摘要:

SummaryIn well velocity surveys made to calibrate Sonic (CV) Logs the calibration survey uses frequencies around 50 Hz whereas the Sonic Logging tool uses frequencies around 20 kHz. There thus exists the possibility of making a direct measure of velocity dispersion. In any one survey the disturbing factors, both instrumental and operational, will often mask any dispersive effect that might exist. Consequently this paper reports on a statistical analysis of the velocity differences resulting from calibration surveys and Sonic logs. Only Borehole Compensated Sonic Logs were used. Four areas were investigated: the North Sea, Abu Dhabi, Libya and Alaska. After rejecting logs and calibration records which were obviously in error there remained 424000 feet (about 130 km) of usable log distributed throughout 66 wells. The four areas were analysed separately and in no case was the estimated dispersion significantly different from zero. However, the mean values did correlate with lithology from (− 0.17 ± 0.18)% for the essentially carbonate section in Abu Dhabi to (+ 0.45 ± 0.25)% for the sand‐shale section in Alaska, a positive sign meaning that the higher frequencies travelled faster. Except for Alaska the calibration surveys were made with a wall‐clamp geophone, and for these areas amplitude measurements were made. After suitable corrections estimates of the absorption parameterQwere obtained. These varied from 20 to 200 with mean values of 63 for Libya, 70 for Abu Dhabi and 88 for the North Sea (excluding the Tertiary). If, as is usually assumed, the absorption mechanism is linear and is described by aQwhich is independent of frequency, then these values would necessarily imply dispersion of several percent. As instanced above no such dispersion was observed. It is possible that the expected dispersion was compensated for by invasion of the mud filtrate into the borehole walls, but it is more likely that the absorption mechanism was substantially non

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1971.tb00583.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

摘要:

AbstractThe increasing use of velocity analysis programs in seismic processing in addition to direct application to normal move out corrections often makes it possible to study the variation of the average velocity versus time and distance. Usually, it is even feasible to compute interval velocities with a good accuracy.We intend to use these velocities to convert the usual time domain displays to the depth domain.An exact conversion requires the knowledge of all interval velocities and all the velocity interfaces.However, when the dips are small, the conversion can be done without considering the dip of all these interfaces, and in some cases migration can even be completely neglected.Three different programs will be described and their use discussed in view of the type of problems to which they are applied.Practical examples of the various methods will be presented.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1971.tb00584.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

摘要:

AbstractLarge memory computers create new possibilities for seismic interpretation in well‐drilled areas, and more generally in problems involving deep sources.Records obtained by well‐to‐surface, surface‐to‐well and/or well‐to‐well propagation contain a large amount of information, owing to the great number of seismic rays travelling throughout the subsurface.This paper considers two successive steps:— direct case: computation of the first arrival times of seismic rays propagating through a two‐dimensional subsurface with given vertical and horizontal velocity variations;— inverse case: computation of a two‐dimensional velocity distribution, knowing the travel times of seismic rays through the subsurface.A computer program has been elaborated to solve both direct and inverse problems. There are no difficulties with the direct case; a certain number of uncertainties remain

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1971.tb00585.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

摘要:

AbstractReddy, I. K., D. Rankin, 1971, Magnetotelluric Effect of Dipping Anisotropies, Geophysical Prospecting 19, 84–97.A solution is obtained for the magnetotelluric effect in the case of multi‐layered earth, each layer containing dipping anisotropy, with the strike of anisotropy oriented at an arbitrary angle to the measuring directions. A technique to compute tensor impedances and tensor apparent resistivities is presented and the importance of anisotropies in magnetotelluric interpretation is discus

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1971.tb00586.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

摘要:

AbstractFontanel, A., G. Grau, 1971, Corrélation optique en lumière cohérente, Geophysical Prospecting 19, 98–117.This paper describes a general bidimensional two‐step method of correlation (or convolution) making use of the theory of holography.In the first step the light diffracted by one of the two plane transparent objects to be correlated interferes with the light diffracted by the other one. The hologram thus generated is photographed in the focal image plane of a convergent lens. Owing to the quadratic detection property of the photographic emulsion, the square of the modulus of the product of the spectra of the two objects considered is recorded on the photographic plate.In the second step the convolution product of the two objects appears when the hologram is illuminated with a beam of coherent light.In its geophysical application this optical method of convolution makes it easy for us to obtain the autocorrelogram of a seismic cross‐section. This method also makes it possible to correlate each of the seismic traces by special precalculated optically‐record

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1971.tb00587.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

摘要:

AbstractSimultaneous observations of electrical earth‐potentials along parallel measuring lines offer a simple but useful method to study telluric and stray‐current potentials. Different correlation‐factors between the records of both observation points can help to separate telluric from industrial stray‐current potentials. A method is suggested to eliminate disturbing impulses of stray currents from continuous telluric current recordings as those made at observatories.The influence of stray currents can also be eliminated from potential readings of an artificial current as applied in geoelectric measurements with the four‐point‐method. The elimination is possible by a simultaneous observation of the stray‐current potentials on an equipotential line of the artificially generated field. The recording examples show an appreciable improvement of the signal‐noise ratio without increase in the current. The method is useful for geoelectrical prospecting in areas which are contaminated by i

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1971.tb00588.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

摘要:

AbstractIn magnetic routine interpretation the comparison of two‐dimensional model curves with measured magnetic anomalies is widely used for an approximate evaluation of the position and depth of magnetic models.Before starting an interpretation of a survey by means of two‐dimensional models, it is very useful to have an idea of the shape of anomalies caused by extended but finite bodies, taking into account various strike directions: Three sets of anomalies of thin plates (horizontal length 19, downward length 9, width 1) dipping 30°, 60°, and 90° resp. for various strike directions and an inclination of 20° were computed. Some of these anomalies, e.g. those with nearly N‐S strike direction look rather complicated, and at the first glance one would not expect that they are caused by such simple bodies.Several profiles crossing the computed anomalies perpendicularly were interpreted two‐dimensionally. For less extended anomalies the depths determined for the top of the plates are 10‐20% too small, the magnetization amounts to 50–75 % of the value of the finite bodies. The interpretation of the profiles covering more extended anomalies gave very accurately the same values for the position, depth and magnetization for the two‐dimensional body as for the original three‐dimensional model.Anomalies of vertical prisms with varying extensions in they‐direction were computed. Their differences in amplitude and in the distance maximum‐minimum show that interpretation of short anomalies by two‐dimensional methods yields depth errors of up to 20 percent.To see the possibilities of the separation of superimposed anomalies dike anomalies were added to the anomaly of a broad body in great depth and several attempts were made to interpret parts of the composite anomalies. The interpreted bodies lie too deep. In complicated cases the depth values can have large errors, but experienced interpreters should be able to keep the errors in the range of one

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1971.tb00589.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

摘要:

AbstractThe interface phenomena for low periods, i.e. the initial oscillations, and the cut‐off periods are two characteristic features in Magnetotelluric Sounding curves. A method is proposed to distinguish between transitional and homogeneous layered earth by means of these two feature

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1971.tb00590.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY