摘要:

AbstractComparison of synthetic seismograms with field records has shown, that, under favorable circumstances, a large part of what is present on the latter may be ascribed to direct or multiple reflections due to velocity contrasts as they appear on a nearby velocity log.Therefore it does not seem unreasonable to submit good field records to a series of transformations, inverse to those which lead from a velocity log to the synthetic seismogram, with the purpose of getting somewhat more detailed and more accurate information on the variation of velocities with depth.The main difficulty in this kind of problem is generally the lack of stability of the results, i.e. the great influence on the final outcome of even small and unavoidable inaccuracies in the data or in the assumptions.For this reason a theoretical case has first been examined, where both the data and the physical hypotheses, as well as the final result were perfectly well known,—as this allows estimation and if necessary improvement in the stability of the method employed.Thus the successive steps of the inverse procedure, leading from a filtered synthetic record with multiples back to the initial velocity log are briefly discussed and the results obtained are shown.Two main stages are distinguished:—suppression of the effect of filtering, called “deconvolution”—discrimination between direct and multiple reflection, called “analysis”.For the former stage, the largely unknown filtering effect of the earth has also to be taken into account. First the way of deducing the total filtering effect from the filtered record itself is examined; second (a certain shape of the “signal”, the impulsional response of this filter, being assumed) an inverse filter is calculated ensuring a compromise between a good recovery of the original impulse and a minimum amplification of noise.A complete example of the results of the inverse procedure is given, in the case of a filtered synthetic recordwithoutmultiples. The only data, supposedly known, were the sampled ordinates of this record, drawn on the usual scale of a field record. Good correlation was nevertheless found between the output of the inverse filtering and the original impulsional record. Integration yielded a pseudo‐velocity as a function of time, showing again good agreement with the true velocities, except for their absolute values. If, in addition, check‐shot data are supposedly available, these pseudo‐velocities can be tied in, and a second integration yields the values of depth in function of time.The analysis itself, discrimination of direct and multiple reflection, starts by a step by step reversal of the recurrent procedure used for introducing multiple reflections (p. 4 ref. 3). The rapidly increasing accumulation of errors, due to the noise on the record and to the approximate nature of the physical assumptions, is partially accounted for by a continous readjusting of the results obtained. Nevertheless, if this method allows, as shown by a last example, a very satisfactory analysis of a rather noisysyntheticrecord, it is still too unstable to be applied to field records. An alternative method of successive approximations is finally outlined.As a conclusion, the necessarily approximate nature of the solutions of these problems is stressed. If some of the methods presented are still too theoretical, others have already been applied with s

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01514.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY

摘要:

AbstractA filtered seismic trace often appears as an almost sinusoidal curve. The reflected energy arrivals are superimposed and interfere with the continuous oscillations of the trace, and are therefore often difficult to distinguish. This is the chief difficulty in picking reflections. The situation is similar to that met with in gravimetry when a strong regional anomaly conceals small local anomalies. However, a regional anomaly is regular and broad, and owing to these two characteristics it can be removed. In reflection seismology batches of energy may also be concealed by oscillations of a continuous character. The main difference lies in the fact that the background of continuous seismic vibrations is not static, as is the case in gravimetry, but variable with time. However, a fairly constant physical quantity corresponds to these vibrations, which are composed partly of noise, partly of undesired secondary reflections. This quantity is the energy of motion of the surface layer of the ground. It is this troublesome energy that we intend to remove, in order to keep only the useful, actually reflected, energy.These remarks clearly demonstrate the importance of an investigation of the energy contained in the surface layer of the ground. After showing that, in the simplest case, the density of this energy can be expressed almost exactly by the formulau′2−uu′′in whichu′ is the velocity of a ground particle, we show how the non‐linear filtering defined by this formula can be realized, and we provide a f

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01515.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY

摘要:

AbstractThe surface layer is replaced by a layer in which the velocity changes linearly with the depth, i.e. by a transition layer. A plane irrotational wave of arbitrary shape travels vertically upward and hits this layer. The multiples produced by differential reflections in the layer and by reflections at the free boundary are computed. From the third order on, these multiples look exactly like empirical reflections. The sum of all multiples is the solution of the differential equations of the problem, and has some formal mathematical similarity to diffraction phenomena known from optics.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01516.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY

摘要:

AbstractFor the propagation of acoustic waves in formations, laboratory experiments yield an expression of the form:in whichAis the amplitude (strain or stress)A0some constant coefficientxthe coordinate according to which propagation takes placeα the attenuation coefficientCvelocityω the angular frequency of sinusoidal oscillation.No analogy between this law and that of electromagnetic waves can be made, since:1) coefficient α is not independent of frequency: its expression is α=Kω (Kconstant);2) however, with present recording technique, no noticeable dispersion can be found.The present paper shows how results are compatible if losses of energy are accounted for by a hysteresis phenomenon that is analyzed (see Figure). StressTis in abscissae, and strain δu/δx, multiplied by a coefficientE′ characteristic of the elastic properties of the solid, is in ordinates. The horizontal part of the curve is θ. It is supposed that the absorption properties of the ground are given by some dimensionless coefficientλ=θ/T−Then one getsα=ωλ/4CIf λ≪ 1, we get the propagation lawAn attenuation takes place without phase shift, and consequentlywithout dispersion.The author reverts to the inapplicability of the superposition principle, and foresees theoretically, for instance, that a strainT′ cos ω′t, whereT′>Tand ω′>ω, c

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01517.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY

摘要:

AbstractThe gravitational attraction of a circular lamina and a right circular vertical cylinder at any axial point can be found by elementary methods. The attraction at a pointPoff the axis is usually estimated as follows. The attraction of a lamina, with the axis throughP, touching the given lamina externally is subtracted from the attraction of a coaxial lamina which encloses the given lamina. The result is multiplied by the ratio of the area of the given lamina to the area of the annulus. The attraction of the cylinder is estimated by a closely analogous method. The answer is, however, valid only ifPis a distant point.As a step towards calculating exactly the attraction of the cylinder at all points of the space external to it, the corresponding problem for the lamina is dealt with first. It is shown that the anomaly can be expressed as an infinite series in even powers of the horizontal distance ofP.The coefficients of the powers consist of exact algebraic expressions containing simple terms of the form (1 +χ2)−n−1/2. These coefficients are determined by exactly summing certain infinite series involving the derivatives of the Legendre polynomials of odd order. The method of summation is believed to be of interest also in other problems where Legendre polynomials are encountered.The attraction of the cylinder is calculated by appropriately integrating the expression for the attraction of a lamina.Numerical calculations show that the approximate method everywhere underestimates the gravity anomaly of the lamina as well as of the cylinder. The error at distances one to two times the radius is upto 20 % for the lamina and 8.5 % for an infinitely long cyli

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01518.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY

摘要:

AbstractFormulae are derived and discussed. They can be applied to magnetometer observations, with negligible computation, to yield limits to the depth and intensity of magnetisation of the bodies causing the anomaly.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01519.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY

摘要:

AbstractModel experiments at a scale of 1:200 were carried out in order to assess the performance of an E. M. system which was rigidly mounted on the wingtips of an aircraft. The experiments were performed over model conductors of various sizes, shapes and resistivities. From the results the detectability of various natural‐sized conductors was calculated. The results obtained over an unmined massive sulphide body in Cyprus are shown and a comparison made with the model result

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01520.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY

摘要:

AbstractThe ratios of induced to remanent magnetic intensities have been determined for samples of Precambrian rock from part of the Canadian Shield in northeastern Alberta.The relatively insignificant contribution of remanent magnetism to the observed field indicates that interpretation techniques based on the assumption of zero remanent intensity can be used.Other implications of the magnetic measurements are discussed briefly.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01521.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY

摘要:

AbstractThe synthetic seismogram is a geophysical instrument. Examples taken from several areas are presented in order to define this tool and to show how it may be utilised in all stages of the prospection.The synthetic seismogram translates the stratigraphic information obtained from exploration wells into seismic terms; it therefore deserves to be taken into consideration when choosing the methods to be employed for solving a given problem.The synthetic seismogram enables one to know the limitations imposed by the reflecting power of the different subsurface layers. The knowledge of these limitations, combined with an understanding of the difficulties inherent in the surface conditions, helps to determine the frame in which the survey and the interpretation must be set.During the actual survey the synthetic seismogram illuminates somehow the work of the seismologist. It is a reference base which defines the correlation criteria which permit him to analize the real traces and, at the same time, to increase their resolving power.The methods of utilisation are examined both with respect to the field work (attempts to determine the technique to be used in any area by reference to the synthetic traces; determination of the volume control; choice of filters) and in the field of interpretation (identification and picking of reflections; analysis of the signals in order to obtain stratigraphic information from them; study of the character of reflections on a regional scale and density of the synthetic seismograms to be made in any region; re‐evaluation of the information contained in old seismic records

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01522.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY

摘要:

AbstractThe study of tectonics of salt and of diapirs in particular is an important problem in the mining field as well as in that of petroleum exploration. This paper describes surveys that were carried out, using the method of the rectangle of resistivities.After stating the principle of the method, the way it is carried out and its advantages, three examples are discussed: two in the Alsace plain (Hettenschlag‐Oberentzen and Blodelsheim), the third in Baden‐Wurtemberg (Weinstetten‐Heitersheim). Geological conditions are practically the same (resistive alluvia, very conductive marls and resistive salt series); the depth at which the top of the salt was found varied from 100 meters to more than 1000 meters. Maps of resistivities (with current electrode spacing of 6000 meters) and graphs of a few electrical soundings are presented, together with the results obtained under the form of isobath maps of the top of the salt series.The surveys lasted 26 months and covered 171 square kilometers. The work presented corresponds to three sections totalling 93 square kilometers and was chosen because of the diversity of the tectonics. Four drill holes have been carried out since then and have confirmed the results quite well.The method may be extended to the survey of the tectonics of a salt series or of a resistive substratum, for instance a limestone formation, that are deeper than the above cases (2000 meters or more), using current electrode spacing of 20 kilometers. Surveys of this type have already been realized in the

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1961.tb01523.x

出版商:Blackwell Publishing Ltd    

年代:1961

数据来源: WILEY