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Understanding the Meaning of the Positive bouguer Anomaly of Waza (Northernmost Cameroon, Central Africa)

Journal of Geoscience and Environment Protection 2019年第6期7卷 55-65页

作者： Eyike Albert Basseka Charles Antoine Lordon Djieto Anatole Eugene Nguimbous-Kouoh Jean Jacques Zanga-Amougou Alain Tabod Charles Tabod Department of Physics Faculty of Science University of Douala Douala Cameroon Department of Earth Science Faculty of Science University of Douala Douala Cameroon Department of Geology Faculty of Science University of Buea Buea Cameroon Department of Gas and Petroleum Exploration Institute of Mines and Petroleum Industries University of Maroua Maroua Cameroon Department of Physics Faculty of Science University of Yaoundé 1 Yaoundé Cameroon

Gravity and magnetic data were analyzed in conjunction with available geological data to determine the origin of observed positive gravity anomaly at Waza and its possible relationship to mafic intrusions. The data analysis includes the construction of a bouguer gravity anomaly, isostatic residual gravity anomaly, total magnetic gradient and Euler solutions maps, and two and one-half dimensional gravity models. A positive gravity anomaly that occurs in Waza has amplitude of 35 mGal and can be separated into two parts. The top of the disturbing body causing the anomaly cannot be deeper than 4 km. We believe the anomaly is probably caused by a body of basaltic rocks lying at the upper surface of the Precambrian basement and originate from a self-propagating disturbance of magmatic loads on a pre-existing zone of lithospheric weakness. Melt migration might have been aided by tectonic and flexural stresses, such that the intrusion is limited in extent to the melting region of the plume and did not reach the surface. The model calculated to satisfy the observed anomaly consists of a 2.5 Km-thick slab underlain by a 10.5 km deep column. This model is interpreted to represent a sheet or saucer-like mafic intrusion that has a thick deep feeder. Detailed geophysical studies would be necessary to locate any possible economic occurrences.

关键词： Northern Cameroon Waza bouguer High Total Magnetic Gradient Basaltic Intrusion

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Structure and seismo-tectonic investigation of the Sar?g?l-Buldan region,Western Anatolia,by using gravity and seismicity data

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Journal of Mountain Science 2022年第10期19卷 2758-2775页

作者： ALTINOGLU Fatma Figen POLAT Gulten Department of Geological Engineering Faculty of EngineeringPamukkale UniversityDenizli 20017Turkey Department of Civil Engineering Yeditepe UniversityKayisdagi Cad 34755IstanbulTurkey

The parameter b(commonly referred to as the b-value)is one of the most significant seismic parameters to describe the seismicity of an investigated *** this study,the structural framework of the Sar?g?l-Buldan region located in the intersection area of the western Anatolian graben system in Turkey was investigated by using seismicity and gravity *** known,western Anatolia is one of the most seismically active regions of the Anatolian ***,seismic activity in this region is very *** of the bouguer gravity data enabled to define the shallow subsurface structure of the study *** from the gravity analysis indicated that the sediment basement depth varied from 0.1 km and 2.1 *** also detected many NW-SE,E-W,and NESW trending lineaments that may be faults or fractures,and the NW-SE trending the Denizli basin lies to the east of the Ala?ehir *** observed that the findings from this analysis seem compatible with the regional geological *** addition to this,the seismicity of the region was analyzed by using the frequency-magnitude distribution to find out the seismic hazard *** most useful way in the analysis of the seismic hazard studies is to reveal the location of the earthquake boundary,which produces devastating big seismic events because such studies make it possible to forecast the location of possible future earthquakes and improve seismic hazard *** b-value for this study region was estimated by using the maximum likelihood *** in the b-value were observed,which range from≈0.2 to 2.A higher b-value was detected in the Buldan horst and surrounding *** contrast to this,lower bvalues were observed in the northeast part of the interaction region between the Denizli and the Ala?ehir *** positive bouguer anomaly values as high as+10 m Gal and low b-values in the north-eastern part of the study region were interpreted as indicating a thinner crustal *** comparison,negative bouguer a

关键词： bouguer Gravity Seismicity Stress bvalue the Sar?g?l-Buldan region

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Precollisional tectonics and terrain amalgamation offshore southern Taiwan:Characterizations from reflection seismic and potential field data

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Science China Earth Sciences 2007年第6期50卷 897-908页

作者： LI ChunFeng ZHOU ZuYi LI JiaBiao CHEN HuanJiang GENG JianHua LI Hui State Key Laboratory of Marine Geology Tongji UniversityShanghai 200092China The Second Institute of Oceanography State Oceanic AdministrationHangzhou 310012China

Sponsored by the Chinese National Fundamental Research and Development Program in 2001,Guang-zhou Marine Geological Survey launched out a long geophysical survey from the northeastern part of the South China Sea (SCS),through the Luzon Arc,to the Huatung Basin and the Gagua Ridge. Based on high-resolution seismic data from this survey,combined with gravimetric and magnetic modeling,a systematic effort is made to the study of the regional geodynamics offshore southern Taiwan. By focusing particularly on precollisional tectonic interactions between adjacent geological units and their tectonic affiliations,this study can help reveal early arc-continent collisional processes that formed the Taiwan orogen. The construction of the Manila accretionary prism and its eastward progressive deformation indicate that the subduction of SCS have experienced multiple phases of increased activity. Active precollisional crustal shortening within the Northern Luzon Trough resulted in tilting of sedimentary layers at angles between 6° and 13°. But the shortening induced by tilting accounts for only a tiny part of regional total crustal compression. The eastern flank of the Luzon Arc appears to be more active than the rest,evidenced by active faulting and folding in the intra-arc basins on the eastern flank. Magnetic modeling/inversion shows that the Luzon Arc may have experienced multiple phases of magmatic activities,causing lateral magnetic inhomogeneity. bouguer gravity anomalies and gravity modeling indicate that the Huatung Basin has anomalously higher crustal and upper mantle densities than those of SCS and the Luzon Arc. In addition,there is a large bathymetric difference between the Huatung Basin and the northeastern part of SCS basin. These observations argue against early hypothesis that the Huatung Basin and the northeastern part of SCS basin may once have belonged to one single oceanic crust,in part or in whole. The Gagua Ridge,as a sliver of uplifted oceanic crust,may be r

关键词： precollisional tectonics Taiwan orogen Luzon Arc gravity and magnetic modeling bouguer gravity anomaly Huatung Basin

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Lineament characteristics using gravity data in the Garoua Zone,North Cameroon: Natural risks implications

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Earth and Planetary Physics 2019年第1期3卷 33-44页

作者： Kokea Ariane Darolle Fofie Fidèle Koumetio Jean Victor Kenfack David Yemele Department of Physics Faculty of Sciences University of Dschang Department of Earth Sciences Faculty of Sciences University of Dschang

The Garoua Zone in North Cameroon, the subject of this study, is known to have undergone tectonic movements during the Cretaceous, but the zone's structural data remain poorly *** study exploits the bouguer anomaly to improve knowledge of Garoua tectonics *** order to characterize these structures, two methods are used: Euler's deconvolution method and the method of the horizontal gradient of the vertical *** of the Euler's solutions map for index N=1 with the map from the horizontal gradient of the vertical derivative method allows determination of gravimetric lineaments, interpreted as faults or as linear contacts, from which we deduce a structural map of the study *** on this map, we identify sixteen lineaments, of which we count eight as linear contacts and eight as *** the faults, we denote one of depth between 4 and 8 km, five faults of depth ranging between 8 and 13 km, and two faults of depths between 13 and 36 *** of these faults shows that the seven deepest faults might present a natural risk in our study *** purposes of civil protection, such deep faults should be monitored and taken into consideration in the implementation of large public *** structural map, established herein from data on the in-depth extension of each fault, thus increases scientific knowledge in the area that can be used to site public works in ways that reduce risk.

关键词： bouguer anomaly horizontal derivative vertical derivative Euler deconvolution natural risks

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Gravity pattern in southeast margin of Tibetan Plateau and its implications to tectonics and large earthquakes

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Earth and Planetary Physics 2019年第5期3卷 425-434页

作者： Yue Wu Yuan Gao Key Laboratory of Earthquake Prediction Institute of Earthquake ForecastingChina Earthquake Administration

There are many active faults in the southeast margin of Tibetan Plateau,where three large active faults zones,the Longmenshan,Xianshuihe and Anninghe,merge to form a"Y"*** crustal deformation and a complicated fault distribution accompany strong earthquake activity in this *** this paper,we investigate a multi-scale gravity anomaly in the southeastern margin of the Tibetan Plateau using the wavelet transform;we find that the pattern of the gravity field is closely related to the fault system in the study *** the characteristics of this bouguer gravity anomaly at different orders indicates that the eastern Himalayan syntaxis has produced a strong eastward push during its northward movement,resulting in a shortening of the crust from west to east and a rapid uplift of the Tibetan *** Songpan–Garzêand Sichuan–Yunnan blocks have been forced to slip and extrude southward and eastward *** distributions of seven large earthquakes from 1970 to 2018 reflects the relationship between large earthquakes and characteristics of the gravity *** the tectonic backgrounds of several earthquakes reveals that the large earthquakes occur usually in the high gravity anomaly gradient zone,which corresponds in general to the boundary zones of the *** infer that large earthquakes occur primarily in high bouguer gravity anomaly zones in the upper crust,while low bouguer gravity anomalies encompass the lower crust and the uppermost mantle.

关键词： southeast margin of Tibetan Plateau bouguer gravity anomaly(BA) wavelet transform(WT)and multiscale analysis(MA) focal mechanism of earthquake gradient zone

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Crustal density structure of the southern segment of the Liaocheng-Lankao fault, China

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Geodesy and Geodynamics 2019年第5期10卷 347-355页

作者： Lei Jiang Lanbo Liu Zhiping Xu Xiaoguo Deng Lipu Yang Wei Xiong Shunqiang Xu Geophysical Exploration Center China Earthquake AdministrationZhengzhou450002China Center for Integrative Geosciences University of Connecticut at StorrsCT06269USA

The 1:200,000 middle-large scale bouguer gravity anomaly data covering the southern segment of the Liaocheng-Lankao fault(SLLF)and its vicinity are analyzed with two ***,the bouguer gravity anomaly data are decomposed by two-dimensional(2 D)wavelet to make the family of multi-scale modes correspond with density structure at different ***,a two and half dimension(2.5 D)human-computer interaction inversion of the bouguer gravity anomaly data are conducted with the constraints provided by two deep seismic sounding profiles(DSS1 and DSS2)crossing the study area to get the crustal density *** on the integrated study,we can draw the following conclusions:1)SLLF appears to be a deep fault with almost vertical dipping and rooted into the uppermost mantle;2)In the middle to upper crust SLLF shows an clear turning patterns and segmentation features;3)In the study area the epicentral distributions of the precisely re-located small earthquakes and the historical large earthquakes have a good correspondence with the turning patterns and segmentation features of SLLF;and 4)The results of the horizontal slices from 2 D wavelet decomposition show that there are significant differences in the density structure on the two sides of the fault.A well-defined concave structure with low density exists in the upper crust of the Dongming Depression on the west side of the fault,with the concave center being estimated at a depth of about 8 *** contrast,the upper crust on the east side presents a relative thinner pattern in depth with a bit higher ***,the low-density structure in the middle crust underneath the fault is presumably caused by the uplift of the upper mantle materials and their intrusion along the deep rupture *** paper clarified the inconsistency of fault system and epicenters of small earthquakes from upper to lower *** results indicated that the fault system plays an important governing role to the seismicity in this

关键词： Southern segment of the Liaocheng-Lankao fault(SLLF) bouguer gravity anomalies Density structure Dongming depression Multi-scale wavelet decomposition Epicenters of small earthquakes

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Density structure of the crust in the Emeishan large igneous province revealed by the Lijiang-Guiyang gravity profile

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Earth and Planetary Physics 2018年第1期2卷 74-81页

作者： Xi Zhang Peng Wang Tao Xu Yun Chen José Badal JiWen Teng State Key Laboratory of Lithospheric Evolution Institute of Geology and GeophysicsChinese Academy of SciencesBeijing 100029China Institute of Earthquake Science China Earthquake AdministrationBeijing 100036China CAS Center for Excellence in Tibetan Plateau Earth Sciences Beijing 100101China Physics of the Earth Sciences BUniversity of ZaragozaPedro Cerbuna 1250009 ZaragozaSpain

The Emeishan large igneous province(hereafter named by its acronym ELIP) is the first accepted large igneous region in *** current study tries to reconstruct the density structure of the crust in this region. For this purpose, we conducted the gravity survey along an 800-km-long profile, which stretched laterally along the latitude 27°N from Lijiang(Yunnan province) to Guiyang(Guizhou province). The fieldwork included 338 gravity measurements distributed from the inner zone to the outer zone of the mantle plume *** a series of gravity reductions, we calculated the bouguer gravity anomaly and then constructed the density model for ELIP by iterative forward modeling from an initial density model tightly constrained by wide-angle seismic reflection data. The topography of the Moho, here physically interpreted as a density discontinuity of ~0.4 g·cm^(–3), gradually rises from the inner zone(~50 km deep) to the outer zone(~40 km), describes a thicker crust in the inner zone than in any other segment of the profile and largely reproduces the shape of the bouguer gravity anomaly curve. Both the bouguer gravity and the density structure show significant differences with respect to the inner zone and the other two zones of ELIP according to the commonly accepted partition of the Emeishan area. A thicker and denser middle-lower crust seems to be the main feature of the western section of the profile, which is likely related to its mafic magmatic composition due to magmatic underplating of the Permian mantle plume.

关键词： bouguer gravity seismic reflection data forward modeling crustal density structure Emeishan large igneous province South China

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Delineation of Lineaments in South Cameroon (Central Africa) Using Gravity Data

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Open Journal of Geology 2013年第5期3卷 331-339页

作者： Owona Angue Marie Louise Clotilde Tabod Charles Tabod Nguiya Séverin Kenfack Jean Victor Tokam Kamga Alain Pierre Department of Physics Advanced Teacher Training College The University of Yaounde I Yaounde Cameroon

Gravity method is among the most applied geophysical methods in mineral and oil exploration. It may help to identify fault networks which are of interest for mineral exploration. Potential field data can give valuable information on the location of faults in the basement. These faults may have propagated into the overlying sedimentary rocks and influenced fluid flow and distribution of hydrocarbon traps and mineralization zones. A study was therefore conducted in south Cameroon with the aim of highlighting the different lineaments of the region, which were completely or partially hidden by the sedimentary cover. Different gravity data processing techniques including horizontal gradient coupled with upward continuation and Euler deconvolution were used. The application of these methods has mapped out a number of lineaments depicting gravity density discontinuities whose directions are NS, NE-SW, EW and NW-SE. The predominant direction for major lineaments is NE-SW. The major lineaments associated to the faults are: the Kribi-Edea faults, Ambam faults, Ebolowa-south of Yaounde faults;Bipindi-Yaounde faults;Pouma-Yaounde fault and the fault system which crosses the east, north, west of Monatele city. Euler solutions indicate depths up to18 km for the roof of the faults. The main results worked out from this study provide with new elements that allow the improvement of the knowledge on the structure of the study area. The structural map obtained shows major tectonic events that are responsible of the structural layout of the study zone. In addition, information related to the dip and depth of the various structures was also obtained. The map of lineaments is a useful tool for the planning of hydrogeological and/or petroleum investigations.

关键词： South Cameroon bouguer Anomaly Upward Continuation Horizontal Gradient Euler Deconvolution Lineament

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Gravity Survey along the Mizuho Traverse Routes, East Antarctica: SEAL Seismic Exploration in 1999-2000

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International Journal of Geosciences 2013年第10期4卷 1392-1400页

作者： Shigeru Toda Hiroki Miyamachi Hiroshi Murakami Tomoki Tsutsui Masaki Kanao Department of Earth and Environmental Sciences Faculty of Science Kagoshima University Kagoshima Japan Department of Earth Sciences Faculty of Education Aichi University of Education Kariya Japan Faculty of Engineering and Resource Science Akita University Akita Japan National Institute of Polar Research Tokyo Japan Technical Center for Seismological Observations Ltd. Tsukuba Japan

A gravity survey was conducted along the Mizuho traverse routes in austral summer of 1999-2000 by the 41st Japanese Antarctic Research Expedition (JARE-41). The main purpose of the survey was to obtain detail gravity anomaly along the traverse routes, where deep seismic exploration was simultaneously conducted in order to obtain a fine crustal structure of the Mizuho Plateau. By using SCINTREX (CG-3M) gravity meter, the survey was carried out at 160 sites with about 1 km interval in a distance of 190 km from S16 to Z20 on the traverse routes. Free-air and bouguer anomalies were calculated using precise locations by GPS measurements, by taking into account an effect of the thick icesheet. The furrowed negative Free-air anomalies are identified around H192, where middle points of the whole traverse routes. Two bedrock elevation models derived both by seismic refraction analyses and radio-echo sounding might correspond to the bottom and the top of a mixture layer composed from ice-moraine rocks over the bedrock surface. Thickness of the crust estimated by bouguer anomalies is 0.5 - 1 km larger toward inland at the terminal point of Z20 along the profile.

关键词： Gravity Survey Mizuho Traverse Routes Bedrock Elevation bouguer Gravity Anomaly Crustal Structure

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Gravimetric Study of Geological Structures of Teboursouk Area, Northern Tunisia

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International Journal of Geosciences 2012年第4期3卷 675-682页

作者： Amira Ayed-Khaled Taher Zouaghi Mohamed Ghanmi Fouad Zargouni Université de Carthage Université Tunis El Manar

Gravity data associated with surface geology in the Northern Tunisian Atlas offer better understand to the underlying structures in Teboursouk area and to highlight other deep or unknown structures in surface. The gravity study was based on qualitative and quantitative analysis including the construction of the gravity bouguer anomaly, upward continuations, residual anomaly, and Horizontal gradient maxima maps. The main results display many positive and negative anomalies as the response of geological structures (J. Cheid Triassic structure, Khalled plain, El Aroussa plain). In addition, the horizontal gradient maxima integrated with geological and structural maps let the identification of major directions of gravimetric lineaments in the study area us NE-SW trending features at the boundaries of J. Cheid structure, NW-SE direction that limit Gaafour plain and Tabet Ech Cherif syncline, and N-S trending that bordered El Aroussa basin. Major results allowed the construction of a new structural map of the study zone.

关键词： Gravimetry bouguer Anomaly Upward Continuations Gradient Structure Northern Tunisia

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