共查询到20条相似文献，搜索用时 62 毫秒
1.
Ulrich Fleischer 《Marine Geophysical Researches》1971,1(3):314-327
A regional survey of the southern Reykjanes Ridge (52°N to 57°N) shows an irregular topography: a rift valley which is only partly recognizable as such, with varying azimuth and some fracturezone-like interruptions. The survey also comprised gravity and magnetic measurements.The course of the axis as well as the perpendicular fractures show up well in the free air anomalies as relative troughs within an area of positive free air gravity (Figure 5). There is no indication of density variations within the topographic masses.The anomaly pattern of total magnetic intensity indicates the exact position of the rift axis and a bifurcation at about 55°N. From the parallel magnetic anomalies south of 55°N (Figure 2) a spreading rate can be deduced of 1.10 cm/yr perpendicular to the rift axis (Figure 3). This spreading rate is at the same time the plate movement involved.A survey of the Iceland-Faeroe Ridge with a 3–5 miles grid shows large gravity and magnetic anomalies over a smooth topography, indicating large pockets of light material, probably of volcanic origin. These areas have normal magnetization. Positive gravity anomalies forming a ring structure along the 200 m isobath are characterized by reversed magnetization.The dissimilarity in morphology, seismicity and inner structure between the two ridges that intersect in Iceland suggest that there is no relation between the two phenomena.Paper presented at the meeting of the International Gravity Commission, Paris, on September 8, 1970. 相似文献
2.
胶州湾地磁场特征及其工程地质意义 总被引：4，自引：2，他引：2
通过对胶州湾进行大比例尺磁力测量,绘制了胶州湾磁力异常图。胶州湾磁异常呈以下特征：在胶州湾中北部为变化较平缓的正磁异常区,梯度变化较小。西部则是NW向条带状强磁变化异常区,磁异常正负变化剧烈,呈明显的条带状展布。胶州湾东北角磁异常则表现为团块状分布,并以正异常为主。南部表现为混合异常,上部(大致为胶州湾最中间地带)磁异常为近东西向条带状展布,而胶州湾南部基本呈NE向正负相间分布。上述磁异常现象表明,胶州湾的断裂构造比较发育,在NE向断裂构造大背景下,从磁异常分布图上可以清楚地判别出NW向及近东西向断裂分布,这些断裂对胶州湾的工程建设将产生不利的影响。 相似文献
3.
The crenulated geometry of the Southeast Indian ridge within the Australian-Antarctic discordance is formed by numerous spreading ridge segments that are offset, alternately to the north and south, by transform faults. Suggested causes for these offsets, which largely developed since ~ 20 Ma, include asymmetric seafloor spreading, ridge jumps, and propagating rifts that have transferred seafloor from one flank of the spreading ridge to the other. Each of these processes has operated at different times in different locations of the discordance; here we document an instance where a small (~ 20 km), young (< 0.2 Ma), southward ridge jump has contributed to the observed asymmetry. When aeromagnetic anomalies from the Project Investigator-1 survey are superposed on gravity anomalies computed from Geosat GM and ERM data, we find that in segment B4 of the discordance (between 125° and 126° E), the roughly east-west-trending gravity low, correlated with the axial valley, is 20–25 km south of the ridge axis position inferred from the center of magnetic anomaly 1. Elsewhere in the discordance, the inferred locations of the ridge axis from magnetics and gravity are in excellent agreement. Ship track data confirm these observations: portions of Moana Wave track crossing the ridge in B4 show that a topographic valley correlated with the gravity anomaly low lies south of the center of magnetic anomaly 1; while other ship track data that cross the spreading ridge in segments B3 and B5 demonstrate good agreement between the axial valley, the gravity anomaly low, and the central magnetic anomaly. Based on these observations, we speculate that the ridge axis in B4 has recently jumped to the south, from a ridge location closer to the center of the young normally magnetized crust, to that of the gravity anomaly low. The position of the gravity low essentially at the edge of normally magnetized crust requires a very recent (< 0.2 Ma) arrival of the ridge in this new location. Because this ridge jump is so young, it may be a promising location for future detailed studies of the dynamics, kinematics, and thermal effects of ridge jumps.The U.S. Government right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged. 相似文献
4.
5.
In this paper, we demonstrate the effectiveness of steerable filters as a method of delineating the boundaries of subsurface geological structures. Steerable filters, generally used for edge detection on 2-D images, have the properties of band pass filters with certain directions and are applied to many image processing problems. We first tested the method on synthetic data and then applied it to the aeromagnetic data of İskenderun Basin and adjacent areas.İskenderun Basin is located in the Northeastern Mediterranean where African–Arabian and Anatolian plates are actively interacting. The basin fill records a complex tectonic evolution since the Early Miocene, involving ophiolite emplacement, diachronous collision of Eurasian and Arabian plates and subsequent tectonic escape related structures and associated basin formation. Geophysical investigations of the tectonic framework of İskenderun Basin of Turkey provide important insights on the regional tectonics of the Eastern Mediterranean and Middle East. In this study we show geological structures, which are responsible for the magnetic anomalies in İskenderun Basin and enlighten the structural setting of the Northeastern Mediterranean triple junction using steerable filters. We obtained a magnetic anomaly map of the region from the General Directorate of Mineral Research and Exploration as raw data and then evaluated this by steerable filters. We determined the magnetic anomaly boundaries for İskenderun Basin by using various types of steerable filters and correlated these to drilling data and seismic profiles from the Turkish Petroleum Corporation. The result of the steerable filter analysis was a clarified aeromagnetic anomaly map of İskenderun Basin. The tectonic structure of İskenderun Basin is divided into regions by an N–S trending oblique-slip fault defined by the steerable filter outputs. We propose a new tectonic structure model of İskenderun Basin and modify the direction of the East Anatolian Fault Zone. In our model, East Anatolian Fault Zone cross-cuts the basin as a narrow fault zone and continues towards the Cyprus arc. 相似文献
6.
7.
Faggioni Osvaldo Beverini Nicolò Carmisciano Cosmo Giori Italiano 《Marine Geophysical Researches》2001,22(2):63-79
We show the results obtained by means of a seabed reduction technique on the intensity of geomagnetic anomaly fields applied
to a synthetic case and then to the real case of a geomagnetic survey of eastern Ligurian Sea (Italy). The eastern Ligurian
Sea has very intense short waves anomaly fields and a sea bed that varies greatly in depth. As a result the geomagnetic space
signal is characterized by a very large spectral content; in these conditions it is not possible to obtain a full sampled
marine survey and vertical continuation analytic procedures and standard numerical bottom reduction based on a single vertical
incremental parameter, whichever is applicable, fails to give accurate results. The present technique, which has been fine-tuned
over 4 years of experimentation in environmental researchs, aims to provide a simple and efficient means to reduce the distortion
of geomagnetic anomalies field caused by the variation of distance between survey plane and magnetic outcrop source position.
The compensation procedure is based on evaluation, by comparison of two measurements carried out at different altitudes, of
the mean vertical increment typical of each anomaly field principal frequency component bands. The component anomaly fields
are then corrected by application of the corresponding vertical increments and lastly, the anomaly geomagnetic field reduced
to the sea-bed is computed as Inverse Fourier Transform of a spectrum built as synthesis of the component anomaly fields'
spectra. The results obtained have shown a notable increase in definition of anomaly field intensity without the production
of appreciable distortions or false geomagnetic echoes.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
8.
Poles of rotation for the North Atlantic have been derived from the results of a new aeromagnetic survey northeast of Newfoundland. Reconstruction of the North Atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the Atlantic from Flemish Cap and Goban Spur to the Azores-Gibraltar Fracture Zone. A group of similar anomalies has also been identified in the Bay of Biscay. North of Goban Spur and Flemish Cap, these anomalies follow the ocean-continent boundary. Poles of rotation derived for this anomaly show that it forms an isochron (100–110 m.y.) during the long Cretaceous normal polarity interval. The cause of this anomaly is not definite, but it may represent an increase in the magnetization of the crust during a limited time within the Cretaceous Magnetic Quiet Zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by Raymond and LaBrecque.The North Atlantic has also been reconstructed at the time of the initial opening in the region between Flemish Cap and the Charlie-Gibbs Fracture Zone, using inferred ocean-continent boundaries on the west and east sides: it has been shown that the entire region could not have saparated at one time, but that spreading between the British Isles and Newfoundland had to progress from south to north. Consequently, when active sea-floor spreading was taking place between Goban Spur and Flemish Cap (about 110 m.y.) the region to the north was still being stretched. The calculated amount of stretching as derived from the reconstructions (about 25%) agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region, and with the results of deep seismic studies. Active spreading in the north started about 100 m.y. ago. 相似文献
9.
Gravity data collected at two different levels, sea-surface and seafloor, are compared and interpreted to characterize the effect of hydrothermal circulation on the sediment density in Middle Valley, a deeply sedimented spreading center on the Juan de Fuca Ridge. The sea-surface gravity data constrain density variations at depth beneath the seafloor, whereas sea-bottom measurements are more sensitive to shallow sources. At least two different types of hydrothermal signatures in the sediments can be distinguished from the gravity data: short-wavelength anomalies associated with sulfide deposits and broader anomalies associated with areas of lithified sediments. In Middle Valley, three distinct gravity anomalies were identified. (a) An anomaly over a sulfide mound, south of Bent Hill, shows that the sulfide body extends to depths of 120 to 180 m and has been fed by several near-surface conduits. (b) An anomaly at the base of the fault bordering the valley to the east is interpreted as a regional anomaly combined with the local effect of lithified sediments and possibly sulfide deposits. (c) An anomaly paralleling an intra-valley fault, that limits the deepest part of the graben, is interpreted as indicating lithification of the upper sediment layer. A high heat flow anomaly is located 1 to 2 km east of this fault, suggesting that sediment lithification occurred in a wide band above the fault and eastward to the current high heat flow area, due to the progressive migration of the hot fluid circulation. 相似文献
10.
Aeromagnetic data collected over the Offshore Mahanadi Basin along the Eastern margin of India display high amplitude magnetic anomalies. The presence of a Cretaceous volcanic sequence masks the seismic response from the underlying basement and results in poor quality seismic data. In this study spectral analysis of the aeromagnetic data collected over this part of the Offshore Mahanadi Basin was carried out. Results of this analysis indicate the presence of a high density, highvelocity (6.45 km/s) mafic layer within the crystalline basement varying from 4–6 km depth. This intra-basement layer seems to have been affected by a number of lineaments, which have played a role in the evolution of the Mahanadi Offshore Basin. The western part of the offshore basin is affected by the volcanism related to the 85°E Ridge, whereas the intense anomaly band (900 nT) offshore Puri, Konark and Paradip is interpreted as a combined effect of crystalline Precambrian basement overlain (i) by Cretaceous volcanic rocks of variable thickness (25–860 m) and (ii) by a mafic layer within the basement. 相似文献
11.
Enderby Land in East Antarctica and its adjacent areas, which are closely related to the Indian Plate in their geological evolution, have become one of the key zones for studies on how the Antarctic continent evolves. Based on the isostasy and flexure theories of the lithosphere and using the CRUST1.0 model as the depth constraint, this paper uses the gravity field model EIGEN-6C4 and topographic data to calculate the isostatic gravity anomalies of Enderby Land and its adjacent areas. Then, the ... 相似文献
12.
Geophysical data (magnetic, gravity, and bathymetric) collected by international and national agencies along intersecting
track lines in the Bay of Bengal show discrepancies that are analyzed and adjusted by crossover techniques. The original magnetic
anomaly map over this region exhibits high gradients over the entire region, which do not fit with the known geological framework.
The corrected map is devoid of artifacts, and the contours became smooth, which can be utilized for meaningful geological
interpretation. The corrected free-air gravity map indicates improvements from the original in resolution with clear isolated
anomalies in high track density areas. Corrected bathymetry does not show much improvement.
Received: 29 May 1998 / Revision received: 23 February 1999 相似文献
13.
A detailed survey of a 1°×1°-square of seafloor 100 miles south-east of the Azores shows a strong correlation between directions of regional topographic and magnetic lineations. The area is dissected by the East Azores Fracture Zone at 36°55N, identified as the active Eurasian-African plate boundary, and by another large, non-active fracture zone at 36°10N. Both fracture zones strike 265° and are accompanied by large amplitude magnetic anomalies. The general strike in the area in between is 000°–015°. The skewing effect at this magnetic latitude is very sensitive to variations in strike of the magnetic contrasts. This effect was eliminated by a non-linear transformation which also gives the positions of magnetic contrasts. Some N-S contrasts were identified as sea floor spreading polarity contrasts (anomalies 31 and 32). Weak contrasts could be identified as topographic effects and gave a magnetization intensity of 5 A m-1. The identified sea floor spreading anomalies to both sides of the fracture zone at 36°10N agree very well, also quantatively, with a three-dimensional model for the fracture zone anomalies. This model describes the non-linear anomalies as end effects of the magnetic layer which is divided in blocks of alternating polarity. 相似文献
14.
Zvi Ben-Avraham Yoram Shoham Eitan Klein Haim Michelson Colette Serruya 《Marine Geophysical Researches》1980,4(3):257-276
A magnetic survey of Lake Kinneret (Sea of Galilee) was conducted on a 1 km grid of north-south and east-west lines. The results indicate that the margins of the lake are associated with large amplitude anomalies, while the centre is quite smooth. The largest anomaly, more than 500 nT, was detected in the vicinity of the entrance of the Jordan River into the lake. Its source is interpreted to be Late Cenozoic basaltic flows. The lake's margins are associated with faults, hot springs and magnetic anomalies. A broad magnetic anomaly trending east-northeast extends from Ginosar Valley into the lake through most of the lake's width. The distribution of basalt flows of different ages and the various structures of the magnetized layers are all contributing to the magnetic anomaly pattern. 相似文献
15.
The Atlantis Fracture Zone (30° N) is one of the smallest transform faults along the Mid-Atlantic Ridge with a spatial offset of 70 km and an age offset of ~ 6 Ma. The morphology of the Atlantis Fracture Zone is typical of that of slow-slipping transforms. The transform valley is 15–20 km wide and 2–4 km deep. The locus of strike-slip deformation is confined to a narrow band a few kilometers wide. Terrain created at the outside corners of the transform is characterized by ridges which curve toward the ridge-transform intersections and depressions which resemble nodal basins. Hooked ridges are not observed on the transform side of the ridge-transform intersections. Results of the three-dimensional inversion of the surface magnetic field over our survey area suggest that accretionary processes are sufficiently organized within 3–4 km of the transform fault to produce lineated magnetic anomalies. The magnetization solution further documents a 15-km, westward relocation of the axis of accretion immediately south of the transform about 0.25 Ma ago. The Atlantis Transform is associated with a band of high mantle Bouguer anomalies, suggesting the presence of high densities in the crust and/or mantle along the transform, or anomalously thin crust beneath the transform. Assuming that all the mantle Bouguer anomalies are due to crustal thickness variations, we calculate that the crust may be 2–3 km thinner than a reference 6-km thickness beneath the transform valley, and 2–3 km thicker beneath the mid-points of the spreading segments which bound the transform. Our results indicate that crustal thinning is not uniform along the strike of the fracture zone. Based on studies of the state of compensation of the transform, we conclude that the depth anomaly associated with the fracture zone valley is not compensated everywhere by thin crust. Instead, the regional relationship between bathymetry and gravity is best explained by compensation with an elastic plate with an effective thickness of ~ 4 km or greater. However, the remaining isostatic anomalies indicate that there are large variations away from this simple model which are likely due to variations in crustal thickness and density near the transform. 相似文献
16.
通过模型正演分析发现,盆地深层火山岩产生的磁异常不是很大,表现为叠加在强背景之上的次级异常。基于积分迭代延拓方法提出了增强盆地深层火山岩磁异常信息的新方法—积分迭代延拓平化曲。该方法不仅能起到增强盆地深层火山岩磁异常的作用,而且还能起到消除火山岩埋深不同对磁异常形态和大小的影响,以达到均衡磁异常的目的。将该方法应用到松辽盆地的深层火山岩预测中见到了明显的应用效果。 相似文献
17.
Approximately 147000 km of low-level (450 m) aeromagnetic tracks were flown over the Arctic Ocean and adjacent Greenland and Norwegian Seas, for the greater part with a digitally recording nuclear precession magnetometer designed and built by Wold (1964). The digital recording feature of the system facilitated numerous data processing and analytical techniques which are described herein. These include: noise filtering coordinate conversion, removal of the regional field, second derivatives, downward continuations, polynomial fits of varying degrees to profiles and surfaces, numerical approximations, and depth to source calculations. Using these data and interpretative techniques some inferences could be made about the geologic structure and evolution of the Arctic Ocean Basin. Salient amongst these are: both gravity and magnetic data suggest that there is a 2 1/2 km basement uplift in the eastern Chukchi Shelf associated with the Tigara structure which truncates the western end of Lisburne Peninsula. A 30–40 km wide basement root encircles the Chukchi Rise and extends over 30 km into the mantle. Within the Canda Basin there is a thickening of sediments from the Asian continental margin toward the Canadian Arctic Archipelago. Sediment thickness in the Makarov Basin is 1–1 1/2 km. There appears to be only about a 1/2 km sediment cover in the Fram and Nautilus Basins. The absence of large amplitude magnetic anomalies over these basins is attributed to a 10 km elevation of the Curie isotherm. The Alpha and Nansen ridges produce magnetic profiles that show axial symmetry and correlate with profiles in the North Atlantic. A quantitative attempt has been made to verify these correlations, which infer that the Alpha Cordillera became inactive 40 mybp when the locus of rifting shifted to the Nansen Cordillera. The absence of significant magnetic anomalies over the Lomonosov Ridge reinforces the hypothesis that it is a section of the former Eurasian continental margin that was translated into the Arctic Basin by sea-floor spreading along the Nansen Cordillera axis. 相似文献
18.
Roger Searle 《Marine Geology》1977,25(4):299-320
A detailed survey of a 1°-square of sea floor 100 miles northeast of the Azores has revealed the presence of scarps over 30 km long striking generally 120°, transverse to the regional topographic and magnetic lineations. They are not associated with a major fracture zone. Sedimentation in the area appears to have been by a mixture of pelagic and turbidity-current processes, with a Miocene increase in sedimentation rate probably indicating the subaerial emergence of the Azores Archipelago. Magnetic anomalies were identified along a number of profiles between the survey area and the Mid-Atlantic Ridge crest, and indicate a short period of increased spreading rate from about 12-3 m.y. ago. 相似文献
19.
利用东海及邻域最新的磁力异常数据,分析东海的磁场特征,并利用该磁力数据计算东海的磁性基底界面,分析解释磁性界面的特征及地质特征。研究表明,从陆区、陆架盆地到冲绳海槽中部,磁力异常呈正负相间变化,最大值出现在福建沿海地区;磁性基底深度在4～11km之间变化。从冲绳海槽中部到琉球群岛,磁异常从正磁异常变为负磁异常;磁性基底深度为7～12km之间变化。从琉球弧前盆地到琉球海沟,磁力异常为正负相间变化,中部磁异常为负值,两侧异常为正值;磁性基底深度为7.5～11km之间变化。 相似文献
20.
提出地磁日变改正中需要顾及地磁长期变化和磁扰的可靠分离,保证磁力数据改正处理具有明确的基准,提高拼接整合的精度.如果日变站能够控制测区,两者可采用时间同步固定的地磁正常场作为磁异常基准,测区的地磁长期变化由日变站实测的地磁长期变化来改正,反之,对日变站和测区应分别采用时间同步变化的地磁正常场作为磁异常基准,测区的地磁长期变化由地磁正常场的地磁长期变化来改正.为此,应该明确定义与时间相关的日变改正基值为地磁正常场与地磁异常之和.通过磁静日总场观测值偏离正常场的某种形式的长期平均值,可以逼近日变站的地磁异常.磁扰的分离应保证磁扰初动和消失相对于磁静日无磁扰的连续过渡.采用综合模型一并进行日变改正、正常场及其长期变化改正,可以更好地解决目前日变改正中遇到的问题. 相似文献