bluuebunny/crossref_metadata_2025_split huggingface dataset:
THREE‐DIMENSIONAL KINEMATIC MIGRATION IN VARIABLE VELOCITY MEDIA*
Authors: WEN JING , McMECHAN G.A. 
Published: April 2006

ABSTRACTA three‐dimensional (3‐D) kinematic migration algorithm for media in which migration velocity varies linearly with depth is developed, implemented and tested. The algorithm is based on the concept that a single reflection or diffraction in a (zero‐ or finite‐offset) trace may have originated at any point on a constant traveltime surface within the Earth defined by the observed two‐way traveltime. The envelope of all such constant time surfaces, for all observed reflections and diffractions produced by one reflector, is the desired migrated 3‐D image. The optimal envelope position in depth is determined, beneath each point on a regular grid, by a statistical imaging condition; an incremental function of depth containing the number of constant time surfaces passing through that depth increment is cross‐correlated with a Gaussian function whose width is chosen to correspond to the vertical scale of the features of interest.The numerical procedures are based on the observation that, in a medium in which velocity varies linearly with depth, ray segments are circular so traveltimes can be computed analytically. Also, traveltimes are independent of azimuth so the 3‐D problem can be collapsed into an equivalent 2‐D problem.The algorithm is illustrated and tested by application to synthetic data and to scale‐model data from the Seismic Acoustics Laboratory at the University of Houston.
TRANSFORMATION OF DIPOLAR, WENNER AND TWO‐ELECTRODE CURVES TO SCHLUMBERGER APPARENT RESISTIVITY SOUNDING CURVES*
Authors: BANERJEE BUDDHADEB , SENGUPTA B. J. 
Published: April 2006

ABSTRACTA simple mathematical technique based on regressional analysis allows the transformation of dipolar, Wenner and two‐electrode apparent resistivity sounding curves to Schlumberger ones. The algorithm is suitable for a programmable pocket calculator and the accuracy is very high. This has been demonstrated by comparing Schlumberger master curves with transformations of master curves for the other configurations for the same model.
A SIMPLE METHOD OF INTERPRETATION OF RESISTIVITY SOUNDING DATA USING EXPONENTIAL APPROXIMATION OF THE KERNEL FUNCTION*
Authors: NIWAS SRI , ISRAIL M. 
Published: April 2006

ABSTRACTA simple unified equation of apparent resistivity for a general four‐electrode array is developed. The main idea is the analytical integration of the Stefanescu expression for potential over a layered earth by writing an exponential approximation for the kernel function. Finally a matrix equation is developed to estimate the kernel function from observed apparent resistivity values. The general equation automatically reduces to the particular configuration once the electrode separations are modified suitably. Examples for Schlumberger and Wenner configurations are numerically calculated to estimate the precision of the method. Good results in a short execution time are obtained, irrespective of the shape of the apparent resistivity curve. Finally, the full interpretation of one theoretical resistivity curve and two field resistivity curves is demonstrated. The more stable ridge‐regression estimation method is used in the identification of layer parameters from the kernel function.
GEOPHYSICAL DETECTION OF MINERAL CONDUCTORS IN TROPICAL TERRAINS WITH TARGET CONDUCTORS PARTLY EMBEDDED IN THE CONDUCTIVE OVERBURDEN*
Authors: BARONGO J.O. 
Published: April 2006

ABSTRACTGround geophysical methods were used in the greenstone belt of Western Kenya as follow‐up of an Input airborne electromagnetic survey. Observation and drilling results showed that the graphite and pyrite bodies, which constitute the main target conductors in the area, either outcrop or reach near to the ground surface. The drilling results further showed that the weathered layer is between 20 m and 60 m thick. Therefore the target conductors are partly embedded in this fairly conductive weathered layer and may be in galvanic contact with it. The geophysical methods were able to delineate the conductors. However, by virtue of the galvanic contact between the conductors and the conductive overburden, the profiles from the horizontal loop EM measurements are highly enhanced. This effect makes interpretation of the profiles possible at low frequencies but difficult at high frequencies. The interpreted EM results show that the target conductors appear shallow and less conductive at higher frequencies. Thus one must choose geophysical instruments and operating frequencies carefully to ensure good results in tropical terrains.
RESISTIVITY INVERSION WITH A PRIORI INFORMATION*
Authors: POUS J. , MARCUELLO A. , QUERALT P. 
Published: April 2006

ABSTRACTAn inversion algorithm for interpreting electrical soundings based on a probabilistic treatment of the a priori information not only includes all the previous ones, but allows consideration of the constraints between the parameters. By introducing the a priori information, a unique solution among all the equivalent ones is obtained which is coherent with the geological background. Several examples dealing with the usual problems in the automatic interpretation of the electrical soundings illustrate the advantages of this algorithm. Good results are obtained with this method.
EXAMPLES OF ELECTROMAGNETIC PROSPECTING FOR KARST AND FAULT SYSTEMS*
Authors: VOGELSANG D. 
Published: April 2006

ABSTRACTElectromagnetic (EM) surveys using the horizontal coplanar loop (‘Slingram’) geometry have been successful in the location of karst aquifer systems in limestone areas of Germany and the Mediterranean. In spite of the rapid changes in shape and resistivity of karst pipes, correlations of significant EM minima can be made by reference to the main structural directions. The EM results have often been verified by drilling. EM prospecting need not be confined to limestone areas. It can be used on all hard rocks where faulting has created vertical zones with a resistivity contrast to the undisturbed rock.EM siting of wells has proved successful in water‐supply schemes for several places. This method is now being applied as a routine, fast and low‐cost procedure to tap ground‐water resources in hard‐rock areas.
DECOMPOSITION AND INVERSION OF SEISMIC DATA–AN INSTANTANEOUS SLOWNESS APPROACH*
Authors: MILKEREIT B. 
Published: April 2006

ABSTRACTInterpretation techniques are presented that aim at the estimation of seismic velocities. The application of localized slant stacks, weighted by coherency, produces a decomposition of multichannel seismic data into single trace instantaneous slowness p(x, t) components. Colour displays support the interpretation of seismic data relevant to the near surface velocity structure. Since p(x, t) is directly related to stacking velocities and the depth of reflection, or bottoming points, in the subsurface, this data transformation provides a powerful tool for the inversion of reflection and refraction data.
THE INTERPRETATION OF TRAVELTIME FIELDS IN REFRACTION SEISMOLOGY*
Authors: BRÜCKL E. 
Published: April 2006

ABSTRACTWe consider multiply covered traveltimes of first or later arrivals which are gathered along a refraction seismic profile. The two‐dimensional distribution of these traveltimes above a coordinate frame generated by the shotpoint axis and the geophone axis or by the common midpoint axis and the offset axis is named a traveltime field.The application of the principle of reciprocity to the traveltime field implies that for each traveltime value with a negative offset there is a corresponding equal value with positive offset. In appendix A procedures are demonstrated which minimize the observational errors of traveltimes inherent in particular traveltime branches or complete common shotpoint sections.The application of the principle of parallelism to an area of the traveltime field associated with a particular refractor can be formulated as a partial differential equation corresponding to the type of the vibrating string. The solution of this equation signifies that the two‐dimensional distribution of these traveltimes may be generated by the sum of two one‐dimensional functions which depend on the shotpoint coordinate and the geophone coordinate. Physically, these two functions may be interpreted as the mean traveltime branches of the reverse and the normal shot. In appendix B procedures are described which compute these two functions from real traveltime observations by a least‐squares fit.The application of these regressed traveltime field data to known time‐to‐depth conversion methods is straightforward and more accurate and flexible than the use of individual traveltime branches. The wavefront method, the plus‐minus method, the generalized reciprocal method and a ray tracing method are considered in detail. A field example demonstrates the adjustment of regressed traveltime fields to observed traveltime data. A time‐to‐depth conversion is also demonstrated applying a ray tracing method.
WEIGHTED STACKING FOR ROCK PROPERTY ESTIMATION AND DETECTION OF GAS*
Authors: SMITH G.C. , GIDLOW P.M. 
Published: April 2006

ABSTRACTAmplitude versus offset concepts can be used to generate weighted stacking schemes (here called geo‐stack) which can be used in an otherwise standard seismic data processing sequence to display information about rock properties.The Zoeppritz equations can be simplified and several different approximations appear in the literature. They describe the variation of P‐wave reflection coefficients with the angle of incidence of a P‐wave as a function of the P‐wave velocities, the S‐wave velocities and the densities above and below an interface.Using a smooth, representative interval velocity model (from boreholes or velocity analyses) and assuming no dip, the angle of incidence can be found as a function of time and offset by iterative ray tracing. In particular, the angle of incidence can be computed for each sample in a normal moveout corrected CMP gather. The approximated Zoeppritz equation can then be fitted to the amplitudes of all the traces at each time sample of the gather, and certain rock properties can be estimated. The estimation of the rock properties is achieved by the application of time‐ and offset‐variant weights to the data samples before stacking. The properties which can be displayed by geo‐stack are: P‐wave reflectivity (or true zero‐offset reflectivity), S‐wave reflectivity, and the reflectivity of P‐wave velocity divided by S‐wave velocity (or ‘pseudo‐Poisson's ratio reflectivity’). If assumptions are made about the relation between P‐wave velocity and S‐wave velocity for water‐bearing clastic silicate rocks, then it is possible to create a display which highlights the presence of gas.
ENGINEERING GEOPHYSICAL STUDIES OF THE LOVIISA NUCLEAR POWER PLANT SITE, FINLAND*
Authors: ROUHIAINEN P.J. 
Published: April 2006

ABSTRACTField studies have been performed in the power plant area of Loviisa in southern Finland to evaluate the suitability of the local bedrock for disposal of low‐level and intermediate‐level reactor wastes. The aim of the borehole geophysical studies was to evaluate the geometry and properties of fracture zones in relatively homogeneous granite.Of the single‐hole methods, the dipmeter method was used to determine the orientation of individual fractures. The sonic log was used to evaluate the openness (width or thickness) of fractures. Cross‐hole methods such as the seismic and the mise‐à‐la‐masse method were used to determine the geometry and continuity of fractured zones between boreholes.It was, in general, impossible to evaluate the continuity of single fractures. However, fracture sets can be identified based on the dipmeter data. The continuity of fracture zones can be evaluated with the combined results of single‐ and cross‐hole methods.
FORWARD MODELLING FOR FREQUENCY DOMAIN MARINE ELECTROMAGNETIC SYSTEMS*
Authors: GOLDMAN M. 
Published: April 2006

ABSTRACTAn algorithm for the accurate evaluation of rapidly oscillating integrals is described. The method is based on deformation of the integration path into the complex plane of the integration variable. Numerical integration is then carried out along appropriate cuts where the oscillating factor is transformed to the decaying factor. Contrary to standard methods, the proposed technique permits accurate evaluation of numerically divergent integrals.The algorithm is especially useful in forward modelling for high‐frequency electromagnetic methods and, in particular, for the new marine electromagnetic system based on measuring signals on the sea bottom at high induction numbers.Results of calculations using both the proposed and standard methods are compared with available analytical solutions.
SIGNAL ADJUSTMENT OF VIBROSEIS AND IMPULSIVE SOURCE DATA*
Authors: BRÖTZ R. , MARSCHALL R. , KNECHT M. 
Published: April 2006

AbstractIn certain areas continuous Vibroseis profiling is not possible due to varying terrain conditions. Impulsive sources can be used to maintain continuous coverage. While this technique keeps the coverage at the desired level, for the processing of the actual data there is the problem of using different sources resulting in different source wavelets. In addition, the effect of the free surface is different for these two energy sources.The approach to these problems consists of a minimum‐phase transformation of the two‐sided Vibroseis data by removal of the anticipation component of the autocorrelation of the filtered sweep and a minimum‐phase transformation of the impulsive source data by replacement of the recording filter operator with its minimum‐phase correspondent. Therefore, after this transformation, both datasets show causal wavelets and a conventional deconvolution (spike or predictive) may be used. After stacking, a zero‐phase transformation can be performed resulting in traces well suited for computing pseudo‐acoustic impedance logs or for application of complex seismic trace analysis. The solution is also applicable to pure Vibroseis data, thereby eliminating the need for a special Vibroseis deconvolution.The processing steps described above are demonstrated on synthetic and actual data. The transformation operators used are two‐sided recursive (TSR) shaping filters.After application of the above adjustment procedure, remaining signal distortions can be removed by modifying only the phase spectrum or both the amplitude and phase spectra. It can be shown that an arbitrary distortion defined in the frequency domain, i.e., a distortion of the amplitude and phase spectrum, is noticeable in the time section as a two‐sided signal.
THE SPECTRAL FUNCTION OF A VERTICALLY INHOMOGENEOUS MEDIUM1
Authors: URSIN B. 
Published: April 2006

ABSTRACTElastic, acoustic and electromagnetic waves in media consisting of vertically inhomogeneous layers are considered in a common formulation. The spectral function of a vertically inhomogeneous medium is the downward energy flux due to an impulsive source at the top of the first layer. A propagation‐invariant form is used to derive several identities for the reflection and transmission matrices. When the top layer is bounded by a free surface, one of the expressions reduces to a formula derived by Kunetz for the one‐dimensional wave equation.A source radiating upwards and downwards gives a discontinuity in the propagation‐invariant form which is equal to the source energy. A new formula is derived for when the source is located just beneath the top interface of the layers.
A NEW APPROACH FOR DERIVING PSEUDOVELOCITY LOGS FROM RESISTIVITY LOGS1
Authors: DOS SANTOS W. L. BRITO , ULRYCH T. J. , DE LIMA O. A. L. 
Published: April 2006

ABSTRACTThis paper describes a method of generating pseudovelocity logs using measurements of electrical resistivity. A theoretical relation between electrical resistivity and transit time, which is applicable to a wide range of lithologies, has been developed. The application of this relation using a method which defines lithoresistivity zones as lithological intervals related to the same formation and showing small resistivity variations, has been tested in the Recôncavo sedimentary basin in Bahia, Brazil. A comparison of derived pseudovelocity logs with actual sonic logs for five wells whows the validity of the present approach.
CRUSTAL REFLECTIVITY PATTERN AND ITS RELATION TO GEOLOGICAL PROVINCES1
Authors: TRAPPE H. , WEVER TH. , MEISSNER R. 
Published: April 2006

ABSTRACTThe statistical treatment of deep seismic reflections from several different geological units has resulted in different reflectivity histograms. Reflectivity in old and cold shields differs significantly from that in younger, warmer areas. In the shields reflectivity is generally poor and concentrated in the upper crust whilst Caledonian and Variscan areas show strong reflecting lamellae in their lower crust. Also the length of the reflecting elements varies with age and heat flow. The lower crust in young areas is a zone of a strong viscosity minimum as derived from model studies with a temperature dependent rheology. The subhorizontal reflecting lamellae in the lower crust are considered to have been created in a large‐scale high‐temperature, low‐viscosity ordering process whose remnants are still preserved today. Local‐scale differences of reflectivity histograms define certain subprovinces which can be distinguished by their specific patterns.
ENERGY‐INTERACTION: THE LONG‐RANGE INTERACTION OF SEISMIC SOURCES1
Authors: LAWS R. , PARKES G. , HATTON L. 
Published: April 2006

ABSTRACTThe quantity of energy that is radiated as sound by marine seismic sources is examined. When more than one source is used, the total radiated acoustic energy depends on the separation of the sources and is not constant, even when the separation is sufficiently large that the individual signatures are substantially unaffected. We believe that this long‐range interaction has not been described before in the literature and have called it energy‐interaction.The existence of energy‐interaction is demonstrated experimentally for airguns. In. the experiment presented, the acoustic output is more than doubled because of energy‐interaction.Two methods are described for computing the energy of the wavefield of an array of sources. One method is simple and direct; the energy that is radiated into the far field is calculated by integrating the directivity pattern over all directions. The other method finds the energy that is radiated into the near field. Because the medium is assumed lossless, these two energies are the same. Although the near‐field method is conceptually more difficult, it is faster, more accurate and provides a more detailed description of the energy budget.
SEISMIC AND ELECTRICAL PROPERTIES OF UNCONSOLIDATED PERMAFROST1
Authors: KING M. S. , ZIMMERMAN R. W. , CORWIN R. F. 
Published: April 2006

ABSTRACTA model has been developed to relate the velocities of acoustic waves Vp and Vs in unconsolidated permafrost to the porosity and extent of freezing of the interstitial water. The permafrost is idealized as an assemblage of spherical quartz grains embedded in a matrix composed of spherical inclusions of water in ice. The wave‐scattering theory of Kuster and Toksoz is used to determine the effective elastic moduli, and hence the acoustic velocities. The model predicts Vp and Vs to be decreasing functions of both the porosity and the water‐to‐ice ratio. The theory has been applied to laboratory measurements of Vp and Vs in 31 permafrost samples from the North American Arctic. Although no direct measurements were made of the extent of freezing in these samples, the data are consistent with the predictions of the model. Electrical resistivity measurements on the permafrost samples have demonstrated their essentially resistive behaviour. The ratio of resistivity of permafrost in its frozen state to that in its unfrozen state has been related to the extent of freezing in the samples.Electromagnetic and seismic reflection surveys can be used together in areas of permafrost: firstly an EM survey to determine the extent of freezing and then the acoustic velocity model to predict the velocities in the permafrost. The necessary transit time corrections can thus be made on seismic reflection records to compensate for the presence of permafrost.
THE MAXIMUM ENTROPY APPROACH TO THE INVERSION OF ONE‐DIMENSIONAL SEISMOGRAMS1
Authors: RIETSCH E. 
Published: April 2006

ABSTRACTDetermination of impedance or velocity from a stacked seismic trace generally suffers from noise and the fact that seismic data are bandlimited. These deficiencies can frequently be alleviated by ancillary information which is often expressed more naturally in terms of probabilities than in the form of equations or inequalities.In such a situation information theory can be used to include ‘soft’information in the inversion process. The vehicle used for this purpose is the Maximum Entropy (ME) principle. The basic idea is that a prior probability distribution (pd) of the unknown parameter(s) or function(s) is converted into a posterior pd which has a larger entropy than any other pd which also accounts for the information. Since providing new information generally lowers the entropy, this means that the ME pd is as non‐committal as possible with regard to information which is not (yet) available. If the information used is correct, then the ME pd cannot be contradicted by new, also correct, data and thus represents a conservative solution to the inverse problem.In the actual implementation, the final result is, generally, not the pd itself (which may be quite broad) but rather the expectation values of the desired parameter(s) or function(s).A general problem of the ME approach is the need for a prior pd for the parameter(s) to be estimated. The approach used here for the velocity is based on an invariance criterion, which ensures that the result is the same whether velocity or slowness is estimated. Unfortunately, this criterion does not provide a unique prior pd but rather a class of functions from which a suitable one must be selected with the help of other considerations.
DESIGN OF MODELS FOR ELECTROMAGNETIC SCALE MODELLING1
Authors: CHAKRIDI R. , CHOUTEAU M. 
Published: April 2006

ABSTRACTElectromagnetic scale modelling for most conductors encountered in prospecting requires models of conductivity in the range 10‐1000 S/m. A polyester composite filled with aluminium fillers is used to design the required materials. The fillers are chosen with a high aspect ratio to achieve the desired conductivities without altering mechanical properties. The design process, consisting of a chemical reaction, is simple, repeatable and easy to realize in a geophysical laboratory.The conductivity of the designed material is estimated by an inductive method based upon the variation of the quality factor of a coil in which the samples are inserted. This method is much less influenced by the distribution and orientation of fillers in the resin matrix than a galvanic method.The conductivities obtained fill the major part of the interval 10‐1000 S/m without requiring unreasonable mixing ratios. The homogeneity of the material was tested and the anisotropy was low at high conductivities. The electrical properties do not vary with time in free air or when immersed in salt water.We propose standard curves giving conductivities of composites filled with aluminium fibres and flakes. A large cylindrical model of fixed resin‐to‐filler ratio was made to show the capability of the technique to produce models of useful size. Its measured conductivity agrees well with the expected conductivity from the established standard curves. A scale modelling experiment with the horizontal loop technique was carried out over a horizontal thin sheet made of the material, and again the conductivity obtained from the results agrees reasonably well with the expected value.The noted difference between the inductively and galvanically measured conductivities of the proposed material presently restricts the use of the technique to EM scale experiments for which the host rock is considered very resistive.