bluuebunny/crossref_metadata_2025_split huggingface dataset:
NUMERICAL VLF MODELING*
Authors: KAIKKONEN P. 
Published: April 2006

AbstractThe 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

AbstractInspired 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractSign 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

AbstractNumerous 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

AbstractThe 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

AbstractConstant 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

AbstractInduced 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

AbstractThe 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

AbstractA 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

AbstractPopper'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

AbstractComplete 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

AbstractApproaches 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 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

AbstractTwo 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

AbstractThe 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

AbstractThe 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

AbstractModern 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

AbstractSource 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

AbstractNonlinear 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

AbstractThree 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.