bluuebunny/crossref_metadata_2025 huggingface dataset:
SYNTHETIC SEISMOGRAMS AT NON‐VERTICAL INCIDENCE*
Authors: DOUZE E. J.
Published: April 2006

Abstract

Synthetic seismograms are usually computed for reflections from vertical incidence of P waves for a horizontally layered medium. In actual practice the angle of incidence departs from the vertical, as receivers are usually located at some distance from the source. At angles other than the vertical, the conversion of P‐ to S‐wave energy and changes in the reflection coefficient affect the shape of the synthetic seismograms. The effect of non‐vertical incidence on synthetic seismograms is examined in this paper.

Seismograms at non‐vertical incidence have been computed using the plane‐wave approach of Haskell (1953) for a layered medium. The use of plane waves is an approximation to the actual case of spherical wavefronts from a surface source.

Using plane‐wave theory, the expected wave forms as a function of angle of incidence were computed numerically for several simple models. The results indicate that the synthetic seismograms do not change significantly for angles of incidence between o and 25 degrees. For larger angles the changes in the wave forms may be severe. The effect is more pronounced for high‐velocity layers than for low‐velocity layers.
THEORETICAL AND EXPERIMENTAL INVESTIGATIONS FOR CAVITY RESEARCH WITH GEOELECTRICAL RESISTIVITY METHODS*
Authors: MILITZER H. , RÖSLER R. , LÖSCH W.
Published: April 2006

Abstract

The exact localization of subterranean cavities and the determination of their dimensions is very important for the planning of geotechnical and mining activities. It is a complicated geophysical task often at the limit of detection. Nevertheless geophysical investigation is the only alternative to a dense and expensive grid of boreholes. This report tests the usefulness of geoelectrical resistivity methods for cavity detection under some new aspects. The basis for evaluation was a theoretical analysis of different conventional and focussing measuring arrays and of special arrays for a geoelectrical research between two boreholes.

The limit of detectability of a cylindrical cavity of defined cross‐section and depth was calculated for the different measuring arrays on the basis of computation of the apparent resistivity ρa. Furthermore, the influence of possible errors (current supply of the electrodes and the distance between the electrodes) is discussed for focussed systems.

The second part of the article is directed at the behaviour of the apparent resistivity ρa, the disturbing potential δVd caused by the cavity and the normal potential δV0 of the measuring array all in relation to a homogeneous earth. Some new results are presented.

In the last part of the article theoretical results are compared with some field measurements.
END CORRECTIONS IN POTENTIAL FIELD MODELING*
Authors: RASMUSSEN R. , PEDERSEN L. B.
Published: April 2006

Abstract

A general treatment of end corrections in gravity and magnetic modeling is presented. The well‐known 2‐D formulae of Talwani and Talwani and Heirtzler become slightly changed to represent a 2 1/2–D body, i.e. a body of polygonal cross section with the tails In the strike direction cut off.
THE STUDY OF THE GENESIS OF SURFACE WATERS BY OBSERVING THEIR ELECTRICAL RESISTIVITIES*
Authors: OGILVY A. A. , BERRY B. L. , KUZMINA E. N.
Published: April 2006

Abstract

Specific electrical resistivity of natural waters contains information on their genesis. The authors propose to conduct mass and regime observations of this parameter in river and stream beds.

The electrical resistivities in streams flowing from under a glacier reveal details formed at the same time as the glacier. Observations in the beds of big rivers show a gradual increase in water salinity overlain by reductions by inflowing glacial waters.

The diurnal and annual trend of changes in the electrical conductivity of water associated with the change in the balance of glacial and ground waters has been established near to glaciers.

Resistivity observations help to locate discharge sites of sub‐permafrost waters, for water.
AUTOMATIC DECONVOLUTION OF GRAVIMETRIC ANOMALIES*
Authors: BICHARA M. , LAKSHMANAN J.
Published: April 2006

Abstract

Existing techniques of deconvolution of gravity anomalies are principally based on upward and downward continuation of measured fields. It can be shown that a unique set of linear filters, depending only on geometrical parameters, relates density distribution at a given depth to gravity measured on the surface. A method to compute the filter coefficients is developed. Very accurate reconstitution of theoretical models of intricate shape, prove the validity of the linear relationship. One of these sets of linear filters is applied to a field case of underground quarries.
NUMERICAL VLF MODELING*
Authors: KAIKKONEN P.
Published: April 2006

Abstract

The VLF response of laterally inhomogeneous and anisotropic models is calculated numerically using the finite element method.

Some results are presented for a slab model in terms both of the polarization parameters, i.e., the tilt angle and ellipticity of the magnetic polarization ellipse, and the amplitude ratio |Hz/Hx|.

On the basis of both the ellipticity and the tilt angle, it is possible to discriminate between a poor conductor and a good one. The direction of the dip can be determined from the anomaly profiles of all diagnostic parameters. The effect of the conductive overburden is most noticeable on the ellipticity profile: one observes attenuation for a poor conductor and “negative attenuation” for a good conductor. The anomaly profiles for anisotropic cases are consistent with the ones of the isotropic cases.
FAST HANKEL TRANSFORMS*
Authors: JOHANSEN H. K. , SØRENSEN K.
Published: April 2006

Abstract

Inspired by the linear filter method introduced by D. P. Ghosh in 1970 we have developed a general theory for numerical evaluation of integrals of the Hankel type:

image

Replacing the usual sine interpolating function by sinsh (x) =a· sin (ρx)/sinh (aρx), where the smoothness parameter a is chosen to be “small”, we obtain explicit series expansions for the sinsh‐response or filter function H*.

If the input function f(λ exp (iω)) is known to be analytic in the region o < λ < ∞, |ω|≤ω0 of the complex plane, we can show that the absolute error on the output function is less than (K(ω0)/r) · exp (−ρω0/Δ), Δ being the logarthmic sampling distance.

Due to the explicit expansions of H* the tails of the infinite summation
image

((m−n)Δ) can be handled analytically.

Since the only restriction on the order is ν > − 1, the Fourier transform is a special case of the theory, ν=± 1/2 giving the sine‐ and cosine transform, respectively. In theoretical model calculations the present method is considerably more efficient than the Fast Fourier Transform (FFT).
IMPROVEMENTS IN THE OBSERVATION OF SHEAR WAVES*
Authors: DOHR G. , JANLE H.
Published: April 2006

Abstract

The observation of shear waves is finding more interest in seismic exploration, especially for the determination of additional lithologic parameters, e.g. Vp/Vs. We demonstrate the observation of converted waves in routine seismic work by means of horizontal geophones. Field technique and data processing permit acquisition and interpretation of converted waves in connection with routine seismic measurements. A special interpretation of the recorded converted waves results in the shear wave velocity and the Vp/Vs relation.
VLF ANOMALIES FROM A PERFECTLY CONDUCTING HALF PLANE BELOW AN OVERBURDEN*
Authors: OLSSON O.
Published: April 2006

Abstract

A theoretical solution to the electromagnetic problem of a perfectly conducting half plane below a conducting overburden has been obtained. The VLF anomalies have been computed for different overburden conductivity and thickness and also for different dip angles of the half plane. In the computations the contribution to the secondary magnetic field from the electric Hertz potential has been neglected. The anomaly curves which are displayed as EM 16 readings, show a fairly complicated behaviour. This is mainly due to the phase shift and attenuation of the field caused by the conductivity of the overburden and the host rock. From the anomaly curves it is possible to define the apparent depth to the top of the conductor as the distance between the peak value and the cross‐over of the real component. The apparent depth is usually larger than the actual depth, but it is possible to determine the actual depth to the conductor from the relation between the peak‐to‐peak anomaly and the apparent depth. When the peak‐to‐peak anomaly is fairly large, it is also possible to make estimates of the dip angle. However, a complete set of master curves will be a necessary tool for interpretation of VLF data when there is need to obtain more accurate estimates of the half plane parameters. In a specific case the theoretical calculations are shown to be in good agreement with measured data.
SEISMIC AND GRAVITY TWO‐DIMENSIONAL INHOMOGENEOUS MODEL OF THE EARTH'S CRUST (ON THE EXAMPLE OF CENTRAL KAZAKHSTAN)*
Authors: OBLOGINA T.I. , FEDYNSKY V.V.
Published: April 2006

Abstract

The interpretation of deep seismic sounding (DSS) data has been made on the basis of a two‐dimensional inhomogeneous model. The refracted first arrivals as well as reflected and diffracted waves on the seismic records have been utilized. The seismic section was modeled in the iso‐veolcity lines v(x, y) = const, taking into account the zones of diffraction associated with deep faults.

Gravity observations have been used to construct a block model of the Earth's crust with vertical boundaries.

It is suggested to define the base of the crust as the zone with velocities between 7.8 and 8.2 km/s. The reflecting boundaries of different length occurring in this zone can be conformal or unconformal with the iso‐velocity lines near the base of the crust.

As an example of our approach to the interpretation of DSS data the folded‐blocky structure of the crust with horizontal inhomogeneities of velocity and density is shown in the Kzyl‐Orda‐Dzheskazgan profile in Central Kazakhstan.
A FIELD OCCURRENCE OF SIGN REVERSALS WITH THE TRANSIENT ELECTROMAGNETIC METHOD*
Authors: SPIES B. R.
Published: April 2006

Abstract

Sign reversals have been observed with the one‐loop version of the TEM method in an area near Cloncurry, Queensland. This is the only area in which such a response has been recorded in seven years of field surveys by the Australian Bureau of Mineral Resources.

The geology of the area consists mainly of pyrrhotitic graphitic shale, which has resistivities as low as 0.1 Ωm and frequency effects up to 30%. The sign reversals could be due to magnetic phenomena, reflections from layers, or complex conductivity effects.
NOISE, TEMPERATURE COEFFICIENT, AND LONG TIME STABILITY OF ELECTRODES FOR TELLURIC OBSERVATIONS*
Authors: PETIAU G. , DUPIS A.
Published: April 2006

Abstract

Numerous electrodes, already used in geophysics or just perfected by us, have been compared by measuring the three main characteristics which interest the user: noise spectrum, temperature coefficient and polarization with its stability versus time.

Among the most used unpolarizable electrodes, silver‐silver chloride (Ag‐AgCl) are the best ones. But a systematic research of all different possible metal‐salt couples, have led us to use lead‐lead chloride (Pb‐PbCl2) for the following reasons: noise as low as the one of Ag‐AgCl at 1 Hz and even lower for the low frequencies (0.4 μV at 1 Hz and 1.2 μV at 0.01 Hz for peak to peak value and ΔF=F), temperature coefficient about ten times weaker (−40 μV/°C instead of −450 μV/°C) and also better long time stability of the polarization (1 mV/month instead of 2 at 10 mV/month).

We have been using these electrodes since 1977 as “tube” electrodes which are very easy to use. They allow us to record correctly the fast variations thanks to their low noise, the very slow variations, their low temperature coefficient and their stability, and this with telluric lines only about 100 m long.
EFFET TOPOGRAPHIQUE EN PROSPECTION ELECTRIQUE*
Authors: CECCHINI A. , ROCROI J.P.
Published: April 2006

Abstract

The computation of the electrical potential created by a source of direct current in a horizontally stratified earth is easy, and it is now a common practice to interpret, by manual or automatic procedures, the maps of apparent resistivity or of mise‐à‐la‐masse surveys.

The study of 3‐D cases requires a computer cost too high for a general use, and the available techniques usually refer only to an anomalous body embedded in a stratified medium. In fact, it often happens that the surface of the ground cannot be regarded as a plane, an assumption which can cause large discrepancies between the observed and the calculated potentials. A first estimate of these discrepancies can be made by assuming that the earth's surface makes a dihedron and the underground consists either of one homogeneous medium or of several media whose plane interfaces pass through the edge of the ground surface dihedron. Although very schematic, this approximate model can provide a useful information on the effect of the relief.
GEOMETRICAL OPTICS AND WAVE THEORY OF CONSTANT OFFSET SECTIONS IN LAYERED MEDIA*
Authors: DEREGOWSKI S.M. , ROCCA F.
Published: April 2006

Abstract

Constant offset sections can be mapped to a fixed offset and compared in order to provide a method of velocity analysis. The direct mapping to zero offset prior to stack might provide an alternative processing procedure to NMO and stack. The main advantage of such a procedure would lie in the correct treatment of cross‐dips, but interpretational advantages might also follow from the performance of partial stacks biased to either high or low offset information.
I.P. AND RESISTIVITY MODEL STUDIES OVER THIN DYKES*
Authors: MAJUMDAR R. K. , DATTA S.
Published: April 2006

Abstract

Induced polarization and resistivity model studies over thin dykes for varying resistivity contrasts, depth of burial, and dip angles show striking parallelism of the surface apparent resistivity contours with the boundary of the body. This effect may be utilized for the estimation of strike length of the body. Results show that intermediate values of the electrode spacing is satisfactory for detection under widely varying conditions of resistivity contrast and depth. The percentage frequency effect (P.F.E.) and the metal factor (M.F.) responses are found to be more sensitive to the variation in the depth of burial than the resistivity responses. Pseudosections for P.F.E. and M.F. are concentrated much closer to the body than the resistivity pseudo‐sections.
SIMPLIFIED EXPRESSIONS FOR THE MAGNETIC ANOMALIES DUE TO VERTICAL RECTANGULAR PRISMS*
Authors: KUNARATNAM K.
Published: April 2006

Abstract

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.
LABORATORY INSTRUMENTATION TO STUDY CHANGES OF ELECTRICAL CONDUCTIVITY OF ROCKS WITH CHANGES OF FREQUENCY, TEMPERATURE AND PRESSURE*
Authors: LOSITO G. , FINZI‐CONTINI G.
Published: April 2006

Abstract

A laboratory instrument was developed to investigate the electrical properties of rock samples with respect to changes of frequency, temperature and pressure. The instrument can be used to obtain general trends and typical values for geological media. It should be of particular interest in geothermal studies and research in the geophysical properties of rocks.

The design intervals for quantities under investigation were 20–300°C, 105‐4 × 107 Pa and 5 × 10−4‐103 Hz. Certain limitations exist on the simultaneous use of the highest values of temperature and pressure.

The main features of the instrument are: analog electrical outputs recorded by an x‐y recorder or observed on a scope with an attached camera (according to the frequency range); pressures produced by an oil hydraulic system in order to ensure electrical insulation of the sample; temperatures obtained by placing the pressure vessel into a thermostatically controlled room. Thus the monitoring instrumentation is directly accessible during the experiment. Information is also given about calibration procedures and examples of rock sample data are given.
FURTHER THOUGHTS ON POPPERIAN GEOPHYSICS—THE EXAMPLE OF DECONVOLUTION*
Authors: ZIOLKOWSKI A.
Published: April 2006

Abstract

Popper's demarcation criterion should be applied to all our theories in geophysics to ensure that our science progresses. We must expose our theories to tests in which they stand some risk of being refuted. But if we have a theory which has no rivals it may be difficult in practice to devise a test in which the theory risks being refuted conclusively.

The example of the deconvolution problem for seismic data is considered for the case where the source wavelet is unknown. It is shown that all our existing theories of deconvolutions are not scientific in Popper's sense; they are statistical models. We cannot compare these models in a way that is independent of the geology, for each model requires the geology to have a different set of statistical properties. Even in our chosen geology it may be extremely difficult to determine the most applicable model and hence determine the “correct” deconvolution theory.

It is more scientific to attempt to solve the deconvolution problem (a) by finding the source wavelet first, deterministically, or (b) by trying to force the wavelet to be a spike—that is, by devising a “perfect” seismic source. A new method of seismic surveying, which has been proposed to tackle the deconvolution problem by the first of these approaches, is based on a theory which is open to refutation by a simple Popperian test. Since the theory makes no assumptions about the geology, the test has equal validity in any geology.

It pays to frame our theories in such a way that they may easily be put at risk. Only in this way will we establish whether we are on firm ground. The alternative is simply to take things on trust.
ANALYTICAL EXPRESSIONS FOR GRAVITY DUE TO HOMOGENEOUS REVOLUTIONAL COMPARTMENTS IN THE GAUSSIAN DIVERGENCE APPROACH*
Authors: OKABE M.
Published: April 2006

Abstract

Complete analytical expressions are developed for the first and second derivatives of the Newtonian gravitational potential in arbitrary directions due to the homogeneous revolutional compartment with a polygonal vertical section by applying the Gaussian divergence theorem in the cylindrical coordinate system. Elementary solutions presented can easily be translated into magnetic anomalies caused by a uniformly magnetized body.

The divergence approach in the polar coordinate system is also described, and gravity attractions in the radial direction are presented in closed form associated with a homogeneous cap compartment. The explicit solutions are tested against well‐known formulae for a cylinder, cone, infinite plate, and sphere.
THE MAGNETOTELLURIC IMPEDANCE TENSOR—ITS RANDOM AND BIAS ERRORS*
Authors: PEDERSEN L. B.
Published: April 2006

Abstract

Approaches to the reduction of bias in the computation of the elements of the magnetotelluric impedance tensor have been proposed in the past by several authors. In this paper a clear distinction is made between random errors and bias errors. No effort is made to reduce either, but the emphasis is on their estimation. Both types of errors depend critically upon the polarization of the magnetic field. The random error increases with increasing noise‐to‐signal ratio in the electrical field, and it is rather insensitive to noise in the magnetic field. The bias error increases with increasing noise‐to‐signal ratio in the magnetic field. Expressions for random errors and maximum bias errors are developed and discussed using a single station set‐up. Random errors with a reference station set‐up are also calculated.
A COMMENT ON “SYNTHETIC SEISMOGRAMS AT NONVERTICAL INCIDENCE”*
Authors: AMINZADEH F.
Published: April 2006

Abstract

The notion of nonvertical incidence plane wave seismograms and nonzero effect point source seismograms are sometimes confused with each other in the literature. This short note attempts to clarify this confusion which is observed in Douze (1979).
A COMPARISON OF TWO METHODS OF ESTIMATING STATIC FORMATION TEMPERATURE FROM WELL LOGS*
Authors: LEBLANC Y. , LAM H‐L. , PASCOE L. J. , JONES F. W.
Published: April 2006

Abstract

Two methods for estimating the true formation temperature from well logs are compared. One method requires knowledge of the circulation time, whereas the other requires an estimate of the thermal diffusivity of the contents of the well. Both methods require three or more successive bottom‐hole temperature measurements. Data from 157 wells have been analyzed, and the calculated formation temperatures from the two methods agree well. By an analysis of best matching of the two methods, it is found that 0.35 × 10‐6 m2 s‐1 is a good estimate for the thermal diffusivity of the well contents.
TRANSIENT COUPLING BETWEEN FINITE CIRCUITS ON AN ANISOTROPIC CONDUCTING HALF‐SPACE*
Authors: NABULSI K. A. , WAIT J. R.
Published: April 2006

Abstract

The frequency domain response of a grounded linear circuit is deduced. The model of the ground is a homogeneous conducting half‐space whose horizontal and vertical conductivities are specified. The source itself is formulated as an insulated current‐carrying wire of finite length lying on the surface that is grounded at its end points. The tangential electric field expressions are used to calculate the induced voltage between two additional electrodes that are connected by an insulated wire. Particular attention is paid to the transient coupling response in the potential circuit when the source current is a step‐function of time. Among other things, it is shown that the voltage response is a step‐function only if the circuits are at right‐angles and if the ground is effectively isotropic and nonpolarizable.
OPTIMIZATION OF SHORT DIGITAL LINEAR FILTERS FOR INCREASED ACCURACY*
Authors: GUPTASARMA D.
Published: April 2006

Abstract

The accuracy of short length digital linear filter operators can be substantially increased if the sampling interval as well as the abscissa shift are properly adjusted. This may be done by a trial and error process of adjustment of these parameters until the error made by the filter operator, applied to a suitably chosen test function, is smallest.

As an illustration of the application of this method, 7‐, 11‐ and 19‐point filters for the calculation of Schlumberger apparent resistivity from a known resistivity transform are designed. Errors with the new 7‐point filter are seen to be less than those with a 19‐point filter of conventional design. The errors with the new 19‐point filter are two to three orders of magnitude smaller than those made by the conventional 19‐point filter.

The new method should provide digital linear operators that allow significant improvements in accuracy for comparable computation efforts, or substantial reduction in computation for comparable accuracy of results, or something of both.
PREDICTING ABNORMALLY PRESSURED SEDIMENTARY ROCKS*
Authors: BILGERI D. , ADEMENO E. B.
Published: April 2006

Abstract

Modern seismic processing techniques developed in recent years have provided the explorationist with more meaningful data than would have been predicted even by optimists. Correct migration of seismic data, relative amplitude preservation of reflections, and seismic trace inversion represent the necessary efforts to ensure that the best possible picture of in situ physical properties of the subsurface section is revealed. Moreover, compacted and over‐pressured zones can be predicted from surface data prior to drilling a well through them.

The basic tool for predicting overpressured zones from the surface is still the velocity analysis derived from good reflection data with few erratic multiples. The extraction of regional normal compaction trends from the seismic velocities allows one—where velocities deviate from the trend—to locate the top of overpressure. Moreover, the statistical behavior of the ratios of the sonic log vs pore pressure in existing boreholes enables one to convert the deviation from the trend of the seismic velocities into overpressure rates expected at the seismic reflection horizon.

This paper presents a field case study showing how the knowledge of well site lithology together with the more detailed information extracted from inverted seismic data enables the prediction to match well conditions with high reliability.
DEVELOPMENT OF MORE EFFICIENT AIRGUN ARRAYS: THEORY AND EXPERIMENT*
Authors: JOHNSTON R. C.
Published: April 2006

Abstract

Source strength of an airgun array may be increased by:

utilizing higher pressure,

increasing total array volume,

employing more guns,

improving gun efficiency.

One measure of gun efficiency is “specific source strength”, Pa*, defined as source strength per unit quantity of air used. Typical units are MPa m/l. Most developments are directed toward increasing gun pressure and/or gun volume to increase source strength of the array. These efforts require that more air compressors be installed onboard the ship. Consequently, a larger ship may be needed for the additional compressors, guns, and auxiliary equipment.

A development program was initiated in 1976 to increase source strength of the array without using a larger ship. New guns were designed and built—one for 41.4 MPa and 7.37 liter (6000 p.s.i./450 in3) operation and another with 13.8 MPa and 4.92 liter (2000 p.s.i./300 in3) capability. Experiments were conducted with these new guns (and existing guns) over a range of pressures from 13.8 to 41.4 MPa (2000 to 6000 p.s.i.).

Design of the new guns was aided by a mathematical model. The model relates physical dimensions of the airgun to acoustic pressure in the water. It consists of four nonlinear differential equations relating

shuttle motion,

bubble pressure,

chamber pressure,

bubble radius.

The last equation is the “free‐bubble‐oscillation equation” and represents the ideal case of a pressurized bubble released instantaneously in water. The three other equations modify this ideal case; the four equations together model an airgun of the type manufactured by Bolt Associates, Inc.
QUADRATIC CORRECTIONS FOR THE Z‐EFFECT APPLIED TO THE INVERSION OF BROADBAND GAMMA RAY LOGS FROM URANIFEROUS ZONES*
Authors: DUFFIN R. H.
Published: April 2006

Abstract

Nonlinear correction parameters have been determined to correct broadband gamma ray logs for the effects of photoelectric absorption in zones of high‐grade uranium mineralization. These corrections have been determined empirically by logging artificial test pits. The corrections are a function of grade of mineralization, hole casing thickness, hole medium, and probe energy thresholds. Comparisons between core assays from a high‐grade mineralized zone and corrected log inversions recorded with both low‐threshold and high‐threshold filtered probes are made. The best correlations were obtained between assays and inversion solutions for the low‐threshold probe. This reflects the superior counting statistics of this type of probe. Filtered probes reduce z‐effect errors but do not eliminate them, and have poorer counting statistics than unfiltered probes.
ON NUMERICAL METHODS FOR MIGRATION IN LAYERED MEDIA*
Authors: DUBRULLE A. A.
Published: April 2006

Abstract

Three current migration methods derived from the wave‐equation model and based on approximations by Fourier series, finite‐differences, and a combination of both (mixed method) are reviewed. Each method is examined in the context of horizontally layered media, where the velocity of wave propagation varies with depth alone. It is shown how, under such conditions, the special properties of the linear equations obtained from finite‐difference and mixed approximations can be exploited for computational efficiency. A numerical algorithm is described which substantially reduces the complexity and the computational cost of the solution. This algorithm is directly applicable to vector processing.

The description of each method includes derivation, dispersion relation, stability properties, and applicable numerical algorithms. Comparative analyses show that, for the same computational cost, the finite‐difference approximation has poor accuracy characteristics and should be used only when the boundary conditions are difficult to express in the frequency domain. The results are illustrated with numerical experiments on synthetic data.
REFLECTION OF ELASTIC WAVES FROM PERIODICALLY STRATIFIED MEDIA WITH INTERFACIAL SLIP*
Authors: SCHOENBERG M.
Published: April 2006

Abstract

A periodically stratified elastic medium can be replaced by an equivalent homogeneous transverse isotropic medium in the long wavelength limit. The case of a homogeneous medium with equally spaced parallel interfaces along which there is imperfect bonding is a special instance of such a medium. Slowness surfaces are derived for all plane wave modes through the equivalent medium and reflection coefficients for a half‐space of such a medium are found. The slowness surface for the SH mode is an ellipsoid. The exact solution for the reflection of SH‐waves from a half‐space with parallel slip interfaces is found following the matrix method of K. Gilbert applied to elastic waves. Explicit results are derived and in the long wavelength limit, shown to approach the results for waves in the equivalent homogeneous medium. Under certain conditions, a half‐space of a medium with parallel slip interfaces has a reflection coefficient independent of the angle of incidence and thus acts like an acoustic reducing mirror. The method for the reflection of P‐ and SV‐waves is fully outlined, and reflection coefficients are shown for a particular example. The solution requires finding the eigenvalues of a 4 × 4 transfer matrix, each eigenvalue being associated with a particular wave. At higher frequencies, unexpected eigenvalues are found corresponding to refracted waves for which shear and compressional parameters are completely coupled. The two eigenvalues corresponding to the transmitted wavefield give amplitude decay perpendicular to the stratification along with up‐ and downgoing phase propagation in some other direction.

Much of this work was performed while the author was at the Department of Geophysics and Planetary Sciences, Tel‐Aviv University, Ramat‐Aviv, Israel. The author is grateful for illuminating discussions with K. Helbig and K. Gilbert.
PROBLEMS IN THE USE OF GEOPHYSICAL INFORMATION WHEN INTERPRETING THE DATA OF SPACECRAFT OBSERVATIONS*
Authors: BOGOSLOVSKY V. A. , IL'INA E. B. , KUZMINA E. N. , OGILVY A. A.
Published: April 2006

Abstract

Observations made from space have become almost a standard method of geologic investigation. However, interpretation of space photographs is not always unambiguous. Particularly, the nature and spatial location of various objects photographed from space remain obscure. It is especially difficult to investigate territories where basement rocks are overlain by sedimentary cover.

From the example of investigations carried out in the central part of the Russian Platform it is shown how separate lineaments and ovoids reflect the peculiar features of geologic, hydrogeologic and geophysical structure of the region. It has been established that—within the sedimentary cover—they correspond usually to zones of increased fissuredness and are associated with low‐amplitude tectonic dislocations.

The authors concentrate on the elucidation of the transfer of deep geologic information to the surface. The sedimentary cover is regarded as a communication channel which makes it possible to employ the concepts of information theory for assessing its “transparency”. The contrast due to heterogeneities of the sedimentary cover in space photographs and, consequently, the degree of their manifestation in geologic and physical fields, depends on the water content of the rocks composing the section. Along with hydrogeologic characteristics, geo‐electric parameters, such as specific electric resistivity, longitudinal conductivity and transverse resistivity, can serve as criteria of the degree of rock moistening.

When the location of lineaments does not coincide with the plane axes of geophysical anomalies, an inclination of the fault plane can be supposed. The angle of inclination is characterized by the deviation of the lineament from the axis of the anomaly.

The data resulting from the investigation of selected standard structures were used to develop a classification based on the degree of their expression in hydrodynamic, geoelectric, gravitational, and magnetic fields and depending on the spatial position and peculiarities of geologic arrangement. For geophysical investigations of unknown structures detected by space observations, this classification can be used as the basis for solving inverse problems by the methods of mathematical statistics.
REGIONAL GEOTHERMAL EXPLORATION IN EGYPT*
Authors: MORGAN P. , BOULOS FOUAD K. , SWANBERG C. A.
Published: April 2006

Abstract

Although Egypt is not characterized by abundant Cenozoic igneous activity, its location in the northeastern corner of the African plate suggests that it may possess geothermal resources, especially along its eastern margin. Regional geothermal exploration has been carried out in Egypt using the thermal gradient/heat flow technique and groundwater temperature/chemistry technique. In the thermal gradient/heat flow study, existing oil‐well bottom‐hole temperature data as well as subsurface temperature measurements in existing boreholes were utilized before special thermal gradient holes were drilled. Groundwater temperature and chemistry data were used to extend the geographic range of the direct subsurface thermal measurements. On a very modest budget, a regional thermal high has been discovered along the eastern margin of Egypt, and a local thermal anomaly has been discovered in this zone. Published geological information suggests that the sandstones of the Nubian Formation may be a suitable reservoir for geothermal fluids. The new data indicate that temperatures of 150°C or higher may be found in this reservoir in the Gulf of Suez and Red Sea coastal zone where it lies at a depth of 4 km and deeper.
MIGRATION BY EXTRAPOLATION OF TIME‐DEPENDENT BOUNDARY VALUES*
Authors: McMECHAN G. A.
Published: April 2006

Abstract

Migration of an observed zero‐offset wavefield can be performed as the solution of a boundary value problem in which the data are extrapolated backward in time. This concept is implemented through a finite‐difference solution of the two‐dimensional acoustic wave equation. All depths are imaged simultaneously at time 0 (the imaging condition), and all dips (right up to vertical) are correctly migrated. Numerical examples illustrate this technique in both constant and variable velocity media.
THE INTEGRAL‐FINITE‐DIFFERENCE METHOD FOR CALCULATING TRANSIENT ELECTROMAGNETIC FIELDS IN A HORIZONTALLY STRATIFIED MEDIUM*
Authors: GOLDMAN M. M.
Published: April 2006

Abstract

A finite‐difference solution and an integral algorithm are developed for computing time‐domain electromagnetic fields generated by an arbitrary source located in horizontally stratified earth.

The finite‐difference problem is first solved for the kernel of an integral Bessel transform of the desired field and then an inverse transformation is performed either by numerical integration or by linear filtering. A description of the appropriate algorithm as well as numerical results are presented for two cases: an axially symmetric source (a vertical magnetic dipole or a circular loop) placed on the surface of a horizontally layered section overlying either an insulating or an ideally conducting basement.

In comparison with the conventional methods used for computing transient electromagnetic fields, the integral‐finite‐difference method is more universally applicable.
GEOELECTRICAL MODELS INVOLVING LAYERS WITH A LINEAR CHANGE IN RESISTIVITY AND THEIR USE IN THE INVESTIGATION OF CLAY DEPOSITS*
Authors: MUNDRY E. , ZSCHAU H.‐J.
Published: April 2006

Abstract

A model has been set up for the interpretation of geoelectrical sounding data for certain kinds of clay deposits containing gradually varying amounts of sand. The model assumes that in the so‐called gradient layer, resistivity varies linearly with depth. Model calculations show how such a layer can be replaced by two homogeneous layers.

An inversion procedure using the Marquardt algorithm has been developed for the interpretation of sounding data obtained with the Schlumberger array; it assumes a gradient layer beneath several covering layers. The procedure is demonstrated on two clay deposits. A comparison is made between the newly developed interpretation, the traditional approach using model curves, and computerized inversion for a homogeneous layer model.
EXPLORATION OF HIGH RESISTIVITY BASEMENT USING ELECTRICAL AND MAGNETIC FIELDS OF QUASI‐STATIC POINT SOURCES*
Authors: SZARKA L.
Published: April 2006

Abstract

Determination of thickness of sediments (usually of high conductivity) overlying a high‐resistivity basement is one of the basic problems of electrical exploration methods. This paper proposes to determine horizontal electrical conductance on the basis of impedance calculated from electrical and magnetic fields of distant quasi‐static (low‐frequency) point sources. Using the proposed method, horizontal conductance of the sediments can be determined also from artificial quasi‐static noise‐impulses coming from sources of unknown position and intensity. The results of analogue modeling and field examples prove the potential of the proposed technique.
SHEAR WAVES BY AN EXPLOSIVE POINT‐SOURCE: THE EARTH SURFACE AS A GENERATOR OF CONVERTED P‐S WAVES*
Authors: FERTIG J.
Published: April 2006

Abstract

The most common source of seismic energy is an explosion at some depth in a borehole. The radiated waves are reflected not only at the subsurface layers but also at the free surface. The earth's surface acts as a generator of both P‐ and S‐waves.

If the source depth is much less than the dominant wavelength the reflected waves resemble closely the waves generated by a single force. Theoretical seismograms were computed with different methods to look for the relevance of the surface‐reflected waves. The numerical experiments show reflected shear waves even for small shotpoint—receiver distances. Due to their polarization these waves can be detected most easily on in‐line horizontal geophones. The existence of these waves was examined during a conventional survey in Northern Germany. Conventional data analysis shows a large variability in the νp/νs ratio. The method used here produced a shear‐wave section with a rather good signal‐to‐noise ratio down to 4 s S‐wave reflection time.
A THREE‐DIMENSIONAL NUMERICAL BOTTOM‐HOLE TEMPERATURE STABILIZATION MODEL*
Authors: JONES F. W. , RAHMAN M. , LEBLANC Y.
Published: April 2006

Abstract

The heat flow equation in cylindrical coordinates is solved numerically for any general distribution of thermal diffusivity. The temperature stabilization of a borehole is considered, and solutions for the case where thermal diffusivity is a function of radial distance from the borehole are obtained and compared to solutions for uniform diffusivity. The results are discussed in terms of thermal diffusivities that are different for the well contents and for the surrounding material. It is found that the approach to formation temperature is affected by differences between well contents and the surrounding region.
A DIGITAL LINEAR FILTER FOR RESISTIVITY SOUNDING WITH A GENERALIZED ELECTRODE ARRAY*
Authors: O'NEILL D. J. , MERRICK N. P.
Published: April 2006

Abstract

This paper presents a digital linear filter which maps composite resistivity transforms to apparent resistivities for any four—electrode array over a horizontally layered earth. A filter is provided for each of three sampling rates; the choice of filter will depend on resistivity contrasts and computational facilities.

Two methods of filter design are compared. The Wiener‐Hopf least‐squares method is preferable for low sampling rate filters. The Fourier transform method is more successful in producing a filter with a high sampling rate which can handle resistivity contrasts of 100 000: 1.
GENERALISATION ET OPTIMALISATION DE LA METHODE DES DIFFERENCES FINIES POUR LA MODELISATION EN MAGNETO‐TELLURIQUE (MT)*
Authors: DOUCET D. , VAN NGOC PHAM
Published: April 2006

Abstract

After a brief discussion of MT modeling methods, we expose a particular application of finite elements that may be assimilated to a finite‐difference method. The resulting linear equations obtained are similar to the transmission network equations for two‐dimensional media. The introduction of resistivity discontinuities is more rigorous than in the usual finite‐difference development. The consequence is an improved accuracy.

We also present some conclusions about various problems encountered in modeling, such as the choice of network boundary conditions, linear system resolution and the final derivation of apparent resistivity. Application of successive over‐relaxation is discussed and we detail rules for mesh design that control result accuracy and iterative convergence.
COMPUTER MODELING OF VERTICAL SEISMIC PROFILING*
Authors: YOUNG T. K. , MONASH C. B. , TURPENING R. M.
Published: April 2006

Abstract

There are numerous modeling techniques commonly employed for the computer simulation of seismic wave propagation. The capabilities of these techniques vary according to the theoretical foundations and subsequent approximations upon which the algorithms are based. This paper constitutes a comparative review of seven modeling methods: geometric ray theory, asymptotic ray theory, generalized ray theory, Kirchhoff wave theory, Fourier synthesis, finite differences, and finite elements.

These methods can be categorized as ray or wave, acoustic or elastic, and can be contrasted according to their relative abilities to simulate such behavior as wave interference effects, diffractions, and mode conversions. As is implied by their names, geometric ray theory and asymptotic ray theory are both ray methods. The other five methods provide wave theory simulations. Geometric ray theory and Kirchhoff wave theory are normally implemented in acoustic form, while the other methods are readily adapted for computing elastic theory solutions. Generalized ray theory and Fourier synthesis are more limited in the complexity of geological model they can accommodate than are the other techniques. The methods which typically demand the greatest computer resources are the finite‐difference and the finite‐element techniques. All methods can incorporate at least some multiple events. Diffractions, however, are only inherent in the solutions computed by Kirchhoff wave theory, finite differences and finite elements. Attenuation is readily incorporated in both the Fourier synthesis and the finite‐element methods.

As an example of the application of seismic modeling, a geological model representative of a typical petroleum exploration target is used to compare vertical seismic profiles calculated by different modeling methods.
SIMULATIONS OF REFLECTIONS AND DIFFRACTIONS FROM TARGETS BURIED IN A MEDIUM WITH LINEAR VARIATION OF VELOCITY WITH DISTANCE*
Authors: CHANDER RAMESH , RADHAKRISHNA BHAT C
Published: April 2006

Abstract

A combination of ray theory and diffraction theory has been used to simulate reflections of seismic pulses from a full plane and reflections and diffractions from a half plane buried horizontally in earth models with linear increase of velocity with depth. Wave forms of signals reflected from a full plane in this case are modifications of the source pulse, even if the medium is assumed to be perfectly elastic. The extent of pulse modification increases with increase of the velocity gradient. The duration of the reflected pulse is always longer than that of the source pulse. The pulses diffracted from the edge of the half plane in such cases do not lie along hyperbolic curves in the time‐offset plane. Diffracted signals can be observed only up to a limited distance from the edge of the half plane equalling half the horizontal range of the ray which is tangential to the half plane and has its end points at the level of the source and the receiver.
SEISMIC REFRACTION METHOD TO STUDY THE FOUNDATION ROCK OF A DAM*
Authors: DUTTA N. P.
Published: April 2006

Abstract

Dynamic elastic moduli like E, μ, K and μ of the foundation rock of a dam have been determined by finding Vp‐ and Vs‐velocities by seismic refraction with a hammer as source. Some parameters such as “fracture frequency” and “rock quality designation” (RQD) of the foundation rock have been derived using “average regression curves” and Vp‐velocities. By comparing K/μ with Vp/Vs, a few locations showing weathered conditions have been demarcated. This compares well with RQD values of those locations.
INVERSION METHOD IN THE SLANT STACK DOMAIN USING AMPLITUDES OF REFLECTION ARRIVALS*
Authors: CARRION Ph.M. , KUO J. T. , STOFFA P. L.
Published: April 2006

Abstract

Almost all ray‐tracing methods ignore the analysis of the amplitudes of seismic arrivals and therefore utilize only half of the available information. We propose a method which is a combination of ray‐tracing imaging and transformation of the amplitudes of wide‐aperture data.

Seismic data in the conventional X‐T domain are first transformed to the domain of intercept time τ and ray parameter p to recover the plane wave response. The next step is the derivation of a series of plane wave reflection coefficients, which are mapped as a function of τ and p. The reflection coefficients R(τ, p) for two arbitrarily chosen traces can then be used in our inversion method to derive a slowness‐depth and a density‐depth profile. It is shown that the inclusion of amplitudes of seismic arrivals (in this method, we consider the acoustic case) makes the inverse method highly stable and accurate. In a horizontally stratified medium one can recover separate profiles of velocity and density. Since this method utilizes large‐offset data, it can be used for separate recovery of velocity and density to a greater depth.
A LABORATORY INVESTIGATION OF THE INDUCED POLARIZATION OF THE TRIASSIC SHERWOOD SANDSTONE OF LANCASHIRE AND ITS HYDROGEOLOGICAL APPLICATIONS*
Authors: OLORUNFEMI M. O. , GRIFFITHS D. H.
Published: April 2006

The time‐domain induced polarization (IP) of saturated Sherwood Sandstone correlates significantly with the intergranular permeability and the matrix conductivity but only at low electrolyte concentrations (< 500 p.p.m. NaCl). An increase in the magnitude of sandstone IP with increase in the valence of the electrolyte cation is pronounced but occurs only at intermediate concentrations, i.e., between 100 and 2500 p.p.m. Surface IP and resistivity depth sounding measurements, supplemented by data from laboratory measurements, can be used to estimate the groundwater conductivity and hence the salinity in a moderate to strongly saline sandstone aquifer.