共查询到20条相似文献，搜索用时 15 毫秒
1.
A. K. GOODACRE 《Geophysical Prospecting》1973,21(1):66-69
The use of arctangents rather than arcsines in the expression for the gravitational attraction of a homogeneous rectangular prism reduces computational difficulties. Once a subroutine is available to compute one component of attraction in a Cartesian coordinate system, the other components may be obtained by cyclic permutation of the field point and body coordinate parameters. This technique also readily provides derivatives of the gravitational attraction and hence forms a compact method for the calculation of a magnetic anomaly due to a homogeneous rectangular magnetic prism. 相似文献
2.
A precise method of gravity interpretation is described which uses tables for the attraction of rectangular prisms in all positions relative to a given point. Hence the attraction of any body or combination of bodies may be assessed without complicated mathematical procedures. 相似文献
3.
The prism method for terrain corrections using digital computers 总被引：1，自引：0，他引：1
Dezső Nagy 《Pure and Applied Geophysics》1966,63(1):31-39
Summary In the prism method of making terrain corrections, the topography is approximated by a model consisting of right rectangular prisms. The vertical component of the gravitational attraction of each prism is calculated and the sum of these components gives the terrain correction.The prism method as programmed has no computational limitations. It can be used on all sizes of computers; it can be applied to a large area with any fine grid interval; it can be processed in a single run and yet provides complete flexibility for both research and routine computations. This has been achieved by breaking up larger areas into regions which fit into the computer memory. The contributions of these regions are automatically summed up for each station. While processing each region, various controls may be used at each station to exclude the contribution of a distant part of the area, to use approximate expressions farther from the station, to print out details around the station. There is also provision to refine the model by using smaller prisms around each computation point. Thus full use of elevation control can be made to calculate the terrain correction, the accuracy of which depends only on the quality of the input data.The prism method has been used to calculate terrain corrections for 130 stations in the New Quebec crater area. For five of these stations terrain corrections were also calculated by usingHammer's template. The two independent sets of values differ by less than four per cent. 相似文献
4.
D. J. WOODWARD 《Geophysical Prospecting》1975,23(3):526-532
Formulae for the gravitational attraction, at a point, of vertical triangular prisms with oblique ends are presented. They are suited to computation on an electronic computer and are useful for determining the thickness of sediments in a sedimentary basin by iterative means. 相似文献
5.
A nomogram has been devised for situations, in which the source of a self-potential anomaly can be approximated by an obliquely polarized sphere or horizontal cylinder embedded in a homogeneous half space. The nomogram can be used for rapid determination of three parameters of the target: (1) depth to the centre, (2) angle between the axis of polarization and the horizontal, (3) shift of the point vertically above the centre of the body from zero potential value. The nomogram has been tested and the parameters determined for SP results obtained over ore bodies Weiss and Süleymanköy in the Ergani Copper district, Turkey. The curves computed for the estimated parameters match the field curves well. 相似文献
6.
K. KUNARATNAM 《Geophysical Prospecting》1981,29(6):883-890
The magnetic anomaly due to a uniformly magnetized vertical rectangular prism and that due to an arbitrary structure which can be divided into a number of such prisms are expressed in forms suitable for rapid computation. Both two-dimensional and three-dimensional cases are considered. The simplified expressions will find use in interpretation techniques where repeated computations have to be made of the anomaly due to prisms as in automated fitting of prism anomalies to observed magnetic anomalies using non-linear optimization techniques or related methods. 相似文献
7.
A brief review of the existing methods of gravity reduction is given and a new method suitable for use on high speed digital computers is described. The method is based on the formula for the gravitational attraction of a frustum of a cone. The topographic contours are represented by polygons and the x and y coordinates of corners of the polygons constitute the input to the computer. The vertical component of the gravitational attraction is calculated by evaluating the cone formula for a number of vertical sections of the topography. Each vertical section is simplified by adopting a procedure of grouping and averaging for the distant points of the section. The effect of the earth's sphericity is taken into account by lowering the distant points of the sections by amounts determined by the curvature. The computations include the area close to the point at which the attraction is required and may be limited to an area defined by a circle centered at this point. The method is therefore compatible with the conventional zone chart methods. As an illustration of the method the gravitational attraction of Caryn Seamount in the Atlantic Ocean is computed. The total Bouguer correction and the Terrain correction are also computed for an area in northwestern South America and comparisons are made with hand computations by a zone chart method. As an example, for work at sea, the Bouguer corrections for an area near the Island of Mauritius in the Indian Ocean are computed and the effects of sphericity and three-dimensionality are calculated. The gravitational attraction of two-dimensional bodies can be computed in a very similar manner. The attraction of the Puerto Rico Trench model is computed and the results are compared with other methods. The effects of sphericity and assumptions involved in extending the models to infinity are discussed. 相似文献
8.
本文对研究区(98°E～103°E, 315°N～34°N) 1∶50万航磁资料的476条南北向剖面采用磁性标量层析成像方法进行了2-D成像反演.每条剖面资料点距1 km，反演的磁性柱体剖面方向宽度1 km，共约277个柱体，深度方向共8层柱体，每层厚度4 km.反演的476个断面的磁性柱体相对磁化强度幅值组成了3-D磁性数据体.对5.5、17.5、21.5、29.5 km深的磁性水平切片和7个北东向的磁性垂直切片表现的磁性特征进行了研究，并进行了地质解释.结合研究区已经公布的地质、地球物理资料，讨论了磁性细结构能提供工程地质灾害数值模拟仿真结构模型的信息，得到了反演的磁性细结构特征比场的特征能提供更多的断层和岩体3.D细节信息的初步认识. 相似文献
9.
Martina Capponi Ahmed H. Mansi Daniele Sampietro 《Studia Geophysica et Geodaetica》2018,62(2):206-222
The precise computation of the vertical gravitational attraction of the topographic masses (terrain correction) is still being studied both for geodetic and geophysical applications. In fact, it is essential in high precision geoid estimation by means of the well-known remove-compute-restore technique, which is used to isolate the gravitational effects of anomalous masses in exploration geophysics. The terrain correction can be evaluated exploiting a Digital Terrain Model (DTM) in different ways, such as classical numerical integration, prisms, tesseroids, polyhedrons, and/or Fast Fourier Transform techniques. The increasing resolution of recently developed DTMs, the increasing number of observation points, and the increasing accuracy of gravity data represent, nowadays, major challenges for the terrain correction computation. Classical point mass approximation and prism based-algorithms are indeed too slow, while Fourier-based algorithms are usually too much approximate when compared to the required accuracy. In this work, we improve the Gravity Terrain Effects (GTE) algorithm, the innovative tool that exploits a combined prism-Fast Fourier Transform approach especially developed for airborne gravimetry, to compute the terrain correction on the surface of the DTM (i.e. corresponding to the ground stations and/or its vicinity). This required development of a proper adjustment of the algorithms implemented within the GTE software and also to define and implement a procedure to overcome the problems of the computation of the gravitational effects due to the actual slope of the terrain close to the stations. The latter problem is thoroughly discussed and solved by testing different solutions like concentric cylindrical rings, triangulated polyhedrons, or ultra-high resolution squared prisms. Finally, numerical tests to prove the temporal efficiency and the computational performances of the improved GTE software to compute terrain correction for ground stations are also presented. 相似文献
10.
A recently proposed method for the computation of the gravitational effect due to the topographic masses defined by a Digital Elevation Model (DEM) involves the representation of the surface relief by means of parts of bilinear surfaces. The so-called bilinear method delivers eventually the mathematical model for the gravitational attraction of a right rectangular prism, whose top is modeled by a bilinear surface. Scope of the paper is to assess the new method by conducting numerical tests using both real and synthetic data. The performance of the bilinear method is evaluated in terms of its computational efficiency as well as its precision by comparing it with other analytical methods available for the practical evaluation of gravitational terrain effects. The techniques considered for the assessment of the bilinear approximation are the vastly applied right rectangular prism method and the polyhedral modeling, a less popular but extremely flexible approach based on the closed expression for the gravity field of an arbitrarily shaped mass distribution defined by planar faces. The different geometric modeling of the topographic relief produces discrepancies to the gravitational attraction of up to several mGal. Thus the choice for the geometric representation of the terrain plays a fundamental role to the numerical computation of potential field quantities especially in the critical region surrounding the computation point. 相似文献
11.
The gravitational attraction produced by a geological body of irregular shape can be easily determined by dividing it into cubes of small size. The exact expression of the effect of a cube is very complicated, but it can be calculated by using an electronic computer. 4851 values of the gravitational attraction were determined for different positions of a cube with the side of 2l and the center in M(x0, y0, z0), for x0∈[0;20], y0∈[0;20] and z0∈[0;10]. Using these values, templates in double logarithmic representation were drawn, with x0 and z0 as parameters and y0 as abscissa, and with x0 and y0 as parameters and z0 in abscissa; this double set of templates permits a good interpolation for all cubes in the considered domain of M. The use of templates was tested to approximate the effect produced by a theoretical model of spherical shape and in a real case of a three-dimensional salt body of known shape based on a large number of boreholes. In both cases very good results were obtained. 相似文献
12.
D. Zidarov 《Pure and Applied Geophysics》1971,86(1):5-12
Summary A method is developed for rapid gravity interpretation of a sedimentary basin using a digital computer. Assuming a linear variable density contrast (both vertically and horizontally), expressions have been derived for the gravitational attraction of a rectangular strip. An interpretation of the structure of Godavari Valley (India) has been made using these expressions. 相似文献
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14.
N.P.J. O&#x;Hora 《Physics of the Earth and Planetary Interiors》1973,7(1):92-96
Deflexions of the vertical due to changes in luni-solar gravitational attraction can be determined by astronomical observations and the measures are of great importance in the derivation of values for parameters defining elastic properties of the earth. The primary purpose of the observations is the determination of time or latitude, or both, and the systematic deflexions of the vertical are small in comparison with the scatter of the observations so that regular observations made over a number of years are required for the detection of these effects. Eleven years of time and latitude observations made with the Herstmonceux Photographic Zenith Tube have been separately analysed and the two solutions reveal with great clarity the magnitudes and phases of the variations associated with tidal activity. The results are complicated because at Herstmonceux the deflexions arising directly from changes in the gravitational attraction of the moon have associated with them deflexions caused by changes in the attraction of the proximate waters of the English Channel which is itself affected by complex tidal activity. 相似文献
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16.
本文利用连云港～临沂～泗水剖面的重力测量资料计算出地壳厚度,并与人工地震测深结果进行比较,讨论了该剖面的重磁异常和地壳厚度分布特征。 相似文献
17.
The forward computation of the gravitational and magnetic fields due to a 3D body with an arbitrary boundary and continually varying density or magnetization is an important problem in gravitational and magnetic prospecting. In order to solve the inverse problem for the arbitrary components of the gravitational and magnetic anomalies due to an arbitrary 3D body under complex conditions, including an uneven observation surface, the existence of background anomalies and very little or no a priori information, we used a spherical coordinate system to systematically investigate forward methods for such anomalies and developed a series of universal spherical harmonic expansions of gravitational and magnetic fields. For the case of a 3D body with an arbitrary boundary and continually varying magnetization, we have also given the surface integral expressions for the common spherical harmonic coefficients in the expansion of the magnetic field due to the body, and a very precise numerical integral algorithm to calculate them. Thus a simple and effective method of solving the forward problem for magnetic fields due to 3D bodies of this kind has been found, and in this way a foundation is laid for solving the inverse problem of these magnetic fields. In addition, by replacing the parameters and unit vectors in the spherical harmonic expansion of a magnetic field by gravitational parameters and a downward unit vector, we have also derived a forward method for the gravitational field (similar to that for the magnetic case) of a 3D body with an arbitrary boundary and continually varying density. 相似文献
18.
The decrease of density contrast in sedimentary basins may be approximated by a quadratic function. A sedimentary basin may be viewed as a number of prisms placed in juxtaposition. Equations in closed form for the gravity anomalies of 3D and 2½ D prismatic models are derived. Approximate equations for these models are also derived for rapid calculations. Efficient methods are developed for anomaly calculation by an appropriate use of the exact and approximate equations, and hence, for 3D and 2½ D modelling. The depths to the basement are adjusted iteratively by comparing the calculated anomalies with the observed anomalies. These methods are applied for analysis of the residual anomaly map of the Los Angeles Basin, California. 相似文献
19.
The response of a rectangular plate to the Crone PEM system has been investigated for different values of depth, dip and conductance parameters. The conductance aperture diagram is presented to estimate the conductance and depth to the top of a vertical sheet. Information regarding dip and depth may be obtained using the appropriate nomogram. 相似文献
20.
This paper describes application of a fast inversion method to recover a 3D susceptibility model from magnetic anomalies. For this purpose, the survey area is divided into a large number of rectangular prisms in a mesh with unknown susceptibilities. Solving the full set of equations is substantially time consuming, and applying an algorithm to solve it approximately can reduce the time significantly. It is shown that the Lanczos bidiagonalization method can be an appropriate algorithm to solve a Tikhonov cost function for this purpose. Running time of the inverse modeling significantly decreases by replacing the forward operator matrix with a matrix of lower dimension. A weighted generalized cross validation method is implemented to choose an optimum value of a regularization parameter. To avoid the natural tendency of magnetic structures to concentrate at shallow depth, a depth weighting is applied. This study assumes that there is no remanent magnetization. The method is applied on a noise-corrupted synthetic data to demonstrate its suitability for 3D inversion. A case study including ground based measurement of magnetic anomalies over a porphyry-Cu deposit located in Kerman providence of Iran, Now Chun deposit, is provided to show the performance of the new algorithm on real data. 3D distribution of Cu concentration is used to evaluate the obtained results. The intermediate susceptibility values in the constructed model coincide with the known location of copper mineralization. 相似文献