bluuebunny/crossref_metadata_2025 huggingface dataset:
BOUNDARY CONDITIONS FOR THIN IMPERFECT CONDUCTORS AND INSULATORS*
Authors: HURLEY D. G.
Published: April 2006

Abstract

Consider a thin, plane, finite sheet of material of thickness 2t whose electrical resistivity is σ2. It is surrounded by a medium of resistivity σ1 in which there is an electric field whose length scale is L. We consider the limit t/L→ o with , or fixed. In the former case the sheet is an imperfect conductor and it is shown that it may be modelled by a surface on which the boundary condition is , where φ and Z are the (nondimensional) electrical potential and distance measured normal to the surface, and the subscripts + and − denote values on either side of the surface.

In the latter case the sheet is an imperfect insulator and the boundary condition is
FACTORS AFFECTING SEISMIC AMPLITUDES*
Authors: SHERIFF R. E.
Published: April 2006

Abstract

The amplitude of seismic energy varies over a tremendous range. Some of the factors responsible for such variation do not contain subsurface information; these include source strength and coupling, geophone sensitivity, array directivity, instrument balance, scattering in the near‐surface, for example. Others depend on subsurface factors but do not convey information about lithology or hydrocarbon accumulation in a form from which we are able to extract it; these include spherical divergence, ray‐path curvature, loss in transmission through intervening reflectors, peg‐leg multiples, reflector rugosity, and curvature. The amplitude‐governing factors we are primarily interested in are reflection coefficient, the interference of reflections from the top and base of a sand, and absorption.
VIBROSEIS DECONVOLUTION*
Authors: RISTOW D. , JURCZYK D.
Published: April 2006

Abstract

Vibroseis deconvolution can be performed either before or after correlation. As regards the deconvolution before correlation, the Vibroseis deconvolution operator can be described as convolution of a spike deconvolution operator with a minimum‐phase filter operator with bandpass properties. As regards the deconvolution after correlation, the deconvolution operator can be shown to be the convolution of three operators: spike deconvolution operator and two‐fold convolution with a minimum phase operator. Time‐varying Vibroseis deconvolution can particularly well be described and performed after correlation.
MICROGRAVIMETRY FOR ENGINEERING APPLICATIONS*
Authors: ARZI A. A.
Published: April 2006

Abstract

Microgravimetric surveys for verifying bedrock soundness at the foundation of a nuclear power plant and for delineating a zone of small cavities and tracing grout emplacement at the foundation of a large cooling tower are described and used to illustrate an analysis of the method.

It is pointed out that the difference between the usual large scale gravity surveys and microgravimetric surveys for engineering applications involves significantly more than a mere scaling down of grid spacings and precision tolerances. In addition to the smaller scale, the special physical environments for such microgravimetric surveys as well as strict timing and economy considerations and the demand for conclusive answers to specific questions require careful revisions of field procedures as well as data reduction and interpretation.

Where properly applied, microgravimetric surveys—usually in conjunction with selective drilling—yield reliable subsurface information with a high resolution that is still limited by the gravimeter noise. Costly extensive drilling can thus be avoided. The method offers special advantages over other subsurface exploration methods in a wide variety of applications.
FLAT SPOT EXPLORATION*
Authors: Backus M. M. , Chen R. L.
Published: April 2006

Abstract

The hydrocarbon‐brine contact produces a flat reflection, unconformable with the lithologic reflections from the trap boundaries, and over a limited area bounded by structural contours. When it can be reliably detected and mapped, the flat spot can provide (i) a reasonably unambiguous indication and areal extent of a reservoir and (ii) an estimate of reservoir thickness.

The gas‐brine contact in thick reservoirs in offshore elastic sections is the easiest target. An example of gas mapping is presented in the paper. Other reservoirs represent a continuum of increasingly elusive targets.

Increasing the range of applicability of flat spot exploration will require (i) increases in signal/noise and multiple ratio, increases in the three dimensional depth point density (or grid density), increased resolution and static and dynamic correction accuracy, and (ii) processing and interpretation aimed directly at flat spot mapping.

Low relief structural and stratigraphic traps should provide the most attractive targets. The approach should be at least theoretically feasible, though not necessarily cost effective, for most major reservoirs with a well‐behaved hydrocarbon brine contact.
SIGN REVERSALS IN THE TRANSIENT METHOD OF ELECTRICAL PROSPECTING (ONE‐LOOP VERSION)*
Authors: LEE T.
Published: April 2006

Abstract

The transient electromagnetic fields observed by the MPPO‐1 equipment in practice over geological structures cannot always be adequately explained by Maxwell's equation when the imaginary part of the conductivity of the rocks is neglected.

If, for any reason, the conductivity of the rocks is a complex function of frequency such that the imaginary part is appreciable the model calculations must include this. This explains why some observed transient electromagnetic decay curves change sign with time.
CONTINUATION OF TOTAL INTENSITY ANOMALY ON AEROMAGNETIC PROFILES WITH CONSTANT ELEVATION CHANGE*
Authors: TSAY L. J.
Published: April 2006

Abstract

A new scheme for continuing aeromagnetic data along a profile with constantly changing elevation to a horizontal level is presented in this paper. It only requires some modifications of the conventional methods which are designed to continue potential field data from one horizontal level to another level.

The new scheme procedes in the following steps: (1) digitization of anomaly along flight level, thus giving distance and elevation to the desired horizontal level at each station. (2) upward continuation at each digitized point from the corresponding elevation to a desired horizontal level. The validity and usefulness of the scheme is shown with theoretical and real data.
THE WAVE EQUATION APPLIED TO MIGRATION*
Authors: LOEWENTHAL D. , LU L. , ROBERSON R. , SHERWOOD J.
Published: April 2006

Abstract

Claerbout's method has been implemented for the migration of stacked seismic data. A simplified description of the method is given together with an account of some of the practical programming problems and the types of inaccuracy encountered. Routine production results are considered to be comparable or superior to the results derived from alternative migration techniques. Particular advantages are 1) the possibility of using a detailed velocity model for the migration and 2) the preservation of the amplitude and character of the seismic events on the migrated time section.
MAGNETOMETRIC AND ELECTROMETRIC METHODS FOR THE INVESTIGATION OF THE DYNAMICS OF LANDSLIDE PROCESSES*
Authors: BOGOSLOVSKY V. A. , OGILVY A. A. , STRAKHOVA N. A.
Published: April 2006

Abstract

Geophysical methods for the investigation of the dynamics of landslide processes are most effective in long‐term observations. Magnetometric and electrometric methods for observing the displacement of man‐made and natural markers are most sensitive and most highly developed at the present time.

A detailed magnetic survey carried out near boreholes where magnets have been emplaced makes it possible to establish the direction and velocities of the displacement of landslide masses at different depths.

Observations of the dislocation of the stable anomalies of the natural electric field allow us to elucidate the character of landslide deformations in the zone of capillary uplift of ground waters.
ANALYSIS OF THE GRAVITY EFFECT OF TWO‐DIMENSIONAL TRAPEZOIDAL PRISMS USING FOURIER TRANSFORM*
Authors: BHIMASANKARAM V. L. S. , MOHAN N. L. , RAO S. V. SESHAGIRI
Published: April 2006

Abstract

The gravity effect of an infinite horizontal trapezoidal prism is derived and its Fourier spectrum is analyzed so as to yield information about four parameters of the causative structure, namely the depths to the upper and lower surfaces, width of the upper surface, and the inclination of the sides. In order to test the applicability of the method, synthetic data are constructed by digitizing the theoretical gravity effect. Subsequently, the corresponding Discrete Fourier Transform (DFT) is obtained. The parameters evaluated from the DFT are observed to be sufficiently close to the chosen values.
COMPUTERIZED ANALYSIS OF VIBROSEIS SIGNAL SIMILARITY*
Authors: RIETSCH E.
Published: April 2006

Abstract

Computerized evaluation of Vibroseis similarity test data is the logical consequence of the increasing quality requirements for signal reproducibility and for synchronization of vibrators. The differences in start time and phase as well as an indication of local phasing problems between any two Vibroseis signals can be obtained from an analysis of the difference of their respective phase spectra. This method appears to be accurate and stable with respect to harmonics which usually plague the signals from transducers monitoring the motion of the vibrators'base plates.
A MAN/COMPUTER INTERPRETATION SYSTEM FOR RESISTIVITY SOUNDINGS OVER A HORIZONTALLY STRAFIFIED EARTH*
Authors: JOHANSEN H. K.
Published: April 2006

Abstract

The proposed system works as follows:

By a trial‐and‐error procedure using a graphic display terminal a geologically relevant layer sequence with parameters (ρj, dj) is adjusted to yield roughly the measured curve.

The resulting layer sequence is used as starting model for an iterative least squares procedure with singular value decomposition. Minimization of the sum of the squares of the logarithmic differences between measured and calculated values with respect to the logarithms of the resistivities and thicknesses as parameters linearizes the problem to a great extent, with two important implications:

a considerable increase in speed (the number of iterations goes down), thus making it cheap to achieve the optimum solution;

the confidence surfaces in parameter space are well approximated by the hyper‐ellipsoids defined by the eigenvalues and eigenvectors of the normal equations.

Since these are known from the singular value decomposition we do in fact know all possible solutions compatible with the measured curve and the geological concept.

It is possible to “freeze” any combination of parameters at predetermined values. Thus extra knowledge and/or hypotheses are easily incorporated and can be tested by rerunning step (2). The overall computing time for a practical case is of the order of 10 sec on a CDC 6400.
THE PROPAGATION OF ATTENUATED SH CHANNEL WAVES*
Authors: BUCHANAN D. J.
Published: April 2006

Abstract

The problem of detecting discontinuities which interrupt coal seams is of great importance to the coal mining industry. One possible method of detecting such discontinuities is that of in‐seam seismology where both source and detectors are placed under‐ ground on the coal face. In this paper the propagation of SH waves from a line source in the seam is investigated. There exists a particular set of waves–channel waves–, confined to the coal seam. These waves are dispersive and have an associated Airy phase whose geometrical attenuation is least. However, if absorption of energy within the coal is included then the Airy phase is the dominant wavegroup only for distances less than a certain maximum. If a detection criterion proposed by Dresen and Freystätter applies, then there is a lower limit to the frequency that should be used for exploration. This requirement may be in conflict with attenuation considerations.
A NOTE ON CONVOLUTION AND PADDING FOR TWO‐DIMENSIONAL DATA*
Authors: GUPTA V. K. , RAMANI N.
Published: April 2006

Abstract

Gupta, V. K., and Ramani, N., 1978, A Note on Convolution and Padding for Two‐Dimensional Data, Geophysical Prospecting 26, 214–217.

Filtering of gridded data by means of two‐dimensional generalized convolution results in a loss of data along the edges of the map. This loss can be counteracted by ‘padding’– filling the surrounding of the input map with some kind of data. Padding by algebraic extrapolation is assumed to be better than surrounding the maps with zeroes. A Føsrtran IV routine for padding by algebraic extrapolation is available from the authors.
SUPERSTACK—AN ITERATIVE STACKING ALGORITHM*
Authors: NÆSS O. E.
Published: April 2006

Abstract

An algorithm for separation of signals according to their coherency is proposed. The algorithm, called Superstack, is used on common depth point data after normal moveout corrections have been applied. The algorithm can be regarded as an iterative stacking procedure. After each stack, input values are changed depending on the consistency of the output of the previous stack.

The Superstack algorithm is able to provide better separation of signals showing a different degree of horizontal consistency (coherency) than the normal horizontal stack.

All noise signals in a CDP‐gather may be assumed to show less horizontal consistency than the primary reflections. On this basis the algorithm will give an improved primary/multiple and an improved primary/noise ratio.

The Superstack is very general, should not be especially expensive in terms of computer time, and can easily be implemented in routine processing.
DETERMINATION OF RESISTIVITY SOUNDING FILTERS BY THE WIENER‐HOPF LEAST‐SQUARES METHOD*
Authors: KOEFOED O. , DIRKS F. J. H.
Published: April 2006

Abstract

It has been found that the Wiener‐Hopf least‐squares method is a very successful tool for the determination of resistivity sounding filters. The values of the individual filter coefficients differ quite appreciably from those obtained by the Ghosh procedure. These differences in the filter coefficients, however, have only a negligible effect on the output of the filter. It seems that these differences in the coefficients correspond to a filter function of a rather narrow frequency band around the Nyquist frequency, which is only very weakly present in the input and output functions.
SEISMIC CLASSIFICATION OF ROCK MASS QUALITIES*
Authors: SJØGREN B. , ØFSTHUS A. , SANDBERG J.
Published: April 2006

Abstract

Correlations between longitudinal velocities and rock mechanic parameters such as fracture frequencies and “Rock Quality Designation” (RQD) values have been studied, based upon velocity data from various rock types and different geographical locations.

The dispersion of values at different sites studied is on average ± 0.8 cracks per meter and for the RQD values ± 3.5%. Within sites the dispersion of individual values relative to the average for the site is ± 1.0 – 2.0 cracks per meter and ± 2 – 6% for the RQD values.

The deviations are rather moderate, especially when considering the variation of rock type involved in the studies: amphibolite, granite, gneiss, meta‐anorthosite, pegmatite, porphyry, quartzite, and mylonite. The studies thus confirmed earlier assumptions that there is a strong correlation between longitudinal velocity and fracturing and that the velocities can be used to give rather accurate predictions of the quality of rock masses for construction purposes. The accuracy of the predictions increases if the velocity level of the more competent rock is taken into account.

The correlation between velocity and fracturing is related to jointed but unweathered igneous and metamorphic rock and cannot be applied without introducing serious errors to a site where the rocks present a higher degree of alteration and weathering.

Comparisons between rock permeability and longitudinal velocity proved that a more reliable general correlation is not likely to be found.

By comparing the elastic moduli Edyn, μ, and k with ø, Vp/V8, and k/μ, indications have been obtained where the optimum rock conditions for a certain site are to be encountered. This has been verified by a similar comparison where the elastic moduli have been replaced by fracturing values.

The value of the longitudinal velocity as a means to evaluate rock quality increases if the position of the velocity in the range of the Poisson's ratio has been established.

The average relationships between longitudinal velocities and the corresponding elastic moduli proved to be:

The values from each site differ from the average values with about ± 2 GPa for Edyn and about ± 1 GPa for μ and k.

It was confirmed that in igneous and metamorphic rocks longitudinal velocities ≤ 4000 m/s generally indicate rock masses where heavier tunnel support will be needed. This velocity limit corresponds to an average fracture frequency of about 10 cracks per meter and a RQD value of about 65 %. The prediction of the tunnel reinforcements needed at a particular site will, however, be improved if the general velocity level of the more competent rock is considered.
ADAPTIVE LONG‐PERIOD MULTIPLE ATTENUATION*
Authors: BRANDSÆTER H. , URSIN B.
Published: April 2006

Abstract

A least squares estimation procedure is used to estimate the pulse‐shape, amplitude function, and arrival time of multiple reflected signals. The estimates of the multiple reflections are subtracted from the data which are subsequently processed by standard methods. The estimation algorithm is applied continuously along the seismic line for each shot point or common datum point. In some cases it is advantageous to apply a pulse‐shaping filter prior to using the estimation algorithm.

The effectiveness of the technique is demonstrated by studying common shot point gathers, velocity analyses, and stacked sections derived from field data.
CONSTRAINED INVERSION OF POTENTIAL FIELD DATA*
Authors: PEDERSEN L. B.
Published: April 2006

Abstract

For constrained inversion of potential field data within the framework of generalized inversion an analysis of data error variances leads to confidence limits for the model parameters. For that purpose Pseudo‐hyper‐ellipsoids can be used to describe the nonlinear behaviour of the given inverse problem, and upper and lower bounds can be added to those parameters of which some independent knowledge is available. A gravity example is treated to show the application of the method.
AN EFFICIENT METHOD OF OPERATING THE AIR‐GUN*
Authors: SAFAR M.H.
Published: April 2006

Abstract

A new technique is developed for generating a short seismic pulse from the bubble pulses which are radiated by an air‐gun. The new technique, which is useful in well velocity surveys and vertical seismic profiling, can be implemented by firing a single air‐gun several times at the same depth but with different chamber pressures.

A record obtained by this procedure from a well‐geophone clamped at a depth of 2450 m gave a maximum peak‐to‐peak amplitude within the first 100 ms of the effective seismic pulse at least ten times any later peak‐to‐peak amplitude.
FACTOR ANALYSIS FOR INTERPRETING PETROPHYSICAL DATA ON RORO ULTRAMAFICS, SINGHBHUM DISTRICT, INDIA*
Authors: RAO B. NARASIMHA , PAL P.C.
Published: April 2006

Abstract

The physical properties of the serpentinized pyroxenitic and saxonitic rocks from stratiform ultramafic complexes of Roro, Singhbhum distrct, India, are examined using principal factor (R‐mode) analysis technique. The variations and inter‐relations of these properties reflect the compound effects of two processes—(a) degree of serpentinization, and (b) mineralogical and other changes attendant to serpentinization. Factor analysis is thus shown to be an effective tool for petrophysical inferences.
PARTIAL COHERENCE MATCHING OF SYNTHETIC SEISMOGRAMS WITH SEISMIC TRACES*
Authors: WHITE R.E.
Published: April 2006

Abstract

A crucial step in the use of synthetic seismograms is the estimation of the filtering needed to convert the synthetic reflection spike sequence into a clearly recognizable approximation of a given seismic trace. In the past the filtering has been effected by a single wavelet, usually found by trial and error, and evaluated by eye. Matching can be made more precise than this by using spectral estimation procedures to determine the contribution of primaries and other reflection components to the seismic trace. The wavelet or wavelets that give the least squares best fit to the trace can be found, the errors of fit estimated, and statistics developed for testing whether a valid match can be made.

If the composition of the seismogram is assumed to be known (e.g. that it consists solely of primaries and internal multiples) the frequency response of the best fit wavelet is simply the ratio of the cross spectrum between the synthetic spike sequence and the seismic trace to the power spectrum of the synthetic spike sequence, and the statistics of the match are related to the ordinary coherence function. Usually the composition cannot be assumed to be known (e.g. multiples of unknown relative amplitude may be present), and the synthetic sequence has to be split into components that contribute in different ways to the seismic trace. The matching problem is then to determine what filters should be applied to these components, regarded as inputs to a multichannel filter, in order to best fit the seismic trace, regarded as a noisy output. Partial coherence analysis is intended for just this problem. It provides fundamental statistics for the match, and it cannot be properly applied without interpreting these statistics.

A useful and concise statistic is the ratio of the power in the total filtered synthetic trace to the power in the errors of fit. This measures the overall goodness‐of‐fit of the least squares match. It corresponds to a coherent (signal) to incoherent (noise) power ratio. Two limits can be set on it: an upper one equal to the signal‐to‐noise ratio estimated from the seismic data themselves, and a lower one defined from the distribution of the goodness‐of‐fit ratios yielded by matching with random noise of the same bandwidth and duration as the seismic trace segment. A match can be considered completely successful if its goodness‐of‐fit reaches the upper limit; it is rejected if the goodness‐of‐fit falls below the lower one.
ESTIMATION OF THE SIGNAL‐TO‐NOISE RATIO OF SEISMIC DATA WITH AN APPLICATION TO STACKING*
Authors: RIETSCH E.
Published: April 2006

Abstract

The amplitude of the signal and the energy of the noise on each of at least three traces can be estimated provided that the signal has the same form (but not necessarily the same amplitude) on these traces and that the noise on any trace is correlated with neither the signal nor the noise on any other trace. This estimation of signal amplitude and noise energy can be achieved by a rather simple algorithm. The accuracy of the estimate depends, of course, on the degree to which the assumption that signal and noise on the different traces are mutually uncorrelated is actually met. The accuracy tends to improve with increasing number of traces.
VERTICAL SEISMIC PROFILES: THEIR APPLICATIONS IN EXPLORATION GEOPHYSICS*
Authors: KENNETT P. , IRESON R.L. , CONN P.J.
Published: April 2006

Abstract

During the past four or five years, Vertical Seismic Profiles have been run in a large number of wells and in a variety of geological provinces with the object of assisting the seismic data processor and improving the geological interpretation.

The special properties of the Vertical Seismic Profile, which allows the separation of the upward and downward travelling components of the recorded waveform, provides a means of studying the acoustic response of the earth in detail. Deconvolution of the data in particular gives a clearer understanding of the mechanics of multiple reflections and the way in which they may obscure primary reflections in the vicinity of the well.

The primary reflection response of the earth can be observed with greater resolution than conventional seismic data permits, making correlation with lithology more precise, and allowing a good estimate of the reflection coefficient series to be determined. Inversion of the seismic traces to produce an acoustic impedance log leads to better definition of the sedimentary sequence and is of particular interest in predicting lithologic variations ahead of the drilling bit.

In addition, Vertical Seismic Profile data can be used to assess parameters in areas such as deconvolution and signal band width for the evaluation of conventional seismic data.
THE LAW OF PARALLELISM IN REFRACTION SHOOTING*
Authors: SJÖGREN B.
Published: April 2006

Abstract

The half‐intercept‐time for in‐line reversed profiles is highly adaptable to multi‐layer cases, but the deep evidence of the intercept time on the depths and the velocities at the shotpoint and detector position poses a problem. The law of parallelism provides a reliable and simple means to solve this problem. The interpretation procedure implies that from a travel‐time curve the refractor segment is projected back to the shotpoint parallel to a refractor curve from an offset shot. The intercept time thus obtained refers only to the geological conditions in the immediate vicinity of the shotpoint, and varying conditions outside this region are automatically climinated. The assumptions involved in the interpretation technique are confined to the region where, in relation to the shotpoint, the rays leave and strike the refractor surface.

The use of the “ABC method” to establish true intercept times at the shotpoint is also discussed briefly as well as the applicability of the method to eliminate or minimize arrival time disturbances in order to improve velocity determinations.
GRAVITY INTERPRETATION USING THE HANKEL TRANSFORM*
Authors: PATELLA D.
Published: April 2006

Abstract

The Hankel transform theorem can be applied to the inversion of gravity data for the buried sphere, the horizontal cylinder, and the vertical rod. This new approach leads to exact solutions of the transforms for the assumed bodies. A comparison with the classical procedure by Fourier transform reveals that for the sphere and the vertical rod, the Hankel transform is preferable.
MODEL STUDIES ON SOME ASPECTS OF RESISTIVITY AND MEMBRANE POLARIZATION BEHAVIOUR OVER A LAYERED EARTH*
Authors: ROY K.K. , ELLIOTT H.M.
Published: April 2006

Abstract

Electrolytic model tank experiments to study resistivity and time domain induced polarization (IP) response over layered earth models were initiated primarily to facilitate the understanding of field results. Alternate layers of clay and sand (or clay‐coated sand) with, in some cases, a surficial layer of water were assembled in the tank and resistivity and IP measurements made for a range of electrode spacings using the Wenner configuration. Graphite and silver‐silver chloride electrodes were used as current and potential electrodes respectively.

Clay‐coated (3% by weight) sand was found to generate stronger polarization than either clay or sand alone. Apparent chargeability ma was observed to be positive for a nonpolarizable surface layer. For a polarizable surface layer, the sign of IP was controlled by the polarizability, the thickness of the second layer, and the spacing of the electrode spreads. The apparent chargeability ma can theoretically change sign from positive to negative and vice versa with a gradual increase in electrode spacing, and such negative IP effects were obtained in a few observations. A simultaneous decrease in IP and an increase in resistivity, which is a qualitative diagnostic feature for the occurrence of clean freshwater sand aquifers, could also be generated in the model tank experiment.

Combined resistivity and IP soundings were carried out near Fredericton Junction and Tracy, New Brunswick, Canada. Field curves are presented along with the model curves for qualitative comparison and understanding of IP behaviour over a layered earth. Twenty‐five out of twenty‐seven soundings show only positive apparent chargeabilities, whereas two show chargeability sign changes (positive/negative/positive). The model study gives reason to believe that surface soils and Quaternary gravel boulder deposits near Fredericton Junction are relatively non‐polarizable.

As an auxiliary experiment, sand and clay were taken in different proportions by weight and mixed thoroughly with water in a cement mixer. The mixtures were then compressed with a suitable die and plunger under 3.6 Pa pressure to prepare cylindrical samples of height 18 cm and diameter 15.5 cm. IP measurements were done on the flat faces using the Wenner configuration with a= 2 cm. Chargeability was found to be negative for 100 and 90% clay mixtures. It reached a positive maximum for an 80% clay‐20% sand mixture and then decreased gradually with increasing sand and decreasing clay content.
RESISTIVITY PROFILING WITH DIFFERENT ELECTRODE ARRAYS OVER A GRAPHITE DEPOSIT*
Authors: BRASS G. , FLATHE H. , SCHULZ R.
Published: April 2006

Abstract

Motivated by several papers by Indian colleagues suggesting that a single‐pole array is the most favorable one for locating electrically conductive targets in the underground, detailed measurements were carried out with different electrode arrays over a well‐known graphite deposit near Pfaffenreuth, Germany. The following arrays were used: (1) Wenner array; (2) single‐pole array; (3) half‐Wenner array; (4) half‐Schlumberger array. Each of the arrays established the location of the main graphite deposit. The half‐Schlumberger array turned out to be the most reliable one since it showed details which were only very poorly indicated by the other arrays or not at all.

Mathematical modeling of three‐dimensional bodies simulating the geological situation for this deposit were carried out. These results show an unexpectedly good correlation with the measured effects.
FOCUSSED ELECTRICAL RESISTIVITY ARRAYS: SOME THEORETICAL AND PRACTICAL EXPERIMENTS*
Authors: JACKSON P.D.
Published: April 2006

Abstract

The “Laterolog 7′’and the “Microlaterolog’ have been studied theoretically for the case of a conducting halfspace containing a single overburden, using a technique based on the method of images. The results have shown these focussed arrays to be more sensitive to the lower medium (i.e., having greater depth of investigation) than unfocussed ones of the same dimensions, when the “correct’ geometric factor is used. The geometric factor of a focussed electrode array is somewhat involved, and is explained with reference to an ideal focussed array; such an array would pass a measuring current of constant intensity into a fixed geometrical shape of conducting material, irrespective of any layering or any other heterogenities that may be present, using auxiliary current sources of variable magnitude. This concept of a constant amount of current flowing in a beam of fixed geometry and current density, is the basis of focussed arrays, and it is shown that the geometric factor, used to calculate the apparent resistivity, is the one derived when considering the homogeneous case. The value of the constant measuring current, alone, is used to convert the measured potential difference into a resistance, the ratio between this value and the combined intensities of the auxiliary current sources, for the homogeneous case, being incorporated into the geometric factor.

Surprisingly good agreement was found between the theoretical models and practical experiments using a focussing seabed resistivity probe which is a substantial modification of the “Microlaterolog”. Both show similar deviations from the ideal case which are explained in terms of refraction at the overburden interface. All experiments indicated that focussed arrays have a greater depth of investigation than similar unfocussed ones for a single overburden, whether it be resistive or conductive.
QUASI‐ANALYTIC CONVOLUTION SOLUTION OF THE ELECTROMAGNETIC FIELD*
Authors: ERCAN A.
Published: April 2006

Abstract

The objective of this study is to generate the separation‐distance‐domain (r‐domain) transformation of the theoretically calculated wave number domain (m‐domain) electromagnetic induction field component Bz(m, ω) of a stratified medium and to search for interpretive information which has been absent in the previously achieved numerical solutions of the problem.

The r‐domain kernel R̃(r, ω) function defining the induction field appears to adequately reflect the layering and electrical properties of the medium if it is expressed as a function of the frequency if the source‐receiver separation r is small with respect to the thickness of the first layer. However, exact values of the conductivity cannot be distinguished from those of the neighboring values unless a resistive basement layer is present. This feature is the result of the truncation in series representation of the kernel function R̃(m, ω). However, this truncation is regarded as significant in the case of a conductive first layer. In m‐domain static‐zone studies, a conductive first layer slightly influences its r‐domain correspondent.

Although the computational cost of obtaining the kernel B(r, ω) by evaluation of the convolution in a cylindrical coordinate system is high, this semi‐analytic solution is still superior to those based on the asymptotic assumptions.
THE OFFSET SYSTEM OF ELECTRICAL RESISTIVITY SOUNDING AND ITS USE WITH A MULTICORE CABLE*†
Authors: BARKER R.D.
Published: April 2006

Abstract

One of the most troublesome problems in resistivity sounding is caused by the spurious effects of near‐surface lateral resistivity variations. It has been found that the effects can be strongly reduced by measuring earth resistances at two electrode array positions such that in one position the lateral effect is a positive contribution to the total measured signal and in the other the contribution is negative. The subsequent combination of the two measurements virtually eliminates the lateral effect.

The same technique can also result in a significant reduction in the total number of necessary electrode positions. Consequently the method can be used with a multicore cable system. In this way, in addition to improving the accuracy of the measured results, time and manpower are considerably reduced.

Details of the method, examples and comparisons with traditional sounding techniques are given.
VELOCITY ANALYSIS FOR WIDE APERTURE SEISMIC DATA*
Authors: STOFFA P. L. , DIEBOLD J. B. , BUHL P.
Published: April 2006

Abstract

Common midpoint data are now being collected with ever increasing source‐receiver offsets. For wide aperture seismic data classical methods of interpretation fail, since velocity analyses and signal‐to‐noise enhancement methods based on hyperbolic traveltime curves are no longer appropriate. Therefore, the goals of increased velocity resolution and signal enhancement, which motivate the increase in offset, are not achieved. Approximate methods, involving higher order traveltime curves or extrapolations, have been developed for velocity analysis but these are ineffective in the presence of refracted arrivals, and lack a physical basis. These problems can be minimized by transforming the observational data to the domain of intercept or vertical delay time τ and horizontal ray parameter p. In this domain headwave refractions are collapsed into points and both near vertical and wide angle reflections can be analyzed simultaneously to derive velocity‐depth information, even in the presence of velocity gradients or low velocity zones.
A RECTANGULAR LOOP SOURCE OF CURRENT ON A TWO‐LAYERED EARTH*
Authors: PODDAR M.
Published: April 2006

Abstract

Using the concept of reciprocity and the known solution for the electric field of a vertical oscillating magnetic dipole source placed over a two‐layered halfspace, an integral expression for the vertical magnetic field produced by a horizontal rectangular loop, carrying an oscillating current and placed on the surface of the same halfspace, is deduced. This expression is such that it can be evaluated by a combination of straightforward numerical integration and digital linear filter techniques. Displacement currents everywhere in space are neglected.

Fields both inside and outside the loop are presented as sounding curves. A little over a decade of frequencies (100–3000 Hz) appears to be the minimum requirement and two decades (100–10000 Hz) are sufficient for most situations. Parametric and geometric sounding curves both show good resolution of subsurface layering. Phase shows better resolution than amplitude. Similarly, resolution is higher when conductivity increases with depth than when it decreases.

Besides being useful for the electromagnetic depth sounding for layered earth structures, such computations can be applied to determine normal corrections to Turam observations, whenever a rectangular loop of finite size is used as a transmitter.
STATISTICAL ANALYSIS OF AIRBORNE ELECTROMAGNETIC SIGNATURES OF DIFFERENT CONDUCTOR GROUPS IN TROPICAL TERRAIN OF KARNATAKA, INDIA*
Authors: CHOUDHURY KALPAN
Published: April 2006

Abstract

AEM (airborne electromagnetic) anomalies caused by massive sulphide conductors and superficial conductors can be recognized with a statistical method, as shown by an analysis of Input AEM data from Karnataka State. The weathering in the survey area is of tropical type. Parameters, such as various amplitude ratios and time parameters (inverse of decay rate) for exponential and power‐law decay were analyzed for sulphide bodies, conducting soil, superficial conductors, and cultural conductors. Time parameters τ1 (exponential decay) is defined as ratio of time differences between the third and fourth channel to the logarithmic value of the relative amplitude of the two channels. Time parameter K1 (power‐law decay) is defined as ratio of the difference of the logarithmic values of the delay times of the third and fourth channels to the logarithmic value of the relative amplitude of the two channels. The two parameters have been useful in recognizing sulphide conductors. Also the first channel Input amplitude and logarithmic plot of the transients appear to be helpful in conductor identification. Channel ratios seem to be the least effective parameters of conductor identification. In the area studied both power‐law and the conventional exponential decay were found equally suitable for approximating Input AEM transients.
THE INFLUENCE OF PERMEABILITY ON COMPRESSIONAL WAVE VELOCITY IN MARINE SEDIMENTS*
Authors: HAMDI F. , SMITH D. TAYLOR
Published: April 2006

Abstract

To investigate the effect of permeability on the propagation of seismo‐acoustic waves through marine sediments, a theoretical model based on Biot's equations is established which relates the compressional wave velocity measured at a fixed frequency to computed velocities at zero and infinite frequencies in terms of sediment porosity and permeability. The model is examined experimentally in a standard soil mechanics consolidation test (itself dependent, among other things, on sediment porosity and permeability) which has been modified to include measurements of compressional wave velocity at 1 MHz and shear‐wave velocity at 5 kHz. This test allows the elastic modulus of the sediment frame to be assessed under different load conditions simultaneous with the velocity determinations.

From a number of tests on different samples, five samples are chosen to typify the range of sediment sizes. The results show that the difference between the measured velocity at 1 MHz and the model‐derived velocity at zero frequency increases with increasing particle size (from clays to fine sand), with decreasing porosity, and with increasing permeability. For sediments coarser than fine sand the simple model breaks down, possibly because of the dominance of scattering/diffraction effects at the high frequency of the experiment. Within this limitation the model seems satisfactory to offer a capability of predicting the permeability of a sea floor sediment to an order of magnitude by the in situ measurement of seismic velocities over a wide range of frequencies; the prediction process requires a good in situ determination of sediment porosity such as that offered by electrical formation factor measurements.
SINGLE‐TRACE PROCESSING USING ITERATIVE CDP‐STACKING*
Authors: NAESS O. E.
Published: April 2006

Abstract

The use of conventional CDP‐stacking in the processing of reflection data imposes restrictions on the horizontal and vertical resolution. Ideally, the final seismic section should consist only of short offset or, in practice, near‐trace primary energy. Through the use of the iterative stacking algorithm, the signal‐to‐noise ratio on a single trace in the CDP‐gather may be improved to an extent comparable to what occurs on a conventional stacked trace. By using this approach and treating the near‐trace section after iterative stacking as the final section, the seismic resolution can be improved.
OFFSET CONTINUATION OF SEISMIC SECTIONS*
Authors: BOLONDI G. , LOINGER E. , ROCCA F.
Published: April 2006

Abstract

Geometrical acoustic and wave theory lead to a second‐order partial differential equation that links seismic sections with different offsets. In this equation a time‐shift term appears that corresponds to normal moveout; a second term, dependent on offset and time only, corrects the moveout of dipping events.

The zero‐offset stacked section can thus be obtained by continuing the section with maximum offset towards zero, and stacking along the way the other common‐offset sections.

Without the correction for dip moveout, the spatial resolution of the section is noticeably impaired, thus limiting the advantages that could be obtained with expensive migration procedures. Trade‐offs exist between multiplicity of coverage, spatial resolution, and signal‐to‐noise; in some cases the spatial resolution on the surface can be doubled and the aliasing noise averaged out.

Velocity analyses carried out on data continued to zero offset show a better resolution and improved discrimination against multiples. For instance, sea‐floor multiples always appear at water velocity, so that their removal is simplified.

This offset continuation can be carried out either in the time‐space domain or in the time‐wave number domain. The methods are applied both to synthetic and real data.
BOREHOLE SEISMIC PROFILING AND TUBE WAVE APPLICATIONS IN A DAM SITE INVESTIGATION*
Authors: HENRIET J. P. , SCHITTEKAT J. , HELDENS Ph.
Published: April 2006

Abstract

Continuous, single‐channel reflection profiling has been carried out in PVC‐lined boreholes, primarily with the aim of ascertaining the position of an old subsurface gas storage tunnel on a proposed dam site.

Tube wave reflection patterns thus generated have been interpreted in terms of sediment rigidity and shear wave velocity, and these results could be compared with some independent data. It is interesting to note that, within the well section penetrating Tertiary clays, the velocity of the hydraulic transients apparently was not affected by the PVC casing, which might be explained by a tight coupling between casing and clay wall. In such situations, tube waves turn out a straightforward tool for the determination of shear wave velocity and the derivation of dynamic elastic moduli of unconsolidated sediments.

Further applications of the study of the distribution of seismic velocities on the dam site dealt with the consolidation history of the clays. A level of abnormally low P‐wave velocities has been detected and interpreted as a gas‐charged horizon which, by its coincidence with the base level of clay diapirs, might be considered to have contributed to clay flowage in past geological times. Data about maximum past burial depth, derived from shear wave velocities, turned out to be in agreement with results from consolidation testing.
THE AUTOBALANCER—AN AUTOMATIC SYSTEM FOR THE REDUCTION OF POWER‐LINE INTERFERENCE WITH SEISMIC SIGNALS*
Authors: ENSING L.
Published: April 2006

Abstract

Since the early years of seismic surveying, field engineers and observers have been faced with the appearance of interference at power‐line frequency (and harmonics) on seismic records: 60 Hz in the Americas, 50 Hz in Eurasia; and 16 2/3 Hz (the train supply frequency in Germany, Austria, and Switzerland) in part of Europe. Such interference is picked up from the ground surface through electric leakage between geophone leads and ground.

To minimize power‐line interference, the autobalancer automatically performs a balancing procedure by adjusting two potentiometers on each channel—essentially the same operation as manually performed by seismic operators for decades. The first 24‐channel system became operational in 1977.

The first‐generation design comprised two 256‐position electronic switching potentiometers. Over 4 years of experience with this system in all parts of the world has shown its value under a variety of field conditions. Improvements up to 30 dB were often found, sometimes even up to 40 dB. Balancing time was about 30 s.

A second‐generation design was field‐tested in 1981. Potentiometer stepping is now performed at three levels: coarse, medium, and fine. This leads to faster operation and better resolution: balancing time is about 10 s; improvements up to 60 dB are occasionally found.

A later modification gives a further reduction of the balancing time to 5 s in 60 Hz areas or 6 s in 50 Hz areas.
A STATISTICAL APPROACH TO THE EXTRACTION OF THE SEISMIC PROPAGATING WAVELET*
Authors: ANGELERI G. P.
Published: April 2006

Abstract

A model of the seismic trace is generally given as a convolution between the propagating wavelet and the reflectivity series of the earth and normally it is assumed that a white noise is added to the trace. The knowledge of the propagating wavelet is the basic point to estimate the reflectivity series from the seismic trace.

In this paper a statistical method of wavelet extraction from several seismic traces, assuming the wavelet to be unique, is discussed. This method allows one to obtain the propagating wavelet without any classical limitative assumptions on the phase spectrum. Furthermore, a phase unwrapping method is suggested and some statistical properties of the phase spectrum of the reflectivity traces are examined.
NON‐NORMAL INCIDENCE INVERSION: EXISTENCE OF SOLUTION*
Authors: SHIVA MOSTAFA , MENDEL J. M.
Published: April 2006

Abstract

An inverse problem is one in which the parameters of a model are determined from measured seismic data. Important to the solution of inverse problems is the issue of whether or not a solution exists. In this paper we show, in a constructive manner, that a solution does exist to the specific inverse problem of determining the parameters of a horizontally stratified, lossless, isotropic and homogeneous layered system that is excited by a non‐normal incidence (NNI) plane wave. Mode conversion between P‐ and S‐waves is included.

We develop a seven‐step layer‐recursive procedure for determining all of the parameters for layer j. These parameters are P‐wave and S‐wave velocities and angles of incidence, density, thickness, traveltimes, and reflection‐ and transmission‐coefficient matrices. Downward continuation of data from the top of one layer to the top of the next lower layer is an important step in our procedure, just as it is in normal incidence (NI) inversion. We show that, in order to compute all parameters of layer j, we need to (and can) compute some parameters for layer j+ 1. This is a non‐causal phenomenon that seems to be necessary in NNI inversion but is not present in NI inversion.