bluuebunny/crossref_metadata_2025_split huggingface dataset:
SOURCE ARRAY SCALING FOR WAVELET DECONVOLUTION*
Authors: ZIOLKOWSKI A.M. 
Published: April 2006

AbstractA seismic source array is normally composed of elements spaced at distances less than a wavelength while the overall dimensions of the array are normally of the order of a wavelength. Consequently, unpredictable interaction effects occur between element and the shape of the far field wavelet, which is azimuth‐dependent, can only be determined by measurements in the far field. Since such measurements are very often impossible to make, the shape of the wavelet—particularly its phase spectrum—is unknown.A theoretical design method for overcoming this problem is presented using two scaled arrays. The far field source wavelets from the source arrays have the same azimuth dependence at scaled frequencies, and the far field wavelets along any azimuth are related by a simple scaling law. Two independent seismograms are generated by the two scaled arrays for each pair of source‐receiver locations, the source wavelets being related by the scaling law.The technique thus permits the far field waveform of an array to be determined in situations where it is impossible to measure it. Furthermore it permits the array design criteria to be changed: instead of sacrificing useful signal energy for the sake of the phase spectrum, the array may be designed to produce a wavelet with desired amplitude characteristics, without much regard for phase.
INMOD—TWO DIMENSIONAL INVERSE MODELING ALGORITHM BASED ON RAY THEORY*
Authors: SATTLEGGER J.W. , ROHDE J. , EGBERS H. , DOHR G.P. , STILLER P.K. , ECHTERHOFF J.A. 
Published: April 2006

AbstractTwo dimensional inverse modeling, a process to be applied after standard processing and interpretation, uses interfaces picked by the user. These interfaces are transformed into an approximate subsurface model.The subsurface model is represented by curved interfaces and interval velocities. The interfaces have to be unique functions of the line coordinate. Otherwise they may be arbitrarily curved and may begin or terminate anywhere along the section, e.g., at faults, pinchouts, salt domes and the like. Interval velocities may vary laterally along the section. The inverse modeling algorithm then modifies the model until traveltimes calculated from this model match the traveltimes observed as closely as possible in a least squares sense.The traveltimes corresponding to the model are obtained through ray tracing taking exact account of refraction. The traveltimes observed are the arrival times of single impulses before stacking contributing to the interfaces. These traveltimes are provided by ANAKON, a continuous interface analysis system.The comparison of INMOD results with those of well measurements and those of classical interval velocity computation from seismic data shows the accuracy of the method. Deviations of INMOD derived interface depths are within 2% of well data.
THE SOLUTION OF THE STATIONARY ELECTRIC FIELD STRENGTH AND POTENTIAL OF A POINT CURRENT SOURCE IN A 2 1/2‐ DIMENSIONAL ENVIRONMENT*
Authors: ESKOLA L. , HONGISTO H. 
Published: April 2006

AbstractA method is given for solving the dc electric field problem of a point current source in an anisotropic 2 1/2‐dimensional structural model. The surface integral equation of the field strength is given. Parallel to the strike the field strength is represented by a Fourier series. On the plane perpendicular to the strike each term of the field strength series is solved by means of the method of sub‐sections, where linear behaviour of field strength over the sub‐sections is assumed. Some numerical examples for different galvanic effects are given.
FLUID‐FLOW MONITORING USING ELECTROMAGNETIC PROBING*
Authors: LYTLE R.J. , LAGER D.L. , LAINE E.F. , SALISBURY J.D. , OKADA J.T. 
Published: April 2006

AbstractHigh‐frequency electromagnetic probing is used to monitor the rate and direction of flow of fluids injected into the ground. This method shows the potential for providing more detailed information than procedures presently used. The experimental technique and the test‐of‐concept experimental results are discussed. This technique has applications in oil‐reservoir engineering and in hydrology studies concerning storage of chemical wastes.
RECURSIVE MIGRATION IN THREE DIMENSIONS*
Authors: BERKHOUT A.J. , DE JONG B.A. 
Published: April 2006

AbstractA simple theory is presented to derive explicit recursive migration schemes in three dimensions which are suitable for situations with lateral velocity variations. The proposed schemes can be realized by one‐dimensional convolutions along the x‐axis and y‐axis. An investigation is made on the errors of each scheme. For each frequency component the amplitude error and the phase error are computed as a function of the immergence angle (dip). Based on the error analysis a critical discussion is given on the stability of explicit schemes.
VELOCITY ANALYSIS USING ITERATIVE STACKING*
Authors: NAESS O.E. , BRULAND L. 
Published: April 2006

AbstractAn iterative stacking algorithm (superstack) has been applied in velocity analysis as a velocity filter prior to the coherency measurements. The effect of this filtering is demonstrated on both synthetic and real data using the following three methods of velocity analysis: Constant velocity stack, constant velocity gather, and velocity spectrum. The experimental results indicate that the velocity analysis can be improved by this approach.
BOUNDARY ELEMENT METHOD FOR THE ARBITRARY INHOMOGENEITIES PROBLEM IN ELECTRICAL PROSPECTING*
Authors: OKABE M. 
Published: April 2006

AbstractThe generalized integral equation for the electric potential governed by a quasi‐harmonic equation can be derived via a variational formulation. For surface current distributions it is not always a Fredholm integral equation of the second kind. Numerical solutions of the general heterogeneous problem can be obtained with the “reciprocal averaging technique”, where the solution is obtained a second time after exchange of source and field points.
THE AIRBORNE ELECTROMAGNETIC METHOD AS A TOOL OF GEOLOGICAL MAPPING*
Authors: PALACKY G.J. 
Published: April 2006

AbstractSince its development some thirty years ago, the airborne electromagnetic (AEM) method has been primarily used as a tool for mineral prospecting. However, advanced AEM systems are capable of other tasks, such as geological mapping and groundwater exploration. Excellent correlation between maps of apparent conductivity and geological maps was observed in several regions of Brazil where AEM surveys were performed. The degree of correlation seems to depend on the local climate. In humid and subhumid tropical regions, a weathered layer develops whose thickness and conductivity depend upon bedrock lithology. Therefore certain lithological types can be recognized from their conductivity signature; e.g., granites and Precambrian coarse clastic rocks are resistive, metavolcanic (particularly mafic) and volcanic rocks are conductive, Phanerozoic sediments are generally highly conductive.Two geophysical surveys are analyzed in the paper. The first was conducted with the time‐domain, towed‐bird AEM system in the Itapicuru greenstone belt in the state of Bahia. The apparent conductivity map correlated better with the local lithology than the magnetic map. Results of the AEM survey were successfully used to improve the regional geological map. A helicopter EM system was used in the second survey, which covered a portion of the Precambrian shield of Rio Grande do Sul. Also in this region ground checks confirmed the usefulness of conductivity surveys in geological mapping.The technique outlined in the paper holds great promise for countries of humid tropical climate, where few outcrops exist and access is often difficult. The tests performed in Brazil indicate that by executing AEM/aeromagnetic surveys during initial stages of mapping and exploration programs, time and expenditure required for geological field work can be considerably reduced. The resulting geological maps are more accurate and the inventory of mineral occurrences becomes more complete.
A NUMERICAL METHOD OF CALCULATING THE KERNEL FUNCTION FROM SCHLUMBERGER APPARENT RESISTIVITY DATA*
Authors: SANTINI R. , ZAMBRANO R. 
Published: April 2006

AbstractA method to calculate the resistivity transform of Schlumberger VES curves has been developed. It consists in approximating the field apparent resistivity data by utilizing a linear combination of simple functions, which must satisfy the following requirements: (i) they must be suitable for fitting the resistivity data; (ii) once the fitting function has been obtained they allow the kernel to be determined in an analytic way.The fitting operation is carried out by the least mean squares method, which also accomplishes a useful smoothing of the field curve (and therefore a partial noise filtering). It gives the possibility of assigning different weights to the apparent resistivity values to be approximated according to their different reliability.For several examples (theoretical resistivity curves in order to estimate the precision of the method and with field data to verify the practicality) yield good results with short execution time independent of shape the apparent resistivity curve.
ON THE SIGNIFICANCE OF AMPLITUDE STUDIES IN SHALLOW REFRACTION SEISMICS*
Authors: KUMAR G. NANDA , RAGHAVA M.S. VIJAYA 
Published: April 2006

AbstractAn analysis of amplitudes of refraction records of some shallow refraction profiles shot primarily for detailing the near‐surface structure in a granitic terrain has yielded information on refractor properties: reduced amplitudes are plotted on amplitude‐distance graphs. The negative power n to which distance should be raised to represent (elastic) amplitude decay with respect to distance due to spreading of the critically refracted wave involved is examined. Computed values of this “spreading index”n are close to n = 2 as predicted by the theory.With this value of n, amplitude data are processed to determine residual attenuation attributable to elastic absorption in the bedrock. A graphical approach for this purpose from comparison of amplitude‐distance graphs with the plots of amplitude decay due to spreading which is applicable to flat and horizontal refractor situations is suggested. Assuming residual attenuation to represent absorption in the granite bedrock, the computed coefficients of absorption, which vary from 0.5 to 3.90 km−1 for a frequency of 50 Hz, are obtained.From amplitude graphs of reversed profiles it is shown that the amplitude differences plot bears a relation to lateral velocity changes in the refractor. From comparison of practical amplitude decay graphs with those computed for different subsurface models, it appears possible to detect fractured rock occurrences in the refractor.
UNDERGROUND GRAVITY SURVEY IN ALPINE REGIONS*
Authors: HUSSAIN A. , WALACH G. , WEBER F. 
Published: April 2006

AbstractAn underground gravity study was carried out under extreme conditions of the Alpine regions. The lead–zinc mine Bleiberg, Carinthia, was selected as an example to show the possibilities and limitations of the subsurface gravity method. For in situ density determinations, gravity measurements were made in two vertical mine shafts passing through Triassic sedimentary rocks of the Bleiberg Unit. The main prblem in gravity data reduction in extremely rugged topography is the accurate calculation of the terrain effect on underground stations. A general discussion of the various corrections required for the gravity measurements in the mine is presented. The mean interval densities in the two shafts, in limestone, dolomite, and schists formations, were determined as 2.76 and 2.77 g/cm3, respectively, with an accuracy of better than 0.01 g/cm3 for a depth interval of 50 m. The interval densities provide valuable information about the lithological and structural changes in the shaft surroundings and also agree well with the representative hand sample densities.In the second part, the applications of subsurface horizontal gravity surveys in exploration are discussed. Since the influence of topography is less underground because of the greater distance to the surface, subsurface surveys have definite advantages over surface surveys and can be very helpful in locating anomalous density zones in the mines. An example of gravity survey with a station spacing of 10 m at a depth of about 540 m is presented.
ELECTROMAGNETIC RESPONSE OF FOLD MODELS IN A UNIFORM FIELD*
Authors: VERMA O.P. 
Published: April 2006

AbstractThe electromagnetic response of anticline fold models in a uniform field has been studied using scale‐modeling. With the increasing conductance the in‐phase anomaly increases initially but then starts to decrease and finally attains a negative saturations, whilst the quadrature anomaly reaches a positive saturation. E.M. response of folds is similar to that of an interaction between two weakly coupled circuits in a dipolar field. The base of the fold is one of the characteristic parameters to determine the measured response.
A TUNNEL RESOLUTION INVESTIGATION USING AN AUTOMATED RESISTIVITY TANK ANALOG*
Authors: KARWATOWSKI J. , HABBERJAM G.M. 
Published: April 2006

AbstractThe examination of apparent resistivity space sections over two‐dimensional targets requires the accumulation of large amounts of data, if the resistivity field is to be adequately described.Whilst such data may be obtained from an intensive sounding traverse in the field, it is desirable for interpretation purposes to be able to generate such space sections for a range of model variations.Within their limitations, tank analogs can be used to provide such interpretation material, but the collection of data can itself be time‐consuming. In this paper, an account is given of the development of an automated tank analog, where an entire space section can be scanned and the output obtained in paper tape format for later processing on a digital computer. The resistance measurement accuracy attained is comparable with normal field measurement accuracy.A practical example of the use of this analog in investigating the problem of resolving and discriminating between two horizontal cylindrical tunnels is given. This problem is basic to an understanding of the minimum size and disposition of targets which can be observed on a space section. The square array has been used in this example because of its good resolution properties in general prospecting rather than its particular suitability for the targets investigated. The directional response of the array is also illustrated and discussed.
SEISMIC REFLECTION APPLIED TO SEDIMENTOLOGY AND GAS DISCOVERY IN THE GULF OF CADIZ*
Authors: DELAPLANCHE J. , LAFET Y. , SINERIZ B. G. , GIL M. A. REMON 
Published: April 2006

AbstractIn the Gulf of Cadiz, a Tertiary basin became filled by clastic series during Miocene and Pliocene times. This terrigenous influx, derived from the Iberic Meseta in the north, is characterized by a sandy episode during the Tortonian and Messinian. The sand deposits were probably connected with uplift and major erosion of the Meseta during the sliding of the olistostrome, which occupied the south of the basin from late Helvetian to middle Tortonian.High resolution seismic techniques produced a good picture of the stratigraphy and of the depositional environment of the sands.A further study, using the amplitude of the reflections, inversion of seismic traces into acoustic impedance traces, and modeling, provides a remarkable example of the possibilities of seismic stratigraphy for depicting the lateral evolution of facies and localizing hydrocarbon occurrences. Out of seven exploratory wells based upon seismic information, six encountered gas‐bearing sands with economic potential.
USE OF ELECTRICAL RESISTIVITY METHODS FOR THE STUDY OF COAL SEAMS IN PARTS OF THE RANIGANJ COALFIELD (INDIA)*
Authors: VERMA R. K. , BANDOPADHYAY T. K. , BHUIN N. C. 
Published: April 2006

AbstractThe surface resistivity method has been used to study a few exposed coal seams located in the northwestern part of the Raniganj Coalfield (belonging to the Damodar Valley Gondwana basin), India. Different electrode configurations, viz., Wenner, two‐electrode and half‐Schlumberger, have been used with different electrode spacings in horizontal profiling. The vertical distribution of resistivity has been studied using Schlumberger configuration at different locations along the profile.Laboratory studies of resistivity (at partial to full water saturation conditions) and porosity of different coal, shaly sandstone and sandstone samples of the survey area had already been carried out before the field survey. The results indicated the presence of a good resistivity contrast between the coal seams and the surrounding formations.The field results did not bear this out: the resistivity responses of the coal formations on the profiles and sounding curves are not as clear as one would expect for such high contents. The Wenner profiles show a broad resistivity anomaly over the coal seams. Two‐electrode profiles are less noisy than Wenner profiles. Sharp peaks have been observed over the coal seams. The half‐Schlumberger configuration seems the best: all the coal seams and their edges can be accurately outlined on the resistivity curves.Borehole data close to the profile have been used for correlating the field results. There is good agreement with vertical electrical soundings.
UNCERTAIN RESISTIVITY SOUNDING AND EQUIVALENT MODELS*
Authors: SZARANIEC E. 
Published: April 2006

AbstractProblems of the resistivity sounding method may be reparametrized by means of Dar Zarrouk—or generalized D.Z.—transformations.A complete set of equivalent models is subdivided into three classes: initial models, intermediate models, and all other models. Equivalent reparametrized models are subdivided into three analogous classes. There is no one‐to‐one mapping of intermediate primary and intermediate reparametrized models. Non‐trivial equivalent models are generated from initial models via intermediate reparametrized models. A method is developed to find equivalent models relative to the uncertainty of resistivity soundings.There are different forms of approximate equivalence laws, apart from those commonly accepted. The equivalence depends considerably on the “flatness” of the sounding curve. For mean flatness, the electric models exhibit the (hp±2)‐equivalence rather than the (hp±1)‐equivalence.
TIME‐TO‐DEPTH MIGRATION USING WAVEFRONT CURVATURE*
Authors: URSIN B. 
Published: April 2006

AbstractA well‐known technique for the migration of normal‐incidence two‐way travel‐time maps is extended to common‐source‐point travel‐time data. The travel time and the travel‐time gradient are used to compute the parameters defining the tangent plane of the reflecting interface. It is also shown how the curvature matrix of the received wavefront can be used to compute the curvature of the reflecting interface. The method is initially derived for common‐source‐point data and then extended to common‐midpoint data.In a three‐dimensional medium the wavefront curvature matrix is computed by solving a 2 × 2 symmetric matrix Riccati equation. In a two‐dimensional medium and in a medium with constant velocity gradient, the wavefront curvature matrix is computed by solving a scalar Riccati equation and two linear equations. The migration procedures are also simplified.When the velocity function is unknown, the migration procedures cannot be used. An inverse modeling algorithm which simultaneously performs the migration and estimates the velocity function must then be applied. Two different inversion schemes are discussed briefly.
BIPOLE‐DIPOLE FIELD DIFFERENCE—A NEW RESISTIVITY SOUNDING TECHNIQUE*
Authors: DAS U. C. , SINGH R. D. 
Published: April 2006

AbstractThe method of field differences using two parallel bipole sources is described. The system possesses much greater resolving power and investigating depth than the conventional Schlumberger configuration. Of the two variations, the longitudinal field difference has about two and half times greater depth of investigation than the transverse one. Practical advantage of the present technique over the Schlumberger system in reducing the ambiguity in interpretation due to equivalent layered structures is demonstrated on a theoretical example.
APPLICATION OF THE GENERAL COMPLEX COMPLIANCE MODEL TO THE DIRECT‐DETECTION PROBLEM*
Authors: STRICK E. 
Published: April 2006

AbstractA methodology is outlined for using general complex compliance functions—which contain the constant‐Q case and any kind of fluid saturant as special cases—to determine the nature of that saturant from seismic reflections observed on seismic records. The important attribute of such a formulation based upon complex compliance functions is that the presence of any kind of saturant is indicated by the values of the parameters contained within the complex compliance function, the latter having the same mathematical form for all rocks under any conditions as long as it remains a crystalline solid.
QUANTITATIVE INTERPRETATION OF HORIZONTAL‐LOOP EM MEASUREMENTS USING A PERMEABLE SPHERE MODEL*
Authors: RAI S. S. , VERMA S. K. 
Published: April 2006

AbstractAn interpretation scheme for horizontal‐loop EM measurements is presented for a permeable sphere model. The induced multipole moments are found to contribute significantly even at very low frequencies for a permeable conductor. The anomaly profiles are computed considering multipole excitation (up to 20) to study the effect of depth of burial and permeability of conductor. The anomaly half‐width along with the inphase and quadrature anomaly amplitudes allow direct interpretation of the parameters of the sphere. The above scheme is suitable for results of Dighem II (coplanar configuration), Slingram and Max Min II measurements.
COMPARISON OF FIVE LEAST‐SQUARES INVERSION TECHNIQUES IN RESISTIVITY SOUNDING*
Authors: HOVERSTEN G. M. , DEY A. , MORRISON H. F. 
Published: April 2006

AbstractA brief history of the development of the inverse problem in resistivity sounding is presented with the development of the equations governing the least‐squares inverse. Five algorithms for finding the minimum of the least‐square problem are described and their speed of convergence is compared on data from two planar earth models. Of the five algorithms studied, the ridge‐regression algorithm required the fewest numbers of forward problem evaluations to reach a desired minimum.Solution space statistics, including (1) parameter‐standard errors, (2) parameter correlation coefficients, (3) model parameter eigenvectors, and (4) data eigenvectors are discussed. The type of weighting applied to the data affects these statistical parameters. Weighting the data by taking log10 of the observed and calculated values is comparable to weighting by the inverse of a constant data error. The most reliable parameter standard errors are obtained by weighting by the inverse of observed data errors. All other solution statistics, such as dataparameter eigenvector pairs, have more physical significance when inverse data error weighting is used.
A CONTRIBUTION TO THE INVESTIGATION OF AMPLITUDE CHARACTERISTICS OF VIBRATOR SIGNALS*
Authors: EDELMANN H. A. K. 
Published: April 2006

AbstractThe characteristic and information content of a seismic signal depends considerably on the bandwidth of the signal. The Vibroseis system gives the possibility of selecting a certain frequency range. This selection is mostly done in the field by visual comparison, mainly taking into account the characteristic of the signal and the signal/noise ratio. A signal with a good characteristic has, in some cases, an arbitrary amplitude and phase characteristic which is not by itself optimum for the transmission of the desired stratigraphic information. The different factors which influence the amplitude characteristic of a Vibroseis signal are discussed. The present situation is illustrated by some results of measurements of vibrators in different areas. An outlook of how the characteristic of vibrators may be improved is given.