共查询到20条相似文献，搜索用时 203 毫秒
1.
B. SJ
GREN 《Geophysical Prospecting》1979,27(3):507-538
For detailed determination of refractor velocities of in-line reversed profiles, two principally different systems have been employed, the ABC method and Hales's method. The two systems differ in the travel-path arrangements used. The more conventional approach to the problem, the ABC method, makes use of critically refracted rays converging on a common surface position, while Hales's method deals with the common position on the refractor surface from which critically refracted rays diverge towards the ground surface. Because of the travel-path system used, Hales's method has proved to be particularly applicable to high-relief structures and to cases where the refractor lies at considerable depth. Some of the ambiguities in more commonly used interpretation techniques can be solved by Hales's method. Some attention has also been paid to errors caused by non-critical refractions and to the diffraction problem. 相似文献
2.
Seismic interferometry is a relatively new technique to estimate the Green's function between receivers. Spurious energy, not part of the true Green's function, is produced because assumptions are commonly violated when applying seismic interferometry to field data. Instead of attempting to suppress all spurious energy, we show how spurious energy associated with refractions contains information about the subsurface in field data collected at the Boise Hydrogeophysical Research Site. By forming a virtual shot record we suppress uncorrelated noise and produce a virtual refraction that intercepts zero offset at zero time. These two features make the virtual refraction easy to pick, providing an estimate of refractor velocity. To obtain the physical parameters of the layer above the refractor we analyse the cross‐correlation of wavefields recorded at two receivers for all sources. A stationary‐phase point associated with the correlation between the reflected wave and refracted wave from the interface identifies the critical offset. By combining information from the virtual shot record, the correlation gather and the real shot record we determine the seismic velocities of the unsaturated and saturated sands, as well as the variable relative depth to the water‐table. Finally, we discuss how this method can be extended to more complex geologic models. 相似文献
3.
E. BRÜCKL 《Geophysical Prospecting》1987,35(9):973-992
We consider multiply covered traveltimes of first or later arrivals which are gathered along a refraction seismic profile. The two-dimensional distribution of these traveltimes above a coordinate frame generated by the shotpoint axis and the geophone axis or by the common midpoint axis and the offset axis is named a traveltime field. The application of the principle of reciprocity to the traveltime field implies that for each traveltime value with a negative offset there is a corresponding equal value with positive offset. In appendix A procedures are demonstrated which minimize the observational errors of traveltimes inherent in particular traveltime branches or complete common shotpoint sections. The application of the principle of parallelism to an area of the traveltime field associated with a particular refractor can be formulated as a partial differential equation corresponding to the type of the vibrating string. The solution of this equation signifies that the two-dimensional distribution of these traveltimes may be generated by the sum of two one-dimensional functions which depend on the shotpoint coordinate and the geophone coordinate. Physically, these two functions may be interpreted as the mean traveltime branches of the reverse and the normal shot. In appendix B procedures are described which compute these two functions from real traveltime observations by a least-squares fit. The application of these regressed traveltime field data to known time-to-depth conversion methods is straightforward and more accurate and flexible than the use of individual traveltime branches. The wavefront method, the plus-minus method, the generalized reciprocal method and a ray tracing method are considered in detail. A field example demonstrates the adjustment of regressed traveltime fields to observed traveltime data. A time-to-depth conversion is also demonstrated applying a ray tracing method. 相似文献
4.
The purpose of this report is to show a method of determining the top of a refractor departing from the times and slopes of the direct and inverse dromocrones. The method does not need topographical correction and can be applied without knowledge of the distance between the geophone and the shot point. These results having been obtained, the commonly accepted point of view is upset: instead of looking for two points on the surface corresponding to one point of the refractor, we try to etablish, starting with only one point from the surface, the two corresponding points from the top of the refractor. This method can be applied to isolated points and does not demand interpretative hypotheses of any kind, excluding the velocity evaluation of the overburden and of the refractor. The necessary calculations can be easily executed by means of a digital computer to which the dromocrone times and the distances between the geophones must be given. These calculations can also be executed by a person having no knowledge of refraction seismology. This report also examines the validity of the approximations involved in the method proposed. 相似文献
5.
An analysis of amplitudes of refraction records of some shallow refraction profiles shot primarily for detailing the near-surface structure in a granitic terrain has yielded information on refractor properties: reduced amplitudes are plotted on amplitude-distance graphs. The negative power n to which distance should be raised to represent (elastic) amplitude decay with respect to distance due to spreading of the critically refracted wave involved is examined. Computed values of this “spreading index”n are close to n = 2 as predicted by the theory. With this value of n, amplitude data are processed to determine residual attenuation attributable to elastic absorption in the bedrock. A graphical approach for this purpose from comparison of amplitude-distance graphs with the plots of amplitude decay due to spreading which is applicable to flat and horizontal refractor situations is suggested. Assuming residual attenuation to represent absorption in the granite bedrock, the computed coefficients of absorption, which vary from 0.5 to 3.90 km?1 for a frequency of 50 Hz, are obtained. From amplitude graphs of reversed profiles it is shown that the amplitude differences plot bears a relation to lateral velocity changes in the refractor. From comparison of practical amplitude decay graphs with those computed for different subsurface models, it appears possible to detect fractured rock occurrences in the refractor. 相似文献
6.
波前重建法折射成像及应用研究 总被引：2，自引：0，他引：2
根据实际工作需要对传统的地震折射资料解释方法的适用范围进行了讨论,指出了传统折射资料解释方法所存在的问题。采用重建波前的方法进行折射成像,通过改进震源函数,并在反演过程中使用有限差分技术解程函方程,进行波场外推,从根本上解决了传统折射资料解释方法存在的问题,计算精度高,速度快。通过理论模型和实际资料的对比计算和验证,效果良好。 相似文献
7.
DERECKE PALMER 《Geophysical Prospecting》1991,39(8):1031-1060
The depth to the surface of a refractor and the seismic velocity within the refractor are very often intimately related. In the shallow environment, increased thicknesses of weathering occur in areas of jointing, shearing or lithological variations, and these zones of deeper weathering can have lower subweathering refractor velocities. This association is important in geotechnical investigations and in the measurement of weathering thicknesses and sub-weathering velocities for statics corrections for reflection seismic surveys. Algorithms, which employ forward and reverse traveltime data and which explicitly accommodate the offset distance through the process known as refraction migration, are necessary if detailed structure on a refractor and rapid lateral variations of the seismic velocity within it are to be resolved. These requirements are satisfied with wavefront construction techniques, Hales’ method and the generalized reciprocal method (GRM). However, these methods employ refraction migration in fundamentally different manners. Most methods compute an offset distance with an often imprecise knowledge of the seismic velocities of the overlying layers. In contrast, the GRM uses a range of offset distances from less than to greater than the optimum value, with the optimum value being selected with a minimum-variance criterion. The approach of the GRM is essential where there are undetected layers and where there are rapid variations in the depth to a refractor and the seismic velocity within it. In the latter situations the offset distance necessary to define the seismic velocities can differ considerably from the value required to define depths. The efficacy of the GRM in resolving structure and seismic velocity is demonstrated with three model studies and two field examples. 相似文献
8.
R.A. van Overmeeren 《Geophysical Prospecting》2001,49(6):687-696
Hagedoorn developed the plus-minus method with the objective of providing the exploration world with a simple and rapid approximation of Thornburgh's wavefront reconstruction method. Straightforward adding and subtracting produces depths and velocities at all geophones where the refracted waves from a set of reciprocal shots are first arrivals. Even though seismic refraction has not kept up with the revolutionary advance of reflection technology during the past decades, the method still has a lot to offer, especially in shallow engineering, and environmental, groundwater and sea-bed surveying. Its strong points are the ease of application, the low costs, the effectiveness in the shallow zone, the unique ability to provide detailed velocity information on the deepest refractor and the capability for producing parameters for layer and rock characterization.Because of its simplicity, the plus-minus method is ideally suited for real-time processing of refraction data in the field, thus monitoring the data quality and the optimal shot configuration. Today, such field processing is easily performed with a laptop and dedicated software. matlab ® is a software package that enables tailor-made processing, offering a combination of a programming language, visualization tools and a large library of ready-made functions. This paper presents a plus-minus program developed in matlab and illustrates its application with a case study in Yemen, where seismic refraction was used in a regional groundwater study. Here, refraction provided not only a detailed section across the recharge area of a coastal plain but also the additional information needed to reduce ambiguity in the interpretation of the vertical electrical soundings made in the area. 相似文献
9.
在城市活断层探测中 ,浅层结构常常表现为强烈的非均匀性 ,界面横向强烈起伏 ,层内速度变化较大 ,传统的基于平界面均匀层模型的折射资料处理方法不能适用。研究开发能应用于复杂介质结构中折射资料处理的方法就显得十分必要。文中基于惠更斯原理 ,用波前扩张法对波场作正演计算 ,根据哈格多恩折射波前成像原理 ,在lecomte算法和Hole有限差分计算程序的基础上 ,开发出 1种复杂介质结构中折射资料的处理方法与软件 ,并用此方法处理了福州城市活断层折射探测试验中在义序完成的 2条折射剖面资料。结果表明 :探测区浅层为 3层结构 ,分别为盖层、强风化层和基岩。基岩顶界面的埋深约为 5 8～ 5 2m ,盖层P波速度变化较大 相似文献
10.
The computation of static corrections requires information about subsurface velocities. This information can be obtained by different methods: surface wave analysis, short refraction lines, downhole times, uphole times and first arrivals from seismograms. For pure shear waves generated by SH sources the analysis of first arrivals from seismograms combined, if necessary, with short refraction lines has proved to be most accurate and economic. A comparison of first-arrival plots from P- and S-wave surveys of the same line measured in areas of unconsolidated sediments in northern Germany illustrates the characteristic differences between the two velocity models. P-waves show a marked velocity increase at the water table from about 600 to 1800 m/s. S-wave velocities of the same strata increase gradually from about 100 to 400 m/s. As a consequence, S-wave models are vertically and laterally more complex and, in general, show no significant velocity increase at a defined boundary as P-wave models do. Therefore, other suitable correction levels with specific velocities must be chosen. A comparison of “tgd-corrections” (correction time between geophone position and datum level) for P- and S-waves in areas of unconsolidated sediments shows that their ratio is different from the P-/S-velocity ratio for the respective correction level because of the greater depth of the S-wave refractor. Therefore, P- and S-waves are influenced by different near-surface anomalies, and time corrections calculated for both wave types are largely independent. 相似文献
11.
Traveltime computation by wavefront-orientated ray tracing 总被引：1，自引：0，他引：1
For multivalued traveltime computation on dense grids, we propose a wavefront‐orientated ray‐tracing (WRT) technique. At the source, we start with a few rays which are propagated stepwise through a smooth two‐dimensional (2D) velocity model. The ray field is examined at wavefronts and a new ray might be inserted between two adjacent rays if one of the following criteria is satisfied: (1) the distance between the two rays is larger than a predefined threshold; (2) the difference in wavefront curvature between the rays is larger than a predefined threshold; (3) the adjacent rays intersect. The last two criteria may lead to oversampling by rays in caustic regions. To avoid this oversampling, we do not insert a ray if the distance between adjacent rays is smaller than a predefined threshold. We insert the new ray by tracing it from the source. This approach leads to an improved accuracy compared with the insertion of a new ray by interpolation, which is the method usually applied in wavefront construction. The traveltimes computed along the rays are used for the estimation of traveltimes on a rectangular grid. This estimation is carried out within a region bounded by adjacent wavefronts and rays. As for the insertion criterion, we consider the wavefront curvature and extrapolate the traveltimes, up to the second order, from the intersection points between rays and wavefronts to a gridpoint. The extrapolated values are weighted with respect to the distances to wavefronts and rays. Because dynamic ray tracing is not applied, we approximate the wavefront curvature at a given point using the slowness vector at this point and an adjacent point on the same wavefront. The efficiency of the WRT technique is strongly dependent on the input parameters which control the wavefront and ray densities. On the basis of traveltimes computed in a smoothed Marmousi model, we analyse these dependences and suggest some rules for a correct choice of input parameters. With suitable input parameters, the WRT technique allows an accurate traveltime computation using a small number of rays and wavefronts. 相似文献
12.
V.B. Piip 《Geophysical Prospecting》2001,49(4):461-482
A method using simple inversion of refraction traveltimes for the determination of 2D velocity and interface structure is presented. The method is applicable to data obtained from engineering seismics and from deep seismic investigations. The advantage of simple inversion, as opposed to ray‐tracing methods, is that it enables direct calculation of a 2D velocity distribution, including information about interfaces, thus eliminating the calculation of seismic rays at every step of the iteration process. The inversion method is based on a local approximation of the real velocity cross‐section by homogeneous functions of two coordinates. Homogeneous functions are very useful for the approximation of real geological media. Homogeneous velocity functions can include straight‐line seismic boundaries. The contour lines of homogeneous functions are arbitrary curves that are similar to one another. The traveltime curves recorded at the surface of media with homogeneous velocity functions are also similar to one another. This is true for both refraction and reflection traveltime curves. For two reverse traveltime curves, non‐linear transformations exist which continuously convert the direct traveltime curve to the reverse one and vice versa. This fact has enabled us to develop an automatic procedure for the identification of waves refracted at different seismic boundaries using reverse traveltime curves. Homogeneous functions of two coordinates can describe media where the velocity depends significantly on two coordinates. However, the rays and the traveltime fields corresponding to these velocity functions can be transformed to those for media where the velocity depends on one coordinate. The 2D inverse kinematic problem, i.e. the computation of an approximate homogeneous velocity function using the data from two reverse traveltime curves of the refracted first arrival, is thus resolved. Since the solution algorithm is stable, in the case of complex shooting geometry, the common‐velocity cross‐section can be constructed by applying a local approximation. This method enables the reconstruction of practically any arbitrary velocity function of two coordinates. The computer program, known as godograf , which is based on this theory, is a universal program for the interpretation of any system of refraction traveltime curves for any refraction method for both shallow and deep seismic studies of crust and mantle. Examples using synthetic data demonstrate the accuracy of the algorithm and its sensitivity to realistic noise levels. Inversions of the refraction traveltimes from the Salair ore deposit, the Moscow region and the Kamchatka volcano seismic profiles illustrate the methodology, practical considerations and capability of seismic imaging with the inversion method. 相似文献
13.
A new 3D wavefield modelling approach based on dynamic ray tracing is presented. This approach is called wavefront construction, and it can be used in 3D models with constant or smoothly varying material properties (S- and P-velocity and density) separated by smooth interfaces. Wavefronts consisting of rays arranged in a triangular network are propagated stepwise through the model. At each time step, the differences in a number of parameters are checked between each pair of rays on the wavefront. New rays are interpolated whenever this difference between pairs of rays exceeds some predefined maximum value. A controlled sampling of the wavefront at all time steps is thus obtained. Receivers are given multiple-event values by interpolation when the wavefronts pass them. The strength of the wavefront construction method is that it is robust and efficient. 相似文献
14.
A useful method for increasing the signal/noise ratio of refracted waves is Common-Midpoint (CMP)-refraction seismics. With this technique the shallow underground can be described in detail using all information (amplitude, frequency, phase characteristics) of the wavetrain following the first break (first-break phase). Thus, the layering can be determined and faults, weak zones, and clefts can be identified. This paper deals with the optimization of CMP-refraction seismics used in combination with the Generalized Reciprocal Method (GRM). Theoretical studies show a close relationship of both methods to the kinematics of wave propagation. Velocities and optimum offsets determined by the GRM can be used directly in the partial Radon transformation in CMP-refraction seismics. The integration of refracted waves leads to an increase in the signal/noise ratio but simultaneously the integration boundaries must be restricted to deal only with selective parts of the investigated refractor. The result of this process is an intercept-time section which can be converted directly to a depth section using standard refraction seismic techniques. Another possibility of depth conversion is the transformation of this intercept-time section to a `pseudo-zero-offset section', known from reflection seismics. Thus, zero-offset sections can be migrated using wave-equation techniques such as Kirchhoff migration. 相似文献
15.
Derecke Palmer 《Geophysical Prospecting》2006,54(5):589-604
The performance of refraction inversion methods that employ the principle of refraction migration, whereby traveltimes are laterally migrated by the offset distance (which is the horizontal separation between the point of refraction and the point of detection on the surface), can be adversely affected by very near‐surface inhomogeneities. Even inhomogeneities at single receivers can limit the lateral resolution of detailed seismic velocities in the refractor. The generalized reciprocal method ‘statics’ smoothing method (GRM SSM) is a smoothing rather than a deterministic method for correcting very near‐surface inhomogeneities of limited lateral extent. It is based on the observation that there are only relatively minor differences in the time‐depths to the target refractor computed for a range of XY distances, which is the separation between the reverse and forward traveltimes used to compute the time‐depth. However, any traveltime anomalies, which originate in the near‐surface, migrate laterally with increasing XY distance. Therefore, an average of the time‐depths over a range of XY values preserves the architecture of the refractor, but significantly minimizes the traveltime anomalies originating in the near‐surface. The GRM statics smoothing corrections are obtained by subtracting the average time‐depth values from those computed with a zero XY value. In turn, the corrections are subtracted from the traveltimes, and the GRM algorithms are then re‐applied to the corrected data. Although a single application is generally adequate for most sets of field data, model studies have indicated that several applications of the GRM SSM can be required with severe topographic features, such as escarpments. In addition, very near‐surface inhomogeneities produce anomalous head‐wave amplitudes. An analogous process, using geometric means, can largely correct amplitude anomalies. Furthermore, the coincidence of traveltime and amplitude anomalies indicates that variations in the near‐surface geology, rather than variations in the coupling of the receivers, are a more likely source of the anomalies. The application of the GRM SSM, together with the averaging of the refractor velocity analysis function over a range of XY values, significantly minimizes the generation of artefacts, and facilitates the computation of detailed seismic velocities in the refractor at each receiver. These detailed seismic velocities, together with the GRM SSM‐corrected amplitude products, can facilitate the computation of the ratio of the density in the bedrock to that in the weathered layer. The accuracy of the computed density ratio improves where lateral variations in the seismic velocities in the weathered layer are known. 相似文献
16.
LUIGI ZANZI 《Geophysical Prospecting》1990,38(4):339-364
The aim of refracted arrivals inversion is the computation of near-surface information, i.e. first-layer thicknesses and refractor velocities, in order to estimate the initial static corrections for the seismic data. The present trend is moving towards totally automatic inversion techniques, which start by picking the first breaks and end by aligning the seismic traces at the datum plane. Accuracy and computational time savings are necessary requirements. These are not straightforward, because accuracy means noise immunity, which implies the processing of large amounts of data to take advantage of redundancy; moreover, owing to the non-linearity of the problem, accuracy also means high-order modelling and, as a consequence, complex algorithms for making the inversion. The available methods are considered here with respect to the expected accuracy, i.e. to the model they assume. It is shown that the inversion of the refracted arrivals with a linear model leads to an ill-conditioned problem with the result that complete separation between the weathering thickness and the refractor velocity is not possible. This ambiguity is carefully analysed both in the spatial domain and in the wavenumber domain. An error analysis is then conducted with respect to the models and to the survey configurations that are used. Tests on synthetic data sets validate the theories and also give an idea of the magnitude of the error. This is largely dependent on the structure; here quantitative analysis is extended up to second derivative effects, whereas up to now seismic literature has only dealt with first derivatives. The topographical conditions which render the traditional techniques incorrect are investigated and predicted by the error equations. Improved solutions, based on more accurate models, are then considered: the advantages of the Generalized Reciprocal Method are demonstrated by applying the results of the error analysis to it, and the accuracy of the non-linear methods is discussed with respect to the interpolation technique which they adopt. Finally, a two-step procedure, consisting of a linear model inversion followed by a local non-linear correction, is suggested as a good compromise between accuracy and computational speed. 相似文献
17.
KLAUS HELBIG 《Geophysical Prospecting》1990,38(2):189-220
Wavefront charts in anisotropic gradient media are a useful tool in ray geometric constructions, particular in shear-wave exploration. They can be constructed by: (i) a family of wavefronts that contains a vertical plane as member - it is convenient to choose constant time increments; (ii) tracing one ray that makes everywhere the angle with the normal to the wavefront that is required by the anisotropy of the medium; (iii) scaling this ray to obtain a set of rays with different ray parameters; (iv) shifting these rays (with wavefront elements attached) so that they pass through a common source point; (v) interpolating the wavefronts between the elements. The construction is particularly simple in linear-gradient media, since here all members of the family of wavefronts are planes. Since the ray makes everywhere the angle prescribed by the anisotropy with the normal of the (plane) wavefronts, the ray has the shape of the slowness curve rotated by ?π/2. For isotropic media the slowness curve is a circle, and thus rays are circular arcs. The circles themselves intersect in the source point and in a second point above the surface of the earth. This provides a simple proof that wavefronts emanating from a point source in an isotropic linear-gradient medium are spheres: inversion of the set of circular rays with the source as centre maps the pencil of circular rays into a pencil of straight lines passing through a point. A pencil of concentric spheres around this point is perpendicular to the pencil of straight lines. On inverting back the pencil of spheres is mapped into another pencil of spheres that is perpendicular to the circular rays. 相似文献
18.
在地震勘探中,地形起伏和近地表速度的剧烈变化会导致地震波旅行时的扰动,通常会通过折射波信息来估算和消除这些扰动.本文在虚折射的基础上提出了逆虚折射干涉法,通过虚折射波场和原始折射波场的互相关,并对所有位于固定相位点上的检波点进行叠加,重构出逆虚折射波场.通过逆虚折射与超级虚折射的叠加,保证了不同偏移距下折射波振幅恢复的一致性,显著提高折射波的信噪比,有效提取弱信号.同时,本文采用反褶积干涉法来压制由于互相关和褶积产生的子波旁瓣的影响,弥补低频和高频能量的损失,改善恢复的折射波场的稳定性和分辨率.该新方法不需要知道近地表复杂速度模型的信息,可以将虚折射的勘探孔径恢复到原始地震记录的最大孔径.合成资料和实际资料的计算结果表明,基于反褶积的逆虚折射干涉法能够从低信噪比的资料中,有效恢复出折射波信息. 相似文献
19.
The numerical tracing of short ray segments and interpolation of new rays between these ray segments are central constituents of the wavefront construction method. In this paper the details of the ray tracing and ray-interpolation procedures are described. The ray-tracing procedure is based on classical ray theory (high-frequency approximation) and it is both accurate and efficient. It is able to compute both kinematic and dynamic parameters at the endpoint of the ray segments, given the same set of parameters at the starting point of the ray. Taylor series are used to approximate the raypath so that the kinematic parameters (new position and new ray tangent) may be found, while a staggered finite-difference approximation gives the dynamic parameters (geometrical spreading). When divergence occurs in some parts of the wavefront, new rays are interpolated. The interpolation procedure uses the kinematic and dynamic parameters of two parent rays to estimate the initial parameters of a new ray on the wavefront between the two rays. Third-order (cubic) interpolation is used for interpolation of position, ray tangent and take-off vector from the source) while linear interpolation is used for the geometrical spreading parameters. 相似文献
20.
Roberto de Franco 《Geophysical Prospecting》2011,59(3):432-454
This paper presents a signal processing procedure to perform refractor velocity analysis. The procedure enables one to obtain the seismic velocity from the refracted wavefield without the picking of refracted arrival times. Two processing procedures are derived, one starting from a seismic interferometric approach and another, from the conventional reciprocal method and generalized reciprocal method approaches. The theoretical equivalence of the two approaches is also demonstrated. The proposed processing procedure is applied to synthetic data in order to test the influence of some procedural parameters and its capability to reconstruct a known velocity model starting from refracted signals, without and with perturbations, in arrival times and noise; finally, it is applied to a field data set. 相似文献