bluuebunny/crossref_metadata_2025_split huggingface dataset:
COMPUTATION OF VLF RESPONSE OVER HALF‐PLANE AND WEDGE MODELS*
Authors: OLSSON O. 
Published: April 2006

AbstractA theoretical solution is presented to the problem where a VLF anomaly is generated by a conducting half‐plane or a perfectly conducting wedge below a stratified overburden. The solution is obtained by the use of a scattering matrix for plane‐wave eigenfunctions. VLF anomalies have been computed for different values of the conductance and dip of the half‐plane. The phase of the VLF anomaly due to a conducting half‐plane depends on the conductance and the distance to the half‐plane. Close to the half‐plane the tilt angle and ellipticity are of opposite sign for a perfect conductor, but the ellipticity will change sign for a poor conductor. The VLF anomaly for a perfectly conducting wedge is essentially determined by the position of the upper surface of the wedge, i.e. the anomaly will closely resemble the anomaly of a perfectly conducting half‐plane in the same position as the upper surface of the wedge.
A THEORY OF ACOUSTIC DIFFRACTORS APPLIED TO 2‐D MODELS*
Authors: DEREGOWSKI S. M. , BROWN S. M. 
Published: April 2006

AbstractFinite‐offset seismic reflection modeling of acoustic waves, propagating in a two‐dimensional depth section of arbitrary complexity, is discussed. The procedure developed employs the principles of simplified (far‐field) diffractor theory and ray tracing. Each reflector is represented by a set of discrete secondary sources or diffractors and the wavefield associated with each diffractor is calculated directly in the time domain by ray tracing. Reflections and diffractions are subsequently built up by the numerical superposition of these wavefields. This superposition is nondispersive for all frequencies for which the Fresnel zones are large compared with the diffractor separation.All primary travel paths connecting the shot to diffractor and diffractor to geophone are accounted for together with phase changes induced by focal events. The method allows the modeling of arbitrary trace gathers for energy originating from selected reflectors. The nonsequential nature of the algorithm makes it suited to machines capable of carrying out many similar operations in parallel or concurrently. Diffractor theory also provides physical insight into wave scattering and focusing. In particular, the half‐differential waveform associated with a line diffractor leads to an explanation of the 90° phase lead induced by a cylindrical focus and, similarly, the full differential waveform of a point diffractor can be used to explain the 180° phase shift induced by a point focus.
THE USE OF AIRGUN PRESSURE PULSES FOR CALIBRATING A LOW‐FREQUENCY STANDARD HYDROPHONE*
Authors: SAFAR M. H. 
Published: April 2006

AbstractA new technique is developed for calibrating a low‐frequency hydrophone. The technique involves the use of pressure pulses radiated by the Par 0.65 liter airgun when fired at a fixed depth but with various values of initial chamber pressure. The sensitivity of a low‐frequency hydrophone, when determined by the proposed technique, is found to be in agreement with that obtained by using the so‐called “impulse method”.
ELECTRONIC ACCELERATION‐SENSITIVE GEOPHONE FOR SEISMIC PROSPECTING***
Authors: KLAASSEN K. B. , VAN PEPPEN J. C. L. 
Published: April 2006

AbstractPreviously ignored characteristics of the seismic recording instrument are presently experienced as limitations as more sophisticated interpretive methods using wider frequency ranges are developed to extract stratigraphic information from seismic land data for hydrocarbon and mineral exploration. Most of these limitations arise from inadequate characteristics of the first element of the seismic instrument: the geophone. A geophone does not faithfully follow the motion of the earth for higher frequencies due to poor geophone‐earth coupling. This filtering effect brings about time shifts that are dependent on the frequency and the soil type. A geophone can also produce spurious outputs, brought about by the motion of the suspended part of the geophone, with a magnitude comparable to that of the desired output. The suspension is made very compliant to obtain the required sensitivity. A compliant suspension, however, gives a large sag. The geophone can therefore only be used in one position, tolerating little tilt. A compliant suspension also widens the traveling range of the movable part. Minor sensitivity changes with travel are then noticeable as nonlinearity, since the surface wave is large with respect to the reflected wave. A compliant suspension is usually realized in the form of thin, spirally shaped spring‐spiders. Such suspensions exhibit transverse or rotational resonances that are in or close to the seismic frequency band. Excited by ground roll, they can produce considerable undesirable output.The novel geophone we describe is a light‐weight (17 g) acceleration‐sensitive transducer which gives good ground coupling and partial correction for the increasing damping in the earth with increasing frequencies. It employs internal hybrid electronics for a magnetodynamic velocity‐nulling feedback system. Velocity nulling makes the movable part of the geophone virtually rigid with respect to the housing. This makes the geophone characteristics independent of the suspension. The springs used are stiff in a transverse and rotational direction so that the suspension resonances are well outside the useful frequency band. This suspension also allows the geophone to be used in any orientation while being only sensitive to the vibration component along the main axis. The feedback system makes the sensitivity flat within 1 dB from 2 Hz to 500 Hz, with a phase tolerance smaller than 5°. The geophone is robust, has no moving internal wires, employs a current output [sensitivity 1 mA/(m s−2)] and internal gain so that the signal‐to‐cable‐noise ratio is improved. This type of output allows parallel connection without any interaction between the geophones.
VLF SURVEY OF THE WEATHERED LAYER IN SOUTHERN INDIA*
Authors: PODDAR M. , RATHOR B. S. 
Published: April 2006

AbstractThe electrical properties of the weathered layer were investigated by means of Geonics VLF‐EM 16/16R equipment in two areas of the Andhra Pradesh State. The resistivity and thickness of the weathered layer were found to be variable even over a small survey area. In the area underlain by Precambrian granite‐gneiss, most of the recorded VLF‐EM anomalies were caused by variations in the resistivity of the weathered layer. Changes in thickness were well reflected in the VLF‐EMR curves. The second area was underlain by Cretaceous basalts and dolerites.Quantitative interpretation of the VLF‐EMR data with a simple one‐dimensional model yielded considerable detail about the weathered layer. For the granitic area, a prior estimate of at least one resistivity parameter of the ground is required. If this is not already available, a limited amount of direct‐current resistivity surveying can provide the required information. A study of the EMR data from the basaltic area revealed the presence of a thin, highly conductive layer between the weathered layer and the bedrock. The parameters of this layer were found to be variable, making it necessary to use a set of diagrams for quantitative interpretation. Due to the presence of this highly conductive layer, the EMR data contain little information on the bedrock resistivity.Our field studies suggest that the VLF‐EMR method can be used as a fast and inexpensive tool for mapping of the weathered layer in tropical regions with hard rock geology. Such mapping is of considerable importance because the weathered layer is an important source of groundwater.
MIGRATION IN THE PRESENCE OF A RUGGED INTERFACE WITH HIGH VELOCITY CONTRAST*
Authors: AMEELY L. , KREY TH. , MUHTADIE F. , RAU H.‐F. , RIST H. 
Published: April 2006

AbstractIn the western coal‐mining area of Ruhrkohle AG, reflection seismic prospecting for the Carboniferous coal measures is severely impaired by structures with halokinetic features. These structures make the interface between Mesozoic and Paleozoic layers, i.e., the top of Zechstein in general, very rugged. Unfortunately the velocity contrast at this interface is very high in that area, the ratio of velocities being 1.5 to 2.0. Therefore, migration and stacking become a problem.Three types of migration are presented:

(f, x)‐time‐migration with vertical time‐to‐depth conversion as a second step.
Kirchhoff migration down to a level determined approximately by the highest points of the top of Paleozoics, i.e., 0.35 s, and Kirchhoff‐downward continuation for all times exceeding 0.35 s. Intermediate static corrections for these latter times with subsequent (f, k)‐time‐migration and final vertical time‐to‐depth conversion.
Direct depth migration in the (f, x)‐domain using three interval velocities.

In all cases an intermediate picking of the velocity interfaces is necessary. In case 2 this occurs at an earlier stage of the process than in case 1, and in case 3 at a still earlier stage.The results of the second and third migration procedures are superior to those of the first. Possibilities for misinterpretation of faults are reduced considerably when the second or third migration procedure is applied.
TRANSFORMATION OF RESISTIVITY SOUNDING MEASUREMENTS OBTAINED IN ONE ELECTRODE CONFIGURATION TO ANOTHER CONFIGURATION BY MEANS OF DIGITAL LINEAR FILTERING*
Authors: BASOKUR A. T. 
Published: April 2006

AbstractThe technique of digital linear filtering is used for transformation of apparent resistivity data from one electrode configuration into another. Usually filter spectra are determined via the discrete Fourier transforms of input and output functions: the filter characteristic is the quotient of the spectra of the output function and input function.In this paper, the transformation of the apparent resistivities is presented for four electrode configurations (Wenner, the two‐electrode, Schlumberger, and dipole configurations). In our method, there is no need to use the discrete Fourier transform of the input and output functions in order to determine the filter spectrum for converting apparent resistivity in one electrode configuration to any other configuration. Sine responses for determination of the derivative of apparent resistivities are given in analytical form.If the filter spectrum for converting the apparent resistivity to the resistivity transform for one electrode configuration is known, the filter spectra for transforming the apparent resistivity to the resistivity transform for any electrode configurations can be calculated by using newly derived expressions.
ROCK‐BURST FORECASTING AND GENETIC RESEARCH IN COAL‐MINES BY MICROGRAVITY METHOD*
Authors: FAJKLEWICZ Z. 
Published: April 2006

AbstractMicrogravity can be used for predicting rock bursts. For the first time gravity anomalies related to rock bursts have been recorded. The methodology developed has lead to the first successful predictions. The depth of the rock‐burst focus might be determined on the basis of analytical downward continuation of related gravity anomalies: the focus is treated as a singular point of the gravity potential and its derivatives.The rock‐burst gravity anomalies might be explained on the basis of the assumed dilatancy process that causes the rock burst. The mean density change of the rock mass threatened with rock bursts can be estimated from the corresponding rock‐burst gravity anomalies.
THE TRANSIENT ELECTROMAGNETIC RESPONSE OF A CONDUCTING SPHERE IN AN IMPERFECTLY CONDUCTING HALF‐SPACE*
Authors: LEE T. 
Published: April 2006

AbstractThe presence of a conducting environment about a spherical ore body must be considered when calculating the transient electromagnetic response of the ore body due to a step current flowing in a large circular loop at the earth's surface. Failure to do this can easily lead to errors in excess of 10% in numerical calculations. Moreover, there is only a limited time interval in which the response of the spherical conductor is easily seen.In a poorly conducting ground the resonance response of the sphere is the first to be excited. Later, however, the non‐resonance or wave‐type response is excited. These waves destructively interfere and finally the response of the sphere decays with time as t−7/2.For a range of times and depths the best loop for detecting the sphere has about the same radius as the sphere.
ON THE INTERPRETATION OF RESISTIVITY SOUNDINGS BY THE LEAST‐SQUARES METHOD*
Authors: CHYBA J. 
Published: April 2006

AbstractAn interactive least‐squares method for the interpretation of VES curves was proposed by Johansen (1977). The method permits one to select some parameters (thicknesses and/or resistivities of individual layers) and to change the rest in such a way that the interpreted model approaches the measured data. This note suggests a modification of Johansen's method, in which not only the individual parameters can be selected but also linear combinations of parameters—in particular, the sum of thicknesses of several layers.
THE WAVEFIELD GENERATED BY TWO VERTICAL VIBRATORS IN PHASE AND IN COUNTERPHASE*
Authors: DANKBAAR J. W. M. 
Published: April 2006

AbstractA theoretical study is presented of the effect of source/receiver patterns with fixed phase difference on the radiation/receiving characteristics of the single elements of the pattern in linear elastic media. The radiation characteristic of two vertical vibrators in phase and in counterphase is investigated in detail.By introducing source/receiver patterns the characteristics become dependent on dimension of the pattern, frequency and angle of observation in the horizontal plane. The types of waves generated by source patterns are, independent of phase differences, equal to those generated by single sources. With two vertical vibrators in phase radiation of P‐waves can be improved relative to that of SV‐waves. With two vibrators in counterphase radiation of SV‐waves in vertical directions is enhanced relative to that of P‐waves; this may be a valuable method of SV‐shear wave generation in exploration seismics. SV‐wave amplitudes larger than P‐wave amplitudes can be expected at angles larger than 5°–10° with the vertical.
VLF GROUND SURVEYS, A POWERFUL TOOL FOR THE STUDY OF SHALLOW TWO‐DIMENSIONAL STRUCTURES*
Authors: FISCHER G. , LE QUANG B. V. , MÜLLER I. 
Published: April 2006

AbstractThe suitability of VLF ground surveys in the investigation of shallow two‐dimensional structures is analyzed. For such structures the polar formalism is derived, necessary in practice since the transmitters are generally not in the structural strike or profile. A simple vertical dike is considered to demonstrate the striking anisotropy which can be expected over such a structure, in particular the high apparent resistivity along the direction of a well‐conducting dike and the low resistivity across it. The theory is then confronted with the practical example of an asymmetrical vertical dike resulting from a strike‐slip fault. Modelling of the survey results is very successful and yields good confirmation of the polar behaviour. VLF ground surveys thus provide a quick and powerful tool for the study of geological accidents within about 100 m of the surface.
A REVIEW OF LEAST‐SQUARES INVERSION AND ITS APPLICATION TO GEOPHYSICAL PROBLEMS*
Authors: LINES L.R. , TREITEL S. 
Published: April 2006

AbstractGeophysical inversion involves the estimation of the parameters of a postulated earth model from a set of observations. Since the associated model responses can be nonlinear functions of the model parameters, nonlinear least‐squares techniques prove to be useful for performing the inversion. A common type of inversion applies iterative damped linear least squares through use of the Marquardt‐Levenberg method. Traditionally, this method has been implemented by solving the associated normal equations in conventional ways. However, Singular Value Decomposition (SVD) produces significant improvements in computational precision when applied to the same system of normal equations. Iterative least‐squares modeling finds application in a wide variety of geophysical problems. Two examples illustrate the approach: (1) seismic wavelet deconvolution, and (2) the location of a buried wedge from surface gravity data. More generally, nonlinear least‐squares inversion can be used to estimate earth models for any set of geophysical observations for which an appropriate mathematical description is available.
A HYBRID METHOD FOR WAVE FIELD COMPUTATION*
Authors: SHTIVELMAN V. 
Published: April 2006

AbstractA hybrid method for wave field computation in two‐dimensional heterogeneous media is proposed. The proposed method is a combination of analytical and numerical techniques. The method is based upon the separation of wave propagation and scattering and upon the description of each process by the most suitable technique. The SH wave scattering problem is used to elucidate the proposed method.Examples of numerical computations using the hybrid method are considered for a number of simple models. The analysis of the results shows that the hybrid method gives both a detailed and a reasonably accurate description of the total wave field.
ON VERTICAL SEISMIC PROFILE PROCESSING*
Authors: TARIEL P. , MICHON D. 
Published: April 2006

AbstractFrom the wealth of information which can be deduced from VSP, the information most directly comparable to well logs is considered: P‐wave and S‐wave interval velocity, acoustic impedance, and the velocity ratio γ=Vs/Vp. This information not only allows better interpretation of surface seismic sections but also improves processing.For these results to be usable a number of precautions must be taken during acquisition and processing; the sampling in depth should be chosen in such a way that aliasing phenomena do not unnecessarily limit the spectra during the separation of upwards and downwards travelling waves. True amplitudes should be respected and checked by recording of signatures, and the interference of upwards and downwards travelling waves should be taken into account for the picking of first arrivals.The different steps in processing and the combination of results in the interpretation of surface seismic results are described with actual records.
DERIVATION OF LAYER PARAMETERS OF AN ELASTIC MEDIUM FROM REFLECTION COEFFICIENT MATRICES*
Authors: AMINZADEH F. 
Published: April 2006

AbstractIn this paper we develop a recursive algorithm to obtain the layer parameters of an elastic medium (density, P‐wave velocity, S‐wave velocity) from reflection coefficient matrices in terms of energy flux ratios for a non‐normal incidence case. We define a layer impedance matrix, analogous to the impedance of an acoustic medium. Next we derive a matrix relationship between the layer impedance matrix of the n+ 1st layer and the reflection coefficient and parameter matrices of the nth layer. This relationship leads to recursively computing the parameters of the subsurface. We show that the elastic case—unlike the acoustic case—allows one to recover the layer parameters from the impedance matrix for non‐normal incidence. The results of this work play a key role in the solution of the inverse problem with non‐normal‐incidence plane‐wave seismic data when using a downward continuation technique.
THREE‐TERM TAYLOR SERIES FOR t2 ‐ x2‐CURVES OF P‐ AND S‐WAVES OVER LAYERED TRANSVERSELY ISOTROPIC GROUND*
Authors: HAKE H. , HELBIG K. , MESDAG C. S. 
Published: April 2006

AbstractThe arrival‐time curve of a reflection from a horizontal interface, beneath a homogeneous isotropic layer, is a hyperbola in the x ‐ t‐domain.If the subsurface is one‐dimensionally inhomogeneous (horizontally layered), or if some or all of the layers are transversely isotropic with vertical axis of symmetry, the statement is no longer strictly true, though the arrival‐time curves are still hyperbola‐like. In the case of transverse isotropy, however, classical interpretation of these curves fails. Interval velocities calculated from t2 ‐ x2‐curves do not always approximate vertical velocities and therefore cannot be used to calculate depths of reflectors.To study the relationship between velocities calculated from t2 ‐ x2‐curves and the true velocities of a transversely isotropic layer, we approximate t2 ‐ x2‐curves over a vertically inhomogeneous transversely isotropic medium by a three‐term Taylor series and calculate expressions for these terms as a function of the elastic parameters. It is shown that both inhomogeneity and transverse isotropy affect slope and curvature of t2 ‐ x2‐curves. For P‐waves the effect of transverse isotropy is that the t2 ‐ x2‐curves are convex upwards; for SV‐waves the curves are convex downwards. For SH‐waves transverse isotropy has no effect on curvature.
ON ERRORS OF FIT AND ACCURACY IN MATCHING SYNTHETIC SEISMOGRAMS AND SEISMIC TRACES*
Authors: WALDEN A. T. , WHITE R. E. 
Published: April 2006

AbstractA synthetic seismogram that closely resembles a seismic trace recorded at a well may not be at all reliable for, say, stratigraphic interpretation around the well. The most accurate synthetic seismogram is, in general, not the one that displays the smallest errors of fit to the trace but the one that best estimates the noise on the trace. If the match is confined to a short interval of interest or if the seismic reflection wavelet is allowed to be unduly long, there is considerable danger of forcing a spurious fit that treats the noise on the trace as part of the seismic reflection signal instead of making a genuine match with the signal itself. This paper outlines tests that allow an objective and quantitative evaluation of the accuracy of any match and illustrates their application with practical examples.The accuracy of estimation is summarized by the normalized mean square error (NMSE) in the estimated reflection signal, which is shown to be(/n)(PN/PS)where PS/PN is the signal‐to‐noise power ratio and n is the spectral smoothing factor. That is, the accuracy varies directly with the ratio of the power in the signal (taken to be the synthetic) to that in the noise on the seismic trace, and the smoothing acts to improve the accuracy of the predicted signal. The construction of confidence intervals for the NMSE is discussed. Guidelines for the choice of the spectral smoothing factor n are given.The variation of wavelet shape due to different realizations of the noise component is illustrated, and the use of confidence intervals on wavelet phase is recommended.Tests are described for examining the normality and stationarity of the errors of fit and their independence of the estimated reflection signal.