Continuous wavelet transform,theoretical aspects and application to aeromagnetic data at the Huanghua Depression,Dagang Oilfield,China

We use the continuous wavelet transform based on complex Morlet wavelets, which has been developed to estimate the source distribution of potential fields. For magnetic anomalies of adjacent sources, they always superimpose upon each other in space and wavenumber, making the identification of magnetic sources problematic. Therefore, a scale normalization factor, a^?n, is introduced on the wavelet coefficients to improve resolution in the scalogram. By theoretical modelling, we set up an approximate linear relationship between the pseudo‐wavenumber and source depth. The influences of background field, random noise and magnetization inclination on the continuous wavelet transform of magnetic anomalies are also discussed and compared with the short‐time Fourier transform results. Synthetic examples indicate that the regional trend has little effect on our method, while the influence of random noise is mainly imposed on shallower sources with higher wavenumbers. The source horizontal position will be affected by the change of magnetization direction, whereas the source depth remains unchanged. After discussing the performance of our method by showing the results of various synthetic tests, we use this method on the aeromagnetic data of the Huanghua depression in central China to define the distribution of volcanic rocks. The spectrum slices in different scales are used to determine horizontal positions of volcanic rocks and their source depths are estimated from the modulus maxima of complex coefficients, which is in good accordance with drilling results.     

A geophysical approach to the granite batholith under the eastern Southern Uplands,Scotland

From our interpretation of the Bouguer gravity and aeromagnetic anomalies in south-east Scotland, we conclude that a massive granite batholith underlies the greater part of the eastern Southern Uplands. The granite model which we computed earlier from gravity anomalies in the Tweeddale area fits the observed magnetic anomalies closely, if a normal magnetization of 0.095 A m^–1 is assigned, similar to values found for exposed local granites. Further gravity modelling shows that, apart from the Tweeddale boss, the granite shallows to less than 1 km near Lammer Law in East Lothian and extends north of the Lammermuir Fault. A model for the East Lothian volcanics was computed from their aeromagnetic anomalies, then their gravitational effect was combined with that estimated for the Devonian and Carboniferous sediments and the result stripped off the observed gravity field. The residual gravity anomalies were used to generate a two-dimensional model for the granite north of the Lammermuir Fault. The expected tectonic consequences of a massive granite batholith in the eastern Southern Uplands are compared with the known development of faults and sedimentary basins around its margins.     

THE INTERPRETATION OF GRAVITATIONAL AND MAGNETIC DATA FOR THE VOLCANIC ROCKS DISTRIBUTION IN KE-BAI FAULT ZONE,JUNGGAR BASIN

This paper firstly discusses the feasibility of delineating the volcanic rocks distribution by gravitational and magnetic methods on the basis of the statistical results of the magnetic susceptibility and density of the different lithologies in the study area. After the separation of gravitational and magnetic fields by bandpass filter, we determined the residual gravitational and magnetic anomalies caused by volcanic rocks. The results of potential field separation show that the residual anomalies are in beaded NE-directed distribution. In the meantime, the boundary enhancement techniques such as horizontal total gradient and vertical derivative are employed to delineate the distribution of the faults, as the result, fifteen faults including four NE-striking main faults which appear as an arc protruding to the southeast and other secondary faults are distinguished. Furthermore, two fault systems with deep and superficial attribution are revealed from the inversed section of the telluric electromagnetic sounding, and their properties, characteristics and roles in the volcanic activity are fully discussed by combining with the tectonic background. Based on the comprehensive analysis of the correlation between the distribution characteristics of the residual gravitational and magnetic anomalies and the location of the faults, three volcanic activity zones are reasonably delineated. Finally, we carry out the inversion of apparent density by taking advantage of the residual gravitational anomalies and acquire the apparent density anomalies of the top part of Paleozoic. Integrated with the apparent density anomalies, the lithologies exposed from the boreholes, the observed density data of different rocks and the residual gravitational-magnetic anomalies, we elaborately delineate the lithologies of the top of Paleozoic and further define the distribution range and the lithology combination of Kekesayi group of Middle Ordovician, Tailegula group of Lower Carboniferous and Jiamuhe group of Lower Permian.     

Detection of potential fields source boundaries by enhanced horizontal derivative method

A high‐resolution method to image the horizontal boundaries of gravity and magnetic sources is presented (the enhanced horizontal derivative (EHD) method). The EHD is formed by taking the horizontal derivative of a sum of vertical derivatives of increasing order. The location of EHD maxima is used to outline the source boundaries. While for gravity anomalies the method can be applied immediately, magnetic anomalies should be previously reduced to the pole. We found that working on reduced‐to‐the‐pole magnetic anomalies leads to better results than those obtainable by working on magnetic anomalies in dipolar form, even when the magnetization direction parameters are not well estimated. This is confirmed also for other popular methods used to estimate the horizontal location of potential fields source boundaries. The EHD method is highly flexible, and different conditions of signal‐to‐noise ratios and depths‐to‐source can be treated by an appropriate selection of the terms of the summation. A strategy to perform high‐order vertical derivatives is also suggested. This involves both frequency‐ and space‐domain transformations and gives more stable results than the usual Fourier method. The high resolution of the EHD method is demonstrated on a number of synthetic gravity and magnetic fields due to isolated as well as to interfering deep‐seated prismatic sources. The resolving power of this method was tested also by comparing the results with those obtained by another high‐resolution method based on the analytic signal. The success of the EHD method in the definition of the source boundary is due to the fact that it conveys efficiently all the different boundary information contained in any single term of the sum. Application to a magnetic data set of a volcanic area in southern Italy helped to define the probable boundaries of a calderic collapse, marked by a number of magmatic intrusions. Previous interpretations of gravity and magnetic fields suggested a subcircular shape for this caldera, the boundaries of which are imaged with better detail using the EHD method.     

给定观测精度下的点(线)源模型重力与磁力垂向识别能力研究

重、磁勘探具有效率高、成本低、工作范围广等优点,已在地球物理勘探中得到了广泛应用.前人大多在不考虑重、磁勘探观测精度的条件下进行了垂向识别能力的研究,但在考虑重、磁观测精度条件下,重力（重力异常、重力张量）与磁力（磁力异常、磁力三分量、磁力张量）对孤立异常的垂向识别能力如何则需要进行深入的理论研究.本文从重、磁场正演理论出发,以球体（点源模型）和无限延伸水平圆柱体（线源模型）为例,考虑给定观测精度条件下,以重力和磁力幅值大小与观测精度的关系来研究垂向识别能力,从而消除了背景场的影响,提高了研究结果的可靠度.通过研究表明,对于孤立异常,重力张量在浅部一定深度内比重力异常的垂向识别能力强,该深度与重力异常和重力张量观测精度的比值成正比;垂直磁化磁力张量在浅部一定深度内比化极磁力异常的垂向识别能力强,该深度与磁力异常与磁力张量观测精度的比值成正比;磁力在浅部一定深度内比重力的垂向识别能力强,该深度与地质体的磁化强度和剩余密度比值、重力观测精度和磁力观测精度比值成正比.通过重力和磁力垂向识别能力的研究将为重、磁勘探的实际应用起到指导作用.     

Characteristics of gravity anomalies and prediction of volcanosedimentary boron deposit distribution in the Xiongba area,Tibet

Volcanosedimentary boron deposits are present within Tertiary lacustrine sediments and volcanic rocks in Xiongba, Tibet. Boron deposits are characterized by low density relative to country rocks; thus, it is possible to locate them by gravity measurements. We conducted a 1:50000 high-precision gravity survey in the Xiongba area, Tibet, and obtained the Bouguer and residual gravity anomalies. We analyzed fault systems and the distribution of sedimentary and volcanic rocks and their relation to the volcanosedimentary boron deposits. The processing of the gravity data revealed local gravity variations and fault structures. We applied preferential downward continuation and wavelet transform to the gravity data, and in conjunction with geological data, we predicted the distribution of volcanosedimentary boron deposits.     

Localized cultural denoising of high-resolution aeromagnetic data

We show how a denoising technique based on the wavelet transform can be used to deal with localized noise related to DC electrified railway lines. This method, which performs localized and sharp filtering of cultural noise, was applied to high‐resolution aeromagnetic data acquired in the Phlegrean volcanic area, southern Italy, in 1999 and 2001. The helicopter‐borne survey was aimed at giving new detailed insights into the distribution of the magnetization of the area and, therefore, into the volcanological characteristics of the region. The surveyed area is characterized by the presence of towns, buildings and DC electrified railway lines whose magnetic effects influenced the measurements and were responsible for some of the measured anomalies. This cultural noise has, therefore, to be minimized as much as possible in order to allow the data to be interpreted accurately. Due to the excellent space‐scale localization properties of the discrete wavelet transform, the cultural disturbance was removed very precisely, leaving the field in the adjacent areas unchanged.     

Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties

The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40-60 mGal (10⁻⁵m/s²) and 800-1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE-SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10⁻² SI units with an average Koenigsberger ratio (Q_n) of almost one and average direction of remanent magnetization of D=147.4° and I=+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m³, respectively and susceptibility of 3.14×10⁻² SI units with a Q_n ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D=337° and 340° and I=−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs.     

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.     

Satellite and surface geophysical expression of anomalous crustal structure in Kentucky and Tennessee

An equivalent layer magnetization model obtained from inversion of long-wavelength satellite magnetic anomaly data indicates a very magnetic source region centered in south central Kentucky. The magnetization maximum nearly coincides with a gravity high elongated north-south and extending into Tennessee. Previous refraction profiles suggest that the source of the gravity anomaly is a large mass of rock occupying much of the crustal thickness. The outline of the source delineated by gravity contours is also discernible in aeromagnetic anomaly patterns. Taken together, the geophysical data suggest a large, localized mass of intracrustal rock which is both dense and very magnetic. A simple magnetization/density model is given which accounts for the gravity and long-wavelength aeromagnetic anomalies due to the body. We interpret it as a mafic plutonic complex, and several lines of evidence are consistent with a rift association. The body is, however, clearly related to the inferred position of the Grenville Front. It is bounded on the north by the fault zones of the 38th Parallel Lineament. The inferred mean magnetization (4 A/m) of the body is large, but not inconsistent with values reported by others for deep crustal bodies associated with long-wavelength magnetic anomalies. Such magnetization levels can be achieved with magnetic mineralogies produced by normal oxidation and metamorphic processes and enhanced by viscous build-up, especially in mafic rocks of alkaline character.     

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.     

南海北部磁异常特征及对前新生代构造的指示

为了研究南海北部前新生代构造,利用新近的船载磁力测量数据,对磁异常进行变纬度化极,并反演计算视磁化强度和磁源重力异常,以及对三条OBS剖面进行重磁拟合.结果认为东沙隆起高磁异常带是浙闽沿海火山岩带向西的延续,其间被NW向古老的转换边界断裂F10错断;NE向的F2断裂是高磁异常带的南界,并限制了底侵活动的北界;F3断裂在...     

Review paper: Exploration geophysics for intrusion‐hosted rare metals

Igneous intrusions, notably carbonatitic–alkalic intrusions, peralkaline intrusions, and pegmatites, represent significant sources of rare‐earth metals. Geophysical exploration for and of such intrusions has met with considerable success. Examples of the application of the gravity, magnetic, and radiometric methods in the search for rare metals are presented and described. Ground gravity surveys defining small positive gravity anomalies helped outline the shape and depth of the Nechalacho (formerly Lake) deposit within the Blatchford Lake alkaline complex, Northwest Territories, and of spodumene‐rich mineralization associated with the Tanco deposit, Manitoba, within the hosting Tanco pegmatite. Based on density considerations, the bastnaesite‐bearing main ore body within the Mountain Pass carbonatite, California, should produce a gravity high similar in amplitude to those associated with the Nechalacho and Tanco deposits. Gravity also has utility in modelling hosting carbonatite intrusions, such as the Mount Weld intrusion, Western Australia, and Elk Creek intrusion, Nebraska. The magnetic method is probably the most successful geophysical technique for locating carbonatitic–alkalic host intrusions, which are typically characterized by intense positive, circular to sub‐circular, crescentic, or annular anomalies. Intrusions found by this technique include the Mount Weld carbonatite and the Misery Lake alkali complex, Quebec. Two potential carbonatitic–alkalic intrusions are proposed in the Grenville Province of Eastern Quebec, where application of an automatic technique to locate circular magnetic anomalies identified several examples. Two in particular displayed strong similarities in magnetic pattern to anomalies accompanying known carbonatitic or alkalic intrusions hosting rare‐metal mineralization and are proposed to have a similar origin. Discovery of carbonatitic–alkalic hosts of rare metals has also been achieved by the radiometric method. The Thor Lake group of rare‐earth metal deposits, which includes the Nechalacho deposit, were found by follow‐up investigations of strong equivalent thorium and uranium peaks defined by an airborne survey. Prominent linear radiometric anomalies associated with glacial till in the Canadian Shield have provided vectors based on ice flow directions to source intrusions. The Allan Lake carbonatite in the Grenville Province of Ontario is one such intrusion found by this method. Although not discovered by its radiometric characteristics, the Strange Lake alkali intrusion on the Quebec–Labrador border is associated with prominent linear thorium and uranium anomalies extending at least 50 km down ice from the intrusion. Radiometric exploration of rare metals hosted by pegmatites is evaluated through examination of radiometric signatures of peraluminous pegmatitic granites in the area of the Tanco pegmatite.     

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...     

龙门山及邻区重、磁异常特征及与地震关系的研究

本文通过研究龙门山构造带及邻区的区域重、磁场特征,以及龙门山断裂带的产状等特征,探讨其与地震关系.研究结果表明,龙门山断裂带是环绕青藏高原的重力梯度带的一部分.其对应密度分界面向西北方向倾斜,向下延深数十公里,切穿莫霍面.推测密度分界面分为两段,深部较陡的为岩石圈块体的边界,浅部较缓.基底隆起与凹陷的界线大体与大地构造单元的界线一致.由西部的岩石圈块体的边界至东部在地表的大地构造单元界线之间的距离约为40~50 km.隶属于中上地壳脆性变形层的地质体由岩石圈块体界线沿缓倾的密度界面推覆至地表的大地构造单元的界线处,在此过程中伴随岩层破碎,从而发生地震.龙门山构造带主要部分位于负磁异常区,这种反磁化和退磁的现象,可能与逆冲推覆作用所引起的深部岩层倒转有关.     

东北地区重磁场与地壳结构特征

分析了东北地区的重、磁场特征,同时对研究区的布格重力异常和航磁异常据进行了小波分析计算.根据分析与计算可知,东北地区重力场以北东走向为主,表现出该地区重力场的主要趋势.根据磁场的分布特征,可将研究区分为六个区域:呼和浩特以北磁场相对平静区;赤峰一带正负磁场交互变化区;海拉尔以南磁场缓变区;齐齐哈尔—依春磁场剧烈变化区;长春—沈阳负磁场平缓区;牡丹江—丹东磁场区.利用重力资料,应用调和级数法对研究区的莫霍界面进行了反演计算,得到了该地区莫霍界面深度分布.根据磁力资料采用遗传算法反演计算了研究区居里界面的深度分布.同时对研究区的地壳结构特征进行了探讨.     

How natural remanent magnetization of basaltic units can dominate the reduced to pole magnetic value: A case study from the Faroe Islands

Early attempts to utilize magnetic data to understand the volcanic and subvolcanic succession on the Faroese Continental Shelf have shown that conventional interpretation and modelling of magnetic data from this area leads to ambiguous results. Interpretation of the aeromagnetic data on the Faroese Continental Shelf shows that some previously identified basement highs coincide with reduced-to-pole magnetic highs, whereas others coincide with negative or mixed magnetic features. Similarly, igneous centres are characterized by different polarity magnetic anomalies. Palaeomagnetic analysis of the onshore volcanic succession has demonstrated that the thermoremanent magnetization of the basaltic lavas is stronger than the induced magnetism, and both reversely and normally magnetized units are present. We have tested this with 2½D profile modelling using the palaeomagnetic information to correlate high-amplitude magnetic anomalies with basalt successions containing changes in magnetic polarity. This approach has enabled us to map the termination of the differently magnetized units offshore and thereby extend the mapping of the Faroe Island Basalt Group on the Faroese Platform and into adjacent areas.     

南海北部陆架西区盆地地质、地球物理场特征及其深部结构

分析了南海北部陆架西区盆地的地质、地球物理场特征，计算了研究海域重、磁资料的1阶小波细节、4阶小波逼近变换。根据分析与计算可知，研究区的布格重力异常以北西低的负值，东南高的正值为特征。在东部及东南部异常等值线走向为北东；西部异常等值线以北西走向为特征；西北地区异常以北东东、北东走向的局部等值线圈闭为特征。磁场的展布十分复杂，按磁异常的变化程度可分为三个变化区，即磁异常平静区、剧变区及缓变区。磁异常的平静区位于研究区的西部，即莺歌海盆地所在位置，这一带磁异常等值线极为稀疏，异常值为负背景异常。剧变区位于海南岛，该地区的磁异常变化极为剧烈，异常特征以局部小圈闭为特征，等值线分布密集。磁异常的平缓区位于平静区及剧变区之外的其它地区。琼东南盆地、北部湾盆地的磁异常具有此特征。根据重、磁场资料以及南海北部盆地钻井取样的测试结果、同时参考穿越南海地学断面的结果，对研究区的地壳结构进行了反演计算，计算表明南海陆架盆地区域地壳结构较为复杂，研究区的地壳厚度在22－33km之间，总的趋势由陆向洋地壳厚度逐渐减薄，反映出该区域地壳具有陆壳、拉伸陆壳、过渡壳的性质，同时存在有上地幔隆起区及凹陷区。磁性底界面厚度在17－24km之间变化，其中在莺歌海盆地较深，在海南岛地区磁性界面较浅。     

Ancient volcanic glasses of the Urals

The existing data on findings of unaltered volcanic glasses in the Paleozoic (from Late Ordovician to Late Devonian inclusive) volcanic strata of the Ural fold belt are systematized. These glasses have compositions that correspond to tholeiitic basalts, potassic alkaline basaltoids, andesites, and rhyolites. Relic portions of glasses of cenotypal appearance are preserved in thick glassy crusts of pillow lava flows, in fragments among hyaloclastites, in bombs from tuffs, and in extrusive bodies and dykes. Chemical analysis showed that the amount of dissolved water was low (1–1.3 wt %) in the primary tholeiitic magma and higher (8–10 wt %) in the magma that formed island-arc hyalobasalts, potassic alkaline basaltoids, andesites and rhyolites.     

Application of the continuous wavelet transform of gravity and magnetic data to estimate sub‐basalt sediment thickness

The Continuous Wavelet Transform was recently proposed for the interpretation of gravity and magnetic potential data. We utilize the Continuous Wavelet Transform of gravity and magnetic data to address one of the most common issues in exploration geophysics: mapping of sub‐basaltic sedimentary strata. We observe that the magnetic response of the basaltic layer is dominant in a three‐layer case of a basalt‐sediment‐basement, whereas the gravity signal is dominated by the base of the sediment. Thus the Continuous Wavelet Transform of the magnetic data is related to the thickness of the basalt and the Continuous Wavelet Transform of the gravity data is related mostly to the bottom of the sediment. These observations are demonstrated with a synthetic model and a few field examples. Derived depths using Continuous Wavelet Transform are in good agreement with known vertical cross‐sections. Therefore, Continuous Wavelet Transform analysis of both gravity and magnetic data offers a possibility for primary information of sub‐basaltic sediment thickness, which can provide a basis for further detailed modelling.