we reject solutions with unreasonable estimated structural indices. This method uses an upward continuation filter which reduces the effect of high frequency noise. In the magnetic case, the advantage is that, in general, the normalized magnetic source strength is relatively insensitive to magnetization direction, thus it provides more reliable information than standard techniques when geologic bodies carry remanent magnetization. For dipping gravity sources, the calculated normalized source strength yields a reliable estimate of the source location by peaking right above the top surface. Application of the method on aeromagnetic and gravity gradient tensor data sets from McFaulds Lake area indicates that most of the gravity and magnetic sources are located just beneath a 20 m thick (on average) overburden and delineated magnetic and gravity sources which can be probably approximated by geological contacts and thin dikes, come up to the overburden.     

An integrated geodynamic model of the Nankai subduction zone and neighboring regions from geophysical inversion and modeling

I investigate large-scale deep crustal structures of the Nankai subduction zone and neighboring region using regional magnetic and gravity anomalies, heat flow measurements, and earthquake hypocenters. It is found that ages, dip angles, and geothermal states of the subducting slab have direct influences on mantle wedge serpentinization. The weakest serpentinization observed in the Nankai forearc region is associated with the youngest downgoing plate of the Shikoku Basin. Conspicuous gravity anomalies identified in the forearc region are coincidental spatially with magnetic anomalies after the reduction to the pole, a mathematical procedure that helps relocate magnetic sources and boundaries, and allows us to more easily interpret magnetic data. It is argued that these patches of magnetic and gravity anomalies are caused by the same sources of anomalous density and magnetization, and are linked directly to preexisting structures such as magnetic anomalies and their boundaries in the subducting oceanic crust. Since the gravity and magnetic anomaly patches are found to be closely related to interplate seismogenic behaviors in the Nankai subduction zone, I suggest that major magnetic boundaries in the Shikoku Basin are likely weak places for slab tears that trigger seismic segmentations along the subduction zone.     

Gravity and magnetic investigation on the distribution of volcanic rocks in the Qinggelidi area,north‐eastern Junggar Basin (north‐west China)

A detailed investigation on the location of magmatic intrusions in the Carboniferous strata of the Qinggelidi area, north‐eastern Junggar Basin, is presented based on the interpretation of gravity and magnetic data constrained by petrophysical data, seismics and surface geology. The wavelet multi‐resolution analysis based on the discrete wavelet transform is adopted to the regional‐residual separation of gravity and magnetic anomalies. A power spectrum analysis is applied to estimate the source depths corresponding to different scales. A comparative analysis on the characteristics of local gravity and magnetic anomalies improved our understanding of volcanic rock distribution in the Carboniferous strata. Generally speaking, in total 75 anomalies are recognized, among which 23 are inferred to be the responses of basalts, diabases and andesites with high density and strong magnetization. Twelve anomalies are assumed to be caused by andesites, rhyolites and volcanic breccias with medium‐low density and high magnetization. There are still five anomalies that are believed to be generated by volcanic tuffs with low density and weak magnetization. Lastly, four cross‐sections in 3D gravity and magnetic modelling are displayed to provide a more thorough image of volcanic rocks in our study area.     

Toward a full multiscale approach to interpret potential fields

The way potential fields convey source information depends on the scale at which the field is analysed. In this sense a multiscale analysis is a useful method to study potential fields particularly when the main field contributions are caused by sources with different depths and extents. Our multiscale approach is built with a stable transformation, such as depth from extreme points. Its stability results from mixing, in a single operator, the wavenumber low‐pass behaviour of the upward continuation transformation of the field with the enhancement high‐pass properties of n‐order derivative transformations. So, the complex reciprocal interference of several field components may be efficiently faced at several scales of the analysis and the depth to the sources may be estimated together with the homogeneity degrees of the field. In order to estimate the source boundaries we use another multiscale method, the multiscale derivative analysis, which utilizes a generalized concept of horizontal derivative and produces a set of boundary maps at different scales. We show through synthetic examples and application to the gravity field of Southern Italy that this multiscale behaviour makes this technique quite different from other source boundary estimators. The main result obtained by integrating multiscale derivative analysis with depth from extreme points is the retrieval of rather effective information of the field sources (horizontal boundaries, depth, structural index). This interpretative approach has been used along a specific transect for the analysis of the Bouguer anomaly field of Southern Apennines. It was set at such scales, so to emphasize either regional or local features along the transect. Two different classes of sources were individuated. The first one includes a broad, deep source with lateral size of 45∼50 km, at a depth of 13 km and having a 0.5 structural index. The second class includes several narrower sources located at shallowest depths, ranging from 3–6 km, with lateral size not larger than 5 km and structural indexes ranging from 1–1.5. Within a large‐scale geological framework, these results could help to outline the mean structural features at crustal depths.     

Gravity and magnetic field characteristics and hydrocarbon prospects of the Tobago Basin

The Tobago Basin, which is located offshore northern Venezuela with a southern margin close to Trinidad and Tobago, has an area of approximately 59,600 km². The Tobago Basin has relatively favourable hydrocarbon prospects, and to date, exploration work has mainly concentrated on small areas of the southwestern portion of the basin. To conduct a comprehensive study of the structural framework of the basin and the characteristics of the basement in order to identify prospective zones for hydrocarbon exploration, shipborne‐measured and satellite‐measured gravity data, shipborne‐measured magnetic data, and aeromagnetic survey data were analysed. A regularisation filtering method was used to separate and obtain regional and residual gravity and magnetic anomalies. Directional gradients of gravity and magnetic anomalies and the total horizontal gradient and vertical second derivative of gravity anomalies were employed to extract information about fault structures. Regression analysis methods were used to determine the basement depth. The geological significance of the gravity and magnetic fields was examined, the structural framework of the basin was assessed, the basement depth was estimated, and favourable hydrocarbon exploration prospects within the basin were identified. The results show that the Tobago Basin contains complex structures consisting mainly of two groups of faults trending in northeasterly and northwesterly directions and that the major northeasterly trending faults control the main structural configuration and depositional system within the basin. The basement of the Tobago Basin has deep rises and falls. It can be divided into the following four secondary tectonic units: the western sub‐basin, the central uplift area, the southern sub‐basin, and the northeastern sub‐basin. The central uplift area and northeastern sub‐basin are most likely to have developed hydrocarbon accumulations and should be targeted for further exploration.     

利用位场及其高阶导数的功率谱计算岩层的深度与厚度

本文导出了利用重力与磁异常及其高阶导数的功率谱计算密度体和磁性体上顶与下底埋深的近似公式,还提供了利用垂向或水平导数谱计算深度的方法。理论模型的试验结果表明,由近似公式计算的精度可以满足实际需要。本文最后根据一条重力剖面资料,计算了密度层上、下界面。     

位场总水平导数极值位置空间变化规律研究

通过对位场总水平导数函数性质的研究表明,位场总水平导数不是位函数,因而利用位场总水平导数构造新的边缘识别方法时会出现"奇点",使得计算结果的稳定性下降.对单一边界、双边界、多边界模型重力异常总水平导数和重力异常垂向导数总水平导数极值位置的空间变化规律研究表明,重力异常垂向导数总水平导数和化极磁力异常总水平导数的极值位置相同,与重力异常总水平导数的极值位置空间变化规律相似.利用位场总水平导数极大值位置能够准确识别单一直立边界地质体的边缘位置,但不能准确识别其它任何形体的边缘位置,其识别结果的偏移量大小随地质体的埋深、水平尺寸以及倾斜程度等变化,但能收敛于某一固定值;重力异常垂向导数总水平导数比重力异常总水平导数的峰值更加尖锐、横向识别能力更强,其极大值位置更靠近地质体上顶面边缘位置,但存在"次极大值"的影响.     

A direct method of interpreting gravity and magnetic anomalies: The case of a horizontal cylinder

Summary A new method of interpreting the gravity and magnetic anomalies is introduced with special reference to the magnetic anomalies of a horizontal cylinder. The method consists of calculating the functions of the anomaly and its distance from an arbitrary point. These form a simple linear equation with coefficients related to the parameters defining the body. Since each observation forms a separate linear equation, the required normal equations are formed by the method of least squares and solved for the coefficients and hence for the various parameters defining the target. The discussion here is confined to the vertical magnetic anomalies. The application of the method to horizontal and total field anomalies of two dimensional bodies is also outlined.     

Mapping basement structures in the northwestern offshore of Abu Dhabi from high‐resolution aeromagnetic data

This paper presents a case study of mapping basement structures in the northwestern offshore of Abu Dhabi using high‐resolution aeromagnetic data. Lineament analysis was carried out on the derivatives of the reduced‐to‐the‐pole magnetic data, along with supporting information from published geologic data. The lineament analysis suggests three well‐defined basement trends in the north–south, northeast–southwest, and northwest–southeast directions. The reduced‐to‐the‐pole magnetic data reveal high positive magnetic anomalies hypothesized to be related to intra‐basement bodies in the deep seated Arabian Shield. Depth to basement was estimated using spectral analysis and Source Parameter Imaging techniques. The spectral analysis suggests that the intruded basement blocks are at the same average depth level (around 8.5 km). The estimated Source Parameter Imaging depths from gridded reduced‐to‐the‐pole data are ranged between 4 km and 12 km with a large depth variation within small distances. These estimated depths prevent a reliable interpretation of the nature of the basement relief. However, low‐pass filtering of the horizontal local wavenumber data across two profiles shows that the basement terrain is characterized by a basin‐like structure trending in the northeast–southwest direction with a maximum depth of 10 km. Two‐dimensional forward magnetic modelling across the two profiles suggests that the high positive magnetic anomalies over the basin could be produced by intrusion of mafic igneous rocks with high susceptibility values (0.008 to 0.016 SI.     

归一化总水平导数法在位场数据解释中的应用

本文提出归一化总水平导数法,通过对总水平导数进行空间归一化计算实现了异常体水平位置和深度的估计,此外还推导出基于归一化总水平导数的欧拉反褶积法来估算地下地质体的空间位置,两种方法反演结果的相互验证可有效地提高反演结果的可信度.理论模型试验证明空间归一化总水平导数法和归一化总水平导数欧拉反褶积法均能有效地完成异常体的水平位置和深度的估计,所获得的位置参数与理论值相一致.在利用归一化总水平导数法进行磁异常解释时,对数据进行化磁极计算可得到更加准确的结果.将其应用于实际航磁数据的解释,获得了岩脉的大致分布特征.     

Interpretation of gravity anomalies of dykes by pseudo-magnetic transformation

Summary A simple transformation called pseudo-magnetic transformation is introduced in which the horizontal derivatives of the observed gravity anomalies of any body will be calculated. These horizontal derivatives, called the pseudo-magnetic anomalies, are proportional to the vertical magnetic anomalies of the same body assumed to be polarised horizontally. It is shown that the interpretation of gravity anomalies of dykes is not unique except by computing the pseudo-magnetic anomalies, which will be interpreted by the logarithmic curve fitting technique indicated earlier [5]²).     

基于磁异常的边界特征增强方法对比研究

基于磁异常进行场源边界识别(如断裂划分)是磁法勘探解释工作的重要内容.由于受到斜磁化、场源埋深、异常叠加以及噪声等多种因素的影响,基于磁异常直接进行场源的边界识别往往难以准确识别出场源体的边界,所以,人们研究了各种场源边界特征增强的方法技术,以便于进行场源边界的识别工作.近年来,国内外出现了很多新的边界特征增强方法,但有些方法的边界特征增强效果与已有方法相近,有的方法效果有待深化检验,存在验证方法的理论模型过于简单,未全面细致地分析出它们的优缺点等问题.针对上述情况,本文运用多种复杂、贴近实际的理论模型,深入对比分析了包括解析信号模法和倾斜角法等在内的15种具有代表性的边界增强方法,分析了它们在不同磁化方向、不同场源埋深、不同场源形态、异常叠加和噪声干扰等多种因素影响下的边界增强效果,重点归纳总结出它们的优缺点和适用条件,以为实际应用时的方法选择提供参考.通过模型实验,我们认为解析信号模垂向导数法、总水平导数法、解析信号模倾斜角法是在多种因素影响下,适用性较强、应用效果较好的三种方法.     

空间域滤波在求二维重磁接触面参数中的应用

1975年、1976年格林和斯坦利发表了求二维接触面参数的一个方法。本文试图在他们工作的基础上,用空间域滤波来代替格林和斯特利用差商求水平导数和用希尔伯特变换求垂直导数的方法。文中给出了应用空间域滤波计算重、磁异常水平和垂直导数的方法。模型试验证实了理论上的推断。与差商和希尔伯特变换方法相比,优点在于它既能同时滤掉高频干扰,又能使求得的垂直导数和水平导数具有相同的精度,进而提高反演结果的精确度。