bluuebunny/crossref_metadata_2025 huggingface dataset:
A STUDY OF ABYSSAL FRACTURES IN THE UKRAINIAN SHIELD BY GEOPHYSICAL METHODS*
Authors: TYAPKIN K.F.
Published: April 2006

Abstract

According to data presented by YU.A. Kosigin, 84% of all endogenous mineral deposits are in some direct or indirect relation with the fractures of the earth's crust. Therefore the discovery and the study of the spatial disposition of the fractures is the most important object for geophysicists. Abyssal fractures are of particular interest.

By geophysical methods one can find the geometrical parameters of abyssal fractures such as their extent, the depth of formation, the breadth of the zones, and the amplitude of the relative displacement of separated blocks.

The methods determining these parameters are widely known. A calculation of the difference in the levels of the erosion cuts of the blocks are of particular interest under shield conditions. A method to calculate this difference by gravity interpretation of “step” anomalies and by using the gradient model of the earth's crust is proposed.

A comparision of the results of gravimetry and magnetometry with those of deep seismic soundings shows that the fractures of the first and the second order in the Ukrainian shield cut the earth's crust and part of the upper mantle so they can be qualified as abyssal fractures.

In the Ukrainian shield the spatial regularities of the fractures determined by geophysical methods can be used for the prognosis of the ore deposits.
THE QUANTITATIVE INTERPRETATION OF DIPOLE SOUNDINGS BY MEANS OF THE RESISTIVITY TRANSFORM FUNCTION*
Authors: PATELLA D.
Published: April 2006

Abstract

In this paper a theorem is demonstrated which allows—after the introduction of a suitable dipole kernel function or dipole resistivity transform function—to write the apparent resistivity function as an Hankel transformable integral expression.

As a practical application of the theorem a procedure of quantitative interpretation of dipole soundings is suggested in which the dipole resistivity transform function obtained after inversion of the original dipole apparent resistivity data is used to control the goodness of the set of layering parameters which have been derived with our previous method of transformation of dipole sounding curves into equivalent Schlumberger diagrams.
THE INTERPRETATION OF THE VERTICAL FAULT PROBLEM IN GEOELECTRICS USING HANKEL INTEGRAL*
Authors: PATELLA D.
Published: April 2006

Abstract

It is shown how to interpret, without curve‐matching, Schlumberger resistivity soundings carried out with the array parallel to the surface trace of a vertical boundary plane separating two media with different resistivities (the vertical fault problem). To this end, it is demonstrated that the apparent resistivity function can be expressed as an Hankel‐transformable integral that allows to invert the apparent resistivity curve into an associated resistivity transform curve. The study of the asymptotical properties of this function and of some mathematical properties of a related reduced transform function allows to realize a simple procedure for deriving the parameters of the model. In practice, this procedure consists in fitting a straight‐line to a semi‐logarithmic plot of the reduced transform function and in evaluating the intercept along the vertical axis and the slope.
REDUCTION OF HARMONIC DISTORTION IN VIBRATORY SOURCE RECORDS*
Authors: RIETSCH E.
Published: April 2006

Abstract

Due to non‐linear effects, the swept frequency signals (sweeps) transmitted into the subsurface by vibrators are contaminated by harmonics. Upon correlation of the recorded seismograms, these harmonics lead to noise trains which are particularly disturbing in the case of down‐sweeps. The method described in this paper—which can be regarded as a generalization of Sorkin's approach to the suppression of even order harmonics—allows elimination, from the final vibratory source seismogram, of harmonics of the sweep up to any desired order. It requires that not one single signal but rather a series of M signals is employed where each signal has an initial phase differing from that of the previous one of the series by the phase angle 2πM. Prior to stacking, the seismograms generated with the different signals have to be brought into the form they would have if they had been generated with the same signal.

The method seems also to be capable of reducing the correlation noise if sign‐bit recording techniques are used.
ZERO MEMORY NON‐LINEAR DECONVOLUTION*
Authors: GODFREY R. , ROCCA F.
Published: April 2006

Abstract

A type of iterative deconvolution that extracts the source waveform and reflectivity from a seismogram through the use of zero memory, non‐linear estimators of reflection coefficient amplitnde is developed. Here, we present a theory for iterative deconvolution that is based upon the specification of a stochastic model describing reflectivity. The resulting parametric algorithm deconvolves the seismogram by forcing a filtered version of the seismogram to resemble an estimated reflection coefficient sequence. This latter time series is itself obtained from the filtered seismogram, and so a degree of iteration is required. Algorithms utilizing zero memory non‐linearities (ZNLs) converge to a family of processes, which we call Bussgang, of which any colored Gaussian process and any independent process are members. The direction of convergence is controlled by the choice of ZNL used in the algorithm. Synthetic and real data show that, generally, five to ten iterations are required for acceptable deconvolutions.
THE AMPLITUDE AND PHASE RESPONSE OF A SEISMIC VIBRATOR*
Authors: Lerwill W.E.
Published: April 2006

Abstract

The amplitude and phase response of a simple model is compared with the performance of a real vibrator working in the field. The field results show a characteristic phase response which confirms that the real drive force applied to the baseplate and its load impedance is faithfully represented by the acceleration of the reaction mass. It follows that all the parameters necessary to calculate the load impedance and the true power dissipated in the earth can be measured at the output of the vibrator. It also follows that the current method of baseplate phase compensation should be reconsidered.
THE KUNETZ RELATIONS*
Authors: EISNER E.
Published: April 2006

Abstract

In discussion of the one‐dimensional wave equation the Kunetz “energy relations’ are often invoked. Since the treatment often involves lengthy algebraic calculations which yield a simple result, it may be of interest to present an elementary derivation from first principles. A generalization to higher‐dimensional wave equations follows readily.
SYSTEMATIC CLASSIFICATION OF LAYER‐INDUCED TRANSVERSE ISOTROPY*
Authors: HELBIG K.
Published: April 2006

Abstract

Waves propagating through a sequence of layers that are thin compared with the wavelength show effects of anisotropy: velocity and displacement direction depend on the angle between the plane of layering and the wave normal, and shear waves split up into two distinct types of different velocity. The layered medium can thus be replaced by a transversely isotrophic medium the parameters of which depend on the parameters of the individual constituent layers.

A survey of the anisotropy effects possible in such a medium is generally done by varying the layer parameters in order to obtain different replacement media. This approach guarantees that the replacement medium is realistic, but it does not guarantee adequate sampling of the set of replacement media. To this end one has to begin by selecting the replacement media and then check whether the chosen media possess stable (and eventually realistic) representations by layer sequences. In general, there is an infinite number of layer representations for any transversely isotropic medium that can at all be represented. However, if one restricts the solutions to those requiring the minimal number of layers and the minimum number of different layer parameters, the set of solutions has only one free parameter (i.e., it is a one‐dimensional manifold), and an important subset even has a unique solution. A simple algorithm exists for the determination of these “simplest representations”.

Aside from sampling the set of representable transversely isotropic media for survey purposes, the method can be applied to the problem of determining the cause of observed anisotropy effects (or lateral changes in such effects). If this method can be applied to real data, it would for instance allow to determine changes in relative thickness or lithology on a scale smaller than the limit of resolution of the seismic method.
DETERMINATION OF THE NATURE OF RESERVOIRS BY USE OF PATTERN RECOGNITION ALGORITHM WITH PRIOR LEARNING*
Authors: BOIS P.
Published: April 2006

Abstract

The determination of the nature of reservoirs containing hydrocarbons is approached via a pattern‐recognition method with prior learning. This method consists of comparing the nature of a reservoir that has been drilled into with an unknown reservoir. The known reservoir plays the part of a monitor for the learning process. To this end an algorithm is compiled using the Burg method of the features represented by different portions of traces contained in the reservoirs. An analysis of the dots representing these different features serves as a criterion for deciding whether the two reservoirs have identical or different natures.
SOIL CONSOLIDATION BEHAVIOR ASSESSED BY SEISMIC VELOCITY MEASUREMENTS*
Authors: HAMDI FALAH A.I. , SMITH D. TAYLOR
Published: April 2006

Abstract

Large gravity platforms are often used as alternatives to the more conventional pilesupported structures in hydrocarbon exploitation. A gravity structure, as opposed to the piled structure, is sitting on the sea floor by virtue of its weight and base width; as such it poses considerable problems for the site investigation engineer. One such problem is the calculation of the settlement of the structure and its time history; these depend upon the permeability and compressibility of the soil and its drainage conditions. The required data are usually obtained by sampling for subsequent laboratory testing. The collection of an undisturbed sample is beset by problems so that the consolidation behavior of the foundation material can only be inadequately assessed by laboratory testing. However, a series of laboratory consolidation experiments during which seismic velocities have been measured on the sample as consolidation proceeds shows that it is possible to reconstruct the stress‐strain and time‐dependent curves from the seismic data, once the initial void ratio and permeability of the soil are known. This leads the way to an in situ technique for predicting settlement using a combination of geophysical techniques (electrical resistivity and seismic velocities) to obtain the required engineering properties.
ASPECTS OF VERTICAL SEISMIC RESOLUTION*
Authors: KOEFOED O.
Published: April 2006

Abstract

It is proposed that the vertical resolving power of a seismic signal is controlled by three aspects: the width of the central lobe, the side lobe ratio, and the side‐tail oscillations. A comparative study of zero‐phase signals covering the same frequency range shows that improvement of any one of these aspects inevitably leads to deterioration of one of the other aspects.

An analytical simulation model is proposed of zero‐phase signals free from side‐tail oscillations, in which both the width of the central lobe and the side lobe ratio are adjustable. Analysis of the spectra of this model shows that, while the high frequency content of the spectrum is essential for obtaining a small width of the central lobe, the low frequency content of the spectrum plays an essential part in causing a low value of the side lobe ratio.
A NOTE ON THE USE OF A NOMOGRAM FOR SELF‐POTENTIAL ANOMALIES*
Authors: BHATTACHARYA B.B. , ROY N.
Published: April 2006

Abstract

A nomogram has been devised for situations, in which the source of a self‐potential anomaly can be approximated by an obliquely polarized sphere or horizontal cylinder embedded in a homogeneous half space. The nomogram can be used for rapid determination of three parameters of the target: (1) depth to the centre, (2) angle between the axis of polarization and the horizontal, (3) shift of the point vertically above the centre of the body from zero potential value.

The nomogram has been tested and the parameters determined for SP results obtained over ore bodies Weiss and Süleymanköy in the Ergani Copper district, Turkey. The curves computed for the estimated parameters match the field curves well.
A STRATIGRAPHIC CASE HISTORY USING THREE‐DIMENSIONAL SEISMIC DATA IN THE GULF OF THAILAND *
Authors: BROWN A. R. , DAHM C.G. , GRAEBNER R.J.
Published: April 2006

Abstract

A three‐dimensional marine seismic survey was conducted in the Gulf of Thailand to aid in the development of a gas field indicated by three wildcat wells. The results and interpretation reported previously demonstrated improved fault resolution and better structural definition.

Five successful appraisal wells have now been drilled, and these show that most of the sands have limited extent. Widespread character changes in the seismic data also support stratigraphic variations in many of the sands.

Several new methods of 3D stratigraphic interpretation have been developed while investigating the depositional history of this area. Anomalous seismic amplitudes, tied to sands penetrated by wells and mapped from SeiscropTM horizontal sections in time and depth, have indicated the distribution of bars and channels. Horizon Seiscrop sections, each sliced through a single bed, have been used to delineate these depositional features directly. G‐LOGTM sections, displaying seismic logs derived by rigorous wave equation inversion, confirm the existence of these features. Sands greater than 10 m thick have proved mappable.
A SIMPLE DERIVATION OF SEIGEL'S TIME DOMAIN INDUCED POLARIZATION FORMULA *
Authors: Roy A. , Poddar M.
Published: April 2006

Abstract

It is advantageous to postulate the phenomenological equivalence of chargeability with a slight increase in resistivities rather than a similar reduction in the conductivities. Substitution of these increments in the expression for the total differential of apparent resistivity leads directly to Seigel's formula. Included also are (i) an equally simple demonstration that, for a homogeneously chargeable ground with arbitrary resistivity distribution, the apparent chargeability ma, equals the true homogeneous value m, and (ii) a direct derivation of the completely general resistivity relation

image

where the symbols have the usual meanings.
DIRECT INTERPRETATION OF RESISTIVITY DATA OVER TWO‐DIMENSIONAL STRUCTURES*
Authors: LEE T.
Published: April 2006

Abstract

A direct interpretation scheme is developed which is capable of determining most of the geological features of a ground which can be assumed to be two dimensional in structure. This scheme extends the earlier work of Pekeris (1940) and Koefoed (1968) to the case where the basal layer of a ground is undulating. It also has a limited use for finding the parameters of a dipping dyke in the lower medium. Though the top and dip of the dyke can be determined, this is not true for the thickness.
MIGRATION OF SEISMIC INTERFACES*
Authors: SATTLEGGER J.
Published: April 2006

Abstract

Most interpretation work is still based on stacked and not on migrated sections. In the case of heavy faulting and considerable velocity contrasts between formations, migration of interpreted interfaces poses a problem. In more detail, the problem may be specified as follows:

— a given interpretation of a number of interfaces along with a given heterogeneous velocity field may not always have a plausible solution in the form of migrated interfaces in depth;

— fault planes, salt boundaries, etc., are, in most cases, not directly interpretable in a section and are plotted by intuition using interface terminations as a guide;

— the velocity field in fault zones is, in most cases, hard to determine.

The interpreter may arrive at a plausible solution by repeating the migration process with various possible interpretations and various velocity assumptions.

The subject of this paper is an algorithm based on ray‐theory which allows one:

— to handle faults and velocity variations at faults properly;

— to perform migration in steps, working a particular geological unit at a time and proceeding to the next unit once the foregoing one has been properly migrated;

— to display ray‐paths, where necessary, for investigation of interface distortions, e.g., below fault areas.

The algorithm is designed and implemented for application in an interactive environment. Inspection of intermediate and final results, investigation of interface distortions and modifications are performed on a graphics screen. Thus, various possible interpretations and velocity assumptions may be investigated within a short time.

Interfaces interpreted on migrated sections may be over‐migrated because of neglection of the influence of refraction in most section migration programs. This over‐migration may also be corrected using the above algorithm in the “image ray” mode.
A FREQUENCY DOMAIN MAPPING APPROXIMATION OF MOVEOUT FILTERS WITH APPLICATIONS*
Authors: ÖZDEM˙IR H.
Published: April 2006

Abstract

Wavenumber aliasing is the main limitation of conventional optimum least‐squares linear moveout filters: it prevents adequate reject domain weighting for efficient coherent noise rejection. A general frequency domain multichannel filter design technique based on a one‐to‐one mapping method between two‐dimensional (2D) space and one‐dimensional (1D) space is presented. The 2D desired response is mapped to the 1D frequency axis after a suitable sorting of the coefficients. A min‐max or Tchebycheff approximation to the desired response is obtained in the 1D frequency domain and mapped back to the 2D frequency domain. The algorithm is suitable for multiband 2D filter design. No aliasing damage is inherent in the linear moveout filters designed using this technique because the approximation is done in the frequency‐wavenumber (f, k)‐domain.

Linear moveout filters designed by using the present coefficient mapping technique achieve better pass domain approximations than the corresponding conventional least‐squares filters. Compatible reject domain approximations can be obtained from suitable mappings of the origin coefficient of the desired (f k)‐response to the 1D frequency axis. The (fk)‐responses of linear moveout filters designed by using the new technique show equi‐ripple behavior. Synthetic and real data applications show that the present technique is superior to the optimum least‐squares filters and straight stacking in recovering and enhancing the signal events with relatively high residual statics. Their outputs also show higher resolution than those of the optimum least‐squares filters.
IN‐SITU INVESTIGATION OF SEISMIC BODY WAVE ATTENUATION IN HETEROGENEOUS MEDIA*
Authors: NEWMAN P. J. , WORTHINGTON M. H.
Published: April 2006

Abstract

Field experiments have been carried out to study the nature and magnitude of seismic wave attenuation for a variety of lithologies. In each survey two three‐component sets of geophones with wall clamping mechanisms were lowered down boreholes and signals originating from surface compressional and shear‐wave sources were recorded. The data collected were corrected for spherical divergence and analyzed to determine intrinsic attenuation. For situations of anomalous wavefront expansion and in cases where multiple reflection losses may be significant, corrections were supplied by a synthetic seismogram programme to improve the estimates of intrinsic attenuation.

Values of attenuation were obtained for pure sandstone, sandstone‐marl sequences and fissured‐unfissured chalk sequences. These formations were all near surface and relatively porous. Significant differences in the relative values of compressional and shear‐wave attenuation in the various lithologies were noted. In particular compressional absorption in the fissured zones of chalk was twice the absorption associated with the seismic velocities and the absorption of shear waves fluctuated much less. A large contrast in P‐wave attenuation was also observed between pure Bunter Sandstone and the sandstone‐marl sequence (absorption was over three times as small in the former). A smaller contrast was noted for shear attenuation.

The results obtained suggest that, for the relatively homogeneous formations such as pure sandstone and “unfissured” chalk, “shear” absorption was dominant over “bulk” absorption. In contrast bulk absorption was larger than shear absorption for other formations, e.g. the “fissured” chalk sequences and a “partially saturated” chalk section. Attenuation was found to be approximately proportional to frequency in all experiments.
THE POTENTIAL OF SHEAR‐WAVE OBSERVATIONS*
Authors: HELBIG K. , MESDAG C. S.
Published: April 2006

Abstract

Shear waves can today be generated and observed, though not with the flexibility and the technical standard of compressional waves, and they can be identified in seismograms by various means. Their potential lies not so much in their lower velocity (corresponding—for the same frequency—to shorter wavelength and higher resolution) but in the fact that they probe the earth with stresses and strains that differ from those of compressional waves. Full utilization of the information potential of shear waves, therefore, requires combined use of P‐and S‐waves.

Complications in the combined use of different wave types should be regarded as opportunities to obtain additional information. A typical example is the observation that the depth of one and the same interface estimated on the bases of P‐ and SH‐reflections, respectively, can differ significantly. This discrepancy may be due to the anisotropy of a finely layered medium. Under favorable circumstances some of the parameters describing this anisotropy can be deduced from the different depth estimates and the curvature of the squared‐offset/squared‐time representation of the different reflections. Since in anisotropic media vertically polarized shear waves are significantly different from horizontally polarized ones, the combined observation of all three waves opens up additional possibilities.
UPSWEEP SIGNALS WITH HIGH‐FREQUENCY ATTENUATION AND THEIR USE IN THE CONSTRUCTION OF VIBROSEIS® SYNTHETIC SEISMOGRAMS*
Authors: GURBUZ B. M.
Published: April 2006

Abstract

An analytical relationship for the autocorrelation function of an upsweep with high‐frequency attenuation is used in the construction of synthetic seismograms.

Field experiments were conducted in two areas to investigate the attenuation of upsweep where the near‐surface materials were different. The results showed that the attenuation of high frequencies occurs at the source point depending on the near‐surface lithology.

The attenuation effect is usually neglected in the construction of the input wavelet of synthetic seismograms for Vibroseis data. In this study, the high‐frequency attenuation of upsweep was considered in the construction of the input wavelet for the synthetic seismogram in an area where the Vibroseis technique was used. The synthetic seismogram generated in this manner had a better correlation with the Vibroseis section than that of corresponding synthetics using minimum‐phase and the unattenuated autocorrelation wavelet of the upsweep.
A LINEAR MODEL FOR VELOCITY ANOMALIES*
Authors: LOINGER E.
Published: April 2006

Abstract

Variations of seismic interval velocities within the cable length cause anomalies in the stacking velocity analyses. Utilizing the approximation of rectilinear ray propagation, i.e. supposing that the velocity changes cause time delays only, it is shown that the stacking velocity anomalies are linearly related to the interval velocity variations. In particular, the stacking velocity anomaly is calculated when the interval velocity of an intermediate layer undergoes a stepwise variation. The amplitude of the anomaly increases with the ratio between horizon depth and cable length.

From the forward model, a program for the inversion is derived in order to identify lateral changes of interval velocities from unsmoothed stacking velocity analyses.

Some examples of the application of this technique to synthetic and real data are presented.
COMPUTATION OF VLF RESPONSE OVER HALF‐PLANE AND WEDGE MODELS*
Authors: OLSSON O.
Published: April 2006

Abstract

A 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

Abstract

Finite‐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

Abstract

A 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

Abstract

Previously 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

Abstract

The 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

Abstract

In 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

Abstract

The 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

Abstract

Microgravity 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

Abstract

The 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

Abstract

An 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

Abstract

A 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

Abstract

The 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

Abstract

Geophysical 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

Abstract

A 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

Abstract

From 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

Abstract

In 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

Abstract

The 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

Abstract

A 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.