bluuebunny/crossref_metadata_2025_split huggingface dataset:
A NEW DERIVATION OF THE WAVEFRONT CURVATURE TRANSFORMATION AT AN INTERFACE BETWEEN TWO INHOMOGENEOUS MEDIA*
Authors: URSIN B. 
Published: April 2006

AbstractThe principles for ray‐tracing and wavefront curvature calculations in a three‐dimensional medium are reviewed. A new derivation of the transformation of the wavefront curvature matrix at an interface between two inhomogeneous media is given. The derivation is based on a Taylor series expansion of the ray refraction equation at the interface between two inhomogeneous media, and only elementary geometric arguments are used. The wave‐front curvature transformation at the interface is obtained by neglecting all terms in the direction of the surface normal.With proper definition of the variables, the derivation is also valid for a reflected wave‐front. A simplified transformation rule is derived for a reflected wave of the same type as the incident wave.
POROSITY PREDICTION FROM SEISMIC DATA*
Authors: ANGELERI G. P. , CARPI R. 
Published: April 2006

AbstractThe inversion of seismic traces allows the estimation of reservoir porosity from an analysis of transit times derived from the pseudo‐velocity logs. A four‐step computational procedure is illustrated consisting of (i) inversion of seismic traces and calculation of interval velocities; (ii) accurate stratigraphic interpretation; (iii) determination of the petrophysical parameters for the porosity evaluation; (iv) analysis of the reliability of the results and final corrections.Both the possibilities and the limits of the method are discussed. One of the causes of error is the fact that impedances—and not velocities—are readily obtainable from seismic data. Moreover, the porosity due to fracturation contributes only slightly to velocity, while it often contributes most importantly to the permeability.Results are shown for two of the most significant reservoir types, i.e. carbonatic and clastic. Two cases belonging to the latter type will be examined. In the first case the primary porosity is dominant. The second case is very complex and both primary and secondary porosity are present.
RECIPROCAL AVERAGING TECHNIQUES IN THE GEOELECTRICAL BOUNDARY ELEMENT APPROACH*
Authors: OKABE M. 
Published: April 2006

AbstractA Fredholm integral equation of the first kind with respect to the surface charge density is presented via a weighted residual formulation in the standard isotropic problem. The surface charge densities are numerically obtained by solving the well‐known Fredholm integral equation of the second kind, where the new equation can be regarded as a constraint. The accuracy of boundary element solutions is examined in connection with the violation of such a constraint and the “modified reciprocal averaging techniques” are proposed.
THE INFLUENCE OF SHALE EFFECTS UPON THE ELECTRICAL RESISTIVITY OF RESERVOIR ROCKS*
Authors: WORTHINGTON P. F. 
Published: April 2006

AbstractThe roots of the so‐called shaly‐sand problem in hydrocarbon evaluation lie in the effect of relatively fine‐grained minerals upon measured electrical parameters of granular reservoirs. This influence manifests itself as an excess conductivity, over and above that due to the purely geometric effects of electrolyte distribution within the pore space. For formations with low shaliness, this excess conductivity is usually insignificant in typical oilfield situations.The influence of shaliness upon observed values of formation resistivity has been appraised by collating core‐sample data from four different reservoirs. It has been demonstrated that during the course of electrical measurement under conditions of full electrolyte saturation, any given lithology can exhibit both negligible and highly significant shale effects depending upon the resistivity of the interstitial aqueous electrolyte. The effects of shaliness are also governed by the degree of water saturation.Because the manifestation of shaliness in electrical data is not a function of lithology alone, recourse is made to a more realistic concept of shale effects whereby a formation, or section of a formation, is classified as “effectively clean” or “effectively shaly” according to whether it obeys or defies, respectively, the fundamental empirical laws of Archie (1942). In particular, since an intrinsic formation factor can be obtained directly in fully‐saturated effectively clean reservoirs, whereas only an apparent quantity may be recorded directly in fully‐saturated effectively shaly reservoirs, the ratio of apparent to intrinsic formation factor serves as a useful conceptual indicator of shale effects, attaining the limiting value of unity only under effectively clean conditions.In the context of electrical measurement the terms “shaliness” and “shale effects” are evidently not synonymous and it is the latter which should be considered when selecting equations for the computation of water saturation. The implications for well‐log analysis follow through formulated guidelines that describe the relative levels of shale effects in different zones of lithologically uniform reservoirs.
APPLICATION OF BOREHOLE GEOPHYSICS AT AN EXPERIMENTAL WASTE STORAGE SITE*
Authors: NELSON P. H. , MAGNUSSON K. A. , RACHIELE R. 
Published: April 2006

AbstractA suite of electrical, radiation, and mechanical borehole probes were run in a 76‐mm‐diameter borehole drilled to a slant depth of 380 m in leptite and granite. The hole is located in Precambrian bedrock in central Sweden where a site is dedicated to in‐situ experiments pertaining to the disposal of radioactive wastes. The challenge to borehole logging methods for such site investigations is to resolve geological features and fluid flow parameters in geological sites which are initially chosen for their homogeneity, low porosity, and minimal fracturing. The Stripa borehole is characterized by high electrical resistivity values in the 20–100 kΩm range, by acoustic velocities around 5800 m s‐1 (which is close to laboratory values on intact specimens), and by total porosity of around one volume percent. In this context, probe resolution was adequate to produce interpretable information on almost all of the logs.Two principal rock types were encountered in the hole: granite, of quartz monzonitic composition, and leptite. The granite and leptite intercepts are subdivided into units characterized by mafic mineral content, sulfide mineral content, and electrical and radiation properties. Iron‐rich zones in the leptite are highly anomalous on the gamma‐gamma and neutron logs; thin mafic zones in the granite can also be distinguished. Occurrences of a few percent pyrite are detected by the electrical, gamma‐gamma, and neutron logs. Although overall porosity is quite low throughout the hole, analysis of the resistivity and neutron logs indicates the porosity increases by a few volume percent at fracture zones. The differential resistance and caliper probes detect borehole diameter roughness of less than 1 mm, helping to confirm acoustic waveform anomalies which are indicative of fracture zones. Compres‐sional wave transit time and shear‐wave interference patterns usually occur coincident with open fractures observed in core, the correlation being especially good at major fracture zones.
DOWNHOLE PERIODIC SEISMIC SOURCES*
Authors: HARDEE H. C. 
Published: April 2006

AbstractDownhole periodic seismic sources have been proposed as a means for studying fluid zones in hydrothermal/magma systems encountered in deep scientific holes drilled as part of the Continental Scientific Drilling Program. Scientific measurements in these holes will require the use of downhole, nondestructive, high resolution, seismic tools. Downhole periodic seismic sources tested in a shallow geothermal zone showed that a pneumatic oscillator could effectively generate downhole low‐frequency (10–100 Hz) seismic waves and that these waves could effectively penetrate a highly fractured formation. An improved version of these downhole sources, capable of swept‐frequency operation, is currently under development. This type of downhole seismic source has applications in crustal studies and in the evaluation of petroleum, groundwater and geothermal reservoirs.
HIGH‐RESOLUTION PREDICTION OF ACOUSTIC IMPEDANCES BELOW BOTTOM‐OF‐HOLE*
Authors: TAL‐VIRSKY B. B. , TABAKOV A. A. 
Published: April 2006

AbstractPrediction of acoustic impedances below bottom‐of‐hole developed in the paper is based on VSP data processing. The pulse form and reflections are deduced from records on the vertical array by the method of subtraction. A deterministic prediction error and a wave‐shaping Wiener filter are used to transform the pulse train and reflections into a short symmetrical pulse. Additional broadening of the pulse spectrum is achieved by stacking of seismograms from shots with various dominant frequencies after zero‐phase transformation of downgoing waves.The inversion of seismograms is made after ideal spiking by means of a subtraction procedure. Acoustic impedances deduced are closely related to lithology of rocks below bottom‐of‐hole.
CALCULATIONS ON THE THERMAL CONDITIONS AROUND A SALT DIAPIR*
Authors: JENSEN P. K. 
Published: April 2006

AbstractTemperature modeling around the Vejrum salt structure in Denmark shows that the heat flow near the top is twice the regional heat flow. The temperature in the vicinity of the top of the diapir is thus anomalously high. Overlying permeable formations could therefore be attractive for geothermal energy exploitation. A negative temperature anomaly of −20°C is calculated around the root of the salt body. Comparisons between calculated and measured temperatures are used to test the model. It is shown that measurements of heat flow (or temperature gradients) in wells penetrating the region of groundwater circulation could be used in identification of salt diapirs.
ELECTROMAGNETIC SOUNDING NEAR A LARGE SQUARE LOOP SOURCE OF CURRENT*
Authors: PODDAR M. 
Published: April 2006

AbstractAn electromagnetic sounding experiment with a large square loop as source was carried out on the dried‐up bed of a water reservoir near the town of Dharmavaram in Andhra Pradesh. The sounding was performed in both geometric and parametric modes, and involved measurements of phase as well as amplitude of the vertical magnetic field inside and outside the loop. The six‐frequency EM system used for the experiment was found to be a workable system for electromagnetic sounding. The results of the experiment more or less confirm the conclusions from theoretical modeling. They also show that even though the earth is not always electrically horizontally layered over a sufficiently large lateral extent for the one‐dimensional model to be strictly valid, it is still possible to apply such modeling to sounding curves taken one part at a time and obtain layer parameters which check qualitatively with the layer parameters obtained from direct current resistivity sounding.
MEASURED ANISOTROPY IN PIERRE SHALE*
Authors: WHITE J. E. , MARTINEAU‐NICOLETIS L. , MONASH C. 
Published: April 2006

AbstractThe five elastic coefficients which characterize a transversely isotropic medium have been measured for the Pierre Shale. The first‐arrival times deduced from two closely spaced vertical seismic profiles provide values of magnitude and direction of the phase velocity at a given depth for different locations of the sources. From these velocities, we can estimate the five elastic coefficients by assuming that the P‐ and SV‐wave velocities have only a small dependence on the angle of propagation. On the other hand, it is shown that the accuracy in the difference between the directions of propagation and particle motion is not sufficient to determine the anisotropy.This work was industrially sponsored through the Integrated Geophysics Project.
LINEAR FILTERING OF VLF DIP‐ANGLE MEASUREMENTS*
Authors: KAROUS M. , HJELT S. E. 
Published: April 2006

AbstractThe suggested interpretation technique is based on discrete linear filtering of VLF data. The output of the described filtering results is expressed in terms of an equivalent current density at a specified depth that would cause the measured magnetic field. The most practical six‐point filter gives an accuracy of 8%. The filter is an extension of the commonly used Fraser filter to process VLF dip‐angle data.Filtering the same data set for various depths gives an idea about the change of current density with depth. Areas with high current‐density correspond to good conductors. The conductor dip can also be determined.The use of the technique is illustrated on theoretical and field examples. In all cases a good correlation with original models and other types of geophysical measurements was obtained. As shown in the last example, the filtering technique is also applicable in interpretation of other electromagnetic methods.
ON THE ORIGIN AND INTERPRETATION OF SELF‐POTENTIAL ANOMALIES*
Authors: KILTY K. T. 
Published: April 2006

AbstractThe renewed interest in the self‐potential method of exploration for mineral deposits gives an understanding of the self‐potential mechanism new importance. The cause of SP anomalies in general lies in the interference between simultaneously occurring nonequilibrium phenomena. However, theories of the mechanism of mineral SP anomalies generally relate the SP anomaly to the equilibrium potential of the chemical reaction supposed to occur on the ore body surface. In this paper, I reformulate these equilibrium mechanisms in terms of nonequilibrium thermodynamics. The result is that the SP anomaly depends not on the equilibrium potential alone, but also on the potential resulting from current transferred across the ore body—electrolyte interface. It is not possible to calculate the overpotential theoretically because of the number of complicating factors, and experimental data are not available. This does not imply that SP data are uninterpretable quantitatively. SP data may be interpreted similarly to other potential field data.
THE CLASSIFICATION OF PHYSICO‐GEOLOGICAL MODELS OF MINERAL DEPOSITS*
Authors: VAKHROMEYEV G. S. , BARYSHEV A. S. 
Published: April 2006

AbstractWe describe the concept of physico‐geological models (PGMs) in geophysical exploration. They represent a “general model”, a spatial combination of a set of particular models (disturbing bodies). The modeling is called complete, incomplete or approximate, depending on the degree of characterization of the PGM by parameters such as dimension, shape and petrophysical property. Each of the three modeling types can be realized as a conceptual, and analytical, or a material PGM. Both deterministic and stochastic PGMs exist; deterministic models are mainly used to investigate the possibilities of a geophysical method, while stochastic models serve to substantiate complex geophysical interpretations.Depending on the geological problem, PGMs are subdivided into multi‐alternative models (geological mapping, prediction, general prospecting) and double alternative models (specialized prospecting).An exploration‐oriented classification of the PGMs of mineral deposits is discussed. According to this classification the variety of known genetic deposit types is reduced to a limited number of generalized PGM types. The development of typical PGMs is illustrated with examples of magnetitic deposits of Siberia.
A MODIFIED MODE OF OPERATION FOR THE TURAM ELECTROMAGNETIC EXPLORATION SYSTEM WITH BENEFITS FOR DEEP EXPLORATION*
Authors: DUCKWORTH K. , BAYS A. R. 
Published: April 2006

AbstractA new mode of operation for the Turam electromagnetic exploration system is proposed in which the transmitter loop is placed across the expected trend of a conductor and the receiver is operated along lines parallel to one side of the transmitter. The concept appears to offer several benefits which include greatly extended traverse length, the use of large coil spacing, rejection of the effects of conductive environments, and consistency in the indication of target dip.
OFFSET SOURCE VSP SURVEYS AND THEIR IMAGE RECONSTRUCTION*
Authors: DILLON P. B. , THOMSON R. C. 
Published: April 2006

AbstractZero‐offset‐source VSP surveys provide information about the subsurface only within the Fresnel zone centered at the well. Offsetting the source location moves the reflection zones away from the well thus providing lateral cover.Conventional processing of this type of data gives rise to a distorted image of the subsurface. Using a simple ray‐tracing scheme, this image may be reconstructed into the more familiar coordinate system of the surface seismic section. This simple data‐independent mapping is based on the assumption of horizontal layering and requires a vertical velocity profile.The technique of placing the source away from the borehole was first applied to the single‐offset‐source VSP survey. However, data from any survey geometry (such as deviated well with rig source, walkaway VSP, etc.) can be mapped to the coordinate system defined by the appropriate seismic section.To obtain the best results from this type of survey the target area must be defined and simple modeling techniques used to optimize the source location(s). These pre‐survey modeling methods may also be used to anticipate—and hence avoid a number of problem areas which experience has highlighted.The data from any VSP survey is the result of a realizable experiment and as such obeys the wave equation. This implies that the wave equation may be used to migrate the data to its true subsurface location. Theoretically, such a process is more secure than ray‐tracing techniques, although its practice presents many difficulties.
ODD‐DEPTH STRUCTURES FOR DETERMINISTIC DECONVOLUTION AND SEISMOGRAM TESTING*
Authors: SZARANIEC E. 
Published: April 2006

AbstractIn odd‐depth structure the two‐way traveltime to each boundary is constrained to be an odd integer. The odd‐depth property of a model is exposed to possible refutation under a seismogram test. Test function is a simple transformation of a synthetic seismogram. For an odd‐depth model the test function has identically the value 1.The testability of a synthetic seismogram over an odd‐depth structure provides a method of deterministic deconvolution. There is no need of specialized assumptions, like the minimum‐phase property, about the source wavelet. The deconvolution may be performed in the absence of the early segment of a seismogram.
AN OPERATOR APPROACH TO FORWARD MODELING, DATA PROCESSING AND MIGRATION*
Authors: KENNETT B. L. N. 
Published: April 2006

AbstractThe response of a seismic model to excitation by a source can be represented in terms of the action of reflection and transmission operators for portions of the structure. This approach provides a flexible framework for both modeling and processing problems.The operator development provides a physical description of the wave propagation process and, via the expansion of reverberation operators, gives a mechanism for assessing the accuracy of approximate developments. The representation suggests new ways of developing modeling algorithms by balancing the computational effort expended on minor and major features of the model.For processing problems, the operator representation shows the relation of processing stages to the seismic wave field and thereby indicates effective sequences of operations. For migration it is possible to specify an ideal pre‐stack migration procedure in terms of the inverse of the propagation operators and to examine the problems which need to be overcome by practical algorithms.
PHASE DISTORTION DUE TO ABSORPTION IN SEISMOGRAMS AND VSP*
Authors: ANGELERI G. P. , LOINGER E. 
Published: April 2006

AbstractAbsorption of seismic energy in the earth reduces amplitudes and changes phases of the propagating seismic waves. Amplitudes are usually recovered according to an estimated exponential decay curve, while phase distortions are generally disregarded. Therefore, accurate processing of seismic data requires a careful investigation of the relationship between absorption and phases.In this paper a procedure is suggested to achieve this goal, and some related topics are worked out. A method is outlined for computing synthetic seismograms and vertical seismic profiles with phase distortion due to absorption.The algorithm works in the frequency domain, and it provides for absorption according to the usual model of exponential decay of amplitude with distance. The absorption coefficient is a linear function of frequency and is related to the quality factor Q of the rocks. Complex seismic velocities are introduced and minimum‐phase delay due to absorption is assumed for all cases considered.Methods for estimating Q profiles from seismic well surveys and seismic data are described. Comparison between field and synthetic data shows the effectiveness and benefits of the procedure. Some applications of the method to phase distortion recovery and wavelet processing are presented.
RESOLUTION ENHANCEMENT OF WELL LOG AND SEISMIC DATA*
Authors: MICHETTE A. G. , DARLING A. M. , FIDDY M. A. , WARD W. A. , FRANKLIN E. 
Published: April 2006

AbstractA new method with general applications for seismic data is presented for spectral extrapolation. The method gives a restored image that is optimum in the sense of minimum norm and can be adapted to incorporate any constraints on the reconstruction. As an illustration of the technique it is first applied to a synthesized (noise‐free) image. Then it is shown that enhancement of a density log (run in a hole drilled entirely in Coal Measures rocks) is possible using constraints taken from the geologist's log. A synthetic trace with significantly improved event arrival times and general character is obtained.
NOMOGRAMS TO INTERPRET CRONE PEM DATA USING A DIPPING SHEET MODEL*
Authors: RAI S. S. , VERMA S. K. 
Published: April 2006

AbstractThe response of a rectangular plate to the Crone PEM system has been investigated for different values of depth, dip and conductance parameters. The conductance aperture diagram is presented to estimate the conductance and depth to the top of a vertical sheet. Information regarding dip and depth may be obtained using the appropriate nomogram.
HOW DO SHEAR WAVE EVENTS AFFECT NORMAL P‐WAVE RECORDS?*
Authors: AMEELY L. , EDELMANN H. A. K. , FERTIG J. 
Published: April 2006

ABSTRACTIt has been known since the beginning of reflection seismics that several disturbing events seen in seismic records are caused by waves with S‐wave velocities instead of P‐wave velocity. When using dynamite and recording with vertical geophones these events are primarily caused by converted waves.On the basis of known P‐ and S‐wave velocities in a certain area a theoretical seismogram is calculated, displaying traveltime as well as energy relation for different wave configurations. By comparison with seismograms recorded in the same area it can be shown that converted wave events can be clearly recognized.These events can be described theoretically. Thus, either more effective computer programs can be applied to eliminate these disturbing events, or these events can be evaluated to get additional information about specific strata.