Gravity data constitute a crucial dataset for probing the deep crustal structure and tectonic evolution of the South China Sea (SCS). Gravity inversion can effectively estimate the depth of Moho interface and crustal thickness. However, the inversion of the Moho interface in the SCS faces significant challenges, primarily due to the complex geological configurations where diverse crustal types coexist. In such settings, the density contrast between the crust and mantle exhibits three-dimensional (3D) variations, characterized by a vertical gradient and horizontal abrupt changes. To address these challenges, we introduce a strategy that employs an enhanced variable-density model, which is composed of multiple polynomials to delineate the complex varying crustal compositions, in conjunction with seismic data as constraints for the inversion of Moho depths. The efficacy of this approach is demonstrated through a synthetic example and its application to the Moho interface inversion in the SCS. The inverted Moho depth, derived crustal thickness, and stretching factor provide valuable insights into the interpretation of major tectonics in the SCS.

The eastern and southwestern sub-basins of the South China Sea (SCS) display starkly contrasting magnetic lineation patterns, yet quantitative 3-D mapping of the subsurface magnetic architecture—essential for deciphering basin evolution—remains challenging due to the dominance of remanent magnetization. We introduce a joint workflow that integrates anomaly separation with Magnetization-Vector Clustering Inversion (MVCI) to resolve this challenge. A low-rank Hankel matrix filter first disentangles co-located seamount and stripe anomalies in the ocean basin; each component is then inverted using MVCI to recover 3-D magnetization intensity and direction without prior orientation constraints, while simultaneously deriving cluster statistics. Synthetic tests replicating the SCS crustal setting demonstrate that seamount-signal removal dramatically enhances inversion fidelity for both anomaly sources. Application to the SCS reveals two distinct vector clusters in the eastern sub-basin, with mean declinations indicating 10–24° counter-clockwise rotation relative to the southwestern sub-basin. Magnetization intensities are slightly stronger in the southwestern sub-basin, where NE-trending magnetic stripes exhibit narrow spacing, whereas the eastern sub-basin shows wider and more variable NE–W to E–W trending stripes. This study provides the first basin-scale quantification of along-strike magnetic heterogeneity, offering new quantitative constraints on late-stage seafloor spreading and the dynamic evolution of the SCS, while delivering a robust, transferable methodology for other remanence-dominated marginal seas.

The identification of igneous rock in sedimentary basins serves as the basis for the exploration of igneous oil and gas reservoirs. The implementation of magnetic exploration in the identification and delineation of igneous rock can often achieve good results. However, when igneous rock and deep magnetic layers are under the influence of remanence, the reduction to the pole of magnetic anomaly and conventional magnetic inversion methods, which require clear magnetization directions, is limited, and special magnetic anomaly processing and inversion methods are necessary. We present a case study on igneous rock imaging through a strategy involving the joint use of a preferential filtering method and amplitude inversion affected by remanence in the Qikou depression in China. We first extract the weak anomalies of igneous rock from the observed total-field anomaly via preferential filtering and calculate their amplitude data. We then perform amplitude inversion to determine the underground three-dimensional magnetism distribution and propose a reasonable interpretation by combining seismic and other data. This work demonstrates the feasibility and effectiveness of the above strategy in delineating the igneous rock buried deep in sedimentary basins.

In mineral exploration, the ores with different magnetic susceptibility can be well delineated by physical property inversion of magnetic data. However, further wide applications of the magnetic inversion method have been seriously restricted by the existence of strong remanence and the computation of large-scale datasets. To solve these two problems, this study innovatively presents an efficient magnetic inversion algorithm in the presence of strong remanence. The weighted data misfit term and regularization term were used to establish the objective function. The detailed related formulas were derived and presented. For numerical computation, the Taylor series approximation was introduced in the objective function to linearize the relationship between the transformed magnetic data and magnetic susceptibility. The objective function was converted to a general Tikhonov form by several matrix transformations. To realize computational efficiency, a fast randomized algorithm was utilized to efficiently solve the Tikhonov problem and to efficiently determine the suitable regularization parameter along with the generalized cross-validation (GCV) method. Comparative tests on synthetic example show the effectiveness and efficiency of the proposed algorithm. The proposed efficient algorithm is successfully applied to two sets of real magnetic data, and the inversion results are verified.

Magnetic data are sensitive to both the induced magnetization in rock units caused by the present earth's magnetic field and the remanent magnetization acquired by rock units in past geologic time. Susceptibility is a direct indicator of the magnetic mineral content, whereas remanent magnetization carries information about the formation process and subsequent structural movement of geologic units. The ability to recover and use total magnetization, defined as the vectorial sum of the induced and remanent magnetization, therefore enables us to take full advantage of magnetic data. The exploration geophysics community has achieved significant advances in inverting magnetic data affected by remanent magnetization. It is now feasible to invert any magnetic data set for total magnetization. We provide an overview of the state of the art in magnetization inversion and demonstrate the informational value of inverted magnetization through a set of case studies from mineral exploration problems. We focus on the methods that recover either the magnitude of the total magnetization or the total magnetization vector itself.

The Dapai polymetallic mining region, located in the south of the Datian-Longyan subdepression belt in southwestern Fujian province, and on the west side of Zhenghe-Dapu deep fault zone, is considered as one of the largest polymetallic reserves in Fujian province. As exploration advances, finding the deeper and concealed deposits is urgently needed in this place and its adjacent areas. However, complex terrain and dense vegetation in Dapai deposit bring difficulties to geophysical investigations, especially for high-precision and large-scale surveys. Therefore, the understandings of the concealed ore-controlling structures, ore-bearing horizons and the distribution of igneous rocks are far from meeting the needs of deep prospecting. Under the guidance of metallogenic theory and prospecting model, the reflection seismic characteristics of the shallow thrust nappe ore-controlling structures and ore-bearing horizons have been extracted by using the reflection seismic method, 2D inversion of controlled source audio frequency magnetotelluric method (CSAMT) data with terrain and 3D constrained inversion of aeromagnetic data. In addition, the seismic and electromagnetic characteristics of the Yanshanian igneous rocks that are closely associated with mineralization, and the aeromagnetic anomaly characteristics of the ore-controlling structures and the plane distribution of the concealed rock masses have also been obtained. Thereafter, a geological model, which contains shallow nappe structures and distribution of ore controlling strata, in a main section of this region have been established with the constraints from the geology and the physical properties of known drilling cores. Moreover, 6 major faults, 16 secondary faults and 3 large concealed rock masses have been deduced. All these results provide effective geophysical response characteristics for further understanding the genesis and distribution of ore deposits in the study area and guide prospecting in the periphery and deep spaces in this area.

Apparent density mapping is a technique to invert the gravity anomaly for obtaining the density distribution in the subsurface layer. The conventional approach for apparent density mapping usually assumes that the top and bottom interfaces of the layer are flat and horizontal, which is not often the case in the real world. We have developed a wavenumber-domain iterative approach for mapping a density distribution of the undulant layer, whose top and bottom interfaces could be a variable relief. The wavenumber-domain approach requires that the observational surface of the gravity data is flat and horizontal. The wavenumber-domain equations for forward calculation of the gravity anomaly due to the undulant layer and its inversion are presented. A damping factor is adopted in the inversion equation for suppressing high-frequency noise in the data and stabilizing the mapping. We use an iterative algorithm for mapping to minimize the difference between the observed and calculated gravity anomalies and to optimize the density values within the layer. The presented approach is simple, fast, stable, and easy to operate. A test on the synthetic data and the real data from the Xuefengshan tectonic zone in central South China verified the feasibility of our approach with higher accuracy than the conventional wavenumber-domain mapping approaches and higher efficiency than the conventional space-domain mapping approach.

PreviousNext No AccessInternational Geophysical Conference, Qingdao, China, 17-20 April 2017The Identification of Magnetic Stripes: Corrected Age of Seafloor Spreading in the South China Sea BasinAuthors: Guanxin Wang*Xiaohong MengChunxiao XiuLianghui GuoShuling LiGuanxin Wang*Key Laboratory of Geo-Detection (China University of Geosciences, Beijing), Ministry of Education, Beijing, and School of Geophysics and Information Technology, China University of Geosciences, BeijingSearch for more papers by this author, Xiaohong MengKey Laboratory of Geo-Detection (China University of Geosciences, Beijing), Ministry of Education, Beijing, and School of Geophysics and Information Technology, China University of Geosciences, BeijingSearch for more papers by this author, Chunxiao XiuKey Laboratory of Geo-Detection (China University of Geosciences, Beijing), Ministry of Education, Beijing, and School of Geophysics and Information Technology, China University of Geosciences, BeijingSearch for more papers by this author, Lianghui GuoKey Laboratory of Geo-Detection (China University of Geosciences, Beijing), Ministry of Education, Beijing, and School of Geophysics and Information Technology, China University of Geosciences, BeijingSearch for more papers by this author, and Shuling LiKey Laboratory of Geo-Detection (China University of Geosciences, Beijing), Ministry of Education, Beijing, and School of Geophysics and Information Technology, China University of Geosciences, BeijingSearch for more papers by this authorhttps://doi.org/10.1190/IGC2017-236 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract It's well known that the South China Sea has been traditionally divided into three subunits, the central, northwestern and southwestern sub-basins. Yet some divergences persist regarding the age of seafloor spreading and formation of the South China Sea due to its complexity of geological structure. Here we reviewed previous geophysical data and added new considerations into modeling, such as: the short-wavelength noise, the long-wavelength background, the topography, the thickness of seafloor sediments and the mixed areas between two inversely magnetized blocks, which led us to revise the age of seafloor spreading in the South China Sea Basin. Our magnetic anomaly model was adapted to include features such as the magnetization and thickness of the source layer, different spreading rates, asymmetric spreading and axial jumps using the Modmag program implementing the CK95 geomagnetic reversal time scale. Based on the research, we proposed a two-step spreading episodes of the South China Sea Basin. The spreading age for the entire basin ranges from 31.4 Ma to 16 Ma with spreading rates of 30–80 km/Ma. The central sub-basin has a similar range of extension ages to that of the entire basin. The early spreading episode of the central sub-basin lasted from 31.4 Ma to 25.2 Ma, whereas the later stage began at 25.2 Ma during or after the spreading ridge centered at Lat.17°N jumped to 15°N with an abrupt drop in spreading rates. Spreading of the southwestern sub-basin, which experienced a similar tectonic process to that of the middle part of central sub-basin, underwent late-stage seafloor extension from 24.2 to 16 Ma. Spreading of the South China Sea basin abruptly ceased around 16 Ma. Keywords: magnetics, seafloor, Asia, marine, geologyPermalink: https://doi.org/10.1190/IGC2017-236FiguresReferencesRelatedDetails International Geophysical Conference, Qingdao, China, 17-20 April 2017ISSN (online):2159-6832Copyright: 2017 Pages: 1525 publication data© 2017 Published in electronic format with permission by the Society of Exploration Geophysicists and Chinese Geophysical SocietyPublisher:Society of Exploration Geophysicists HistoryPublished Online: 31 May 2017 CITATION INFORMATION Guanxin Wang*, Xiaohong Meng, Chunxiao Xiu, Lianghui Guo, and Shuling Li, (2017), "The Identification of Magnetic Stripes: Corrected Age of Seafloor Spreading in the South China Sea Basin," SEG Global Meeting Abstracts : 935-938. https://doi.org/10.1190/IGC2017-236 Plain-Language Summary KeywordsmagneticsseafloorAsiamarinegeologyPDF DownloadLoading ...

PreviousNext No AccessInternational Workshop and Gravity, Electrical & Magnetic Methods and their Applications, Chenghu, China, 19-22 April 2015Application of amplitude inversion in identification of igneous rocks in a superimposed basinAuthors: Shuling LiXiaohong MengYaoguo LiShuling LiKey Laboratory of Geo-detection (China University of Geosciences, Beijing), Ministry of EducationSearch for more papers by this author, Xiaohong MengKey Laboratory of Geo-detection (China University of Geosciences, Beijing), Ministry of EducationSearch for more papers by this author, and Yaoguo LiCenter for Gravity, Electrical, and Magnetic Studies, Department of Geophysics, Colorado School of MinesSearch for more papers by this authorhttps://doi.org/10.1190/GEM2015-047 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract We present a case study on the use of magnetic amplitude inversion for identification of igneous rocks that are buried deep in a superimposed sedimentary basin and have remanent magnetization. To tackle the difficulty due to the presence of the remanent magnetization in igneous rocks and weak anomalies due to large source depths in the basin, we combine the use of the preferential continuation for magnetic anomaly separation with magnetic amplitude inversion. The resultant distribution of effective susceptibility not only correlates well with the locations of Cenozoic igneous rocks known previously through drilling and seismic imaging, but also identifies the large scale distribution of Mesozoic igneous rocks at greater depth in the west of the basin. Keywords: magnetic, amplitutde, inversion, 3DPermalink: https://doi.org/10.1190/GEM2015-047FiguresReferencesRelatedDetails International Workshop and Gravity, Electrical & Magnetic Methods and their Applications, Chenghu, China, 19-22 April 2015ISSN (online):2159-6832Copyright: 2015 Pages: 520 publication data© 2015 Published in electronic format with permission by the Society of Exploration Geophysicists and the Chinese Geophysical SocietyPublisher:Society of Exploration Geophysicists HistoryPublished Online: 22 Apr 2015 CITATION INFORMATION Shuling Li, Xiaohong Meng, and Yaoguo Li, (2015), "Application of amplitude inversion in identification of igneous rocks in a superimposed basin," SEG Global Meeting Abstracts : 181-184. https://doi.org/10.1190/GEM2015-047 Plain-Language Summary Keywordsmagneticamplitutdeinversion3DPDF DownloadLoading ...

Directly interpreting total-field magnetic anomaly data in the South China Sea (SCS) can be difficult because of the complex patterns associated with low-latitude anomaly projection and the presence of remanent magnetization. Additional difficulty arises from the fact that the ambient field direction, thus, the total-field anomaly projection direction, varies over a wide range in the area. To alleviate these difficulties, we present a strategy by using magnetic amplitude data analyses and inversion. Equivalent source processing is used to calculate the amplitude data in the space domain since the wavenumber-domain method is no longer applicable due to low and highly variable inclination. The amplitude data serve the role of reduction-to-pole (RTP) transformation for structural interpretation. We then carry out the amplitude inversion to generate a 3D subsurface distribution of effective susceptibility. The inversion results show that this approach is feasible and effective in SCS.

The basin structure and distribution of tectonic units in the Huanghua depression are affected and controlled by deep faults and the basement. These features can be revealed on large scales using gravity and magnetic anomalies. However, the magnetic data processing and interpretation are faced with challenges from macroscopic magnetic differences of the basement and remanence effects present in ignores rocks and the basement. To tackle these challenges, and to develop a new methodology for tectonic structure interpretation from magnetic data when the RTP (reduction to the pole) is not applicable, we present a study on gravity and magnetic amplitude data which is only weakly dependent on the direction of magnetization to delineate traversal tectonic transform zones and analyze trichotomous structure of the basement in the Huanghua depression. The results show that five NW-SE trending tectonic transform zones traverse at high angles to dominate geological structures and "T-shaped" intersection of two hidden deep faults may be the suture zone between three types of basements in this area. These characteristics reveal that a basin structure is featured macroscopically by east-west zoning and north-south partitioning, and that the distribution of the sedimentary tectonic units is controlled by the differences of basement properties and regional tectonic transform zones.

We study the inversion of magnetic data acquired over a rugged observation surface and where the buried source bodies have strong remanent magnetization that leads to unknown total magnetization directions. These factors pose significant challenges for processing and inversion of such data. To tackle the challenges from both a rugged observation surface and an unknown magnetization direction, we propose a strategy through the joint use of the equivalent source technique and 3D amplitude inversion to obtain 3D magnetization strength. We use equivalent source processing to calculate the amplitude data in the space domain because the use of the wavenumber-domain method is invalid due to large variations in the data elevation. We then carried out an amplitude inversion to generate a 3D subsurface distribution of the magnitude of the total magnetization vector. The results from a synthetic example and aeromagnetic data in Daye Mine in China showed that this approach is effective and images the magnetic units whose contact zones with the limestone country rock host the mineralization. The method is general and can be applied to a variety of cases with similar challenges.

We present the preferential filtering method for gravity anomaly separation based on Green equivalent-layer concept and Wiener filter. Compared to the conventional upward continuation and the preferential continuation, the preferential filtering method has the advantage of no requirement of continuation height. The method was tested both on the synthetic gravity data of a model of multiple rectangular prisms and on the real gravity data from a magnetite area in Jilin Province, China. The results show that the preferential filtering method produced better separation of gravity anomaly than both the conventional low-pass filtering and the upward continuation.

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2012Inversion of magnetic anomaly affected by strong remanent magnetization over rugged terrain: A case study from Daye, ChinaAuthors: Shu-Ling LiYaoguo LiShu-Ling LiKey Laboratory of Geo-detection, Ministry of Education, China University of Geosciences (Beijing)Center for Gravity, Electrical and Magnetic Studies, Department of Geophysics, Colorado School of MinesSearch for more papers by this author and Yaoguo LiCenter for Gravity, Electrical and Magnetic Studies, Department of Geophysics, Colorado School of MinesSearch for more papers by this authorhttps://doi.org/10.1190/segam2012-0515.1 SectionsSupplemental MaterialAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract We present a case study on inverting magnetic data acquired over rugged terrain and influenced by strong remanent magnetization. These factors pose significant challenges for processing and inversion of such data. To tackle these challenges, we propose a strategy through the joint use of the equivalent source technique and 3D amplitude inversion to perform 3D magnetization inversions. Equivalent source processing is used to calculate the amplitude data in the space domain since use of the wavenumber-domain is difficult due to large elevation relief. We then carry out the amplitude inversion to generate a 3D subsurface distribution of effective susceptibility when strong remanent magnetization is present and the general magnetization direction cannot be estimated. The inversion results of aeromagnetic data in Daye Mine in China show that this approach is feasible and effective. Permalink: https://doi.org/10.1190/segam2012-0515.1FiguresReferencesRelatedDetailsCited ByQuantifying the error level in computed magnetic amplitude data for 3D magnetization inversionCamriel Coleman and Yaoguo Li21 September 2018 | GEOPHYSICS, Vol. 83, No. 5 SEG Technical Program Expanded Abstracts 2012ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2012 Pages: 4609 Publisher:Society of Exploration Geophysicists HistoryPublished: 25 Oct 2012 CITATION INFORMATION Shu-Ling Li and Yaoguo Li, (2012), "Inversion of magnetic anomaly affected by strong remanent magnetization over rugged terrain: A case study from Daye, China," SEG Technical Program Expanded Abstracts : 1-5. https://doi.org/10.1190/segam2012-0515.1 Plain-Language Summary PDF DownloadLoading ...

Understanding the continental margin of the Northeastern South China Sea is critical to the study of deep structures, tectonic evolution, and dynamics of the region. One set of important data for this endeavor is the total-field magnetic data. Given the challenges associated with the magnetic data at low latitudes and with remanent magnetism in this area, we combine the equivalent-source technique and magnetic amplitude inversion to recover 3D subsurface magnetic structures. The inversion results show that this area is characterized by a north-south block division and east-west zonation. Magnetic regions strike in EW, NE and NW direction and are consistent with major tectonic trends in the region. The highly magnetic zone recovered from inversion in the continental margin differs visibly from that of the magnetically quiet zones to the south. The magnetic anomaly zone strikes in NE direction, covering an area of about 500 km × 60 km, and extending downward to a depth of 25 km or more. In combination with other geophysical data, we suggest that this strongly magnetic zone was produced by deep underplating of magma associated with plate subduction in Mesozoic period. The magnetically quiet zone in the south is an EW trending unit underlain by broad and gentle magnetic layers of lower crust. Its magnetic structure bears a clear resemblance to oceanic crust, assumed to be related to the presence of ancient oceanic crust there.

Gravity anomalies can be used to infer the deep crustal structure in the South China Sea(SCS) where seismic data are limited to some specific points or profiles.This paper presents a comprehensive study of crustal structure by using gravity inversion for interface with seismic data constraints to invert depth of the Moho and to calculate crustal thickness,and by performing gravity anomalies correlative imaging method without constraints to construct the 3D subsurface distribution of equivalent density in the South China Sea.We also focus on the regional linear features of gravity anomalies and jointly use the continuation,horizontal gradient and linear feature enhancement filter to highlight the linear tectonics which always involve the density changes in horizontal,such as major deep faults,boundary of continental-oceanic crust,spreading axes of oceanic basin.The integrated geophysical analysis along the Guangzhou-Palawan geosciences transect demonstrates that the characteristics of the gravity anomalies and their inversion results allow us to reveal crustal structure and regional tectonics features in the large scale of the South China Sea.

Wavelet analysis in the current mathematics,physics field and geological field,is a fast-growing analytical tool,it also possesses the theoretical profundity and extensive applicability.Because of its excellent local time-frequency analysis and multi-analysis,wavelet transform has been used well in the field of geophysical problems.This paper comprehensively introduces wavelet theory,developing history,and its application in Chinese geophysical research progress.On the basis of geophysical fields of wavelet analysis We analysis the existing problems and further prospects of applications.