bluuebunny/crossref_metadata_2025 huggingface dataset:
A PULSED PLASMA JET ACOUSTIC SOURCE FOR PROFILING THE OCEAN FLOOR1
Authors: PITT P. L. , SMITH R. D. , SHEFFLER G. C. , WARREN R. D. , CLEMENTS R. M. , HAMILTON T. S.
Published: April 2006

A plasma gun source has been successfully used to obtain sub‐bottom profiles. The profiles show better penetration than with a 3‐5 kHz source and more resolution than with an air gun. The plasma gun source is compact, self‐contained, and requires no complex auxiliary equipment. The device was deployed from large research vessels and a small boat. The plasma gun produces sufficient acoustic energy and with its characteristically short pulse length and broad bandwidth it is an attractive sub‐bottom profiler, especially in shallow fresh or salt water.
ELASTIC WAVE PROPAGATION IN MEDIA WITH PARALLEL FRACTURES AND ALIGNED CRACKS1
Authors: SCHOENBERG M. , DOUMA J.
Published: April 2006

A model of parallel slip interfaces simulates the behaviour of a fracture system composed of large, closely spaced, aligned joints. The model admits any fracture system anisotropy: triclinic (the most general), monoclinic, orthorhombic or transversely isotropic, and this is specified by the form of the 3 × 3 fracture system compliance matrix. The fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy. To compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 × 3 matrices. When the fractures are assumed on average to have rotational symmetry (transversely isotropic fracture system behaviour) and the background is assumed isotropic, the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters (in addition to the two isotropic parameters of the isotropic background). Dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints, i.e. there are two additional parameters due to the presence of the microcracks which play roles in the stress‐strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints. Thus, knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation. As the aligned microcracks, which were assumed to be ellipsoidal, with very small aspect ratio are allowed to become non‐fiat, i.e. have a growing aspect ratio, the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks. The divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approaching 0.3.
ARTEFACTS IN ZERO‐OFFSET MIGRATION1
Authors: MÆLAND E.
Published: April 2006

Migration of zero‐offset data from a point diffractor gives an elliptic or hyperbolic ‘smile’ when the velocity is too high or too low, respectively (over‐ and undermigration).

The phase‐shift within the smile is positive (approximately +π/4) when the velocity is too high, and negative (approximately –π/4) when the velocity is too low. The phase‐shift is calculated from the stationary phase approximation of the Kirchhoff integral.
ELASTIC MODELLING AND ITS APPLICATION TO AMPLITUDE VERSUS ANGLE INTERPRETATION1
Authors: KINDELAN M. , SERIANI G. , SGUAZZERO P.
Published: April 2006

Abstract

For the correct interpretation of data gathered in the seismic prospecting of complex heterogeneous structures, elastic effects must often be taken into consideration. The use of the elastic wave equations to model the seismic response of an hypothesized geological structure is a valuable tool for relating observed seismic data to the earth's inhomogeneities and verify an interpretation.

Several methods may be used to integrate numerically the partial differential equations describing elastic wave propagation. Pseudospectral (Fourier) methods represent the leading numerical integration technique. Their main advantage is high accuracy and suitability to vector and parallel computer architectures, while their main drawback is high computational cost. However, for a given accuracy, the required grid size with pseudospectral methods is smaller than that required by finite‐difference schemes, thus balancing the computational cost. We describe a two‐dimensional pseudospectral elastic model implemented on the vector multiprocessor IBM 3090 VF. The algorithm has been suitably adapted to fully exploit the computer architecture and thereby maximize the performance.

The elastic model has been validated in a variety of problems in geophysics and, in particular, in the amplitude‐versus‐offset analysis which has proved to be an effective technique to extract additional information from the recorded (prestack) data. With proper conditioning and processing of seismic data, and separating amplitude variations due to changes in reflectivity from variations due to other effects, the resulting offset signatures have been successfully used, for instance, to distinguish true bright spots due to gas‐bearing sands, from false ones associated with lithological changes. To interpret the observed amplitude‐versus‐offset signatures, it is necessary to know the reflection coefficients as a function of angle and frequency for planar interfaces, as well as for other structures of geological interest.

The modelling is first validated by computing the reflection coefficients for planar interfaces, and then used to analyse the reflection signatures of thin beds, corrugated interfaces and multilayers. Their implications, as well as impact on amplitude‐versus‐offset analysis, are discussed. We conclude that elastic modelling is an effective and valuable tool to further our understanding of the amplitude anomalies observed in field data.
EFFECTIVENESS OF WIDE MARINE SEISMIC SOURCE ARRAYS1
Authors: LYNN W. , LARNER K.
Published: April 2006

Abstract

In recent years, the use of wide source arrays in marine seismic surveys has been a topic of interest in the seismic industry. Although one motivation for wide arrays is to get more guns in a source array without increasing the in‐line array dimension, wide arrays can also provide the benefit of suppressing side‐scattered energy. Comparisons of common midpoint (CMP) stacks of data acquired offshore Washington and Alaska with wide and conventional‐width source arrays, however, show only small and sometimes inconsistent differences. These data were acquired in areas where side‐scattered energy is a problem. Comparisons of pre‐stack data, however, show substantial differences between the wide and conventional source array data.

The disparity between the stacked and prestack data is explained by analysing the effective suppression of back‐scattered energy by CMP stacking. Energy reflected from scatterer positions broadside to a given CMP location has a lower stacking velocity than that of the primary reflection events. Thus, CMP stacking attenuates the side‐scattered energy. In both survey areas the action of CMP stacking was so powerful in suppressing the broadside energy that the additional action of the wide array was inconsequential in the final stacked sections. In other areas, where the scattering velocity is comparable to the primary stacking velocity, wide arrays could provide considerable advantage.

Even though CMP stacked data from wide and conventional‐width arrays may appear similar, the reduced amount of side‐scattered energy in wide‐array prestack data may provide a benefit for data dependent processes such as predictive deconvolution and velocity analysis. However, wide arrays cannot be used indiscriminately because they can degrade cross‐dipping primary events. They should be considered primarily as a special tool for attacking severe source‐generated noise from back‐scattered waves in areas where the action of CMP stacking is insufficient.
IMPROVEMENT OF MULTICHANNEL SEISMIC DATA THROUGH APPLICATION OF THE MEDIAN CONCEPT1
Authors: NÆSS O. E. , BRULAND L.
Published: April 2006

Abstract

Different types of median‐based methods can be used to improve multichannel seismic data, particularly at the stacking stage in processing. Different applications of the median concept are described and discussed. The most direct application is the Simple Median Stack (SMS), i.e. to use as output the median value of the input amplitudes at each reflection time. By the Alpha‐Trimmed Mean (ATM) method it is possible to exclude an optional amount of the input amplitudes that differ most from the median value. A more novel use of the median concept is the Weighted Median Stack (WMS). This method is based on a long‐gapped median filter. The implicit weighting, which is purely statistical in nature, is due to the edge effects that occur when the gapped filter is applied. By shifting the traces around before filtering, the maximum weight may be given to, for example, the far‐offset traces. The fourth method is the Iterative Median Stack (IMS). This method, which also includes a strong element of weighting, consists of a repeated use of a gapped median filter combined with a gradual shortening of the filter after each pass. Examples show how the seismic data can benefit from the application of these methods.
A STABLE AND FLEXIBLE PROCEDURE FOR THE INVERSE MODELLING OF SEISMIC FIRST ARRIVALS1
Authors: OLSEN KIM BAK
Published: April 2006

ABSTRaCT

Multiple coverage reflection seismic data provide an important source of information concerning the subsurface. However, due to the stacking and migration techniques used in the processing, the first arrivals are muted and details about the upper part of the sections are generally lost.

This paper describes a computerized method for the inverse modelling of laterally varying velocities and shallow depths which are not sufficiently resolved in the reflection seismic processing. The method minimizes, in a least‐squares manner, the difference between the observed first arrivals, picked from the reflection traces, and a set of synthetic traveltimes, calculated by ray tracing in a cell model. An initial model, e.g. from a priori knowledge or the application of a conventional interpretation method, is refined iteratively until no further essential improvement can be achieved. Traditional first‐arrival inversion methods cannot, in general, provide such flexible modelling. The technique is successfully tested on synthetic data as well as on first arrivals picked automatically from the records of a reflection seismic survey in North Jutland, Denmark.
BLOCKING SURFACE WAVES BY A CUT PHYSICAL SEISMIC MODEL RESULTS1
Authors: PANT D. R. , GREENHALGH S. A.
Published: April 2006

High resolution seismic reflection exploration for minerals places severe demands on field practice so as to maximize the signal‐to‐noise bandwidth. In particular, all horizontally propagating coherent noise, especially ground roll, must be attenuated.

The blocking effect of a trench between source and receiver has been investigated by means of two‐dimensional physical seismic model experiments. Rectangular, circular and wedge‐shaped saw‐cuts of various dimensions were studied. The results show that thin rectangular cuts of depth equal to one‐quarter of the Rayleigh wave noise wavelength produce a 12 dB or better improvement in the signal‐to‐noise ratio. Rayleigh wave attenuation is greater than 30 dB at a cut depth of one wavelength. In the field applications envisaged, this corresponds to trenches up to a few metres deep. The trenches should be filled with foam or loose sand to dampen out mode conversion and diffraction noise. There are obvious practical difficulties of implementing such a technique in routine CMP operations.

The technical effectiveness of the saw‐cut is illustrated by imaging a deeply‐buried small hole (diffractor) in an aluminium plate. Without the saw‐cut between source and receiver, the seismic record is dominated by Rayleigh wave noise, masking P‐wave arrivals from the target diffractor. However, with a saw‐cut of depth three‐quarters of a Rayleigh wave wavelength, the improvement is dramatic, making it easy to detect and identify the hole. When scaled to the field situation, this is equivalent to imaging a 6 m tunnel at a depth of 400 m, using a surface trench of depth 2 m to block ground roll.
SYNTHETIC APERTURE RADAR:A NEW APPLICATION FOR WAVE EQUATION TECHNIQUES1
Authors: ROCCA F. , CAFFORIO C. , PRATI C.
Published: April 2006

The techniques of downward continuation and imaging invented for seismic waves can be applied to other types of waves. We show how they can be applied to electromagnetic surveys conducted with Synthetic Aperture Radar (SAR). The algorithms used closely follow those used for seismic waves. Differences are induced by alternate wavelengths, wave velocities, distances between sources and reflectors, etc. We analyse in detail a survey carried out using a satellite; difficulties arise from because the orbit of the satellite cannot be approximated by a simple straight line if the spatial resolution of the survey is high. We determine appropriate techniques for the correction of the distortion induced by the latter and we delimit the resolution of the observed data, as seen from a satellite. Finally we show examples of the application of the technique of seismic migration to satellite data that were irradiated to Earth during the short but productive life of Seasat.
THE REPRESENTABILITY OF CRACKED MEDIA BY PERIODICALLY LAYERED MEDIA1
Authors: DOUMA JAN
Published: April 2006

Abstract

Media containing aligned cracks or ellipsoidal inclusions as well as media consisting of sequences of isotropic layers show transverse isotropy with respect to elastic wave propagation. However, the transversely isotropic media which are equivalent to media containing aligned inclusions do not necessarily have to be representable by sequences of stable isotropic layers. These transversely isotropic media can be modelled by such sequences if ‐ and only if ‐ several stability conditions are satisfied. Important parameters determining whether these conditions are satisfied are the aspect ratio of the inclusions and the material filling the inclusions, the‘fluid’. An analytical expression describing the range of aspect ratios for which the constraints are satisfied can be derived. This expression (which is a good approximation for several crack models) and numerical calculations show that media containing water‐filled inclusions can be represented by sequences of stable isotropic layers if the inclusions have aspect ratios less than 0.1. The limiting aspect ratio decreases for a decreasing ratio of the bulk modulus of the fluid to the shear modulus of the matrix material. Finally, media containing dry inclusions of any aspect ratio cannot be modelled by thin isotropic layering. These results depend only weakly on the crack density and on the matrix material. The representation of crack‐induced anisotropy by layer‐induced anisotropy can be used to classify crack‐induced anisotropy and might be useful in the separation of the cause of anisotropy and the determination of the nature of the fluid.
AUTOMATIC EDITING OF NOISY SEISMIC DATA1
Authors: ANDERSON RICHARD G. , McMECHAN GEORGE A.
Published: April 2006

Seismic data often contain traces that are dominated by noise; these traces should be removed (edited) before multichannel filtering or stacking. Noise bursts and spikes should be edited before single channel filtering. Spikes can be edited using a running median filter with a threshold; noise bursts can be edited by comparing the amplitudes of each trace to those of traces that are nearby in offset‐common midpoint space. Relative amplitude decay rates of traces are diagnostic of their signal‐to‐noise (S/N) ratios and can be used to define trace editing criteria. The relative amplitude decay rate is calculated by comparing the time‐gated trace amplitudes to a control function that is the median trace amplitude as a function of time, offset, and common midpoint. The editing threshold is set using a data‐adaptive procedure that analyses a histogram of the amplitude decay rates.

A performance evaluation shows that the algorithm makes slightly fewer incorrect trace editing decisions than human editors. The procedure for threshold setting achieves a good balance between preserving the fold of the data and removing the noisiest traces. Tests using a synthetic seismic line show that the relative amplitude decay rates are diagnostic of the traces’S/N ratios. However, the S/N ratios cannot be accurately usefully estimated at the start of processing, where noisy‐trace editing is most needed; this is the fundamental limit to the accuracy of noisy trace editing.

When trace equalization is omitted from the processing flow (as in amplitude‐versus‐offset analysis), precise noisy‐trace editing is critical. The S/N ratio of the stack is more sensitive to type 2 errors (failing to reject noisy traces) than it is to type 1 errors (rejecting good traces). However, as the fold of the data decreases, the S/N ratio of the stack becomes increasingly sensitive to type 1 errors.
EFFICIENT MULTICHANNEL FILTERING OF SEISMIC DATA1
Authors: ÖZDEMIR HÜSEYIN , SAATÇILAR RUHI
Published: April 2006

Abstract

Multichannel filters are used to eliminate coherent noise from surface seismic data, for wavefield separation from VSP stacks, and for signal enhancement. Their success generally depends on the choice of the filter parameters and the domain of application. Multichannel filters can be applied to shots (monitors), common‐receiver traces, CDP traces and stacked sections. Cascaded applications in these domains are currently performed in the seismic industry for better noise suppression and for signal enhancement. One‐step shot‐domain filtering is adequate for some applications. However, in practice, cascaded applications in shot‐and common‐receiver domains usually give better results when the S/N ratio is low. Multichannel filtering after stacking (especially after repeated applications in shot and/or receiver domains) may create undesirable results such as artificial continuations, or smearing and smoothing of small features such as small throw faults and fine stratigraphic details. Consequently, multichannel filtering after stacking must be undertaken with the utmost care and occasionally only as a last resort.

Multichannel filters with fan‐shaped responses (linear moveout filters) should be applied after NMO correction. These are the filters commonly used in the seismic industry where they have such names as velocity filters, moveout filters, f‐k filters and coherency filters. Filtering before NMO correction may result in break‐up and flattening especially of those shallow reflection events with relatively higher curvatures and diffractions. NMO correction is needed prior to wavefield separation from VSP stacks for the same practical reasons outlined above whenever source‐receiver offsets are involved.

Creation of artificial lineup and smearing at the outputs of multichannel filters is presently the common practical concern. Optimum multichannel filters with well‐defined pass, reject and transition bands overcome the latter problems when applied before stacking and after NMO correction. The trace dimension of these filters must be kept small to avoid such lineups and the smoothing of small structures. Good results can be obtained with only five traces, but seven traces seems to be a better compromise both in surface and well seismic applications. The so‐called f‐k filtering and τ‐p domain filtering are no exceptions to the above practical considerations.

Residual static computations after multichannel filtering also need special consideration. Since multichannel filtering improves spatial continuity, residual static algorithms using local correlation, i.e. nonsurface‐consistent algorithms, may be impractical especially after multichannel filtering.
RAYS AND WAVEFRONT CHARTS IN GRADIENT MEDIA1
Authors: HELBIG KLAUS
Published: April 2006

Abstract

Wavefront charts in anisotropic gradient media are a useful tool in ray geometric constructions, particular in shear‐wave exploration. They can be constructed by: (i) a family of wavefronts that contains a vertical plane as member ‐ it is convenient to choose constant time increments; (ii) tracing one ray that makes everywhere the angle with the normal to the wavefront that is required by the anisotropy of the medium; (iii) scaling this ray to obtain a set of rays with different ray parameters; (iv) shifting these rays (with wavefront elements attached) so that they pass through a common source point; (v) interpolating the wavefronts between the elements.

The construction is particularly simple in linear‐gradient media, since here all members of the family of wavefronts are planes. Since the ray makes everywhere the angle prescribed by the anisotropy with the normal of the (plane) wavefronts, the ray has the shape of the slowness curve rotated by −π/2.

For isotropic media the slowness curve is a circle, and thus rays are circular arcs. The circles themselves intersect in the source point and in a second point above the surface of the earth. This provides a simple proof that wavefronts emanating from a point source in an isotropic linear‐gradient medium are spheres: inversion of the set of circular rays with the source as centre maps the pencil of circular rays into a pencil of straight lines passing through a point. A pencil of concentric spheres around this point is perpendicular to the pencil of straight lines. On inverting back the pencil of spheres is mapped into another pencil of spheres that is perpendicular to the circular rays.
PROCESSING OF PS‐REFLECTION DATA APPLYING A COMMON CONVERSION‐POINT STACKING TECHNIQUE1
Authors: TESSMER GISA , KRAJEWSKI PAUL , FERTIG JÜRGEN , BEHLE ALFRED
Published: April 2006

Abstract

Converted waves require special data processing as the wave paths are asymmetrical. The CMP concept is not applicable for converted PS waves, instead a sorting algorithm for a common conversion point (CCP) has to be applied. The coordinates of the conversion points in a single homogeneous layer can be calculated as a function of the offset, the reflector depth and the velocity ratio vP/ vs. For multilayered media, an approximation for the coordinates of the conversion points can be made.

Numerical tests show that the traveltime of PS reflections can be approximated with sufficient accuracy for a certain offset range by a two‐term series truncation. Therefore NMO corrections can be calculated by standard routines which use the hyperbolic approximation of the reflection traveltime curves.

The CCP‐stacking technique has been applied to field data which have been generated by three vertical vibrators. The in‐line horizontal components have been recorded. The static corrections have been estimated from additional P‐ and SH‐wave measurements for the source and geophone locations, respectively. The data quality has been improved by processes such as spectral balancing.

A comparison with the stacked results of the corresponding P‐ and SH‐wavefield surveys shows a good coherency of structural features in P‐, SH‐ and PS‐time sections.
INVERSION OF REFRACTED ARRIVALS: A FEW PROBLEMS1
Authors: ZANZI LUIGI
Published: April 2006

Abstract

The aim of refracted arrivals inversion is the computation of near‐surface information, i.e. first‐layer thicknesses and refractor velocities, in order to estimate the initial static corrections for the seismic data. The present trend is moving towards totally automatic inversion techniques, which start by picking the first breaks and end by aligning the seismic traces at the datum plane.

Accuracy and computational time savings are necessary requirements. These are not straightforward, because accuracy means noise immunity, which implies the processing of large amounts of data to take advantage of redundancy; moreover, owing to the non‐linearity of the problem, accuracy also means high‐order modelling and, as a consequence, complex algorithms for making the inversion.

The available methods are considered here with respect to the expected accuracy, i.e. to the model they assume. It is shown that the inversion of the refracted arrivals with a linear model leads to an ill‐conditioned problem with the result that complete separation between the weathering thickness and the refractor velocity is not possible. This ambiguity is carefully analysed both in the spatial domain and in the wavenumber domain. An error analysis is then conducted with respect to the models and to the survey configurations that are used.

Tests on synthetic data sets validate the theories and also give an idea of the magnitude of the error. This is largely dependent on the structure; here quantitative analysis is extended up to second derivative effects, whereas up to now seismic literature has only dealt with first derivatives. The topographical conditions which render the traditional techniques incorrect are investigated and predicted by the error equations.

Improved solutions, based on more accurate models, are then considered: the advantages of the Generalized Reciprocal Method are demonstrated by applying the results of the error analysis to it, and the accuracy of the non‐linear methods is discussed with respect to the interpolation technique which they adopt. Finally, a two‐step procedure, consisting of a linear model inversion followed by a local non‐linear correction, is suggested as a good compromise between accuracy and computational speed.
A PHYSICAL SCALE MODEL STUDY OF THE COMPARATIVE PERFORMANCE OF TWO MODES OF OPERATION FOR FIXED‐LOOP TURAM‐TYPE EM SYSTEMS1
Authors: DUCKWORTH K. , CUMMINS C.
Published: April 2006

Abstract

The results of a physical scale model study of the conventional mode of operation of fixed loop electromagnetic systems and an alternative mode called the tx‐parallel mode in which traverses are run parallel to the long axis of the rectangular transmitter loop are presented. The results show that over thick or dipping conductors, the tx‐parallel configuration provides coupling with the target which is comparable with that provided by the conventional configuration. In addition, the tx‐parallel configuration is shown to provide more consistent indications of the direction and magnitude of conductor dip. Over wide conductors, where separate conventional surveys are needed to define the opposite edges of such conductors, it is shown that only a single tx‐parallel survey is needed to locate both edges of the conductor.

The tx‐parallel results were found to allow better resolution of the individual anomalies due to closely spaced parallel conductors.

The tx‐parallel response of identical sheet conductors of opposite dip indicated that the response of the separate sheets could be recognized even when the two sheets were placed at zero separation. This was found to be due in part to spatial displacement of the individual current vortices within each conductor owing to their mutual repulsion.
DECOMPOSITION OF MULTICOMPONENT SEISMIC DATA INTO PRIMARY P‐ AND S‐WAVE RESPONSES1
Authors: WAPENAAR C. P. A. , HERRMANN P. , VERSCHUUR D. J. , BERKHOUT A. J.
Published: April 2006

Abstract

Inversion of multicomponent seismic data can be subdivided in three main processes: (1) Surface‐related preprocessing (decomposition of the multicomponent data into ‘primary’ P‐and S‐wave responses). (2) Prestack migration of the primary P‐ and S‐wave responses, yielding the (angle‐dependent) P‐P, P‐S, S‐P and S‐S reflectivity of the subsurface. (3) Target‐related post‐processing (transformation of the reflectivity into the rock and pore parameters in the target). This paper deals with the theoretical aspects of surface‐related preprocessing.

In a multicomponent seismic data set the P‐ and S‐wave responses of the subsurface are distorted by two main causes: (1) The seismic vibrators always radiate a mixture of P‐ and S‐waves into the subsurface. Similarly, the geophones always measure a mixture of P‐ and S‐waves. (2) The free surface reflects any upgoing wave fully back into the subsurface. This gives rise to strong multiple reflections, including conversions.

Therefore, surface‐related preprocessing consists of two steps: (1)Decomposition of the multicomponent data (pseudo P‐ and S‐wave responses) into true P‐ and S‐wave responses. In practice this procedure involves (a) decomposition per common shot record of the particle velocity vector into scalar upgoing P‐ and S‐waves, followed by (b) decomposition per common receiver record of the traction vector into scalar downgoing P‐ and S‐waves. (2) Elimination of the surface‐related multiple reflections and conversions. In this procedure the free surface is replaced by a reflection‐free surface. The effect is that we obtain ‘primary’ P‐and S‐wave responses, that contain internal multiples only.

An interesting aspect of the procedure is that no knowledge of the subsurface is required. In fact, the subsurface may have any degree of complexity. Both the decomposition step and the multiple elimination step are fully determined by the medium parameters at the free surface only. After surface‐related preprocessing, the scalar P‐ and S‐wave responses can be further processed independently by existing scalar algorithms.
ENHANCED MIGRATION OF SEISMIC DATA1
Authors: CARRION PHILIP M.
Published: April 2006

Abstract

The so‐called ‘enhanced migration’ which uses diffraction tomography as the ‘repair tool’ for correction of amplitudes (reflection coefficients) of migrated sections is discussed. As with any linearized procedure, diffraction tomography requires knowledge of the initial model. It is suggested that the initial model is taken as the migrated image. It will be demonstrated that diffraction tomography applied to the data residuals improves the amplitudes of the migrated images. Migration is redefined as the reconstruction of the wavefront sets of distributions (reflection interfaces), and the inversion process as tomographic correction of migrated images.
NUMERICAL MODELLING OF STANDARD AND CONTINUOUS VERTICAL ELECTRICAL SOUNDINGS1
Authors: MOLANO C. E. , SALAMANCA M. , VAN OVERMEEREN R. A.
Published: April 2006

Abstract

Analytical solutions of vertical electrical soundings (VES) have mostly been applied to groundwater exploration and monitoring groundwater quality on terrains of fairly simple geology and geomorphology on which the electrode arrays are symmetrical (e.g. Schlumberger or Wenner configurations). The sounding interpretation assumes flat topography and horizontally stratified layers. Any deviations from these simple situations may be impossible to interpret analytically. The recently developed GEA‐58 geoelectrical instrument can make continuous soundings along a profile with any colinear electrode configuration.

This paper describes the use of finite‐difference and finite‐element methods to model complex earth resistivity distributions in 2D, in order to calculate apparent resistivity responses to any colinear current electrode distribution in terrains in which the earth resistivities do not vary along the strike. The numerical model results for simple situations are compared with the analytical solutions. In addition, a pseudo‐depth section of apparent resistivities measured in the field with the GEA‐58 is compared with the numerical solution of a real complex resistivity distribution along a cross‐section. The model results show excellent agreement with the corresponding analytical and experimental data.
MEASURED UNDERWATER NEAR‐FIELD E‐PATTERNS OF A PULSED, HORIZONTAL DIPOLE ANTENNA IN AIR: COMPARISON WITH THE THEORY OF THE CONTINUOUS WAVE, INFINITESIMAL ELECTRIC DIPOLE1
Authors: WENSINK W. A. , GREEUW G. , HOFMAN J. , VAN DEEN J. K.
Published: April 2006

Abstract

At Delft Geotechnics the technique of ground‐penetrating radar is in use for the detection of buried objects such as pipes. To enable us to give our ‘measurements in the field’ a more quantitative interpretation than can be deduced from these alone, a series of experiments has been started under well‐defined conditions. A cylindrical vessel containing water simulates wet soil. Mounted horizontally above the water surface is a pulsed triangular half‐wave dipole which is used as a transmitting antenna (TA). It has a carrier‐frequency of about 160 MHz and a pulse repetition‐frequency of about 50 kHz.

A movable receiving dipole (‘probe’) in the water measures the transverse, mutually orthogonal Eφ,‐ and Eθ‐components of the pulses as a function of probe‐position (r, θ, φ) and of the height h of the TA above the water surface. When these pulses are Fourier‐transformed, the transverse electric fields Eφ and Eθ at 200 MHz are obtained. The resulting field patterns are compared with computational results on the basis of the theory of the continuous wave, infinitesimal electric dipole (‘point dipole’). It can be concluded that:

Far‐field conditions have not fully developed at a depth of about 2.50 m, the largest value of the radius r at which field patterns were measured, although it represents a distance of about 15 wavelengths.

The attenuation constant of the tapwater used, as deduced from E‐field measurements for θ= 0, 2.50 m < r < 2.75 m, is slightly less than the value measured using a network analyser and air line combination, in agreement with (1).

E
φ field patterns calculated using the value of the conductivity σ corresponding to the former value of the attenuation constant agree reasonably well with the measured patterns for r≤ 2.50 m and for θ < 20° at all antenna heights considered. Calculated Eφ patterns do not agree so well with the measured patterns when h is close to zero. With increasing height the agreement inproves.

In accordance with the theory of the point‐dipole, the angular distribution of the radiation patterns of the TA becomes wider as the frequency decreases.

The normalized underwater pulse‐spectra shift to lower frequencies with increasing r. This can be explained since the attenuation constant of the water rises with rising frequency.
THE USE OF MARINE SEISMIC PROFILING FOR ENVIRONMENTAL ASSESSMENT1
Authors: McGEE T. M.
Published: April 2006

Abstract

One application of marine seismic profiling that has an especially auspicious future is the delineation, and possible identification, of anomalously reflective material accumulated in the sediments of rivers, bays and estuaries. Analogue field data are presented which illustrate that such sediments are associated with strong sequences of multiple reflections. The rate of decay of these sequences indicates the magnitude of the reflection coefficients and the relative polarity of wavelets indicates whether they are positive or negative. If zero‐offset field geometry and digital recording are used, it is possible to compare wavelets within a sequence and thereby determine acoustic impedance contrasts from which sediment density can be inferred. If the source is sufficiently broadband, it becomes possible to determine absorption characteristics as well. Two principal problems arise when analysing multiple sequences; adequate data acquisition and suitable geometric corrections. Having solved these, this type of analysis could lead to the seismic identification of sea‐floor materials. Since anomalous material is often associated with effluent, the method can become an important tool in the growing effort to monitor and improve environmental quality.
SEISMIC MODELLING WITH CHANNEL WAVES IN SEAM STRUCTURES INFLUENCED BY MYLONITE ZONES1
Authors: GELDMACHER I. M. , DRESEN L. , STÜRZNICKEL T.
Published: April 2006

Abstract

Channel waves generated in coal‐seams and their reflections from discontinuities are widely used to indicate the tectonic and stratigraphic features of coal deposits, resulting in greater efficiency and safety in coal‐mining. In the mining area of Ibbenbüren (F.R.G.) seam structures sometimes contain so‐called mylonite zones, which are crushed coal deposits capable of binding gas. If mining hits a mylonite zone this would probably not only reduce output of the mine, but could even cause gas explosions. To investigate the influence of a mylonite zone on the propagation of channel waves, Rayleigh channel wave measurements for 2D analogue models were performed and synthetic seismograms of Love channel waves were calculated.

Analogue modelling of the mylonite zone using Rayleigh seam waves within the seam was carried out using a perforation technique. Calculations were made to obtain an estimate of velocity reduction due to perforation. The results agree well with velocity values measured up to a perforation of 25% in a 2D epoxy resin model. Reflected channel wave energy was found by applying dispersion analysis in the case where the impedance reduction between the mylonite seam structure and the undisturbed seam was approximately 5%. The horizontal width of the mylonite structure was detectable from the time lag between reflected channel wave signals from both in‐seam borders of the mylonite zone. Resolution of two discrete borders was possible for a width of 1.5 λ's. The influence of a vertical fault, positioned within the mylonite zone, could only poorly be resolved.

Numerical model investigations of Love seam waves were concerned mainly with the variation of the horizontal width of the mylonite zone. Mylonite zones with dimensions of the order of 0.4 λ's allow definite statements about their widths from dispersion and spectral analyses. For zones with smaller widths down to 0.2 λ's, it was found that reflectivity and transmissivity analyses give a qualitative criterion for distinguishing a mylonite structure surrounding a fault from a pure fault.
AN ANALYTICAL RAYPATH APPROACH TO THE REFRACTION WAVEFRONT METHOD1
Authors: AK M. ALI
Published: April 2006

Abstract

Seismic refraction surveying is still an important tool for determining the geometries and elastic wave propagation velocities of near‐surface layers. Many analytical and graphical methods have been developed over the years for refraction interpretation, and these can be classified into two basic groups. The first group visualizes critically refracted rays converging on a common surface position, while the second group, which includes the wavefront methods, makes use of the critical rays emerging from a common point on the refractor.

The method described in this paper is an analytical approach to the wavefront methods. The reverse refracted ray received by a geophone is intersected by the forward refracted rays received by subsequent geophones and a common critical refraction point on the refractor is estimated after a series of comparisons. This process is repeated for each geophone to yield the geometry and the velocity of the refractor. Several interpolations are performed to achieve a better accuracy.

Palmer's models are used to test the efficiency of the algorithm. The results are presented together with those of other methods applied to the same models.
THE DETERMINATION OF THE SEISMIC QUALITY FACTOR Q FROM VSP DATA: A COMPARISON OF DIFFERENT COMPUTATIONAL METHODS1
Authors: TONN RAINER
Published: April 2006

Abstract

Ten methods for the computation of attenuation have been investigated, namely: amplitude decay, analytical signal, wavelet modelling, phase modelling, frequency modelling, rise‐time, pulse amplitude, matching technique, spectral modelling and spectral ratio. In particular, we have studied the reliability of each of these methods in estimating correct values ofQusing three synthetic VSP seismograms for plane P‐waves with different noise contents.

The investigations proved that no single method is generally superior. Rather, some methods are more suitable than others in specific situations depending on recording, noise or geology. The analytical signal method has been demonstrated to be superior if true amplitude recordings are available. Otherwise spectral modelling or, in the ‘ noise‐free’ case the spectral ratio method, is optimal.

Finally, two field VSPs in sediments are investigated. Only in the case of the highest quality VSP can significant information be deduced from the computed attenuation.
SEISMO‐ELECTRIC EXPLORATION: EXPECTED SIGNAL AMPLITUDES1
Authors: RUSSELL R. D. , BARKER A. S.
Published: April 2006

Abstract

For more than 20 years, Soviet scientists have published papers and registered patents describing the conversion of seismic to electromagnetic energy in geological environments and the detection of the electric or magnetic signals as a method of geophysical exploration. Because of the potential importance of a reliable geophysical technique for locating quartz veins, we have been conducting extensive laboratory and field tests of the phenomena.

For the purposes of designing appropriate field tests we need to know approximate signal magnitudes, but little has been published on them. The present paper describes a simplified model from which order‐of‐magnitude estimates of expected electric and magnetic signal strengths can be made with sufficient accuracy for such purposes. For mathematical convenience we model the target as a homogeneous sphere in which the seismic input induces uniform, time‐varying electric polarization. More realistic configurations can be described by linear superposition of the potentials of appropriate sub‐elements.

True piezo‐electricity is, by definition, linear. Therefore, responses should have the same frequency content as the seismic input. Combining the low‐frequency form of our results with the assumption that the entire thickness of a target vein responds in phase to a seismic excitation, we obtain the following estimates of the maximum electric and magnetic fields at a distance r from the target:
image

where p is the resistivity and μ0 the dielectric constant of the ground in which E is measured, V is the effective volume of the target, P is the polarization of the vein, s is the seismic stress at the target, and a is the effective piezo‐electric coefficient. Signals observed in experiments at the Erickson Mine, British Columbia, gave electric fields in acceptable agreement with our theoretical predictions.

We conclude by considering plausible relationships for the high‐frequency signals observed from sulfide minerals, assuming that they represent the release of stored stress triggered by the seismic arrival.
τ‐p SEISMIC DATA FOR VISCOELASTIC MEDIA – PART 1: MODELLING1
Authors: MARTINEZ RUBEN D. , McMECHAN GEORGE A.
Published: April 2006

Abstract

Viscoelastic modelling reveals that the interaction of compressional‐wave velocity Cp, compressional‐wave quality factor Qp, shear‐wave velocity Cs, shear‐wave quality factor Qs and Poisson's ratio as a function of time intercept τ and ray parameter p, is complicated; however, distinct, potentially diagnostic behaviours are seen for different combinations of viscoelastic parameters.

Synthetic seismograms for three viscoelastic reservoir models show that variations in the Poisson's ratio produce visible differences when compared to the corresponding elastic synthetic seismograms; these differences are attributable to interaction of the elastic parameters with Qp and Qs.

When the P‐wave acoustic impedance contrast is small, viscoelastic effects become more apparent and more useful for interpretation purposes. The corresponding amplitude and net phase spectra reveal significant differences between the elastic and the viscoelastic responses. When P‐wave reflectivities are large, they tend to dominate the total response and to mask the Q reflectivity effects. The attenuation effects are manifested as an amplitude decay that increases with both time and ray parameter.

The sensitivity of the computed seismic responses for various combinations of viscoelastic parameters suggests the opportunity for diagnostic interpretation of τ‐p seismic data. The interpretation of the viscoelastic parameters can permit a better understanding of the rock types and pore fluid distribution existing in the subsurface.
EXTENSIVE‐DILATANCY ANISOTROPY: RELATIVE INFORMATION IN VSPs AND REFLECTION SURVEYS1
Authors: YARDLEY GARETH S. , CRAMPIN STUART
Published: April 2006

Abstract

Shear‐waves have complicated interactions with the free surface, particularly in the presence of low‐velocity surface layers, topographic irregularities, and the expected near‐surface crack and stress anomalies. Consequently, it has been suggested that shear‐waves should be recorded subsurface in vertical seismic profiles (VSPs), in order to extract accurate information about the in situ crack and stress geometry contained in shear‐wave splitting. This paper compares the information in synthetic shear‐waves in reflection gathers and VSPs, in order to assess the relative merits of the two techniques for investigating shear‐wave splitting. Synthetic seismograms demonstrate that in the presence of even very simple surface layers, shear‐waves recorded in reflection surveys at the surface have polarizations which may not indicate crack and stress geometry at depth. In contrast, shear‐waves recorded in VSPs are relatively unaffected by surface layers and near‐surface stress and crack anomalies, and the behaviour of shear‐wave splitting is dominated by the structure of the rock mass in the vicinity of subsurface geophones. Matrix rotations of multicomponent‐multisource shear‐wave reflection data to extract the information contained in the split shear‐waves, are found to be directly meaningful only in situations where crack orientations do not change with depth.
TUNNELLING AND THE GENERALIZED RAY METHOD IN PIECEWISE HOMOGENEOUS MEDIA1
Authors: DRIJKONINGEN GUY G.
Published: April 2006

Abstract

The contributions of the tunnelled constituents to a seismic wave are analysed in two different configurations pertaining to homogeneous acoustic media: a thin high‐velocity layer, present in a plane‐layered configuration, and a thin layer in media separated by dipping interfaces. The generalized ray method in the far‐field is used to determine them. We expand around the relevant ray parameters in order to determine the characteristics of the tunnelling ray and find that the most important feature of this type of ray is a phase (in terms of asymptotic ray theory) which has a real and an imaginary part. Numerical results illustrate this.
TOMOGRAPHIC INVERSION OF NORMALIZED DATA: DOUBLE‐TRACE TOMOGRAPHY ALGORITHMS1
Authors: DOBROKA MICHAEL , DRESEN LOTHAR , GELBKE CHRISTOPH , RÜTER HORST
Published: April 2006

Abstract

Tomography is widely used in geophysics as a technique for imaging geological structures by means of data that are line integrals of physical characteristics. In some transmission measurements, due to various kinds of normalization, the measured data are related to two (the current and the reference) raypaths and can be expressed as a function of differences between line integrals. This is the case, for example, in seismo‐acoustic emission measurements, when (since the exact start time is unknown) only the differences between traveltimes (differences between line integrals of the slowness) can be determined. Similarly the use of normalized Fourier amplitudes results in data dependent upon the difference between line integrals of the absorption coefficient (computed along the actual and the reference raypaths).

In order to invert these data the ordinary tomography algorithms should be modified. Some generalizations are presented for series expansion tomography methods in order to make them applicable to reconstruction problems in which the input data are differences between two line integrals. The conjugate gradient and the simultaneous iterative reconstruction technique (SIRT) methods were adapted and tested. It is shown that the modified tomography algorithms are stable and sufficiently accurate for practical use. In the reconstruction of noise‐free difference data, the conjugate gradient algorithm is found to be faster and more accurate while, in the case of noisy difference data, the modified SIRT algorithm is more stable and insensitive to noise.