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1.
20 magnetotelluric (MT) soundings were collected on the Isle of Skye, Scotland to provide a high-resolution three-dimensional (3-D) electrical resistivity model of a volcanic province within the framework of a project jointly interpreting gravity, seismic, geological and MT data. The full 3-D inversion of the MT data jointly interpreted with gravity data reveals upper crustal structure. The main features of the model are interpreted in conjunction with previous geological mapping and borehole data. Our model extends to 13 km depth, several kilometres below the top of the Lewisian basement. The top of the Lewisian basement is at approximately 7–8 km depth and the topography of its surface was controlled by Precambrian rifting, during which a 4.5 km thick sequence of Torridonian sediments was deposited. The Mesozoic sediments above, which can reach up to 2.2 km thick, have small-scale depocentres and are covered by up to 600 m of Tertiary lava flows. The interpretation of the resistivity model shows that 3-D MT inversion is an appropriate tool to image sedimentary structures beneath extrusive basalt units, where conventional seismic reflection methods may fail.  相似文献   

2.
A new algorithm is presented for the integrated 2-D inversion of seismic traveltime and gravity data. The algorithm adopts the 'maximum likelihood' regularization scheme. We construct a 'probability density function' which includes three kinds of information: information derived from gravity measurements; information derived from the seismic traveltime inversion procedure applied to the model; and information on the physical correlation among the density and the velocity parameters. We assume a linear relation between density and velocity, which can be node-dependent; that is, we can choose different relationships for different parts of the velocity–density grid. In addition, our procedure allows us to consider a covariance matrix related to the error propagation in linking density to velocity. We use seismic data to estimate starting velocity values and the position of boundary nodes. Subsequently, the sequential integrated inversion (SII) optimizes the layer velocities and densities for our models. The procedure is applicable, as an additional step, to any type of seismic tomographic inversion.
We illustrate the method by comparing the velocity models recovered from a standard seismic traveltime inversion with those retrieved using our algorithm. The inversion of synthetic data calculated for a 2-D isotropic, laterally inhomogeneous model shows the stability and accuracy of this procedure, demonstrates the improvements to the recovery of true velocity anomalies, and proves that this technique can efficiently overcome some of the limitations of both gravity and seismic traveltime inversions, when they are used independently.
An interpretation of field data from the 1994 Vesuvius test experiment is also presented. At depths down to 4.5 km, the model retrieved after a SII shows a more detailed structure than the model obtained from an interpretation of seismic traveltime only, and yields additional information for a further study of the area.  相似文献   

3.
Elastic finite element models are applied to investigate the effects of topography and medium heterogeneities on the surface deformation and the gravity field produced by volcanic pressure sources. Changes in the gravity field cannot be interpreted only in terms of gain of mass disregarding the ground deformation of the rocks surrounding the source. Contributions to gravity changes depend also on surface and subsurface mass redistribution driven by dilation of the volcanic source. Both ground deformation and gravity changes were firstly evaluated by solving a coupled axisymmetric problem to estimate the effects of topography and medium heterogeneities. Numerical results show significant discrepancies in the ground deformation and gravity field compared to those predicted by analytical solutions, which disregard topography, elastic heterogeneities and density subsurface structures. With this in mind, we reviewed the expected gravity changes accompanying the 1993–1997 inflation phase on Mt Etna by setting up a fully 3-D finite element model in which we used the real topography, to include the geometry, and seismic tomography, to infer the crustal heterogeneities. The inflation phase was clearly detected by different geodetic techniques (EDM, GPS, SAR and levelling data) that showed a uniform expansion of the overall volcano edifice. When the gravity data are integrated with ground deformation data and a coupled FEM modelling was solved, a mass intrusion could have occurred at depth to justify both ground deformation and gravity observations.  相似文献   

4.
The VLF-R (very low frequency-resistivity) data, i.e. the apparent resistivity ( ρ a ) and phase ( φ ) data, were inverted individually and jointly using the VFSA (very fast simulated annealing) global inversion approach. Global inversion results for synthetic data without and with various amounts of random and normally distributed Gaussian noise reveal that the inversion of neither the ρ a nor φ data alone yields the true parameters of the structures. However, the joint inversion of the ρ a and φ data yields very good estimates of the model parameters. Five models, representing typical subsurface structures in the shield areas, are studied here. Various models achieved after 10 VFSA runs were used to compute the mean model and the corresponding covariance and correlation matrices, which were used to estimate the uncertainties in the mean model parameters and correlations between the model parameters. We observe that these correlations follow the physics associated with the problem. VLF-R field data due to a nearly vertical contact structure and a very thick dyke-like structure were also inverted to demonstrate the efficacy of the approach in the delineation of the parameters of 2-D structures.  相似文献   

5.
The rifting history of the Atlantic continental margin of Newfoundland is very complex and so far has been investigated at the crustal scale primarily with the use of 2-D seismic surveys. While informative, the results generated from these surveys cannot easily be interpreted in a regional sense due to their sparse sampling of the margin. A 3-D gravity inversion of the free air data over the Newfoundland margin allows us to generate a 3-D density anomaly model that can be compared with the seismic results and used to gain insight into regions lacking seismic coverage. Results of the gravity inversion show good correspondence with Moho depths from seismic results. A shallowing of the Moho to 12 km depth is resolved on the shelf at the northern edge of the Grand Banks, in a region poorly sampled by other methods. Comparisons between sediment thickness and crustal thickness show deviations from local isostatic compensation in locations which correlate with faults and rifting trends. Such insights must act as constraints for future palaeoreconstructions of North Atlantic rifting.  相似文献   

6.
Seafloor compliance is the measure of seafloor deformation under a pressure signal. Our new 2-D finite-difference compliance modelling algorithm presents several advantages over the existing compliance models, including the ability to handle any gridded subsurface structure with no limitations on the gradients of the material properties, as well as significantly improved performance. Applying this method to some of the problems inaccessible to previously existing methods, demonstrates that lateral variations in subsurface structure must be accounted for to adequately interpret compliance data. In areas with significant lateral variations, the utilization of 1-D modelling and inversion is likely to result in high interpretation errors, even when additional subsurface structure information is available. We find that flattened pure melt bodies have a significantly higher compliance than cylindrical melt bodies with the same cross-sectional area. The compliance created by such bodies often has side peaks over their edges, which are as strong as or stronger than the central peak, requiring a series of measurements to best constrain their size and shear velocity. Finally, we find that the compliance data are far and away most sensitive to the broad, thick, lower-crustal partial melt zone. Our simple data fitting model for the compliance measurements on the East Pacific Rise at 9°48'N required shear velocities as low as 700 m s−1 in the centre of this zone, far below the values previously estimated using 1-D model based inversions, suggesting higher melt percentages than those previously estimated, while small melt bodies in the upper part of the crust were found to have little or no effect on the measured compliance.  相似文献   

7.
珠三角地区地表温度与土地利用类型关系   总被引：4，自引：1，他引：3  
牟雪洁  赵昕奕 《地理研究》2012,31(9):1589-1597
随着城市化进程的加快,城市气候与环境问题日益显现,尤以城市热岛效应最为突出。通过监督分类方法对TM遥感影像进行了土地利用分类,并运用TM热红外波段线性拟合模型进行近地表气温反演,分析城市热岛的空间分布及地域性差异,以及与土地利用类型的关系。结果表明:珠三角地区近地表气温与土地利用类型紧密相关,城市建设区形成高温中心,是热岛的主要贡献因子,植被和水体则有明显的冷岛效应;研究区热岛具有区域性集中与分散分布的特点,且以区域性热岛为中心向周边扩展;分析热岛强度剖面线发现,由于地形、植被覆盖度等因素影响,研究区热岛强度的南北差异较大,而东西差异较小;热岛分布与土地利用类型分布格局较为一致,但也有分布不一致性的区域,表现为城市热岛向非城市建设用地扩展。  相似文献   

8.
Although the galvanic distortion due to local, near-surface inhomogeneities is frequency-independent, its effect on the magnetotelluric data becomes, in a 3-D structure, frequency-dependent. Therefore, both the apparent resistivity and the phase responses are disturbed, and a correction should be carried out prior to the 3-D interpretation in order to retrieve the 3-D regional impedance tensor. In many cases, the structure is 2-D for depths corresponding to a first range of periods and 3-D for longer periods (called 2-D/3-D). For these cases, a simple method which allows us to retrieve the 3-D regional impedance tensor (except the static shift) is presented. The method proposed uses the Groom & Bailey decomposition of the distortion matrix for the short periods. Three examples are presented: two using synthetic data and one employing real data. These examples show the effect of the galvanic distortion over a regional 2-D/3-D model and the retrieval of the regional transfer functions from the distorted ones.  相似文献   

9.
This paper describes a method for determining Moho depth, lithosphere thinning factor (γ= 1 − 1/β) and the location of the ocean–continent transition at rifted continental margins using 3-D gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere. The lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of −100 mGal, and for which a correction must be made in order to determine Moho depth accurately from gravity inversion. We describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present-day temperature field from which we calculate the lithosphere thermal density and gravity anomalies. For continental margin lithosphere, the lithosphere thermal perturbation is calculated from the lithosphere thinning factor (γ= 1 − 1/β) obtained from crustal thinning determined by gravity inversion and breakup age for thermal re-equilibration time. For oceanic lithosphere, the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere. A correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading. We investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted Moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the North Atlantic.  相似文献   

10.
国内外区域空间相互作用研究进展   总被引：4，自引：0，他引：4  
区域空间相互作用是区域科学、经济地理学关注的重点。文章对国内外空间相互作用的相关理论、研究内容及测度模型进行梳理。从研究内容来看,区域联系特征及区域空间结构的关注较多,而针对区域间地缘经济关系的研究还有待深化,尤其是在测度方面需要更为深入的探讨,质性方法在研究中的分量也需要强化。经典的引力模型在实践应用中不断调整和深化,而对于城市流模型和地缘经济关系模型的支撑理论较为薄弱,模型应用过程中呈现出新技术、新方法运用的趋势。传统的研究还集中于地理意义上的区域空间及城市组织,针对虚拟空间中的信息技术流动和技术创新等因素的影响应作出积极回应。随着全球城市网络的显现,借助各种属性数据探索垂直层面的联系将是一个重要方向,不同尺度下的城市与区域间的网络结构、功能和关系研究将成为关注的重点。  相似文献   

11.
Surface wave tomography of the Barents Sea and surrounding regions   总被引：1，自引：0，他引：1  
The goal of this study is to refine knowledge of the structure and tectonic history of the European Arctic using the combination of all available seismological surface wave data, including historical data that were not used before for this purpose. We demonstrate how the improved data coverage leads to better depth and spatial resolution of the seismological model and discovery of intriguing features of upper-mantle structure. To improve the surface wave data set in the European Arctic, we extensively searched for broad-band data from stations in the area from the beginning of the 1970s until 2005. We were able to retrieve surface wave observations from regional data archives in Norway, Finland, Denmark and Russia in addition to data from the data centres of IRIS and GEOFON. Rayleigh and Love wave group velocity measurements between 10 and 150 s period were combined with existing data provided by the University of Colorado at Boulder. This new data set was inverted for maps showing the 2-D group-velocity distribution of Love and Rayleigh waves for specific periods. Using Monte Carlo inversion, we constructed a new 3-D shear velocity model of the crust and upper mantle beneath the European Arctic which provides higher resolution and accuracy than previous models. A new crustal model of the Barents Sea and surrounding areas, published recently by a collaboration between the University of Oslo, NORSAR and the USGS, constrains the 3-D inversion of the surface wave data in the shallow lithosphere. The new 3-D model, BARMOD, reveals substantial variations in shear wave speeds in the upper mantle across the region with a nominal resolution of 1°× 1°. Of particular note are clarified images of the mantle expression of the continent-ocean transition in the Norwegian Sea and a deep, high wave speed lithospheric root beneath the Eastern Barents Sea, which presumably is the remnant of several Palaeozoic collisions.  相似文献   

12.
From ACH tomographic models to absolute velocity models   总被引：2，自引：0，他引：2  
The ACH method, a widely used tomographic inverse method, is characterized by the use of relative residuals in order to avoid possible biases coming from outside the target volume. The ACH method thus does not really retrieve the 3-D structure of the target volume, but instead leads to velocity contrasts relative to the layer average of the velocity, this average value remaining unknown ( Aki et al. 1977 ). Two artefacts derive from this particularity: (1) velocity contrasts are known only in the horizontal direction and it is not possible, in a strict mathematical sense, to estimate the contrasts in the vertical direction with ACH alone; (2) negative anomalies are often interpreted as low velocities, whereas negative anomalies may correspond to high velocities if the average value of the corresponding layer is sufficiently high. The converse is true of positive anomalies. We show with synthetic data how these artefacts can affect the interpretation of tomographic images. We propose to correct the artefacts by reintroducing the 1-D regional average model, and show in synthetic experiments how effective this correction can be.
  The application of this procedure to data recorded in the Kunlun region shows that the retrieval of the absolute values of the 3-D velocity model is helpful for interpreting the tomographic images and better defining which features are anomalous.  相似文献   

13.
刘锡清  刘洪滨 《地理研究》2008,27(1):119-127
岛屿对于发展海洋经济,确定海洋权属,以及国家安全等方面都具有重要地位。因此,岛屿的成因分类研究具有重要理论意义和应用价值。传统上将岛屿分为大陆岛、海洋岛(火山岛与珊瑚岛)和冲积岛。这种分类已经不能适应现代地球学科的新进展。本文根据板块构造理论和大洋地貌体系,提出新的岛屿分类意见,即分为内力和外力两个成因系列,包括近岸大陆岛、隆起大陆岛、大陆火山岛、岛弧陆块岛、岛弧火山岛、俯冲增生岛、无震海岭火山岛、微型陆块岛、海山火山岛、中脊火山岛、构造断层岛、河口沙岛、障壁岛、侵蚀沙岛、珊瑚岛15个类型。  相似文献   

14.
Topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric (MT) data. Three-dimensional (3-D) topographic effects have been investigated for a trapezoidal hill model using an edge finite-element method. The 3-D topography generates significant MT anomalies, and has both galvanic and inductive effects in any polarization. This paper presents two different correction algorithms, which are applied to the impedance tensor and to both electric and magnetic fields, respectively, to reduce topographic effects on MT data. The correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography, and improve the quality of subsurface interpretation. Nonlinear least-squares inversion of topography-corrected data successfully recovers most of structures including a conductive or resistive dyke.  相似文献   

15.
The Massif Central, the most significant geomorphological unit of the Hercynian belt in France, is characterized by graben structures which are part of the European Cenozoic Rift System (ECRIS) and also by distinct volcanic episodes, the most recent dated at 20 Ma to 4000 years BP. In order to study the lithosphere-asthenosphere system beneath this volcanic area, we performed a teleseismic field experiment.
During a six-month period, a joint French-German team operated a network of 79 mobile short-period seismic stations in addition to the 14 permanent stations. Inversion of P -wave traveltime residuals of teleseismic events recorded by this dense array yielded a detailed image of the 3-D velocity structure beneath the Massif Central down to 180 km depth. The upper 60 km of the lithosphere displays strong lateral heterogeneities and shows a remarkable correlation between the volcanic provinces and the negative velocity perturbations. The 3-D model reveals two channels of low velocities, interpreted as the remaining thermal signature of magma ascent following large lithospheric fractures inherited from Hercynian time and reactivated during Oligocene times. The teleseismic inversion model yields no indication of a low-velocity zone in the mantle associated with the graben structures proper. The observation of smaller velocity perturbations and a change in the shape of the velocity pattern in the 60–100 km depth range indicates a smooth transition from the lithosphere to the asthenosphere, thus giving an idea of the lithosphere thickness. A broad volume of low velocities having a diameter of about 200 km from 100 km depth to the bottom of the model is present beneath the Massif Central. This body is likely to be the source responsible for the volcanism. It could be interpreted as the top of a plume-type structure which is now in its cooling phase.  相似文献   

16.
Summary. The existence of an anomalously low-velocity, low-density zone within the upper mantle beneath the Kenya Dome has been deduced on the basis of previous gravity and seismic studies. This paper describes an experiment to measure teleseismic delay times across the Gregory Rift near the equator and along a SE radius of the Kenya Dome. The delay times have been determined with good relative accuracy and provide further independent evidence for the existence of the anomalous zone. The pattern of delay times along the two profiles and at other stations indicates that the zone thins rapidly to the SE away from the rift axis, mirroring the attenuation observed, from Kaptagat, for the same zone to the NW. The trend is for the thinning to become very much less rapid with distance, but there is also clear evidence for localized thickening of the zone under the Kilimanjaro–Chyulu volcanic area.
Significantly smaller delay times are measured at the centre of the rift than at the edges. This is shown to indicate that the anomalous zone penetrates the crust to form an intrusion of relatively high-velocity material along the rift axis. The clear correlation of the delay time low with the axial Bouguer high indicates that they are both manifestations of the same underlying structure. Thus the delay time results provide independent confirmation of the existence of the axial intrusion previously inferred from gravity data. The width of this intrusion at the normal base of the crust is well defined by the data as 30 km.  相似文献   

17.

Our study interprets large-scale gravity data to delineate concealed banded iron formation (BIF) iron mineralization in India's Rajasthan province. The study area belongs to the Bharatpur, Dausa, and Karauli districts of Rajasthan. We measured 1462 gravity readings to understand the rock types, depth and geometry of the different rock formations in the proposed study area. We also collected representative lithologies from more than 100 locations in the study area and calculated their density values. The measured gravity datasets are investigated via qualitative (e.g., Bouguer anomaly, first derivative and second derivative) and quantitative (radially averaged power spectrum, 3D Euler deconvolution, and 3D inversion) approach. The qualitative methods suggest a general NE–SW orientation of the BIFs, controlled by the general trend of the study area's structural setting. The lithological contact between the Bhilwara and Vindhyan Supergroups is demarcated by a NE–SW trending steep gravity gradient zone. In this area, representative lithologies yield high densities (about 3.746 gm/cc), and the samples identified as BIF represent exploration targets for iron ore. We have also developed our own in-house 3D gravity inversion code in this study. A model space inversion algorithm is converted into a data space using the identity relationship. It makes inversion algorithm very user-friendly on conventional desktop computers. The outcomes from the 3D inversion suggest that the concealed iron ore thickens to the west. This interpretation is also in good correlation with Euler 3D deconvolution of the gravity data.

  相似文献   

18.
地球重力场的变化是导致陆地水储量变化的重要因素之一,利用GRACE(Gravity Recovery and Climate Experiment)重力场恢复与气候实验重力卫星数据,结合GLDAS(Global Land Data Assimilation Systems)全球陆面数据同化系统和实测地下水位数据,反演和田地区克里雅河流域11年间四季和田地区的陆地水储量动态变化,模拟计算地下水等效水高变化趋势,构建了地下水水位估算模型。研究结果表明：和田地区春、夏两季的陆地水储量呈现出增加趋势,而秋、冬两季出现亏损状态;GRACE地球重力卫星所反演的陆地水储量比GLDAS同化系统所模拟的水资源变化更为剧烈,但2类数据的动态变化拟合度很高;GLDAS水资源等效水高二阶微分、GLDAS水资源变化倒数一阶微分、GRACE陆地水储量变化倒数变化、地下水储量变化一阶微分的敏感程度最高,构建的多元逐步回归模型明显优于线性函数,且水位深度越浅,该估算模型的适用性越高。  相似文献   

19.
Summary. A novel method is proposed for retrieving the 3-D orientation of axes of symmetry of near-source anisotropy by a non-linear inversion of observed radiation patterns of seismic displacement spectra of Rayleigh waves.
If faulting is generated within an anisotropic source region, body force equivalents for the faulting are in general not a double couple but the sum of three orthogonal dipole forces (Kosevich; Kawasaki & Tanimoto). As a result of the third dipole force, radiation patterns of Rayleigh waves are deformed, the deformation amounting to several per cent of those for an isotropic source medium. The non-linear inversion is carried out to find the optimum fault plane solutions giving the minimum square residual between observed and theoretical radiation patterns in some period range. In order to remove effects of heterogeneity along propagation paths, a pair-event scheme is involved in the inversion, which denotes taking spectral amplitude ratios and differential phases of the seismic displacement spectra of the pair-events having close hypocentres and different fault plane solutions. The uniqueness of the fault plane solutions of the non-linear inversion is afforded a proof by the Monte-Carlo experiment.
The non-linear inversion is repeated for some possible types of symmetry of the near-source orthotropic anisotropy due to the preferred orientation of olivine crystals as mantle materials. Square residuals thus obtained are compared with each other to see which orientation gives the minimum.
The method is applied to pair-events which occurred in the anomalous mantle beneath the Mid-Atlantic Ridge. This leads to a discovery that one type of symmetry of the preferred orientations with a -, b - and c-axes aligned vertical, parallel to and perpendicular to the trend (N11E) of the ridge axis, respectively, is most likely existing in the anomalous mantle.  相似文献   

20.
《Basin Research》2018,30(4):766-782
This paper proposes a new methodology to improve the location of potential karstified areas by gravity inversion of a 3D geological model. A geological 3D model is built from surface observations, 2D seismic reflection profiles and well data. The reliability of this geological 3D model obtained from integration, interpretation and interpolation of such data is first tested against the structural consistency of the model. Its theoretical gravimetric response is compared to gravity field during the forward problem in order to evaluate the validity/robustness of the geological model. The coherency between the gravity field and the gravimetric response is tested. The litho‐inversion modelling quantifies the distribution of rock density in a probabilistic way, taking into account the geology and physical properties of rocks, while respecting the geological structures represented in the 3D model. The result of the inversion process provides a density distribution within carbonate formations that can be discussed in term of karstification distribution. Thus, lower densities correlate with areas that are strongly karstified. Conversely, higher than mean densities are found in carbonate formations mostly located under marly and impervious formations, preserving carbonate from karstification and paleokarstification.  相似文献   

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