共查询到20条相似文献，搜索用时 62 毫秒
1.
The existing concept of the gradient of the potential field anomaly over a 3D source has been generalized. An observed anomaly is modified through a filter based on an assumed source geometry. The first-order derivatives of this modified anomaly in three mutually orthogonal directions form the components of a vector termed the resultant gradient. The gravity anomaly over a point mass, a vertical line mass and the gravity/magnetic anomaly reduced-to-pole over a bottomless right rectangular prism have been suitably modified to yield a specific shape for the amplitude of the resultant gradient in order to decipher the depth of the source centroid/corner. The applicability of the proposed technique is demonstrated by the analyses of a simulated example over a composite source and a real example from published literature with drill-hole information. 相似文献
2.
Euler's homogeneity equation has been used to develop a new technique to interpret the gravity anomalies over some simple geometrical sources, namely a finite horizontal line/vertical line, a finite vertical ribbon, a semicircular dome/basin and an isosceles triangle approximating an anticline/syncline. A linear over-determined system of equations has been solved to compute the depth, the horizontal location and the structural index, all treated as free parameters. The concept of a variable structural index provides better depth estimates and helps to identify the source geometry. Nomograms have been prepared to compute an additional model parameter, namely the horizontal/vertical extent of a line, the vertical extent of a ribbon and the radius of a dome/basin. The efficacy of the proposed method has been evaluated using two real field examples. 相似文献
3.
Fourier transformation of gravity and magnetic anomalies from space to the frequency domain provides aready method for source depth estimation since progressively deeper sources are indicated by the lower frequency components of an anomaly. Two examples are presented here to demonstrate the efficacy of this approach. One example pertains to the interpretation of vertical intensity magnetic anomalies over a layered ultramafic body for estimating the thickness of relatively non-magnetic layer in it and map the dispositions of the layers. The other example is the estimation of crustal thickness in the Indian region from the Bouguer anomaly map. The spectral method is shown, by these two examples, to provide a rapid and elegant tool for the source depth estimation for magnetic and gravity data. 相似文献
4.
《Journal of Geodynamics》2003,35(1-2):209-220
The occurrence of swarm earthquakes in the Vogtland/NW-Bohemia area results probably from the physical interactions of fluids, the stress field and the geometry of the geological units. Therefore the present study aims at the development of a 3-D density model of the region with a vertical range of 35 km. A new Bouguer anomaly map is presented containing about 17 000 gravity data points. Prominent Bouguer anomalies are produced by the granites of Eibenstock and Karlovy Vary (low with −75 mGal), the metabasites near Mariánzké Lázně (high with 5 mGal) and the Münchberg Gneiss Massif (gravity high of Hof with 10 mGal). The geometry of the internal model structures correspond to geological units and, thus, the modelled gravity fits well the observed Bouguer anomaly. The 3-D gravimetric modellings indicate detailed geometries of the geological settings. With regard to the periodic occurrence of swarm earthquakes in the Vogtland region the existence of an upwelling mantle or a magmatic body is investigated. Precise information only can be given, if the vertical extension of the near surface bodies is known. 相似文献
5.
Magnetic anomalies are often disturbed by the magnetization direction, so we can’t directly use the original magnetic anomaly to estimate the exact location and geometry of the source. The 2D analytic signal is insensitive to magnetization direction. In this paper, we present an automatic method based on the analytic signal horizontal and vertical derivatives to interpret the magnetic anomaly. We derive a linear equation using the analytic signal properties and we obtain the 2D magnetic body location parameters without giving a priori information. Then we compute the source structural index (expressing the geometry) by the estimated location parameters. The proposed method is demonstrated on synthetic magnetic anomalies with noise. For different models, the proposed technique can both successfully estimate the location parameters and the structural index of the sources and is insensitive to noise. Lastly, we apply it to real magnetic anomalies from China and obtain the distribution of unexploited iron ore. The inversion results are consistent with the parameters of known ore bodies. 相似文献
6.
The spectrum of a magnetic or a gravity anomaly due to a body of a given shape with either homogeneous magnetization or uniform density distribution can be expressed as a product of the Fourier transforms of the source geometry and the Green's function. The transform of the source geometry for any irregularly-shaped body can be accurately determined by representing the body as closely as possible by a number of prismatic bodies. The Green's function is not dependent upon the source geometry. So the analytical expression for its transform remains the same for all causative bodies. It is, therefore, not difficult to obtain the spectrum of an anomaly by multiplying the transform of the source geometry by that of the Green's function. Then the inverse of this spectrum, which yields the anomaly in the space domain, is calculated by using the Fast Fourier Transform algorithm. Many examples show the reliability and accuracy of the method for calculating potential field anomalies. 相似文献
7.
Subsalt exploration for oil and gas is attractive in regions where 3D seismic depth-migration to recover the geometry of a salt base is difficult. Additional information to reduce the ambiguity in seismic images would be beneficial. Gravity data often serve these purposes in the petroleum industry. In this paper, the authors present an algorithm for a gravity inversion based on Tikhonov regularization and an automatically regularized solution process. They examined the 3D Euler deconvolution to extract the best anomaly source depth as a priori information to invert the gravity data and provided a synthetic example. Finally, they applied the gravity inversion to recently obtained gravity data from the Bandar Charak (Hormozgan, Iran) to identify its subsurface density structure. Their model showed the 3D shape of salt dome in this region. 相似文献
8.
M. K. PAUL 《Geophysical Prospecting》1972,20(1):118-129
The interpretation of the gravity anomaly on a horizontal plane over a causative body having circular symmetry about a vertical axis is considered from a rather unconventional approach. As the analytical expression for the gravity effect of a circular body assumes a closed form only on the axis of symmetry, the interpretation in this approach is carried out with the anomaly profile along the axis—which leads to simpler and faster computation. A numerical method is developed for computation of the anomaly profile along the vertical axis from the horizontal radial profile of the symmetric anomaly by upward continuation. Provision is also made for an end correction when the radial profile has only a limited extension. Some simple geometrical shapes are assumed for the causative body. Its parameters are then determined from least squares fitting of its gravity effects to the observed (upward continued) vertical profile (i) by the steepest descent method and (ii) by the Newton-Raphson method. Some applications of these methods are demonstrated. 相似文献
9.
Pierre Keating 《Geophysical Prospecting》1995,43(4):569-580
Gravity data are often acquired over long periods of time using different instruments and various survey techniques, resulting in data sets of non-uniform accuracy. As station locations are inhomogeneously distributed, gravity values are interpolated on to a regular grid to allow further processing, such as computing horizontal or vertical gradients. Some interpolation techniques can estimate the interpolation error. Although estimation of the error due to interpolation is of importance, it is more useful to estimate the maximum gravity anomaly that may have gone undetected by a survey. This is equivalent to the determination of the maximum mass whose gravity anomaly will be undetected at any station location, given the data accuracy at each station. Assuming that the maximum density contrast present in the survey area is known or can be reasonably assumed from a knowledge of the geology, the proposed procedure is as follows: at every grid node, the maximum mass whose gravity anomaly does not disturb any of the surrounding observed gravity values by more than their accuracies is determined. A finite vertical cylinder is used as the mass model in the computations. The resulting map gives the maximum detection error and, as such, it is a worst-case scenario. Moreover, the map can be used to optimize future gravity surveys: new stations should be located at, or near, map maxima. The technique is applied to a set of gravity observations obtained from different surveys made over a period of more than 40 years in the Abitibi Greenstone Belt in eastern Canada. 相似文献
10.
Summary A numerical method is proposed which enables a direct computation of the vertical gravity anomaly due to three-dimensional bodies of arbitrary shape and with an inhomogeneous density distribution. The bodies are approximated by an assembly of appropriate curved finite elements. Gravity effects of such elements are calculated using the Gaussian quadrature formula. The integration domain is simply a cube, into which all the finite elements are transformed. The accuracy of the method is tested on a numerical example. 相似文献
11.
A generic gravity source moment is an integral, over the source volume, of the product of the density distribution by a polynomia
in the Cartesian coordinates of a point belonging to this volume. We obtained a formal expression for a generic moment in
terms of integrals involving the gravity anomaly and the gravity potential. By analyzing the conditions under which this expression
is valid, we conclude that, without usinga priori information regarding the sources, it is possible to determine, from the gravity anomaly, any moment or linear combination
of moments whose associated polynomial has null Laplacian and depends only on the coordinates defining the measurement plane.
Additionally, no moment whose associated polynomial has a nonnull laplacian can be determined without usinga priori information of the source. 相似文献
12.
以河北省流动重力测点覆盖区域内邢台地区4条断裂为例,将卫星布格重力异常与实测布格重力异常进行对比,发现流动重力资料可更为详细地反映测点周围构造的空间分布特征,为精细刻画华北平原地区深部构造背景提供可靠资料。采用向上延拓方法,对实测布格重力异常进行处理,获得不同上延高度的布格重力异常图像,结合当地地震地质与构造资料进行分析,并对流动重力资料在构造解释上的应用前景进行展望,认为该方法可提高现有资料的利用价值,适合在流动重力测量地区推广。 相似文献
13.
对鲜水河断裂带重磁异常进行向上延拓,通过计算观测面高度异常与延拓后异常之间的相关系数得出最佳向上延拓高度,该延拓高度所对应的延拓结果即为研究区构造背景产生的异常值。向上延拓结果显示布格重力异常值沿鲜水河断裂带自北西向南东逐渐增大,反映出下地壳底边界沿该方向呈升高趋势,可能由青藏高原地壳软弱物质 “东向逃逸” 所致。化极后ΔT磁异常延拓结果表明鲜水河断裂带南东段的康定—石棉以东为强磁性刚性基底。以鲜水河断裂带为分界的不同地块之间基底岩石及地层物性的不同是断裂带南北磁性差异的主导原因。对鲜水河断裂带两侧各约50 km范围内地震的震源深度进行统计,经投影至剖面及线性拟合求出鲜水河断裂带的三维几何形状。结果表明鲜水河断裂带总体倾向南西,倾角近乎直立,范围约为57°—88°。 相似文献
14.
We invert 2D surface gravity data constrained both by geological and seismic information. We use a number of pre-processing tools in order to reduce the general multi-body inversion into several single-body inversions, whereby we can reduce the overall complexity of the inversion task. This is done with as few assumptions as possible. Furthermore, for a single-body inversion we uncouple the determination of the shape of the causative sources from the determination of their mass density contrast to the surroundings. The inversion for the geometrical shape of the source body is done in steps. Firstly, a rough 3D shape of the source is modelled—a model consisting of the vertical mass columns of equal height. The horizontal extension is implied by the surface gravity signal. Subsequently, the shape of each source body is modified to obtain a better fit to the surface gravity data. In each modification step, the overall change of the shape of the source body is followed by an update of the mass density contrast to the surroundings. The technique was applied to a set of gravity data from the Eastern Goldfield area in Western Australia. The area has been widely studied in the past. In 1999, two seismic profiles that cross-sect the area were measured. Furthermore, an extensive geological modelling for the area has been conducted. The practical goal of this work was to verify the geological interpretation using the potential field data (mainly the gravity data although magnetic data were also available) and only weakly constrained by the seismic information. The result was the reconstruction of the ‘rough’ 3D geometry of the source bodies and the estimation of a constant mass density contrast to the surroundings. A possible extension of this technique for detailed studies of the geological model is briefly discussed. 相似文献
15.
Shallow structure beneath the Central Volcanic Complex of Tenerife from new gravity data: Implications for its evolution and recent reactivation 总被引：1，自引：0，他引：1
J. Gottsmann A.G. Camacho J. Martí L. Wooller J. Fernndez A. García H. Rymer 《Physics of the Earth and Planetary Interiors》2008,168(3-4):212-230
We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain, constructed from the amalgamation of 323 new high precision gravity measurements with existing gravity data from 361 observations. The new anomaly map images the high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera in greater detail than previously available. Mathematical construction of a sub-surface model from the local anomaly data, employing a 3D inversion based on “growing” the sub-surface density distribution via the aggregation of cells, enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture. We find the resultant density distribution in agreement with geological and other geophysical data. The modelled sub-surface structure supports a vertical collapse origin of the caldera, and maps the headwall of the ca. 180 ka Icod landslide, which appears to lie buried beneath the Pico Viejo–Pico Teide stratovolcanic complex. The results allow us to put into context the recorded ground deformation and gravity changes at the CVC during its reactivation in spring 2004 in relation to its dominant structural building blocks. For example, the areas undergoing the most significant changes at depth in recent years are underlain by low-density material and are aligned along long-standing structural entities, which have shaped this volcanic ocean island over the past few million years. 相似文献
16.
Gary Barnes 《Geophysical Prospecting》2014,62(3):646-657
When anomalous gravity gradient signals provide a large signal‐to‐noise ratio, airborne and marine surveys can be considered with wide line spacing. In these cases, spatial resolution and sampling requirements become the limiting factors for specifying the line spacing, rather than anomaly detectability. This situation is analysed by generating known signals from a geological model and then sub‐sampling them using a simulated airborne gravity gradient survey with a line spacing much wider than the characteristic anomaly size. The data are processed using an equivalent source inversion, which is used subsequently to predict and grid the field in‐between the survey lines by means of forward calculations. Spatial and spectral error analysis is used to quantify the accuracy and resolution of the processed data and the advantages of acquiring multiple gravity gradient components are demonstrated. With measurements of the full tensor along survey lines spaced at 4 × 4 km, it is shown that the vertical gravity gradient can be reconstructed accurately over a bandwidth of 2 km with spatial root‐mean square errors less than 30%. A real airborne full‐tensor gravity gradient survey is presented to confirm the synthetic analysis in a practical situation. 相似文献
17.
在使用电偶极发射源的可控源电磁法(CSEM)勘探中,发射源的方位、长度、形状等对观测数据有重要的影响,然而现有的大部分三维数值模拟方法没有全面地将这些因素考虑进来,很多都只能应对非常简单的场源形态,例如单一方位的点电偶极子,这有可能显著降低模拟结果的准确性.本文实现了基于交错网格有限体积(FV)离散的海洋CSEM三维正演算法,能够模拟形态相对复杂的场源,包括任意方位的有限长直导线和弯曲导线发射源.该算法使用一次场/二次场方法,只需对二次场使用FV法求解,避免了场源的奇异性问题;一次场的计算为一维正演问题,使用准解析法求解,并且只要在计算一次场时考虑复杂的场源形态便可以实现同样场源的三维正演.通过与一维理论模型的解析解对比验证了三维程序的准确性,并针对三维理论模型进行了一系列正演测试,初步考察了场源形态对三维正演结果的影响. 相似文献
18.
The influence of a partially conducting overburden/host rock on the electromagnetic response of a horizontal, tabular conducting ore body, investigated with the aid of quantitative scale model experiments, was discussed in an earlier paper (Gaur, Verma and Gupta 1971), which will be referred to as I. This paper presents the results of more comprehensive experiments subsequently carried out to study the combined effect of various geological parameters, namely: the dip and depth of burial of the ore body, its electrical conductivity and that of the zone surrounding it. These results obtained for four different transmitter-receiver configurations confirm the general enhancement of response in varying degrees, brought about by a conducting overburden. However, the transformation of the shapes of the anomaly profiles with increasing overburden conductivity is observed to depend on the dip of the ore body, being more drastic for gently dipping ones. Variations in the inphase and quadrature components as well as in the phase of the anomaly have been studied for varying depths of burial of the ore body and for a number of values of the solution conductivity. Anomaly index diagrams have been constructed with a view to predicting possible values of the geological parameters from a knowledge of the anomaly components. It is felt that the notable overburden effects are caused by a drastic redistribution and concentration of currents, mostly in a narrow loop at the top of the ore body, brought about by its galvanic contact with an extended medium of relatively poorer conductivity. 相似文献
19.
20.
A set of parabolae is defined to approximate the observed gravity profile for specifying the lateral density variation in a two dimensional causative body. This variation separates the “biased” as well as “unbiased” residual anomaly structure. The method may be directly incorporated into existing numerical and graphical techniques of interpretation for subsequent delineation of structural configuration. The simple relations derived by inspection of the observed gravity profile permit universal application of the technique and in particular for the determination of the size and shape of sub-shelf structures. The suitability of the method in various problems of two dimensional interpretation has been indicated through an illustration of the Godavari Basin (India). 相似文献