摘要:

AbstractIn seismic refraction surveys, in particular those using first arrival recording techniques, the hidden layer problem occurs where energy from a refractor of higher velocity arrives at the surface before energy from an overlying refractor. The maximum thickness of the hidden layer is referred to as the blind zone.Hypothetically, every recorded refractor has an associated blind zone which may or may not contain a hidden layer. For an assumed earth model of plane constant‐velocity layers and stepwise increase of velocity with depth, the effect of a blind zone on an interpreted depth section may be evaluated by defining an intercept time for a blind zone of assumed or known velocity and by using standard time‐term equations for layer thicknesses and depths.The treatment covers an arbitrary number of blind zones embedded within a multilayer sequence of horizontal or dipping refractors. Model calculations affirm the benefits of this approach compared with previous methods which, in general, have been restricted to the case of two horizontal layers with one intermediate blind z

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1978.tb01630.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

AbstractThe use of arrays to separate primary reflections from unwanted coherent seismic events is common practice in land seismic surveys. Very long source and receiver arrays have been used recently to reduce the effects of waterbottom multiples on marine seismic data.The source array consists of five uniformly spaced identical subarrays, each with five different airguns, where the distance between the subarrays may vary from 20 m56 m. The volume of each subarray is 10.3 1 (630 cu.in.) which gives a total volume of the array of 51.5 1 (3150 cu.in.) operated at a pressure of 14 MPa (2000 psi). In order to have a flexible receiver system it was decided to implement the extended receiver array in data processing by computing a weighted sum of two to five traces. The hydrophone cable consists of fifty‐four channels with a group length of 50 m.Data shot with the superlong airgun array are processed by a combination of standard techniques and special procedures. In particular, the quality of the stack section is improved by using a weighted stack. The stack weights are computed by a program which takes into account the primary‐to‐multiple ratio.Comparisons with conventional data show significant improvements in data quality obtained by using the superlong airgun array. Examples show that the waterbottom multiples have been strongly attenuated and the deep seismic events have been enhanced.The combined array response function for dipping events is given in an app

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1978.tb01631.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

AbstractThe problem of the propagation of acoustic waves in a two‐dimensional layered medium can be easily solved in the frequency domain if the Dix approximation is used, i.e. when only the primary reflections are considered. The migrated data at a depthzare obtained by convolving the time section with a proper two‐dimensional operator dependent onz.The same result can be obtained by multiplying their two‐dimensional spectra and summing for all the values of the temporal frequency.The aspect of the operator in the time‐space domain has the classic hyperbolic structure together with the prescribed temporal and spatial decay.The main advantages of the frequency domain approach consist in the noticeable computer time savings and in the better approximation. On the other hand lateral velocity variations are very difficult to be taken into account. This can be done if a space variant filter is used in the time‐space domain.To reduce computer time, this filter has to be recursive; the problem has been solved by Claerbout by transforming the hyperbolic partial differential equation into a parabolic one, and using the latter to generate the recursion operator.In the presentation a method is given for the generation of recursive filters with a better phase characteristics that have a pulse response with the requested hyperbolic shape instead of the parabocli one. This allows a better migration of ste

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1978.tb01632.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

AbstractFinite difference migration has been developed and popularized by J. F. Claerbout of Stanford University and is now widely used in seismic processing. For most sections finite difference migration gives results comparable to those obtained by conventional Kirchhoff migration and, where events are not dipping too much, a cleaner appearance is often apparent. However, there are two practical limitations to the method, and these occur in regions of very steep dip and where there is a large variation of the velocity in the lateral direction.It is possible to develop successively more accurate equations to deal with the steep dip problem, but above third order these schemes become prohibitively expensive to implement. The finite difference method itself introduces errors and so imposes further limitations on the angle of dip. For the effective treatment of steeply dipping beds there appears to be no method available in the time domain which does not suffer from dispersion inaccuracies. However, by developing wavenumber migration, an exact one‐way wave equation can be used, and this eliminates any error except that caused by finite sampling.The other difficulty with wave migration is the correct migration in regions with lateral velocity variation. A number of approaches are possible of which three are discussed here. The first uses an exact theory, the second is based on the deviation from a depth stratified model, and the third uses a transformation to a depth co‐ordinate system. All methods are discussed with their advantages and limitations. Finally, some examples are shown of wave migration applied to synthetic and real d

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1978.tb01633.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

AbstractThe method of finite differences is applied to the computation of multi‐dimensional synthetic seismograms. This paper gives a study of the mathematical and numerical formulations of the problem, the boundary conditions, the convergence conditions and how to simulate the source in both one solid or a liquid. It is shown that the numerical formulation chosen is valid both for direct and inverse problems (i.e. for modeling and migration). This formulation makes it possible to use the normal incidence reflection coefficients for P and S waves, whether they travel horizontally or vertically. The examples shown have been chosen on purpose in order to be easily interpreted. They do not give a full idea of the possibilities of the algorithm which allows to consider non‐planar interfaces, except close to the vertical a

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1978.tb01634.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

AbstractA spatial analysis of both continuous and discrete operators for upward continuation help us realize the problems and limitations which have been encountered before (Henderson 1960, Kontis 1971) but remained unsolved in practical application of upward continuation computation due to the finite length of data and operator in spatial domain. Various numerical examples show that an improvement of accuracy of continuation computations can be achieved through proper sampling and sufficient length of data.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1978.tb01635.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

AbstractWe start from the Hankel transform of Stefanescu's integral written in the convolutionintegral form suggested by Ghosh (1971). In this way it is possible to obtain the kernel function by the linear electric filter theory. Ghosh worked out the sets of filter coefficients in frequency domain and showed the very low content of high frequencies of apparent resistivity curves.Vertical soundings in the field measure a series of apparent resistivity values at a constant increment Δxof the logarithm of electrode spacing. Without loss of information we obtain the filter coefficient series by digital convolution of the Bessel function of exponential argument with sine function of the appropriate argument. With a series of forty‐one values we obtain the kernel functions from the resistivity curves to an accuracy of better than 0.5%. With the digital method it is possible to calculate easily the filter coefficients for any electrode arrangement and any cut‐off frequ

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1978.tb01636.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

AbstractIn marine gravity interpretation, the density required for the Bouguer correction is not easily measured, and, therefore, one often uses either standard values or extrapolates density from a station or a profile to an entire survey. If the real density varies, this leads to inconsistencies. In this paper a method is proposed to eliminate as much as possible the gravimetrical effects of real density variations. The principle is to apply Nettleton's method to the whole space, but also to use a continuously variable density in the Bouguer gravity computation. It is possible to avoid affecting real gravity anomalies by using stations situated on suitable area elements the optimal size of which is determined by the program. The density that is obtained is called “processing density’ because it is not a real one, but rather the resultant of the densities affect on each station. Use of the method is demonstrated on an example from the southeastern part of Bay of Bis

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1978.tb01637.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

AbstractThe reflectivity method for computing synthetic seismograms can be applied to seismic prospecting problems with special focus on coal‐mining problems. The method allows the calculation of reflected waves for point‐source excitation and non‐vertical incidence. It automatically includes all possible conversions of wave types and all inner multiple reflections. Synthetic horizontal‐ and vertical‐component seismogram sections are given for a simple two‐seam model and for a realistic seam sequence which is represented by 48 layers; the source‐receiver distances range from 100 m to 1000 m. These seismograms show prominent PS reflections already at moderate source‐receiver distances. These waves complicate the vertical‐component records by producing arrivals of similar strength as the PP reflections. From this it is concluded that PS reflections in strongly layered media can cause problems in routine CDP stacking. On the horizontal‐component records the PS reflections are dominant. Because of the lower velocities of S waves the time resolution of PS reflections is better than that of PP reflections. This suggests that horizontal‐component recording may be useful in the investigation of subsurface regions with strong velocity contrasts, even with conventional energy sources pro

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1978.tb01638.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY