bluuebunny/crossref_metadata_2025_split huggingface dataset:
THE CROSS‐HOLE MAGNETOMETRIC RESISTIVITY (MMR) RESPONSE OF A DISC CONDUCTOR*
Authors: EDWARDS R. N. 
Published: April 2006

AbstractThe cross‐hole variant of the magnetometric resistivity (MMR) method requires two bore holes in the vicinity of a conductive target. In the first, two fixed current electrodes are located, one above the other. They are linked to a low frequency current source by cables, the whole system forming a vertical current bipole. In the second, a sensitive coil measures the axial magnetic field as a function of depth. For a uniform earth, if both holes are vertical, the measured component vanishes by symmetry. However, the presence of a local conductor channels the current and causes an anomalous magnetic component which is interpreted to indicate the position, shape and relative conductance of the target.Mineral deposits are often lamellar in form. The conductive disc is the simplest bounded lamella for which MMR responses may be computed. It is excited by a single current source on its axis. The second source and the surface of the earth are assumed to be far away, a valid assumption for down‐hole measurements.The numerical method introduces a new integral equation describing the interaction of current dipoles located in the plane of the disc. The equation is solved analytically for a disc of infinite radius, a layer, and the result is compared with a corresponding known boundary value solution.The computed radial current in the disc and the magnetic field generated by it are described in terms of a current channelling number. The magnitude of the computed field is of the order of one nanoTesla for a typical mining problem.
LINEARIZED INVERSION OF SEISMIC REFLECTION DATA*
Authors: TARANTOLA A. 
Published: April 2006

AbstractThis is the first of a series of papers giving the solution of the inverse problem in seismic exploration. The acoustic approximation is used together with the assumption that the velocity field has the form
image
. The forward problem is then linearized (thus neglecting multiple reflected waves) and the inverse problem of estimating δ is set up. Its rigorous solution can be obtained using an iterative algorithm, each step consisting of a classical Kirchhoff migration (hyperbola summation) plus a classical forward modeling step (circle summation).
VELOCITY ANALYSIS IN THE p—x‐PLANE FROM A SLANT STACK WAVEFIELD*
Authors: TANG R. , CARSWELL A. , MOON WOOIL 
Published: April 2006

AbstractClassical methods of interpretation of reflection seismic data are such that interpretation and processing usually occur in the “collected” frame of reference. However, in recent times other data planes have gained increasing acceptance in seismology as a viable alternative. Through linear transformations applied to a record section, both the t—p‐ and p—x‐planes can be produced. The r—p‐domain may be obtained from the t—x‐plane by a transformation known as slant stacking. Normal practice has been to do most of the data processing in the t—x‐plane and then transforming to the r—p‐plane. However, many of the procedures used in the t—x‐domain can be modified for use in the t—p‐plane to increase the coherence.Velocity inversion may be carried out either in the r—p‐domain or further transformed to the p—x‐plane where the modified Herglotz‐Wiechert inversion may be applied. To perform the inversion, the t—p‐wavefield is converted to a p—x‐representation by the use of a new linear transformation technique, the cross‐stack. By a simple sampling process along a particular p—x‐trajectory, the Herglotz‐Wiechert method can be used to reconstruct an acceptable velocity model of the subsurface. A comparison of derived velocity structures is made between that produced by the Herglotz‐Wiechert technique and that of the Dix method.
A NUMERICAL DIRECT INTERPRETATION METHOD OF RESISTIVITY SOUNDINGS USING THE PEKERIS MODEL*
Authors: BASOKUR A. T. 
Published: April 2006

AbstractA numerical method is presented for direct interpretation of resistivity sounding measurements. The early part of the resistivity transform curve derived from field observations by standard methods is approximated by a two‐layer curve. The resistivity of the first layer is determined from the arithmetic mean of the successive computations which are carried on each of three successive discrete values of the resistivity transform curve. Using this mean value of the resistivity, the thickness of the first layer is computed from the sample values in pairs of the resistivity transform curve. After these determinations, the top layer is removed by Pekeris's reduction equation. The parameters of the second layer are obtained from the discrete values of the reduced transform curve (which corresponds to the second part of the resistivity transform curve) by the same procedure as described for the first layer.The same computational scheme is repeated until the parameters of all intermediate layers are obtained. The resistivity of the substratum is determined from the reduction equation.
ON THE DETERMINATION OF THE DOWNGOING P‐WAVES RADIATED BY THE VERTICAL SEISMIC VIBRATOR*
Authors: SAFAR M. H. 
Published: April 2006

AbstractIt is well recognized that in order to realize the full potential of the Vibroseis technique, one needs to ensure accurate phase locking and a meaningful cross‐correlation. To achieve these two important objectives we require an accurate estimate of the compressional stress wave radiated by the vibrator into the ground.In this paper a simple method (subject of a patent application) is developed for predicting the compressional stress waves radiated by a vertical vibrator. The main feature of the proposed method is that it involves the field measurement of the acceleration of the reaction mass and the baseplate, respectively.The method is illustrated by computing the compressional stress waves generated by a typical vertical vibrator radiating into ice, chalk, sand, and mud. It is shown that for a seismic vibrator radiating into hard ground the pressure of the downgoing P‐wave is 180° out of phase with the baseplate velocity. It is also shown that when the driving force of the seismic vibrator has a flat amplitude spectrum, the amplitude spectrum of the downgoing P‐wave falls off by 6 dB/octave towards low frequencies.
INFLUENCE OF AMPLITUDE AND PHASE ERRORS ON MIGRATION RESULTS*
Authors: VAN DER WAL L. F. , BERKHOUT A. J. 
Published: April 2006

AbstractRandom amplitude and phase errors in seismic input data introduce a coherent distribution of migration half‐circles or “smiles”, the occurrence of which may cause a significant decrease of signal‐to‐noise ratio. In addition, the effect of quantization errors is discussed for different wordlengths, used both during acquisition and during data processing. Results of sign‐bit recordings are shown.
INTERVAL VELOCITY ANALYSIS BY WAVE EXTRAPOLATION*
Authors: GAZDAG J. , SGUAZZERO P. 
Published: April 2006

AbstractA method for interval velocity analysis is formulated on the basis of wavefield extrapolation, i.e., on the basis of wave‐equation migration. When this scheme is applied to multioffset seismic sections or to an ensemble of CMP gathers, it allows for the proper treatment of dipping events. The underlying assumptions are that local velocities should be derived from data associated with events within the interval under consideration. To minimize the effect of the region above the layer of interest, the data are first extrapolated to the top of the analysis interval. Subsequent analysis of these data then pertains to the events within this interval. Velocity estimation consists of repeated wavefield extrapolations through the analysis interval using a set of trial velocities. The optimal velocity is chosen on the basis of coherency measures designed to express the collective phase agreement among a set of offset Fourier modes. The reliability of this approach to interval velocity estimation is demonstrated on synthetic multi‐offset data.
THE EFFECT OF A SUPERPARAMAGNETIC LAYER ON THE TRANSIENT ELECTROMAGNETIC RESPONSE OF A GROUND*
Authors: LEE T. 
Published: April 2006

AbstractA thin superparamagnetic layer on the earth's surface greatly affects the transient electromagnetic response of a conducting ground. The effect of the layer is most evident for singleloop transient electromagnetic data where transient voltages decay as 1/t. Even when a separate transmitter and receiver are used, the effect of the superparamagnetic layer is still pronounced. In this case the effect of the 1/t term in the equation is much less. More dominant now is a 1/t2 term. The effect of the superparamagnetism can readily be seen in the analytical expressions for the apparent resistivities. If the presence of the superparamagnetic layer is not recognized, then the apparent resistivities decrease with time rather than approach the true value of the host rock.
TUTORIAL MINIMUM PHASE FOR CONTINUOUS TIME AND DISCRETE TIME FUNCTIONS*
Authors: EISNER E. 
Published: April 2006

AbstractThe concept of minimum phase is clarified for geophysicists by collecting in one place the properties of minimum phase functions. The “earliest energy arrival” property in the time domain, the “minimum phase‐slope property” in the frequency domain, and of the role of causal all‐pass filters are demonstrated. The emphasis is placed on keeping the mathematics within the realm familiar to geophysicists and on making clear the somewhat imperfect match between physical continuous time functions and their associated discrete time representations.
RESISTIVITY AND MAGNETIC SURVEYS IN GROUNDWATER PROSPECTING IN VOLCANIC AREAS—CASE HISTORY MAAR OF BEAUNIT, PUY DE DOME, FRANCE*
Authors: AUBERT M. , CAMUS G. , FOURNIER C. 
Published: April 2006

AbstractVolcanic maar structures sometimes contain important groundwater reserves. The volcanic formations of the Beaunit maar have a much higher magnetization than their granite substrata. The structure of the maar and its volcanoclastic infill can therefore be defined by magnetic surveying and resistivity soundings. Resistivities of the geological formations present in the Beaunit maar are comparable to those recorded in other volcanic systems. The geophysical data, confirmed by drilling tests, contain useful information on the hydrogeological regime of the volcanic system under investigation.
VSP INTERVAL VELOCITIES FROM TRAVELTIME INVERSION*
Authors: STEWART R. R. 
Published: April 2006

AbstractA principal use of the vertical seismic profile (VSP) is to determine the variation of seismic velocity with depth. Presented here is a discussion of the errors involved with the time picks of a VSP survey and several methods currently used to calculate a velocity section from these time picks. Another technique is proposed, based on the least‐squares inversion of the traveltimes, to arrive at a better estimate and statistical description of the velocity section. This technique uses the Levenberg‐Marquardt damped least‐squares formulation and ray tracing through a horizontally‐layered medium to iteratively refine the velocity section. The accuracy and robustness of the procedure are investigated by inverting noisy traveltime curves and comparing these results to the original model velocity section. Agreement is found to be good. One interesting feature of the inverse procedure is that, for certain geometries, it can resolve a few velocity layers, even though there are no measurements made inside those layers. Three actual VSP surveys are analyzed and compared to their corresponding sonic logs. In two of the surveys, it appears that there is some velocity dispersion. Velocity changes associated with gas saturation are evident on one of the surveys.
IN‐SEAM SEISMIC LOVE WAVE SCATTERING MODELED BY THE FINITE ELEMENT METHOD*
Authors: ASTEN M. W. , DRAKE L. A. , EDWARDS S. 
Published: April 2006

AbstractThe dynamic finite‐element method allows frequency‐dependent reflection and transmission coefficients to be computed for Love waves scattered by two‐dimensional inhomogeneities in coal seams.Clean faults of zero hade angle show a throw‐dependent cross‐over frequency in reflection spectra, and throw‐dependent conversion from fundamental to first higher mode energy in transmission spectra. Oblique faults show mode conversion in reflection spectra which is relatable to the fault hade angle by simple Huygens‐theory models. Thin fracture zones or dykes normal to the seam show a reflection maximum when thickness of the zone is of order one quarter of the seam wave wavelength.Published field data from two known faults and a dyke are compared with the modeling results and support the belief that broad‐band seam‐wave data are capable of characterizing a seam discontinuity (throw, hade angle, dyke thickness) as well as locating it. Development of such procedures will require extensions to existing field practice and processing.This research was funded in part by the National Energy Research Development and Demonstration Council of Australia. The authors thank The Broken Hill Proprietary Company Limited for permission to quote from company case histories.
PRACTICAL ASPECTS OF THE DETERMINATION OF 3‐D STACKING VELOCITIES*
Authors: LEHMANN H. J. , HOUBA W. 
Published: April 2006

ABSTRACTProper stacking of three‐dimensional seismic CDP‐data generally requires the knowledge of normal moveout velocities in all source‐receiver directions contributing to a CDP‐gather. The azimuthal variation of the stacking velocities mainly depends on the dip of the seismic interfaces. For a single dipping plane a simple relation exists between the dip and the azimuthal variation of NMO‐velocity. Varying strike and dip of subsequent reflectors, however, result in a complex dependency of the seismic parameters.Reliable information on the spatial distribution of the normal moveout (NMO)‐velocity can be derived from a wavefront curvature estimation using a 3‐D ray‐tracing technique. These procedures require additional information, e.g. reflection time gradients or depth maps to show interval velocities between leading interfaces. Moreover, their application to an extended 3‐D data volume is restricted by high costs.The need for a routine 3‐D procedure resulted in a special data selection to create pseudo 2‐D profiles and to apply existing velocity estimation routines to these profiles. At least three estimates in different directions are necessary to derive the full azimuthal velocity variation, characterized by the large and the small main axis and the orientation of the velocity ellipse.Errors are estimated by means of computer models. Stacking velocities obtained by mathematical routines (least‐squares fit) and by seismic standard routines (NMO‐correction and correlation) are compared.Finally, a general 3‐D velocity procedure using cross‐correlation of preliminarily NMO‐corrected traces is proposed.
COMPUTATION OF ZERO‐OFFSET VERTICAL SEISMIC PROFILES INCLUDING GEOMETRICAL SPREADING AND ABSORPTION*
Authors: URSIN B. , ARNTSEN B. 
Published: April 2006

ABSTRACTSynthetic vertical seismic profiles (VSP) provide a useful tool in the interpretation of VSP data, allowing the interpreter to analyze the propagation of seismic waves in the different layers. A zero‐offset VSP modeling program can also be used as part of an inversion program for estimating the parameters in a layered model of the subsurface.Proposed methods for computing synthetic VSP are mostly based on plane waves in a horizontally layered elastic or anelastic medium. In order to compare these synthetic VSP with real data a common method is to scale the data with the spherical spreading factor of the primary reflections. This will in most cases lead to artificial enhancement of multiple reflections.We apply the ray series method to the equations of motion for a linear viscoelastic medium after having done a Fourier transformation with respect to the time variable. This results in a complex eikonal equation which, in general, appears to be difficult to solve. For vertically traveling waves in a horizontally layered viscoelastic medium the solution is easily found to be the integral along the ray of the inverse of the complex propagation velocity. The spherical spreading due to a point source is also complex, and it is equal to the integral along the ray of the complex propagation velocity.Synthetic data examples illustrate the differences between spherical, cylindrical, and plane waves in elastic and viscoelastic layered media.
TUTORIAL DISTRIBUTION OF ALTERNATING ELECTRICAL CHARGES IN A CONDUCTING MEDIUM*
Authors: KAUFMAN A. A. 
Published: April 2006

ABSTRACTThe main features of the distribution of volume and surface charges in a conducting medium can be described separately for direct and alternating electromagnetic fields. The density of charges depends on the conductivity of a medium and on the electrical field. The relation is particularly simple for the quasi‐stationary field, i.e., when the influence of displacement currents is negligible. Conditions are formulated under which electrical charges arise in a conducting medium: electrical charges are shown to exist for direct and quasi‐stationary fields when there is a component of electric field parallel to the gradient of conductivity. The density of these charges is proportional to the applied electric field.
A NONITERATIVE PROCEDURE FOR INVERTING PLANE‐WAVE REFLECTION DATA AT SEVERAL ANGLES OF INCIDENCE USING THE RICCATI EQUATION*
Authors: BREGMAN N. D. , CHAPMAN C. H. , BAILEY R. C. 
Published: April 2006

ABSTRACTVarious exact methods of inverting the complete waveform of vertical seismic reflection data to produce acoustic impedance profiles have been suggested. These inverse methods generally remain valid for nonvertical, plane‐wave data, provided total reflection does not occur. Thus, in principle, the “seismogram” at each ray parameter in a slant stack can be interpreted separately.Rather than invert each plane‐wave seismogram separately, they can all be interpreted simultaneously and an “average” model thus obtained. Inversion for both the velocity and the density also becomes possible when two or more plane‐wave seismograms are simultaneously inverted. The theory for a noniterative inversion method, based on the time‐domain Riccati equation, is discussed. Numerical examples of inversions using this technique on synthetic data demonstrate its numerical stability and the advantage of simultaneous inversion of several seismograms to reduce the effect of noise in the data and increase the stability of the inversion process.
STABILIZATION OF NORMAL‐INCIDENCE SEISMOGRAM INVERSION REMOVING THE NOISE‐INDUCED BIAS*
Authors: FERBER R.‐G. 
Published: April 2006

ABSTRACTA main problem in computing reflection coefficients from seismograms is the instability of the inversion procedure due to noise. This problem is attacked for two well‐known inversion schemes for normal‐incidence reflection seismograms. The crustal model consists of a stack of elastic, laterally homogeneous layers between two elastic half‐spaces. The first method, which directly computes the reflection coefficients from the seismogram is called “Dynamic Deconvolution”. The second method, here called “Inversion Filtering”, is a two‐stage procedure. The first stage is the construction of a causal filter by factorization of the spectral function via Levinson‐recursion. Filtering the seismogram is the second stage. The filtered seismogram is a good approximation for the reflection coefficients sequence (unless the coefficients are too large).In the non‐linear terms of dynamic deconvolution and Levinson‐recursion the noise could play havoc with the computation. In order to stabilize the algorithms, the bias of these terms is estimated and removed. Additionally incorporated is a statistical test for the reflection coefficients in dynamic deconvolution and the partial correlation coefficients in Levinson‐recursion, which are set to zero if they are not significantly different from noise.The result of stabilization is demonstrated on synthetic seismograms. For unit spike source pulse and white noise, dynamic deconvolution outperforms inversion filtering due to its exact nature and lesser computational burden. On the other hand, especially in the more realistic bandlimited case, inversion filtering has the great advantage that the second stage acts linearly on the seismogram, which allows the calculation of the effect of the inversion procedure on the wavelet shape and the noise spectrum.
FORWARD MODELING IN THE FREQUENCY‐SPACE DOMAIN*
Authors: KELAMIS P. G. , KJARTANSSON E. 
Published: April 2006

ABSTRACTForward modeling of zero‐offset data is performed in the frequency‐space domain using a one‐way extrapolation equation. The use of the frequency domain offers several advantages over conventional time domain methods. The greatest advantage of the frequency domain is that all time derivatives are evaluated exactly by a simple multiplication. Synthetic zero‐offset sections are computed with a high degree of accuracy for arbitrary velocity and reflectivity structures. Examples are shown for realistic complicated models and compared with results from physical modeling.
INVERSION FROM FINITE OFFSET DATA AND THE SIMULTANEOUS RECONSTRUCTION OF THE VELOCITY AND DENSITY PROFILES*
Authors: EIGES R. , RAZ S. 
Published: April 2006

ABSTRACTRecursive inversion algorithms are described which facilitate (a) a direct inversion of a one‐dimensional velocity distribution from finite offset data and (b) the related simultaneous reconstruction of a two‐parameter, say velocity‐density, medium. The suggested procedures are based on an extended format of the WKBJ‐Bremmer scattering model. The attained accuracy levels and the limitations are highlighted by computer simulations against synthetic data.
ON THE IMPROVEMENT IN PENETRATIO ACHIEVED BY USING EXTENDED MARINE SOURCE ARRAYS*
Authors: SAFAR M. H. 
Published: April 2006

ABSTRACTIn dealing with the problem of large amplitude multiple reflections arising from a hard water‐bottom, it has been found that the use of extended source array techniques resulted in a considerably better penetration than that obtained using either computer simulated long arrays or the conventional air‐gun array systems. The purpose of this paper is to use the concept of the array directivity factor in discussing the reason for the improvement in penetration achieved by using extended marine source arrays.Examples are given showing that the low frequency power radiated within the so called “penetration window” can be increased by a factor of two by choosing the correct spacing of the point sources forming the extended array. It is concluded that to ensure that most of the low frequency energy is concentrated within the penetration window to achieve deep penetration, a source array with spacing comparable with the wavelength is required.