bluuebunny/crossref_metadata_2025 huggingface dataset:
GEOTHERMAL PROSPECTING BY GEOELECTRIC SOUNDINGS IN NE GREECE*
Authors: THANASSOULAS C. , TSELENTIS G.‐A. , KOLIOS N.
Published: April 2006

In total 77 direct current resistivity soundings were carried out during a geothermal exploration survey of the Genisea, NE Greece, geothermal field. The data revealed a high electrical conductivity zone at the center of the investigated area and suggested that an anomalous heat source lay beneath the study area.

This was confirmed by subsequent drilling data. Temperature measurements, from 11 boreholes, were used for the construction of isotherms that correlated very closely with the geoelectric data.
GEOPHYSICAL MAPPING BY STATIONARY ELECTRIC AND MAGNETIC FIELD COMPONENTS: A COMBINATION OF POTENTIAL GRADIENT MAPPING AND MAGNETOMETRIC RESISTIVITY (MMR) METHODS*
Authors: SZARKA L.
Published: April 2006

A combined application of two independent direct current geoelectric methods, potential gradient mapping (PM) and magnetometric resistivity (MMR), is proposed for the investigation of high‐resistivity basement structures.

From a one‐dimensional relation between horizontal electric and magnetic fields (due to currents originating from point electrodes placed on the surface of a two‐layered half‐space having a high‐resistivity basement) several advantages of the combined PM‐MMR method are shown for two‐dimensional and three‐dimensional basement structures. Three‐dimensional models have been studied by analogue modeling experiments.

This new mapping method embraces two parameters: the horizontal electric conductance map, derived from the ratio of the related electric and magnetic components, and the gradient map of their product. A close relation between structural forms and anomaly maps is obtained by using two orthogonal source‐electrode layouts.

Based on model results a two‐channel measuring instrument was developed. The method and the instrument were tested near Sopron where a gradual deepening of, and faults in, the Paleozoic basement were found.
INVERSE Q‐FILTERING. A SPECTRAL BALANCING TECHNIQUE*
Authors: GELIUS L.J.
Published: April 2006

Inverse Q‐filtering (IQF) is a technique designed to correct for transmission losses due to inelastic attenuation. It is based on the constant‐Q model and is derived from a Taylor series solution of a standard convolutional‐trace model (primaries only). To avoid a non‐causal solution, the attenuation is assumed to be minimum‐phase. Band limitation is introduced to make IQF a stable process in the presence of noise. The main features of IQF are demonstrated using both synthetic and field data.
CORRECTIONS FOR CONDUCTIVITY ESTIMATES IN INDUCTION PROSPECTING OF SULPHIDE DYKES IN A LAYERED ENVIRONMENT*
Authors: NEGI J.G. , GUPTA O.P. , JOSHI M.S.
Published: April 2006

In a weathered environment estimates of depth and conductance of metallic sulphide dykes from conventional anomaly index diagrams for a vertical half‐plane in air have to be corrected, besides the usual corrections, for:

1. moderate conductivity of the host rocks, and

2. finiteness of strike length S and depth extent D.

Model experiments have been carried out to evaluate the response variation of a vertical planar conductor with varying depth extent and strike length for both insulating and conductive surroundings. The results indicate:

1. A conductor with finite depth extent (D/L < 2.5) or strike length (S/L < 5.0) in an insulating medium yields a lower estimate of conductance (mineralization) and a greater depth.

2. A moderately‐conductive host rock enhances the anomaly and rotates the phase so that the conductor appears to be more resistive (less mineralized) and shallower.

The results have practical significance since in weathered surroundings a highly‐mineralized body of finite size could be missed, or misjudged, because of low estimates of conductivity and depth.
TOWARDS OPTIMUM ONE‐WAY WAVE PROPAGATION1
Authors: HOLBERG O.
Published: April 2006

Numerical wavefield extrapolation represents the backbone of any algorithm for depth migration pre‐ or post‐stack. For such depth imaging techniques to yield reliable and interpretable results, the underlying wavefield extrapolation algorithm must propagate the waves through inhomogeneous media with a minimum of numerically induced distortion, over a range of frequencies and angles of propagation.

A review of finite‐difference (FD) approximations to the acoustic one‐way wave equation in the space‐frequency domain is presented. A straightforward generalization of the conventional FD formulation leads to an algorithm where the wavefield is continued downwards with space‐variant symmetric convolutional operators. The operators can be precomputed and made accessible in tables such that the ratio between the temporal frequency and the local velocity is used to determine the correct operator at each grid point during the downward continuation.

Convolutional operators are designed to fit the desired dispersion relation over a range of frequencies and angles of propagation such that the resulting numerical distortion is minimized. The optimization is constrained to ensure that evanescent energy and waves propagating at angles higher than the maximum design angle are attenuated in each extrapolation step. The resulting operators may be viewed as optimally truncated and bandlimited spatial versions of the familiar phase shift operator. They are unconditionally stable and can be applied explicitly. This results in a simple wave propagation algorithm, eminently suited for implementation on pipelined computers and on large parallel computing systems.
DETECTION AND RESOLUTION OF THIN LAYERS: A MODEL SEISMIC STUDY1
Authors: KROLLPFEIFER D. , DRESEN L. , HSIEH C. H. , CHERN C. C.
Published: April 2006

The detection and resolution of a thin layer closely situated above a high‐impedance basement are predominantly determined by both the frequency content of the incident seismic wavelet and the existence of the nearby high‐impedance bedrock.

The separation of the thin layer and the basement arrivals is investigated depending on the low‐frequency content of the wavelet. The high‐frequency content of the wavelet is kept constant. The initial wavelet spectrum with low frequencies has a rectangular shape. All wavelets used have zero‐phase characteristics. Numerical and analogue seismic modelling techniques are used. The study is based on the geology of the Pachangchi Sandstone in West Taiwan.

Firstly the resolution of a thin layer between two half‐spaces is examined by applying the Ricker and De Voogd‐Den Rooijen criteria. The lack of low‐frequency components of the incident seismic wavelet reduces the shortest true two‐way traveltime by about 20%. In addition, low‐frequency components of the wavelet diminish the deviation between true and apparent two‐way traveltime by about 65% for layer thicknesses in the transition from a thick to a thin layer.

The second step deals with the influence of a high‐impedance basement just below a thin layer on the detection and resolution of that thin layer. Reflected signal energies and apparent two‐way traveltimes are considered. The reflected signal energy depends on the low‐frequency content of the incident wavelet, the layer's thickness and the distance between the basement and the layer. This applies only to layers with thicknesses less than or equal to one‐third of the mean wavelength in the layer, and a distance to basement in the range of one to one‐half of the mean wavelength in the rock material between layer and basement.

The minimum thin‐layer thickness resolvable decreases with increasing distance to the basement; i.e. for a layer thickness of one‐third of the mean wavelength in the layer the relative error of the two‐way traveltime increases from 5% to 30%, if the distance is reduced from one to one‐half of the mean wavelength in the material between the basement and the thin layer.

Finally, a combination of vertical seismic profiling and downward‐continuation techniques is presented as a preprocessing procedure to prepare realistic data for the detection and resolution investigation.
VELOCITY ANALYSIS OF THE SH‐CHANNEL WAVE IN THE SCHWALBACH SEAM AT ENSDORF COLLIERY1
Authors: COX K. B. , MASON I. M.
Published: April 2006

An in‐seam fan shoot was conducted in 1981 over a 300 m × 500 m panel of the Schwalbach seam at Ensdorf by a team from Prakla‐Seismos AG of Hannover under contract to Saarbergwerke AG, Saarbrücken. The object was to study SH‐mode propagation in the coal seam waveguide. The high quality dataset retrieved provides a general and valuable test bed with which to compare the performance of in‐seam seismic velocity analysers.

Five different dispersion analysers are demonstrated using the Schwalbach data. They are all based on the a priori assumption of coal seam homogeneity and isotropy. Space or time windows limit the resolution of the Fourier moving‐window analyser, the migration based phase‐velocity analyser, and the double Fourier transformer. The other two analysers, the maximum entropy moving‐window analyser and the phase‐moveout analyser, achieve noise‐limited super‐resolution by predicting the probable behaviour of the wavefield outside the window.

The coal seam's characteristics conform to those predicted for a simplified model based on proposals by Elsen, Rüter and Schott of Westfälische Berggewerkschaftskasse, Bochum. The slight discrepancy between theoretical and actual dispersion characteristics could be reduced by increasing the model's complexity. However, there would be no material gain without testing the validity of the signal processing assumptions of seam isotropy and homogeneity.
ON THE EFFECTS OF THICKNESS OF THE HALF‐PLANE MODEL IN HLEM INDUCTION PROSPECTING OVER SULPHIDE DYKES IN A HIGHLY RESISTIVE MEDIUM1
Authors: JOSHI M. S. , GUPTA O. P. , NEGI J. G.
Published: April 2006

In the quantitative data interpretation for HLEM induction prospecting, a vertical half‐plane model in an insulating medium is widely employed. For this assumption to be valid, the steeply dipping massive sulphide dykes must have large strike lengths and depth extents, but small thickness.

We report investigations, using the laboratory scale‐modelling method, on the response variation of large vertical conductors as the thickness is varied. We conclude that a steeply dipping large dyke can be approximated by a half‐plane model only if its thickness is less than half the skin depth. An inductively thick conductor produces larger amplitudes and relatively higher quadrature compared to a thin conductor, even if both have the same induction number.
STUDY AND APPLICATIONS OF SPATIAL DIRECTIONAL FILTERING IN THREE‐COMPONENT RECORDINGS1
Authors: BENHAMA A. , CLIET C. , DUBESSET M.
Published: April 2006

Three‐component recordings permit the construction of particle trajectories. These three‐dimensional pictures of particle motion show successive predominant directions of polarization and allow wave modes with distinct polarization directions to be recognized. A polarization selection, called ‘spatial directional filtering’, can be accomplished by several methods; four techniques are described. The application of these polarization filters to a noise shot, offset VSPs and CDP stack are also presented. This type of filtering is shown to cancel waves with undue polarization and to enhance the signal‐to‐noise ratio.
EM MODELLING USING SURFACE INTEGRAL EQUATIONS1
Authors: DOHERTY J.
Published: April 2006

The theory by which the Surface Integral Equation method may be applied to the solution of electromagnetic transmission boundary value problems is presented. For a 3D target of arbitrary electrical property contrast with its host medium excited by an arbitrary time‐harmonic source, two integral equations are derived which need to be simultaneously solved for tangential electric and magnetic source density on the target's surface. If the target is 2D, though still excited by an arbitrary source (the 2½ D case), the problem is best solved in the transform domain for a number of different wavenumbers in the target's strike direction. Then a set of four simultaneous scalar integral equations needs to be solved for the components of the surface source density transforms in the target's strike direction and in the direction of the tangent vector to the target's cross‐sectional contour.

Examples are presented in which the 2½D problem is solved numerically using the method of moments with piecewise linear basis functions. Although the results generally compare well with analytical solutions, or solutions obtained numerically by other means, errors appear in the calculation of the real response of these targets to excitation by a magnetic dipole source at low frequencies. This is attributed to ill‐conditioning of the system resulting from a non‐unique solution at zero frequency.
OPTIMUM EXPRESSION FOR COMPUTATION OF THE GRAVITY FIELD OF A HOMOGENEOUS POLYHEDRAL BODY1
Authors: POHÁNKA V.
Published: April 2006

The formula for the computation of the gravity field of a homogeneous polyhedral body is derived and transformed into the form which is most suitable when considering speed and simplicity of numerical calculation.
TIME‐DOMAIN COMPUTATION OF NON‐NORMAL INCIDENCE WAVEFIELDS IN PLANE LAYERED MEDIA1
Authors: FERBER R.‐G.
Published: April 2006

A new time‐domain method is introduced for the calculation of theoretical seismograms which include frequency dependent effects like absorption. To incorporate these effects the reflection and transmission coefficients become convolutionary operators. The method is based on the communication theory approach and is applicable to non‐normal incidence plane waves in flat layered elastic media. Wave propagation is simulated by tracking the wave amplitudes through a storage vector inside the computer memory representing a Goupillaud earth model discretized by equal vertical transit times. Arbitrary numbers of sources and receivers can be placed at arbitrary depth positions, while the computational effort is independent of that number. Therefore, the computation of a whole plane‐wave vertical seismic profile is possible with no extra effort compared to the computation of the surface seismogram. The new method can be used as an aid to the interpretation of plane‐wave decomposed reflection data where the whole synthetic vertical seismic profile readily gives the interpreter the correct depth position of reflection events.
BAYESIAN ESTIMATION IN SEISMIC INVERSION. PART I: PRINCIPLES1
Authors: DUIJNDAM A. J. W.
Published: April 2006

This paper gives a review of Bayesian parameter estimation. The Bayesian approach is fundamental and applicable to all kinds of inverse problems. Its basic formulation is probabilistic. Information from data is combined with a priori information on model parameters. The result is called the a posteriori probability density function and it is the solution to the inverse problem. In practice an estimate of the parameters is obtained by taking its maximum. Well‐known estimation procedures like least‐squares inversion or l1 norm inversion result, depending on the type of noise and a priori information given. Due to the a priori information the maximum will be unique and the estimation procedures will be stable except (in theory) for the most pathological problems which are very unlikely to occur in practice. The approach of Tarantola and Valette can be derived within classical probability theory.

The Bayesian approach allows a full resolution and uncertainty analysis which is discussed in Part II of the paper.
INTERPRETATION OF RESISTIVITY MEASUREMENTS OVER 2D STRUCTURES1
Authors: SCHULZ R. , TEZKAN B.
Published: April 2006

Resistivity measurements were carried out in a survey area in the south of Germany. This area is characterized by complicated subsurface geology. Schlumberger full‐arrays and their respective half‐arrays were recorded simultaneously. The results obtained by the one‐dimensional (1D) interpretation of the full‐array measurements were incorrect because of a resistivity discontinuity. This discontinuity, under a relatively thick overburden, could only be located by the half‐array soundings. Its exact location and the resistivity distribution in the subsurface were ascertained by comparing the sounding curves with 2D model curves, which are calculated by a finite‐difference method.
FURTHER DEVELOPMENT OF A HIGH‐FREQUENCY SEISMIC SOURCE FOR USE IN BOREHOLES1
Authors: BARIA R. , JACKSON P. D. , McCANN D. M.
Published: April 2006

Abstract

A borehole sparker seismic source enclosed in a semi‐flexible tube has been developed to produce a short compressional seismic pulse with a frequency content in the range 250 Hz to 3.5 kHz with a peak power at 570 Hz. The pulse shape and frequency content are shown to be a function of the input power, the diameter of the spark chamber, the salinity of the electrolyte, the material of the spark chamber and the electrode configuration. When in a borehole, the source produces a vertically polarized shear wave but, being similar to a small explosive charge does not allow phase reversal as a means of identifying the shear wave in the received pulse train. The source is shown to be ideal for tomographic imaging surveys because of its repetitive nature, high frequency content and reliability. Very high resolution seismic reflection surveys are also shown to be possible under favourable circumstances.
CONTRIBUTION DES MESURES DE L'ANISOTROPIE ÉLECTRIQUE À LA RECHERCHE DES AQUIFÈRES DE FRACTURE EN MILIEU CRISTALLIN AU BÉNIN1
Authors: DARBOUX‐AFOUDA R. , LOUIS P.
Published: April 2006

Abstract

A particular methodology adapted to crystalline formations with a thin weathered zone was developed for a village hydrological project, in Benin. A combination of electrical profiles, Schlumberger and square arrays, was able to locate the most fractured zones in the basement. We present some results obtained from theoretical models as well as from field data.

The suggested methodology uses both measurements of resistivity and anisotropy. Strong anisotropy and low resistivity indicate the most productive hydrogeological areas.
DECOMPOSED VIBRATOR PATTERNS TO IMPROVE SEISMIC SURVEY RESULTS1
Authors: EDELMANN H. A. K. , KIRCHHEIMER F. , SCHULTE L.
Published: April 2006

Abstract

Effective noise reduction of single‐sweep recorded data is achieved by application of a velocity filter process on a decomposed vibrator pattern. This technique promises high resolution results with a minimum effect on signal characteristic. A comparison of the stacked section of records vertically stacked in the field with the stacked section of velocity‐filtered receiver gathers shows a significant increase in resolution and signal‐to‐noise ratio.
MATRIX METHOD FOR THE TRANSFORMATION OF RESISTIVITY SOUNDING DATA OF ONE ELECTRODE CONFIGURATION TO THAT OF ANOTHER CONFIGURATION1
Authors: NIWAS SRI , ISRAIL M.
Published: April 2006

Abstract

Matrix equations are derived to transform the resistivity sounding data obtained in one type of a four‐electrode array to the corresponding resistivity sounding data that would be obtained using a different four‐electrode array. These expressions are based primarily on recent work in which we have established a linear relation between the apparent resistivity and the kernel function by using a powerful exponential approximation for the kernel function. It is shown that the resistivity sounding data of two different four‐electrode arrays have a linear relation through an essentially non‐singular matrix operator and, as such, one is derivable from the other for a one‐dimensional model and it can also be extended to two‐dimensions.

Some numerical examples considering synthetic data are presented which demonstrates the efficiency of the method in such transformations. Two published field examples are also considered for transformation giving a reliable interpretation.
APPARENT DENSITY MAPPING AND 3D GRAVITY INVERSION IN THE EASTERN ALPS1
Authors: GRANSER H. , MEURERS B. , STEINHAUSER P.
Published: April 2006

Power spectrum analysis of the Bouger gravity values in the Eastern Alps suggests that the gravity field may be separated into long and short wavelength components. The long wavelength component is assumed to be caused by Alpine crustal thickening. This long wavelength component was subjected to a gravimetric single density‐interface inversion procedure, giving a gravimetric Mohorovičić model which is generally in good agreement with Moho‐depths derived by refraction and reflection seismology.

The residual high‐frequency gravity component correlates well with the main surface geological units in the Eastern Alps.

Apparent density mapping by applying an inverse density deconvolution filter to the short wavelength gravity component gives density values for the upper crust which correspond well with averaged density values from rock samples.
EFFICIENT 2D AND 3D SHOT RECORD REDATUMING1
Authors: KINNEGING N. A. , BUDEJICKY V. , WAPENAAR C. P. A. , BERKHOUT A. J.
Published: April 2006

In order to make 3D prestack depth migration feasible on modern computers it is necessary to use a target‐oriented migration scheme. By limiting the output of the migration to a specific depth interval (target zone), the efficiency of the scheme is improved considerably. The first step in such a target‐oriented approach is redatuming of the shot records at the surface to the upper boundary of the target zone. For this purpose, efficient non‐recursive wavefield extrapolation operators should be generated. We propose a ray tracing method or the Gaussian beam method. With both methods operators can be efficiently generated for any irregular shooting geometry at the surface. As expected, the amplitude behaviour of the Gaussian beam method is better than that of the ray tracing based operators.

The redatuming algorithm is performed per shot record, which makes the data handling very efficient. From the shot records at the surface‘genuine zero‐offset data’are generated at the upper boundary of the target zone. Particularly in situations with a complicated overburden, the quality of target‐oriented zero‐offset data is much better than can be reached with a CMP stacking method at the surface. The target‐oriented zero‐offset data can be used as input to a full 3D zero‐offset depth migration scheme, in order to obtain a depth section of the target zone.
CURRENT CHANNELLING IN SQUARE PLATES WITH APPLICATIONS TO MAGNETOMETRIC RESISTIVITY1
Authors: CHEESMAN S. J. , EDWARDS R. N.
Published: April 2006

The magnetometric resistivity (MMR) method uses a sensitive magnetometer to measure the low‐level, low‐frequency magnetic fields associated with the galvanic current flow between a pair of electrodes. While the MMR anomalies of simple structures such as dikes and vertical contacts have been determined analytically, there is a lack of systematic information on the expected responses from simple three‐dimensional bodies. We determine the characteristic anomalies associated with square, plate‐like conductors, which are excellent models of many base metal mineral deposits.

The anomalies of plates of finite size are determined numerically using an integral equation method. A plate is subdivided into many sections and the current flow within each section is solved by equating the electrical field within each section to the tangential electrical field just outside it. When the plate size is small in relation to either the depth or the transmitter spacing, the shape and amplitude of the anomaly produced is closely approximated by a current dipole model of the same length and depth. At the other extreme, a large plate is represented by a half‐plane. The dipole and half‐plane models are used to bracket the behaviour of plates of finite size.

The form of a plate anomaly is principally dependent on the shape, depth and orientation of the plate. A large, dipping plate near the surface produces a skewed anomaly highly indicative of its dip, but the amount of skew rapidly diminishes with increased depth or decreased size. Changes in plate conductivity affect the amplitude of the anomaly, but have little effect on anomaly shape. A current channelling parameter, determined from the conductivity contrast, can thus be used to scale the amplitude of an anomaly whose basic shape has been determined from geometrical considerations.

The separation into geometrical and electrical factors greatly simplifies both the interpretation and modelling of MMR anomalies, particularly in situations with multiple plates. An empirical formula, using this separation, predicts the anomaly of two or more parallel plates with different conductances. In addition, the relation between the resolution of two vertical, parallel plates of equal conductance and their separation is determined.

The ability of the integral equation method to model plate‐like structures is demonstrated with the interpretation of an MMR anomaly in a survey conducted at Cork Tree Well in Western Australia. The buried conductor, a mineralized graphitic zone, is modelled with a vertical, bent plate. The depth to the top of the plate, and the plate conductance, is adjusted to fit the anomaly amplitude as closely as possible. From the modelling it would appear that this zone is not solely responsible for the observed anomaly.
3D MODELLING OF DIFFRACTIONS OBSERVED ON DEEP REFLECTION LINE DEKORP 2‐S1
Authors: SADOWIAK P. , VOSS J. , MEISSNER R.
Published: April 2006

DEKORP 2‐S is the first profile carried out in the German continental reflection seismic programme. Besides numerous reflections in the lower crust, the seismic section is characterized by strongly curved events that are interpreted as diffractions. These diffractions occur as clusters, mainly in two areas of the profile: the Dinkelsbühl and the Spessart area. This paper deals with the Dinkelsbühl diffractions where three‐dimensional control is available. The control is provided by two additional profiles P‐1 and Q‐40 which run parallel and perpendicular to the main line, DEKORP 2‐S. The type and the location of the diffractors are determined by traveltime‐modelling using crustal velocity functions derived from in‐line wide‐angle observations. A model with inclined line diffractors provides the best fit to the data for all three profiles. Projections of these line diffractors to the surface show that they are aligned parallel to the strike direction of the Variscides. This suggests that the diffractions are associated with the suture zone between the Saxothuringian and Moldanubian geological provinces.
3D SEISMIC MODELLING OF EDGE DIFFRACTIONS1
Authors: PEDERSEN H. M. , GELIUS L.‐J. , STAMNES J. J.
Published: April 2006

An accurate, fast, and simple algorithm for 3D modelling of seismic edge diffractions is presented. It is based on a generalized Kirchhoff theory that applies also to inhomogeneous (non‐uniform) media. Both the boundary values and the Green functions in the Kirchhoff diffraction integral are determined by dynamic ray tracing, and each ray event is treated separately to obtain a description with clear physical interpretation. For each event the resulting Kirchhoff diffraction integral is evaluated by means of a uniform asymptotic technique that remains valid for receiver points near shadow boundaries. Since all parameters needed in the computations are obtained from dynamic ray tracing, the algorithm can readily be incorporated in existing software packages for 3D seismic ray modelling.
TWO METHODS FOR CONTINUOUS MONITORING OF HARMONIC DISTORTION IN VIBROSEIS SIGNALS1
Authors: MARTIN J. E. , WHITE R. E.
Published: April 2006

The quality of Vibroseis survey data can be improved by continuously monitoring the vibrator's baseplate and reaction mass accelerations. Equipment failures can be detected as they occur, rather than relying on similarity trials at the beginning and end of the day's production. Equipment faults can then be corrected as they happen and thus would not have a detrimental effect on the quality of the survey data. Source efficiency can be optimized by monitoring the amount of harmonic distortion generated by the vibrator at different drive levels on the different surfaces which may be encountered during a survey. Phase problems introduced by poor coupling of the baseplate to the ground can also be identified and addressed in the field.

Rapid analysis of vibrator signals is required if continuous monitoring is to be useful. Frequency‐time (f‐t) analyses of vibrator signals are often used in processing centres, but are slow and require a large storage capacity which makes the technique unsuitable for a field analysis system.

The two methods proposed to analyse vibrator signals entail the use of hodograms and time‐varying notch filters. Hodograms provide a qualitative analysis of harmonic distortion and vibrator performance. A fast, time‐varying notch filter gives quantitative and qualitative information about the harmonic distortion present in the signal and can be used to identify problems with vibrator behaviour. Both the hodogram and fast, time‐varying notch filter methods can analyse the vibrator's reaction mass and baseplate accelerations as it progresses through its sweep and can present automatically interpreted results to the operator before moving to the next vibrator point.
3D TABLE‐DRIVEN MIGRATION1
Authors: BLACQUIÈRE G. , DEBEYE H. W. J. , WAPENAAR C. P. A. , BERKHOUT A. J.
Published: April 2006

An efficient full 3D wavefield extrapolation technique is presented. The method can be used for any type of subsurface structure and the degree of accuracy and dip‐angle performance are user‐defined. The extrapolation is performed in the space‐frequency domain as a space‐dependent spatial convolution with recursive Kirchhoff extrapolation operators.

To get a high level of efficiency the operators are optimized such that they have the smallest possible size for a specified accuracy and dip‐angle performance. As both accuracy and maximum dip‐angle are input parameters for the operator calculation, the method offers the possibility of a trade‐off between these quantities and efficiency. The operators are calculated in advance and stored in a table for a range of wavenumbers. Once they have been calculated they can be used many times.

At the basis of the operator design is the well‐known phase‐shift operator. Although this operator is exact for homogeneous media only, it is assumed that it may be applied locally in case of inhomogeneities. Lateral velocity variations can then be handled by choosing the extrapolation operator according to the local value of the velocity. Optionally the operators can be designed such that they act as spatially variant high‐cut filters. This means that the evanescent field can be suppressed in one pass with the extrapolation. The extrapolation method can be used both in prestack and post‐stack applications. In this paper we use it in zero‐offset migration. Tests on 2D and 3D synthetic and 2D real data show the excellent quality of the method. The full 3D result is much better then the result of two‐pass migration, which has been applied to the same data.

The implementation yields a code that is fully vectorizable, which makes the method very suitable for vector computers.
ESTIMATION OF THE DEGREE OF POLYNOMIAL FITTED TO GRAVITY ANOMALIES AND ITS APPLICATION1
Authors: ZENG HUALIN
Published: April 2006

A method for estimating the degree of polynomial fitted to gravity anomalies to evaluate the regional anomaly is presented. The anomaly can be fitted by polynomials of different degrees with the least‐squares method and the optimum degree of the polynomial evaluating this anomaly can be estimated from the point of discontinuity of the gradient on a graph of variance against the polynomial degree. The Bouguer gravity is initially separated by upward continuation to a proper height and then the degree of regional polynomial to fit the Bouguer anomaly can be estimated. Theoretical and field examples show the effectiveness of the method.
SEISMIC TOMOGRAPHIC IMAGING OF A BURIED CONCRETE TARGET1
Authors: BISHOP I. , STYLES P.
Published: April 2006

Abstract

This paper describes a field evaluation of an algebraic reconstruction technique for the tomographic imaging of sub‐surface velocity anomalies. We describe the construction of a three‐dimensional concrete model and the acquisition and processing of seismic traveltime data through the model. Image reconstructions of the data sets, using an algebraic reconstruction technique and incorporating prior knowledge are presented and these are compared with the actual model. Reconstructions show that it is essential that accurate data are obtained as we demonstrate that relatively small errors in the traveltime data can seriously degrade the reconstruction. We also show that raypath effects are very important limiting factors to the analysis.
CROSS‐BOREHOLE OBSERVATION OF MODE CONVERSION FROM BOREHOLE STONELEY WAVES TO CHANNEL WAVES AT A COAL LAYER1
Authors: ALBRIGHT JAMES N. , JOHNSON PAUL A.
Published: April 2006

Abstract

Conversion of borehole Stoneley waves to channel waves was observed in data from a seismic cross‐borehole experiment conducted between wellbores penetrating a thin coal layer at 2022 m depth, near Rifle, Colorado. Traveltime moveout observations show that borehole Stoneley waves underwent partial conversion to channel waves at the coal layer. The channel waves were detected directly in an adjacent borehole 35 m away at receiver positions within the coal. Stoneley waves, subsequently produced by partial conversion of channel waves, were also detected at receiver positions located up to 50 m above and below the coal layer in the adjacent borehole. We infer the channel wave to be the first‐higher Rayleigh mode by comparing the observed group velocity with theoretically derived dispersion curves. Identifying the conversion between borehole and stratigraphically guided waves is significant because coal penetrated by multiple wells may be detected without placing a transmitter or receiver in the coal itself.
SEISMIC INTERPRETATION OF UPPER ELK POINT (GIVETIAN) CARBONATE RESERVOIRS OF WESTERN CANADA1
Authors: BROWN R. J. , ANDERSON N. L. , HILLS L. V.
Published: April 2006

Abstract

The seismic signatures of three reefs of the Upper Elk Point Subgroup (Givetian Stage) of the Western Canada Sedimentary Basin are documented and analysed on the basis of variations in seismic image of particular lithologic units, lateral amplitude and/or phase changes, structural relief and velocity‐generated relief, as rendered by the reflection data. The effects on seismic signatures of spatial geological variations resulting from such phenomena as differential compaction, reef‐focused salt dissolution, palaeotopography, lateral and vertical facies variations, regional dip, and reservoir morphology are discussed. The usefulness of seismic data in clarifying relationships between reefs and their adjacent sedimentary sections, particularly in cases where well control is sparse, is also considered. Such documentation of seismic signatures from known reefs using geophysical and geological analysis can establish criteria to enable recognition of similar buildups elsewhere.

Three example reefs are presented, each typical of a particular area and environment of W. Canada. The first is from the Winnipegosis Formation of SE Saskatchewan, the second and third from the Rainbow Member and Upper Keg River Reef Member, respectively, of the Keg River Formation of NW Alberta. All three of these carbonate buildups developed in the evaporitic Elk Point Basin. However, the degree of salt encasement and subsequent dissolution varied greatly, as do the resulting seismic effects. For these three reef types, the typical elements of their seismic signatures have been compiled and are here summarized.
AN ADVANCED CMP RAY‐TRACING1
Authors: KOREN ZVI
Published: April 2006

Abstract

The ray‐tracing algorithm presented in this paper is based on formulae derived for the common reflecting element (CRE) stacking method. A 2D, smooth, laterally‐varying media is assumed where offset rays and traveltimes are evaluated from normal‐incidence (central) rays. The method uses a second‐order asymmetrical approximation for rays and an additional oblique spherical approximation of the central wavefronts for calculating offset traveltimes. In order to solve the two‐point ray‐tracing problem for the common midpoint (CMP) configuration of source‐receiver pairs located symmetrically around the CMP stations, the central rays are perturbed to satisfy the above‐mentioned asymmetrical distribution. Although the accuracy of the calculations is limited for far offsets, it is still good for distances of the order of the reflecting depths. Since only a few normal‐incidence rays are traced through the medium, the method is very fast and is found to be most attractive for iterative inversions in macromodel estimation.
DIRECTIONAL DECONVOLUTION OF MARINE SEISMIC REFLECTION DATA: NORTH SEA EXAMPLE1
Authors: ROBERTS G. A. , GOULTY N. R.
Published: April 2006

Abstract

Directional deconvolution of the signature from a marine seismic source array may be achieved in combination with prestack migration or dip moveout (DMO) processing. The benefit is demonstrated using an example profile from the southern North Sea. In particular, shallow, dipping reflectors have improved continuity and frequency content. The method could be extended to 3D data to remove both in‐line and cross‐line directivity effects.
ANISOTROPY DETECTED IN SHALLOW CLAYS USING SHEAR‐WAVE SPLITTING IN A VSP SURVEY1
Authors: DOUMA JAN , DEN ROOIJEN HENK , SCHOKKING FLORIS
Published: April 2006

Abstract

Shear waves can provide valuable information about seismic anisotropy. On entering an anisotropic medium, a shear wave generally splits (shear‐wave splitting) into a fast and a slow quasi‐shear wave with polarizations fixed by the elastic properties of the medium and direction of travel. If the medium contains planar discontinuities with common normals, the fast shear wave will be suitably propagated if its polarization lies in the plane of the discontinuities. Measuring this polarization, using a VSP geometry with oriented three‐component geophones in the borehole, offers the possibility of monitoring the orientation and density of the discontinuities as a function of depth.

Such a shear‐wave VSP was carried out in an uncased 0.3 m diameter borehole drilled to a depth of 120 m in the north of The Netherlands. The upper 80 m of the sequence, consisting of a glacial till and sands and clays of Pleistocene age, was studied. The clays in this sequence have been subjected to glacial deformation and as a result are overconsolidated and locally fissured.

In our shallow VSP experiment, shear‐wave splitting and therefore anisotropy was identified at various geophone depths for one source offset. Hodograms showed a consistent polarization of the fast shear‐wave component over a large depth interval. Under the assumption that the anisotropy was caused by planar discontinuities with common normals, this polarization direction gives the strike of the fissures in this interval. The polarization direction of the fast S‐wave did not correspond exactly with the strike which was obtained from geological information on the fissures. The geological information was from undisturbed oriented 70 mm core samples taken at 3 m intervals in the borehole. The discrepancy, however, could be explained in terms of dipping fissures, and such a dip was confirmed by the geological and geotechnical information.

The orientation of fissures is an important factor in the directional deformation and strength characteristics of clays as far as geotechnical behaviour is concerned. This study thus illustrates a practical application of shear‐wave splitting observed in shallow shear‐wave VSP for geology and geotechnical engineering.
3D GRAVITY INVERSION OF THE CHESHIRE BASIN1
Authors: ABDOH A. , COWAN D. , PILKINGTON M.
Published: April 2006

Abstract

Three‐dimensional inverse gravity modelling is used to determine the structure of the Carboniferous sediments underlying the Jurassic and Permo‐Triassic formations in the Cheshire Basin. The gravity effect of the Jurassic and Permo‐Triassic rocks was first removed from the observations and the resulting residual gravity field was then inverted to map variations in depth of the Carboniferous formations. The inversion technique is performed in the frequency domain, enabling rapid computation of solutions for large gridded data sets. Evaluation of results shows that there is good agreement between calculated and observed values, with interpreted depths to the base of the Carboniferous in excess of 6 km. Gravity inversion results also delineate areas of faulting on the south‐western and eastern edges of the basin.
PULSE DISTORTION IN CAUSTIC REGIONS1
Authors: GELIUS L.‐J. , STAMNES J. J. , HEIER H.
Published: April 2006

Abstract

It is well known that there is a phase shift of π associated with the propagation of a seismic pulse through a focus. But this simple behaviour is true only if the two points at which the pulse is observed lie sufficiently far from the focus, one on either side. Near the focus matters are more complicated and the shape of the pulse may be severely distorted because of diffraction. To examine this problem we consider a pulse generated by a point source and focused by a spherical reflector. Using a general combined method of ray tracing and diffraction, we compute seismograms for receivers lying both inside and outside the focal region, and determine how the shape of the pulse changes on propagation through the focus. To assist the physical interpretation of the numerical results, we also present relevant exact and asymptotic results for comparison.
RESOLUTION ANALYSIS OF BAND‐LIMITED AND OFFSET‐LIMITED SEISMIC DATA FOR PLANE‐LAYERED SUBSURFACE MODELS1
Authors: VAN RIJSSEN E. P. F. , HERMAN G. C.
Published: April 2006

Abstract

A singular‐value decomposition technique is presented for quantifying the information content of band‐limited and offset‐limited seismic reflection data for the case of plane‐layered subsurface models. With the aid of this method, vertical resolution, tuning effects and ambiguities between different types of lithological parameters can be analysed simultaneously. The method is applied in a model study for quantifying the effects of offset‐limitation and the presence of different wavetypes on the ability to discriminate different types of parameters.
INTEGRATED EXPLORATION FOR LOW‐TEMPERATURE GEOTHERMAL RESOURCES IN THE HONEY LAKE BASIN, CALIFORNIA1
Authors: SCHIMSCHAL ULRICH
Published: April 2006

Abstract

An integrated exploration study is presented to locate low‐temperature geothermal reservoirs in the Honey Lake area of northern California. Regional studies to locate the geothermal resources included gravity, infra‐red, water‐temperature, and water‐quality analyses. Five anomalies were mapped from resistivity surveys. Additional study of three anomalies by temperature‐gradient and seismic methods was undertaken to define structure and potential of the geothermal resource. The gravity data show a graben structure in the area. Seismic reflection data indicate faults associated with surface‐resistivity and temperature‐gradient data. The data support the interpretation that the shallow reservoirs are replenished along the fault zones by deeply circulating heated meteoric waters.
ANALYSIS OF THE COMMON MIDPOINT GATHER BY DECOMPOSITION INTO ELEMENTARY WAVEFRONTS1
Authors: THORE PIERRE , DE BAZELAIRE ERIC
Published: April 2006

Abstract

The imaginary wavefront of a common midpoint gather of seismic data is analysed in terms of elementary wavefronts. The wavefronts emitted by the sources are split into a sequence of elementary circular wavefronts. Each such wavefront is independently transmitted through the subsurface and received by a group of receivers associated with the sources. The theory is first demonstrated in the case of a one‐layer medium and is then extended to the case of a multilayer medium.

We show that pairs of imaginary stationary points F and F' occur in the first medium through which all the rays, corresponding to the group of sources and receivers associated with the elementary wavefront pass. The location of the points is determined by 2n equations with 2n unknowns; (2n ‐ 1) of these equations correspond to n layers of the medium, and an additional equation is given by the geometry of the CMP gather. Applying Fermat's principle between the points F and F', we obtain an algorithm for CMP ray tracing. The wavefront sampling problem is considered but not theoretically solved. Results obtained in the general case (a multilayer medium with lateral velocity variation) are presented and compared with those obtained by other modelling techniques.
PARAXIAL RAY TRACING IN 3D INHOMOGENEOUS, ANISOTROPIC MEDIA1
Authors: GIBSON RICHARD L. , SENA ARCANGELO G. , TOKSÖZ M. NAFI
Published: April 2006

Abstract

Many investigations of the propagation elastic waves within the earth require a technique for producing synthetic seismograms which is capable of modelling 3D propagation effects. Ray methods are an excellent option for these problems, because they can be made fully 3D and allow a relatively quick and flexible computation of synthetic seismograms. However, the two point problem of finding the ray which connects exactly a specific source and receiver, may still be difficult and time consuming. Therefore, application of the paraxial method, which allows extrapolation of the information on a given ray to nearby receiver locations, is very valuable. With this approach, great savings in computation time and significant simplification of computer codes are possible.

We investigate the application of the paraxial ray method to two problems in which the effects of 3D seismic wave propagation are important. The first is a model of a reef structure. In this case, we consider synthetic seismograms for a VSP experimental configuration. When the SV source and well are located along the axis of the reef, only 2D propagation effects are observed. If the source‐receiver plane is located to the side of the reef, however, the 3D shape of the reef causes significant amplitudes to be predicted for shear‐wave arrivals on the transverse component of the synthetic seismograms.

The second example is a ID, layered earth model, but it includes two layers which are azimuthally anisotropic due to the presence of aligned, vertical fractures. This anisotropy leads to 3D raypaths. Synthetic seismographs are presented for a cross‐hole geometry both for an equivalent isotropic model and for the direction parallel to the fractures and at an angle of 45° to the fractures in the anisotropic case. These synthetics show that the differences between the isotropic case and the case for source and receivers aligned with the plane of the fractures are small and subtle. On the other hand, the predictions for the direction at 45° to the cracks show shear‐wave splitting and significant transverse component signal.

These results have important implications for both modelling and for applications such as tomography. It is clear that for some of the cases considered, a 2D algorithm will lead to errors in interpretation of data. In addition, ray‐based tomographic techniques will have great difficulty in obtaining a well‐defined 2D planar image when the signals are propagating in regions outside the image plane.