bluuebunny/crossref_metadata_2025 huggingface dataset:
MEASUREMENTS OF EARTH ATTENUATION FROM DOWNHOLE AND SURFACE SEISMIC RECORDINGS*
Authors: RAIKES S. A. , WHITE R. E.
Published: April 2006

Abstract

Frequency‐dependent attenuation of compressional waves within the earth has been estimated in the vicinity of wells from

spectral power ratios of the coherent events in separate time gates on the seismic section

matching a broadband synthetic trace with seismic data at the well, and

determining the operator that transforms one down(up) going pulse recorded in the well into another recorded at a deeper (shallower) level.

The accuracy of estimation of all three methods was insufficient to estimate attenuation over small depth intervals, and it was not possible to distinguish between the contribution due to internal multiples and that of genuine absorption with much confidence. Spectral ratios from (1) showed a smoother variation with frequency—and one more consistent with other estimates—when they were compensated for the spectra of the reflectivities over the time gates employed, but they did not provide more than a broad indication of attenuation over a substantial depth interval. Approach (2) was hampered by the restricted durations over which synthetic trace and seismic data can be reliably matched; approach (3) gave the best results. Here matching is a much more powerful tool than the spectral‐ratio techniques that are commonly applied since it can yield the form of the attenuation operator, i.e., both its amplitude and phase response, together with properly defined measures of its accuracy, while at the same time it minimizes the influence of noise and local interference effects at each recording level.

For seismic target depths where internal multiple activity was low the logarithms of the amplitude responses of the estimated attenuation operators decreased approximately linearly with frequency and the phase responses showed no significant dispersion. Application of approach (3) to downgoing and upgoing waves estimated from a vertical seismic profile revealed the importance of changes in frequency‐dependent geophone coupling and their effect on values of Q determined from downgoing pulses only.
INTERPRETATION OF AEROMAGNETIC DATA FROM THE NORWEGIAN SECTOR OF THE NORTH SEA*
Authors: HOSPERS J. , RATHORE J. S.
Published: April 2006

Abstract

Total magnetic intensity contour maps for the study region (between 2°E to 10°E and 56°N to 60°N) were digitized and converted to a regular grid of 285 × 285 points. The study area measures approximately 444 km × 444 km and the grid spacing is thus 1. 56 km. The International Geomagnetic Reference Field for 1975 was gridded for the above‐used net, and from the two data sets a further grid of the ▵T field was generated. A large number of profiles were constructed which were suitable for depth determinations. The regular grid ▵T data is also convenient for the computation of the second vertical derivative. Using the method of vertical prisms of Vacquier et al. (1963), a large suite of curvature‐depth indices was measured to complement the depths obtained from the intensity slopes and from boreholes which reach the crystalline basement. The depth to the magnetic basement has been contoured, and the resulting map is shown to be in good agreement with what is known about the deeper geology of the study area.

The work reported here is part of a research project supported by Amoco Norway, BP Petroleum Development Ltd, Elf Aquitaine, Esso Exploration and Production, Norwegian Gulf, Norsk Hydro, Mobil Exploration Norway, Norwegian Petroleum Directorate, Royal Norwegian Council for Scientific and Industrial Research (NTNF), Norske Shell, and Statoil.
HIGH‐FREQUENCY SHALLOW SEISMIC REFLECTION MAPPING IN TIN MINING*
Authors: SINGH SURENDRA
Published: April 2006

Abstract

Field results of shallow seismic reflections obtained with a propane‐oxygen detonator (POD) are presented. The survey site was in a tin‐mining area of the Kinta Valley in Malaysia. The shallow and irregular limestone bedrock is overlain by alluvial “tailing” and virgin sediments. Sizes of such mining areas can range from about 320 ± 320 m2 to 900 ± 900 m2. The survey was intended to delineate the topography of the bedrock, which is of vital importance in tin ore exploration and exploitation. The equipment included single‐ and 12‐channel signal enhancement seismographs, the POD, a hammer and thumper. The inexpensive and portable POD generates directional waves of reproducible form, variable energy of high frequency, and only a few surface waves at short offsets. Reflections at around 200 Hz were obtained from the shallow bedrock at about 25 m as well as from very shallow lithological interfaces. The interpretation of seismograms is supported by drill‐hole lithological sections and synthetic seismograms. The data illustrate the successful use of shallow reflections for mapping irregular bedrock. Reflection seismics can provide better horizontal and vertical details than the refraction method. Further improvements based on the data‐processing flexibility of new signal enhancement seismographs and synthetic seismograms are suggested.
INDUCED POLARIZATION IN ELECTROMAGNETIC INDUCTIVE SCHEMES*
Authors: WAIT J. R. , DEBROUX P.
Published: April 2006

Abstract

Using homogeneous full‐space and half‐space models, we show that induced‐polarization properties of the medium influence the inductive coupling between two circuits. It is suggested that existing methods to interpret electromagnetic sounding data should be viewed with caution if the electro‐chemical dispersion is not taken into account.
A FIELD EXAMPLE OF THE USE OF ANISOTROPY PARAMETERS DERIVED FROM RESISTIVITY SOUNDINGS*
Authors: MATIAS M. J. S. , HABBERJAM G. M.
Published: April 2006

Abstract

In areas where steep dips are encountered conventional practice in resistivity work has involved orienting arrays favourably in relation to the geological strike. In concealed conditions, however, the geological strike may not be known; moreover, strike may change with depth. Considerable advantage is to be gained, in such circumstances, by the use of crossed square arrays in that these yield orientationally insensitive resistivity measurements and also allow strike determinations and measurements of the effective vertical anisotropy.

Two traverses of crossed square array observations are presented, together with one deeper sounding. The results show that, in favourable circumstances, reliable data on concealed strike directions can be obtained, and that the anisotropy findings greatly assist the subsequent interpretation. Model results pertinent to the field material are presented and discussed.
COMPARISON OF METHODS TO DETERMINE Q IN SHALLOW MARINE SEDIMENTS FROM VERTICAL REFLECTION SEISMOGRAMS*
Authors: JANNSEN D. , VOSS J. , THEILEN F.
Published: April 2006

Abstract

Four methods for the determination of Q in marine sediments are compared: two traditional methods, i.e. the risetime and the spectral ratio method, and two newly established ones, the spectrum modeling and the wavelet modeling method. In the latter one Q and the reflection time T are determined simultaneously, which gives a much better accuracy for T than reading it from the seismogram. The risetime and the spectral ratio methods are used for obtaining Q directly from the data. The principle of the modeling methods is to calculate the effect of absorption and dispersion on a reference wavelet or its spectrum for various values of Q, and the best fit between the observed and the calculated data leads to the optimum result. Numerical tests on synthetic data show that a precision of more than 25% for data containing noise or superposed arrivals can hardly be achieved; in any case, wavelet modeling is the superior method. Application to data from a vertical reflection profile in the Baltic Sea yields Q in the range of 15–100 for different layers, which is to be expected in the sedimentary environment of this area.

The computations were performed in the Computer Center of Kiel University. The authors thank R. Meissner for his comments on the manuscript.
NORMAL‐INCIDENCE WAVEFIELD COMPUTATION USING VECTOR‐ARITHMETIC*
Authors: FERBER R.‐G.
Published: April 2006

Abstract

The time‐domain discrete state‐space models for lossless layered media, characterized by equal one‐way traveltimes and normal‐incidence reflection coefficients, can be formulated in a vector‐arithmetic notation. This approach allows the computation of the seismic wavefield for arbitrary source and sensor location and is well suited for implementation on modern array processors. Included is an extension to a vector‐arithmetic notation for the computation of synthetic vertical seismic profiles.
A SYSTEMATIC APPROACH FOR EVALUATING AIRBORNE ELECTROMAGNETIC SYSTEMS*
Authors: BEST M. E.
Published: April 2006

Abstract

Airborne electromagnetic methods have been in use for more than 30 years. The selection of the most effective airborne system for a given geological/geographical environment is difficult since there are only a limited number of theoretical comparisons and very few comparative field evaluations of the systems.

Shell Canada Ltd carried out a field evaluation using several systems to provide such a comparison. For this evaluation five different airborne systems were flown over 22 conductive features. Some lines were flown at various directions to strike, some at various elevations, and some with different line spacings. The airborne systems were then evaluated target by target for navigational control, line‐to‐line correlation, resolution, lateral coverage, signal strength, noise etc.

The paper examines the philosophy behind our field evaluation and outlines steps required for setting up this comparison. Pitfalls are discussed as well as examples which illustrate the methodology. Two field examples are given from the Bathrust mining camp in New Brunswick and the Timmins Clay belt in northern Ontario (both in Canada). Some general conclusions which help our understanding of present day airborne electromagnetic systems are discussed at the end of the paper.
SOME REMARKS ON THE BEHAVIOR OF THE MAGNETOTELLURIC PHASE*
Authors: FISCHER G.
Published: April 2006

Abstract

The magnetotelluric phase in the medium of incidence over a good conductor varies from its value at the conductor surface, usually close to 45°, to the characteristic standing‐wave phaseshift of 90° over a distance of a few conductor skin‐depths, much shorter than the vacuum wavelength at the frequencies considered. Over the top of a graben or at the surface of a two‐layer structure the basement can be looked upon as the matrix. If the graben or the overburden is a better conductor it will carry an extra current density in phase with the electric field, thus giving rise to an additional magnetic field also in phase with the electric field. This drives the argument of the surface impedance toward smaller values. For a more resistive graben or overburden the current is depressed, corresponding to a reduction of the surface magnetic field by an amount in phase with the electric field. This effect drives the phase of the impedance to values above the characteristic 45° of a uniform conductor.
EM AND IP RESPONSE OF A STEEL WELL CASING FOR A FOUR‐ELECTRODE SURFACE ARRAY. PART I: THEORY*
Authors: WAIT J. R. , WILLIAMS J. T.
Published: April 2006

Abstract

The basic formulation for the mutual coupling between grounded circuits in the presence of a vertical conductor is used to examine the full electromagnetic and induced polarization response for a homogeneous half‐space model. Both interfacial polarization at the steel/electrolyte boundary and induced eddy currents in the casing are accounted for in the analysis. It is shown that for mild steel casings the eddy currents, hitherto neglected, play an important role because of their strong frequency dependence. Calculated curves of the appar‐ ent complex resistivity as a function of frequency show that the overall response can be influenced by the state of corrosion at the surface of the casing.
A NOTE ON THE INTERPRETATION OF MAGNETIC ANOMALIES OF INFINITE THIN SHEETS BY USING RELATION FIGURES*
Authors: CH. RAO RAMA , VIJAYAKUMAR V. , VIRUPAKSHI G. , RAO M. B. S. V.
Published: April 2006

Abstract

The concept of relation figures is utilized in the case of a thin infinite sheet for identification and determination of the causative source geometry, and its various parameters. The plot of ▵T versus ▵V is a conic, the plots between the respective symmetric components and the respective asymmetric components are straight lines, and the resulting diagram between the first horizontal versus the first vertical derivative of either ▵T or ▵V is a cardioid.

The authors are grateful to Shri V. Babu Rao and Dr R. K. Verma for their encouragement. They would like to thank the Director of the N. G. R. I. for permission to publish this paper. The neat drawing of Sri P. Sundara Rao is appreciated.
IDENTIFICATION OF SEISMIC REFLECTIONS USING SINGULAR VALUE DECOMPOSITION*
Authors: URSIN B. , ZHENG Y.
Published: April 2006

Abstract

Singular value decomposition (SVD) is applied to the identification of seismic reflections by using two different models: the impulse response model, where a seismic trace is assumed to consist of a known signal pulse convolved with a reflection coefficient series plus noise, and the delayed pulse model, where the seismic signal is assumed to consist of a small number of delayed pulses of known shape and with unknown amplitudes and arrival times.

SVD clearly shows how least‐squares estimation of the reflection coefficients may become unstable, since a division by the singular values is required. Two methods for stabilizing this procedure are investigated. The inverse of the singular values may be replaced by zeros when they are less than a given threshold. This is called the SVD cut‐off method. Alternatively, we may use ridge regression which in filter design corresponds to assuming white noise. Statistical methods are used to compute an optimal SVD cut‐off level and also to compute an optimal weighting parameter in ridge regression. Numerical studies indicate that the use of SVD cut‐off or ridge regression stabilizes the least‐squares procedure, but that the results are inferior to maximum‐likelihood estimation where the noise is assumed to be filtered white noise.

For the delayed pulse model, we use a linearization procedure to iteratively update the estimates of both the reflection amplitudes and the arrival times. In each step, the optimal SVD cut‐off method is used. Confidence regions for the estimated reflection amplitudes and arrival times are also computed. Synthetic data examples demonstrate the effectiveness of this method. In a real data example, the maximum‐likelihood method assuming an impulse response model is first used to obtain initial estimates of the number of reflections and their amplitudes and traveltimes. Then the iterative procedure is used to obtain improved estimates of the reflection amplitudes and traveltimes.
INSUFFICIENTLY SAMPLED Q: A CAUSE OF WAVELET DISTORTION IN THE GENERATION OF SYNTHETIC SEISMOGRAMS?*
Authors: MOSEGAARD K.
Published: April 2006

Abstract

The effect of sampling of Q‐values on the generation of one dimensional synthetic seismograms including absorption and dispersion is investigated. For the well data considered, a decrease in the Q‐sampling interval results in an elongation of the estimated reflection waveform.

Two explanations are given. The first refers to the way multiple energy is absorbed in the stratified medium. The second is based on the general observation that geological media with high velocity (e.g. limestones) often have high Q‐values, whereas media with low velocity (e.g. shales) often have low Q‐values.
SEISMIC SOURCES FOR SHALLOW REFLECTION SURVEYING*
Authors: McCANN D. M. , ANDREW E. M. , McCANN C.
Published: April 2006

Abstract

A standard seismic reflection profile was shot along a disused railway track at Onley, near Rugby, U. K. Four different seismic sources including explosives, the propane/oxygen gas‐gun, the Bolt airgun, and the borehole sparker were used and compared with each other in terms of output energy, penetration and resolution. The results indicated that the resolution of the borehole airgun and the gas‐gun was slightly higher than that possible with gelignite. Both these sources had an output energy which was equivalent to 30 g of gelignite.

The borehole sparker was only useful for obtaining seismic information on the nearsurface weathered layer, since its output power (1 kJ) was very limited. However, McCann and McCann (1982) used a high‐power sparker source (14 kJ) on the nearby Grand Union Canal for a wide angle seismic reflection survey and achieved a maximum penetration of 250 m, which is comparable with the results obtained on land with the seismic sources mentioned above.

The seismic reflection profile, which was interpreted in the light of borehole information in the area and the results of McCann and McCann (1982), successfully identified the surface of the Palaeozoic rocks. The problem of detecting the presence of thin, high‐velocity layers in a seismic refraction survey without the availability of other information to calibrate the seismic section is also highlighted.
AN EXTENSION OF THE BORN INVERSION METHOD TO A DEPTH DEPENDENT REFERENCE PROFILE*
Authors: BLEISTEIN N. , GRAY S. H.
Published: April 2006

Abstract

An extension of the multidimensional Born inversion technique for acoustic waves is described. In earlier work, a perturbation in reference sound velocity was determined by assuming that the reference velocity was constant. In this extension, we allow the reference velocity to be a function of the depth variable z. The output of this method is a high‐frequency bandlimited reflectivity function of the subsurface. The reflectivity function is an array of bandlimited singular functions scaled by the normal reflection strength. Each singular function is a Dirac delta function of a scalar argument which measures distance normal to a reflecting interface. Thus, the reflectivity function is an indicator map of subsurface reflectors equivalent to the map produced by migration. In addition to the assumption of small perturbation, the method requires that the reflection data reside in the high frequency regime in a well‐defined sense.

The method is based on the derivation of an integral equation for the perturbation in sound velocity from a known reference velocity. When the reference velocity is constant, the integral equation admits an analytic solution as a multifold integral of the reflection data. Further high frequency asymptotic analysis simplifies this integral considerably and leads to an extremely efficient numerical algorithm for computing the reflectivity function. The development of a computer code to implement this constant‐reference‐velocity solution is published elsewhere.

For a reference velocity c(z) we can no longer invert the integral equation exactly. However, we can write down an asymptotic high‐frequency approximation for the kernal of the integral equation and an asymptotic solution for the perturbation. The computer implementation of this result is designed along the same lines as the code for constant background velocity. In tests the total processing time for this algorithm with depth‐dependent background velocity is usually considerably less than that required by a standard Kirchhoff migration algorithm. The method is implemented as a migration technique and compared with alternative migration algorithms on the flanks of the salt dome.
A CROSS‐HOLE AND FACE‐TO‐BOREHOLE IN‐SEAM SEISMIC EXPERIMENT AT INVINCIBLE COLLIERY, AUSTRALIA*
Authors: GREENHALGH S. A. , BURNS D. , MASON I.
Published: April 2006

Abstract

An experimental cross‐hole and face‐to‐borehole in‐seam seismic survey was carried out at Invincible Colliery in the Western Coalfield of New South Wales. Objectives were to determine propagation characteristics of the Lithgow seam and to establish the infrastructure for seismic mapping hole‐to‐hole in Australia.

The seam supports leaky P‐, S‐ and P‐SV‐modes. These modes propagate with group velocities (at 60 Hz) of 3.1, 1.5 and 1.2 km/s respectively. Particle motion polarization is well developed, as is dispersion of the SH‐mode. Attenuation rates are high. The seam is lossy (Q of approximately 20). Two prominent structures were mapped by mode conversion. One is believed to be a fracture zone, the other a zone of intense roof thrusting.

The old workings and a minor strike‐slip fault, which intersected raypaths, were found to be relatively transparent to P‐ and S‐waves at 60 Hz.

Telemetry delay and shot‐break timing errors of the exploder box are significant. The resulting traveltime scatter is reduced by means of a least‐squares “statics” procedure. The group velocity estimation algorithm (based on Fourier transform) yields dispersion characteristics which can be matched with theoretical results for a simple model of a coal seam waveguide.

The experiment demonstrates the capability to retrieve in‐seam seismic data of diagnostic quality over an appreciable distance (2 km). The experience gained in both survey layout and data processing will be beneficial to future seam wave surveying of Australian coal mines.
DETECTION OF GAS BUBBLE BOUNDARY MOVEMENT*
Authors: BLONDIN E. , MARI J.L.
Published: April 2006

High‐resolution seismic surveys were performed on an underground gas storage of Gaz de France at Gournay‐sur‐Aronde (Oise) in order to obtain information about gas bubble boundaries within the reservoir. For that purpose, a light seismic source Soursile and spreads with small spacing between geophone groups (10 m) and small geophone group extension (10 m or 20 m) were used. In December 1982, a line recorded on the top of the anticline structure of the gas reservoir provided an estimation of the lateral extension of the bubble based on an amplitude anomaly (dim spot) associated with reservoir seismic horizons. In order to estimate the displacement of the gas bubble within the reservoir, three lines were recorded on the southeastern part of the anticline at different time periods (April 1981 and January 1982) with different volumes of gas in the reservoir. It is assumed that a variation of gas volume introduces a seismic velocity variation and as a result a travel‐time variation for the seismic waves that pass through the reservoir. A method based on the residual time shifts observed on reflectors below the reservoir from one survey to the other was developed and implemented. This method permitted a detection of a movement gas bubble boundary between April 1981 and January 1982 which agrees with drill hole data (water level measurements).
ANISOTROPY AND TRANSVERSE ISOTROPY*
Authors: CRAMPIN S.
Published: April 2006

A number of authors in the exploration literature have written about anisotropy, but have restricted their discussions to wave propagation through rock having transverse isotropy with a vertical symmetry axis. This note shows that there are fundamental differences between transverse isotropy when the symmetry axis is vertical (normal to the free surface) and more general anisotropy with an azimuthal variation of properties. These differences are important now that effective azimuthal shear‐wave anisotropy resulting from aligned cracks and pores is becoming recognized as a significant property of crustal rocks.
THE SCATTERING OF SH‐CHANNEL WAVES BY A FAULT IN A COAL SEAM*
Authors: BUCHANAN D.J.
Published: April 2006

The imaging of faults in coal seams by the in‐seam seismic method has now become standard practice. In the UK over 300 surveys have been undertaken and the technique is now part of the exploration arsenal of colliery planners. From these users comes the pressure for two major improvements, namely an increase in range and target identification. This paper is directed towards the latter problem.

It has long been recognized that the reflected channel waves must contain information on the fault structure that caused the reflection, and model experiments have been undertaken to investigate the reflection process. Only recently, however, have attempts been made to quantify the reflection process. Calculations using both the finite‐difference and finite‐element techniques have been carried out, and estimates of the reflection coefficient as a function of frequency have been obtained. The object of this paper is to extend these considerations by calculating analytically the scattering matrix of an SH‐channel wave after interaction at a fault plane. The scattering matrix is calculated as a function of frequency, hade angle, and fault throw.

The method employed is based on the decomposition of the incident SH‐channel wave into Fourier components, the calculation of plane wave reflection and transmission coefficients within the constraints of geometrical acoustics, and finally the synthesis of the scattering matrix by application of the Helmholtz‐Kirchhoff integral. The calculation throughout is restricted to normal modes.
TUNING EFFECTS, LITHOLOGICAL EFFECTS AND DEPOSITIONAL EFFECTS IN THE SEISMIC RESPONSE OF GAS RESERVOIRS*
Authors: BROWN A.R. , WRIGHT R.M. , BURKART K.D. , ABRIEL W.L. , McBEATH R.G.
Published: April 2006

Synergism of 3‐D seismic data, wavelet processing, colour display and interactive interpretation has been exploited in the study of a Gulf of Mexico gas reservoir. Seismic amplitude has been used as a measure of the proportion of a sand/shale reservoir capable of producing gas. This has led to the mapping of net producible thickness of gas sand. The tuning phenomena resulting from geometric effects alone were studied in detail, and tuning curves of various levels of sophistication were used as the basis for amplitude editing. Statistical tuning curves were derived by interactive cross‐plotting and deterministic curves by wavelet extraction. Multiple wavelet side lobes cause multiple maxima in the tuning curve. Depositional effects and intrareservoir communication have also been studied by interactive cross‐plotting.
COMPUTER TECHNIQUES FOR THE ZONING AND CORRELATION OF WELL‐LOGS*
Authors: HOYLE I. B.
Published: April 2006

This paper reviews computer techniques used in the automatic zoning and correlation of well‐logs. Prior to correlating, well‐logs are to be segmented–or ‘zoned’–so as to delineate sections that have similar properties. Techniques discussed include statistical methods such as variance tests and Student's t‐test, linguistic analysis, the use of Walsh functions and spectral analysis.

Well‐log correlation, which may be between traces from different wells or between traces from the same hole (as in dip logs), is used in basin studies and the determination of structural dip. A variety of methods are reviewed including conventional time and frequency correlation, sequence slotting, pattern recognition and frequency analysis. Future directions for investigation are proposed.
SHEAR‐WAVE REFLECTION PROFILING FOR NEAR‐SURFACE LIGNITE EXPLORATION*
Authors: MILKEREIT B. , STÜMPEL H. , RABBEL W.
Published: April 2006

We present the results of a shear‐wave reflection experiment and in situ measurements in opencast lignite exploration. Near‐surface coal seams have lower shear‐wave velocities (∼ 200 m/s) and lower densities than sand and clay layers. Due to strong reflection coefficients, a shear‐wave reflection survey provides a powerful tool in lignite prospecting. Due to shorter seismic wavelengths shear waves will yield a higher resolution of shallow subsurface structure than compressional waves. Low shear‐wave velocities and strong lateral velocity variations, however, require a dense data acquisition in the field. The variation of stacking velocities can exceed ± 15% within a profile length of 300 m. The different steps in processing and interpretation of results are described with actual records. The final CMP‐stack shows steep‐angle fault zones with maximum dislocations of 20 m within a coal seam.
SYNTHESIS OF ACOUSTIC WELL‐LOGGING WAVEFORMS ON AN IRREGULAR GRID*
Authors: SALERNO M. , ORLANDI G. , MARTINELLI G. , BURRASCANO P.
Published: April 2006

The need for synthetic seismograms arises in several acoustic well‐logging contexts. We propose a numerical algorithm for computing the seismograms, characterized by a very reduced computational cost. Unlike previous contributions appearing in the technical literature, the proposed algorithm is based on a non‐uniform sampling step and on linear interpolation for computing the integrals involved in the synthesis.
EXPERIMENTAL STUDIES OF ELASTIC‐WAVE PROPAGATION IN A COLUMNAR‐JOINTED ROCK MASS*
Authors: KING M.S. , MYER L.R. , REZOWALLI J.J.
Published: April 2006

Results are presented of a series of cross‐hole acoustic measurements made between four horizontal boreholes drilled from a near‐surface underground opening situated in a basaltic rock mass. The objectives of the program were to assess the extent of blast damage around the opening, and to evaluate the rock mass characteristics and their spatial variation around the opening. The acoustic velocity and attenuation data are indicative of an anisotropic, jointed rock mass, with a greater intensity of jointing along travel paths in the horizontal than the vertical direction. Low acoustic P‐ and S‐velocities are indicative of blast damage and of zones of intense jointing or fracturing. In this case blast damage extends to approximately 1.5 m from the face. Attenuation data appear to be less sensitive in distinguishing between the blast‐damaged zone and intense vertical jointing and fracturing in the virgin rock mass. Taken together with field data, laboratory measurements of P‐ and S‐wave velocities on intact core samples suggest that the rock mass is probably water saturated.
SOME NEW MAGNETIC TRANSFORMATIONS*
Authors: ROY A. , AINA A.O.
Published: April 2006

All magnetic transformations are governed by one simple differential relation between the observed and the transformed quantities. A magnetic map for any component, at any location, and for any given direction of magnetization can be converted into one for which any one, two, or all three parameters differ.

Three new magnetic transformations are introduced: (i) reduction to equator, (ii) orthogonal reduction, and (iii) elimination of remanence. The first eliminates (or minimizes) the asymmetry and the lateral shift of the measured total field anomalies, exactly as in Baranov's reduction to pole. The second produces perfect asymmetry so that a symmetrical target lies vertically below the zero anomaly point, midway between the maximum and minimum. When remanence is a contributing factor, the direction of resultant magnetization must be known a priori in all cases, except for transformation of one component into another in the same area.

Explicit working formulae are presented for reduction to equator and pole, and orthogonal reduction.
DISPERSION CALCULATIONS FOR SH AND P‐SV WAVES IN MULTILAYERED COAL SEAMS*
Authors: BUCHANAN D.J.
Published: April 2006

Recent work by Räder, Schott, Dresen and Rüter has provided a means of calculating dispersion and amplitude distribution curves for SH channel waves in multilayered media. An alternative calculation procedure is presented and the P‐SV wave type is also included. Numerical results are given for a coal seam containing a band of dirt of either higher or lower seismic velocities than the coal itself.
DATA ACQUISITION AND PROCESSING OF CONVERTED pS‐WAVES*
Authors: FERTIG J. , HENTSCHKE M.K.
Published: April 2006

The earth's surface can be an effective means of generating converted pS‐waves. Due to their nearly symmetrical ray path, conventional processing techniques can be used. As the wave is generated by reflection at the surface or at the base of surface layers one can expect a general filtering effect in the data for individual ray paths of a single shot gather.

To balance the spectra of the traces a multiple‐trace filter was used. This filter can be fully determined in the time domain using the prediction‐error operators of the individual traces. The preferred mean spectrum to colour the traces was the geometric mean.

As the process of spectral balancing requires a minimum‐delay wavelet, the recording instrument was replaced by its corresponding minimum‐phase equivalent. This process can also be carried out effectively in the time domain.

Results of the application of minimum‐delay transform and spectral balancing are discussed for single shot gathers and for the general improvement of the final stack.
DETECTION OF THIN BEDS WITH THE PSEUDO‐ACOUSTIC IMPEDANCE SECTION*
Authors: SZULYOVSZKY I.
Published: April 2006

The conventional seismic response of a thin bed approximates the time derivative of the incident wavelet, whereas the pseudo‐impedance response approximates the incident wavelet. Consequently the pseudo‐impedance response of a geological sequence composed of thin beds is simpler and easier to interpret than the conventional response.

By calibrating the sonic log data with check‐shot data and performing zero‐phase seismic processing, the fit of the sonic log and pseudo‐velocity section is improved. Discrepancies in amplitude and phase, however, generally remain.

A five‐step processing and interpretation procedure, which benefits from multichannel interpretation along the model seismic section generated from the sonic logs, is described.

The method has been tested with field data. In the test the detection of thin beds and the estimation of the natural gas content was more reliable with the proposed procedure than with the conventional method.
MODELING OF SH‐WAVE PROPAGATION IN AN IRREGULARLY LAYERED MEDIUM‐APPLICATION TO SEISMIC PROFILES NEAR A DOME*
Authors: CAMPILLO M.
Published: April 2006

An approach that relies on a discrete representation of seismic wavefields allows the computation of synthetic SH‐seismograms in a laterally varying medium with plane and curved interfaces in the two dimensional (2‐D) case. The diffracting interface is represented by an array of body forces located along the interface at equal spacing. The numerical treatment is limited to the irregular boundary while the propagation in flat layered zones is obtained by the reflection‐transmission matrix method. As an example we have studied the case of a dome in a stratified medium. The solutions obtained verify the reciprocity theorem with good accuracy. The computation of vertical profiles and of surface reflection profiles illustrates the effects of diffraction and the importance of lateral propagation in such a structure.
A METHOD FOR DETECTION OF DIFFRACTED WAVES ON COMMON‐OFFSET SECTIONS*
Authors: LANDA E. , SHTIVELMAN V. , GELCHINSKY B.
Published: April 2006

A method of detection of diffracted waves on common‐offset sections is proposed. The method utilizes the main kinematic and dynamic properties of the diffracted waves. The detection algorithm is defined by an automatic procedure including phase correlation of the diffracted waves and the application of certain statistical criteria. This procedure enables us to make decisions with regard to the presence of the diffracted waves and also to estimate parameters of the scattering objects. The method is applied to synthetic and field data and, even for a relatively low signal‐to‐noise ratio, it gives reliable results.
TEMPERATURE ANOMALIES OVER UNDERGROUND CAVITIES*
Authors: MOŚCICKI W.J.
Published: April 2006

Theoretical and practical aspects of a new method for underground cavity localization are presented. The method is based on shallow temperature measurements. It is shown that such measurements can complement the geophysical methods most commonly used for cavity location.

The results from finite‐difference numerical calculations indicate possible origins of temperature anomalies–the existence of which is confirmed experimentally, primarily by field measurements. Temperature anomalies up to 1°C have been recorded over underground cavities. A satisfactory agreement between measurements and theory is found.
COMPUTATIONAL ASPECTS OF THE CHOICE OF OPERATOR AND SAMPLING INTERVAL FOR NUMERICAL DIFFERENTIATION IN LARGE‐SCALE SIMULATION OF WAVE PHENOMENA*
Authors: HOLBERG O.
Published: April 2006

Conventional finite‐difference operators for numerical differentiation become progressively inaccurate at higher frequencies and therefore require very fine computational grids. This problem is avoided when the derivatives are computed by multiplication in the Fourier domain. However, because matrix transpositions are involved, efficient application of this method is restricted to computational environments where the complete data volume required by each computational step can be kept in random access memory.

To circumvent these problems a generalized numerical dispersion analysis for wave equation computations is developed. Operators for spatial differentiation can then be designed by minimizing the corresponding peak relative error in group velocity within a spatial frequency band. For specified levels of maximum relative error in group velocity ranging from 0.03% to 3%, differentiators have been designed that have the largest possible bandwidth for a given operator length.

The relation between operator length and the required number of grid points per shortest wavelength, for a required accuracy, provides a useful starting point for the design of cost‐effective numerical schemes. To illustrate this, different alternatives for numerical simulation of the time evolution of acoustic waves in three‐dimensional inhomogeneous media are investigated. It is demonstrated that algorithms can be implemented that require fewer arithmetic and I/O operations by orders of magnitude compared to conventional second‐order finite‐difference schemes to yield results with a specified minimum accuracy.
EXPERIMENTAL COMPARISON BETWEEN SPECTRAL RATIO AND RISE TIME TECHNIQUES FOR ATTENUATION MEASUREMENT*
Authors: TARIF P. , BOURBIE T.
Published: April 2006

Two techniques for the measurement of attenuation–spectral ratio and rise time techniques–were tested and compared in the laboratory. The spectral ratio technique proved to be reliable and easy to implement for intermediate values (5 < Q < 50) of attenuation. For low (Q > 50) and high attenuations, the spectral ratio technique is inaccurate. Calculating the rise time on simulated signals, we found a relation between rise time τ and the ratio travel‐time to quality factor T/Q which could be approximated in intervals by the linear relation τ=τ+C*T/Q. The constants τ and C depend on the absolute value of T/Q and on the initial source signal. The rise time technique, performed on the first quarter period of the signal, enables high attenuations (Q < 5) to be measured. The determination of the relation between τ and T/Q is possible if one knows the initial source. We theoretically approximate this relation through a simulation using a realistic propagation model. With laboratory measurements made on Fontainebleau sandstone, we show that the rise time technique using the theoretical relation τ=τ(T/Q) gives comparable values of Q to those obtained from the spectral ratio technique. In borehole seismics, where it is often difficult to remove undesired signals, the rise time technique applied with the right (τ, T/Q) relation is the best method to use.
THREE‐DIMENSIONAL INTERPRETATION OF GRAVITY DATA FROM SEDIMENTARY BASINS USING AN EXPONENTIAL DENSITY‐DEPTH FUNCTION*
Authors: GRANSER H.
Published: April 2006

In mapping the topography of the basement of deep sedimentary basins by gravity modelling, the accuracy can be improved by incorporating an exponential increase in density with depth. For calculating the gravity effect of a three‐dimensional (3D) structure with such an exponential density‐depth relation a frequency‐domain forward algorithm based on series expansion is presented, the numerical evaluation of which can be performed efficiently by fast Fourier transform. The algorithm can be applied in a recursive procedure to give the inverse solution in terms of basement relief.

The inversion procedure is satisfactorily tested on a 2D synthetic example and a 3D field example of gravity data from the western margin of the Pannonian Basin in eastern Austria, where up to 2.2 km of Tertiary sediments overlie an igneous or metamorphic basement. The results are confirmed by basement intersections in several wells.
CORRECTING FOR COLOURED PRIMARY REFLECTIVITY IN DECONVOLUTION1
Authors: WALDEN A. T. , NUNN K. R.
Published: April 2006

Statistical deconvolution, as it is usually applied on a routine basis, designs an operator from the trace autocorrelation to compress the wavelet which is convolved with the reflectivity sequence. Under the assumption of a white reflectivity sequence (and a minimum‐delay wavelet) this simple approach is valid. However, if the reflectivity is distinctly non‐white, then the deconvolution will confuse the contributions to the trace spectral shape of the wavelet and reflectivity.

Given logs from a nearby well, a simple two‐parameter model may be used to describe the power spectral shape of the reflection coefficients derived from the broadband synthetic. This modelling is attractive in that structure in the smoothed spectrum which is consistent with random effects is not built into the model. The two parameters are used to compute simple inverse‐ and forward‐correcting filters, which can be applied before and after the design and implementation of the standard predictive deconvolution operators. For whitening deconvolution, application of the inverse filter prior to deconvolution is unnecessary, provided the minimum‐delay version of the forward filter is used.

Application of the technique to seismic data shows the correction procedure to be fast and cheap and case histories display subtle, but important, differences between the conventionally deconvolved sections and those produced by incorporating the correction procedure into the processing sequence. It is concluded that, even with a moderate amount of non‐whiteness, the corrected section can show appreciably better resolution than the conventionally processed section.
LARGE‐OFFSET APPROXIMATIONS IN THE MODIFIED CAGNIARD METHOD FOR COMPUTING SYNTHETIC SEISMOGRAMS: A SURVEY1
Authors: DE HOOP A. T.
Published: April 2006

Large horizontal offset and large vertical offset approximations in the modified Cagniard method for computing synthetic seismograms in a horizontally layered model of the earth are derived. They apply to each of the generalized‐ray constituents into which the seismic wave motion is decomposed. For the results applying to large horizontal offset, which are known in the seismological literature, a simplified derivation is given. The results for large vertical offset, which are of particular interest to vertical seismic profiling, are new. The use of the large‐offset approximations, both horizontal and vertical, leads to a considerable reduction in computation time for synthetic seismograms compared with the full three‐dimensional version of the modified Cagniard method.
MIGRATION OF MIXED‐MODE VSP WAVEFIELDS1
Authors: DILLON P. B. , AHMED H. , ROBERTS T.
Published: April 2006

Two‐dimensional VSP surveys are often conducted to provide structural illumination of the subsurface away from the borehole. The illumination is achieved through offsetting the source with respect to the downhole geophone. This inevitably gives rise to mode‐conversions in both downgoing and upgoing wavefields.

Migration of mixed‐mode wavefields is complex because the velocity profile used for wavefield extrapolation is valid only for a particular propagation mode; the other mode always propagates at a different velocity. It is therefore advisable to separate the wave‐types (P‐wave and SV‐wave) prior to migration. This may be achieved through wavemode filtering, a multichannel process which exploits the relation between propagation velocity, slowness of events at the recording array and particle motion. The necessary information about particle motion is available only if the VSP data are acquired with a three‐component downhole geophone assembly.

The wavemode filter partitions wave‐types at the recording array; it provides no information about the various changes of propagation mode experienced by the energy as it travels from source to geophone. For the purpose of migration, the intermediate modes of propagation must be deduced.

Much of the energy arriving at the receivers is P‐wave which has followed the P‐wave velocity profile from the source. It can therefore be imaged by conventional (Kirchhoff) migration. As an example of SV‐wave imaging, a common mode‐code is P‐wave from source to reflector and SV‐wave from reflector to geophone. Migration of such data calls for back‐propagation of the geophone array wavefield, at SV‐wave velocity, to the point in the subsurface where it is time‐coincident with the forward propagated downwave, at P‐wave velocity.
BAYESIAN ESTIMATION IN SEISMIC INVERSION. PART II: UNCERTAINTY ANALYSIS1
Authors: DUIJNDAM A. J. W.
Published: April 2006

A parameter estimation or inversion procedure is incomplete without an analysis of uncertainties in the results. In the fundamental approach of Bayesian parameter estimation, discussed in Part I of this paper, the a posteriori probability density function (pdf) is the solution to the inverse problem. It is the product of the a priori pdf, containing a priori information on the parameters, and the likelihood function, which represents the information from the data. The maximum of the a posteriori pdf is usually taken as a point estimate of the parameters. The shape of this pdf, however, gives the full picture of uncertainty in the parameters. Uncertainty analysis is strictly a problem of information reduction. This can be achieved in several stages. Standard deviations can be computed as overall uncertainty measures of the parameters, when the shape of the a posteriori pdf is not too far from Gaussian. Covariance and related matrices give more detailed information. An eigenvalue or principle component analysis allows the inspection of essential linear combinations of the parameters.

The relative contributions of a priori information and data to the solution can be elegantly studied. Results in this paper are especially worked out for the non‐linear Gaussian case. Comparisons with other approaches are given. The procedures are illustrated with a simple two‐parameter inverse problem.
STOCHASTIC INVERSION BY RAY CONTINUATION: APPLICATION TO SEISMIC TOMOGRAPHY1
Authors: HAAS A. , VIALLIX J. R.
Published: April 2006

Abstract

The conventional tomographic inversion consists in minimizing residuals between measured and modelled traveltimes. The process tends to be unstable and some additional constraints are required to stabilize it. The stochastic formulation generalizes the technique and sets it on firmer theoretical bases.

The Stochastic Inversion by Ray Continuation (Sirc) is a probabilistic approach, which takes a priori geological information into account and uses probability distributions to characterize data correlations and errors. It makes it possible to tie uncertainties to the results.

The estimated parameters are interval velocities and B‐spline coefficients used to represent smoothed interfaces. Ray tracing is done by a continuation technique between source and receivers. The ray coordinates are computed from one path to the next by solving a linear system derived from Fermat's principle. The main advantages are fast computations, accurate traveltimes and derivatives.

The seismic traces are gathered in CMPs. For a particular CMP, several reflecting elements are characterized by their time gradient measured on the stacked section, and related to a mean emergence direction.

The program capabilities are tested on a synthetic example as well as on a field example. The strategy consists in inverting the parameters for one layer, then for the next one down. An inversion step is divided in two parts. First the parameters for the layer concerned are inverted, while the parameters for the upper layers remain fixed. Then all the parameters are reinverted.

The velocity‐depth section computed by the program together with the corresponding errors can be used directly for the interpretation, as an initial model for depth migration or for the complete inversion program under development.