bluuebunny/crossref_metadata_2025_split huggingface dataset:
VELOCITY ANALYSIS FOR WIDE APERTURE SEISMIC DATA*
Authors: STOFFA P. L. , DIEBOLD J. B. , BUHL P. 
Published: April 2006

AbstractCommon midpoint data are now being collected with ever increasing source‐receiver offsets. For wide aperture seismic data classical methods of interpretation fail, since velocity analyses and signal‐to‐noise enhancement methods based on hyperbolic traveltime curves are no longer appropriate. Therefore, the goals of increased velocity resolution and signal enhancement, which motivate the increase in offset, are not achieved. Approximate methods, involving higher order traveltime curves or extrapolations, have been developed for velocity analysis but these are ineffective in the presence of refracted arrivals, and lack a physical basis. These problems can be minimized by transforming the observational data to the domain of intercept or vertical delay time τ and horizontal ray parameter p. In this domain headwave refractions are collapsed into points and both near vertical and wide angle reflections can be analyzed simultaneously to derive velocity‐depth information, even in the presence of velocity gradients or low velocity zones.
A RECTANGULAR LOOP SOURCE OF CURRENT ON A TWO‐LAYERED EARTH*
Authors: PODDAR M. 
Published: April 2006

AbstractUsing the concept of reciprocity and the known solution for the electric field of a vertical oscillating magnetic dipole source placed over a two‐layered halfspace, an integral expression for the vertical magnetic field produced by a horizontal rectangular loop, carrying an oscillating current and placed on the surface of the same halfspace, is deduced. This expression is such that it can be evaluated by a combination of straightforward numerical integration and digital linear filter techniques. Displacement currents everywhere in space are neglected.Fields both inside and outside the loop are presented as sounding curves. A little over a decade of frequencies (100–3000 Hz) appears to be the minimum requirement and two decades (100–10000 Hz) are sufficient for most situations. Parametric and geometric sounding curves both show good resolution of subsurface layering. Phase shows better resolution than amplitude. Similarly, resolution is higher when conductivity increases with depth than when it decreases.Besides being useful for the electromagnetic depth sounding for layered earth structures, such computations can be applied to determine normal corrections to Turam observations, whenever a rectangular loop of finite size is used as a transmitter.
STATISTICAL ANALYSIS OF AIRBORNE ELECTROMAGNETIC SIGNATURES OF DIFFERENT CONDUCTOR GROUPS IN TROPICAL TERRAIN OF KARNATAKA, INDIA*
Authors: CHOUDHURY KALPAN 
Published: April 2006

AbstractAEM (airborne electromagnetic) anomalies caused by massive sulphide conductors and superficial conductors can be recognized with a statistical method, as shown by an analysis of Input AEM data from Karnataka State. The weathering in the survey area is of tropical type. Parameters, such as various amplitude ratios and time parameters (inverse of decay rate) for exponential and power‐law decay were analyzed for sulphide bodies, conducting soil, superficial conductors, and cultural conductors. Time parameters τ1 (exponential decay) is defined as ratio of time differences between the third and fourth channel to the logarithmic value of the relative amplitude of the two channels. Time parameter K1 (power‐law decay) is defined as ratio of the difference of the logarithmic values of the delay times of the third and fourth channels to the logarithmic value of the relative amplitude of the two channels. The two parameters have been useful in recognizing sulphide conductors. Also the first channel Input amplitude and logarithmic plot of the transients appear to be helpful in conductor identification. Channel ratios seem to be the least effective parameters of conductor identification. In the area studied both power‐law and the conventional exponential decay were found equally suitable for approximating Input AEM transients.
THE INFLUENCE OF PERMEABILITY ON COMPRESSIONAL WAVE VELOCITY IN MARINE SEDIMENTS*
Authors: HAMDI F. , SMITH D. TAYLOR 
Published: April 2006

AbstractTo investigate the effect of permeability on the propagation of seismo‐acoustic waves through marine sediments, a theoretical model based on Biot's equations is established which relates the compressional wave velocity measured at a fixed frequency to computed velocities at zero and infinite frequencies in terms of sediment porosity and permeability. The model is examined experimentally in a standard soil mechanics consolidation test (itself dependent, among other things, on sediment porosity and permeability) which has been modified to include measurements of compressional wave velocity at 1 MHz and shear‐wave velocity at 5 kHz. This test allows the elastic modulus of the sediment frame to be assessed under different load conditions simultaneous with the velocity determinations.From a number of tests on different samples, five samples are chosen to typify the range of sediment sizes. The results show that the difference between the measured velocity at 1 MHz and the model‐derived velocity at zero frequency increases with increasing particle size (from clays to fine sand), with decreasing porosity, and with increasing permeability. For sediments coarser than fine sand the simple model breaks down, possibly because of the dominance of scattering/diffraction effects at the high frequency of the experiment. Within this limitation the model seems satisfactory to offer a capability of predicting the permeability of a sea floor sediment to an order of magnitude by the in situ measurement of seismic velocities over a wide range of frequencies; the prediction process requires a good in situ determination of sediment porosity such as that offered by electrical formation factor measurements.
SINGLE‐TRACE PROCESSING USING ITERATIVE CDP‐STACKING*
Authors: NAESS O. E. 
Published: April 2006

AbstractThe use of conventional CDP‐stacking in the processing of reflection data imposes restrictions on the horizontal and vertical resolution. Ideally, the final seismic section should consist only of short offset or, in practice, near‐trace primary energy. Through the use of the iterative stacking algorithm, the signal‐to‐noise ratio on a single trace in the CDP‐gather may be improved to an extent comparable to what occurs on a conventional stacked trace. By using this approach and treating the near‐trace section after iterative stacking as the final section, the seismic resolution can be improved.
OFFSET CONTINUATION OF SEISMIC SECTIONS*
Authors: BOLONDI G. , LOINGER E. , ROCCA F. 
Published: April 2006

AbstractGeometrical acoustic and wave theory lead to a second‐order partial differential equation that links seismic sections with different offsets. In this equation a time‐shift term appears that corresponds to normal moveout; a second term, dependent on offset and time only, corrects the moveout of dipping events.The zero‐offset stacked section can thus be obtained by continuing the section with maximum offset towards zero, and stacking along the way the other common‐offset sections.Without the correction for dip moveout, the spatial resolution of the section is noticeably impaired, thus limiting the advantages that could be obtained with expensive migration procedures. Trade‐offs exist between multiplicity of coverage, spatial resolution, and signal‐to‐noise; in some cases the spatial resolution on the surface can be doubled and the aliasing noise averaged out.Velocity analyses carried out on data continued to zero offset show a better resolution and improved discrimination against multiples. For instance, sea‐floor multiples always appear at water velocity, so that their removal is simplified.This offset continuation can be carried out either in the time‐space domain or in the time‐wave number domain. The methods are applied both to synthetic and real data.
BOREHOLE SEISMIC PROFILING AND TUBE WAVE APPLICATIONS IN A DAM SITE INVESTIGATION*
Authors: HENRIET J. P. , SCHITTEKAT J. , HELDENS Ph. 
Published: April 2006

AbstractContinuous, single‐channel reflection profiling has been carried out in PVC‐lined boreholes, primarily with the aim of ascertaining the position of an old subsurface gas storage tunnel on a proposed dam site.Tube wave reflection patterns thus generated have been interpreted in terms of sediment rigidity and shear wave velocity, and these results could be compared with some independent data. It is interesting to note that, within the well section penetrating Tertiary clays, the velocity of the hydraulic transients apparently was not affected by the PVC casing, which might be explained by a tight coupling between casing and clay wall. In such situations, tube waves turn out a straightforward tool for the determination of shear wave velocity and the derivation of dynamic elastic moduli of unconsolidated sediments.Further applications of the study of the distribution of seismic velocities on the dam site dealt with the consolidation history of the clays. A level of abnormally low P‐wave velocities has been detected and interpreted as a gas‐charged horizon which, by its coincidence with the base level of clay diapirs, might be considered to have contributed to clay flowage in past geological times. Data about maximum past burial depth, derived from shear wave velocities, turned out to be in agreement with results from consolidation testing.
THE AUTOBALANCER—AN AUTOMATIC SYSTEM FOR THE REDUCTION OF POWER‐LINE INTERFERENCE WITH SEISMIC SIGNALS*
Authors: ENSING L. 
Published: April 2006

AbstractSince the early years of seismic surveying, field engineers and observers have been faced with the appearance of interference at power‐line frequency (and harmonics) on seismic records: 60 Hz in the Americas, 50 Hz in Eurasia; and 16 2/3 Hz (the train supply frequency in Germany, Austria, and Switzerland) in part of Europe. Such interference is picked up from the ground surface through electric leakage between geophone leads and ground.To minimize power‐line interference, the autobalancer automatically performs a balancing procedure by adjusting two potentiometers on each channel—essentially the same operation as manually performed by seismic operators for decades. The first 24‐channel system became operational in 1977.The first‐generation design comprised two 256‐position electronic switching potentiometers. Over 4 years of experience with this system in all parts of the world has shown its value under a variety of field conditions. Improvements up to 30 dB were often found, sometimes even up to 40 dB. Balancing time was about 30 s.A second‐generation design was field‐tested in 1981. Potentiometer stepping is now performed at three levels: coarse, medium, and fine. This leads to faster operation and better resolution: balancing time is about 10 s; improvements up to 60 dB are occasionally found.A later modification gives a further reduction of the balancing time to 5 s in 60 Hz areas or 6 s in 50 Hz areas.
A STATISTICAL APPROACH TO THE EXTRACTION OF THE SEISMIC PROPAGATING WAVELET*
Authors: ANGELERI G. P. 
Published: April 2006

AbstractA model of the seismic trace is generally given as a convolution between the propagating wavelet and the reflectivity series of the earth and normally it is assumed that a white noise is added to the trace. The knowledge of the propagating wavelet is the basic point to estimate the reflectivity series from the seismic trace.In this paper a statistical method of wavelet extraction from several seismic traces, assuming the wavelet to be unique, is discussed. This method allows one to obtain the propagating wavelet without any classical limitative assumptions on the phase spectrum. Furthermore, a phase unwrapping method is suggested and some statistical properties of the phase spectrum of the reflectivity traces are examined.
NON‐NORMAL INCIDENCE INVERSION: EXISTENCE OF SOLUTION*
Authors: SHIVA MOSTAFA , MENDEL J. M. 
Published: April 2006

AbstractAn inverse problem is one in which the parameters of a model are determined from measured seismic data. Important to the solution of inverse problems is the issue of whether or not a solution exists. In this paper we show, in a constructive manner, that a solution does exist to the specific inverse problem of determining the parameters of a horizontally stratified, lossless, isotropic and homogeneous layered system that is excited by a non‐normal incidence (NNI) plane wave. Mode conversion between P‐ and S‐waves is included.We develop a seven‐step layer‐recursive procedure for determining all of the parameters for layer j. These parameters are P‐wave and S‐wave velocities and angles of incidence, density, thickness, traveltimes, and reflection‐ and transmission‐coefficient matrices. Downward continuation of data from the top of one layer to the top of the next lower layer is an important step in our procedure, just as it is in normal incidence (NI) inversion. We show that, in order to compute all parameters of layer j, we need to (and can) compute some parameters for layer j+ 1. This is a non‐causal phenomenon that seems to be necessary in NNI inversion but is not present in NI inversion.
SIMULTANEOUS INTERACTIVE MAGNETIC AND GRAVITY INVERSION*
Authors: MENICHETTI V. , GUILLEN A. 
Published: April 2006

AbstractGravity and magnetic anomalies may have the same source but it is always difficult to achieve correlated solutions if interpretations are carried out separately. Therefore it is useful to invert both anomalous gravity and magnetic profiles at the same time, so that the solution mav be more constrained.Existing inversion techniques do not lend themselves equally to this kind of extension, since the parameters—such as density and susceptibility contrasts—should not be related. The algorithms more easily adaptable to simultaneous inversion are those which look for the shape and the position of anomalous bodies with constant unknown density or susceptibility contrasts.In this study, we use the generalized inverse method in the 2 1/2 D case. Parameters are the coordinates of the vertices of the polygonal cross‐sections of the anomalous bodies and both density and susceptibility contrasts of each body. The two types of profile to be processed must be superposable but may have different lengths, spacings, and coordinate origins. Both synthetic examples and field data from geologically known areas have been processed, and it appears that the simultaneous inversion technique may provide an important additional degree of control in the interactive interpretation process.
ELECTROMAGNETIC FIELDS DUE TO A THIN RESISTIVE LAYER*
Authors: PASSALACQUA H. 
Published: April 2006

AbstractUsing approximate boundary conditions, expressions for electromagnetic fields have been derived for a thin, highly resistive layer lying between two homogeneous layers excited by an electric dipole grounded on the surface of the earth. The variations of the fields with the parameter T/T1 (ratio of the transverse resistance of the thin layer to the transverse resistance of the first layer) were studied in relation to frequency, time, the normalized separation source—receiver, and the angle between the source and the radius to the observation point. For a value of h2/h1 (ratio of thickness of second layer to the thickness of the first layer) approximately equal to 0.2, the general three‐layer medium case gives the same results as this approach. It was found that the electric fields have a very strong dependence on the parameter T (transverse resistance) which characterizes the thin, highly resistive layer. However, the magnetic fields depend only very weakly on this parameter.
A METHOD FOR CALCULATING IP ANOMALIES FOR MODELS WITH SURFACE POLARIZATION*
Authors: ESKOLA L. , ELORANTA E. , PURANEN R. 
Published: April 2006

AbstractA numerical method is given for calculating resistivity and induced polarization anomalies produced by a surface polarization model. Surface polarization is generated when a purely electronic conductor is located in an electrolyte environment. The system that develops on the boundary between the conductor and the electrolyte is described macroscopically by a net surface charge distribution and an electric double layer. An integral equation is derived for the potential by assuming that the electronic conductor forms an equipotential system and that the polarization impedance across the boundary is linear. The integral equation is solved by means of the method of subsections. As an application some numerical modeling results are presented. The surface impedance values used in calculations are based on laboratory measurements that are briefly described. Implications of the results for scale modeling are discussed.
THE USE OF TWO‐ELECTRODE AND SCHLUMBERGER FILTERS FOR COMPUTING RESISTIVITY AND EM SOUNDING CURVES*
Authors: BAŞOKUR A. T. 
Published: April 2006

AbstractDifferent sets of filter coefficients for the linear filter technique for the computations of resistivity and EM sounding curves are evaluated for several electrode and coil configurations. Instead of this procedure, the two‐electrode filter can be used for computations of Wenner, Schlumberger, and dipole—dipole apparent resistivity model curves by defining convolutional expressions which contain the new input functions in terms of the resistivity transform function. Similarly, the Schlumberger filter performs the computations of dipole—dipole apparent resistivity model curves. The Wenner, Schlumberger, and dipole—dipole filter functions are defined in terms of the two‐electrode filter using the new convolutional expressions. A relationship between the Schlumberger and dipole—dipole filter functions is given.The above arguments are adopted for the computations of EM sounding curves. It is shown that the EM filter for the horizontal coplanar loop system (which is identical to the two‐electrode filter) performs the computations of the mutual coupling ratios for perpendicular, vertical coplanar, and vertical coaxial loop systems. In the same way, the Schlumberger filter can be used to compute vertical coaxial sounding curves. The corresponding input functions are defined in terms of the EM kernel for all convolutional expressions presented.After these considerations, integral expressions of the mutual coupling ratios involving zero‐order Bessel function are derived. The mutual coupling ratio for the vertical coaxial loop system is given in the same form as the mutual coupling ratio for the vertical coplanar loop system.
INTERPRETATION OF LANDSAT MSS DATA IN AN IRON‐ORE‐BEARING ZONE IN GOA, INDIA*
Authors: MURTHY Y. V. S. , MALLICK K. 
Published: April 2006

AbstractIn South and Central Goa iron ore occurs in two parallel belts with the general NW‐SE Dharawar trend. The ore occurrence, however, is not continuous. There are barren zones as well as zones of very high concentration in some of which there are mining activities.Landsat MSS data have been interpreted over a zone covering both mineral belts in order to delineate the ore occurrences. As a guide line a known ore‐bearing area has been considered along with the unknown zones.On the basis of two‐dimensional plotting of gray level values it has been found that the MSS bands 4 and 7 are most suitable for the studies over iron‐rich areas in Goa. Two techniques are described here for the processing of the MSS data; the separation of residual from the regional and MSS band‐ratioing. It is observed that (i) the gray level residual maps of MSS bands 4 and 7 are of use in demarcating the iron‐ore‐bearing zones, and (ii) an existing mine, an abandoned mine, and a proved iron ore zone could be delineated by MSS band‐ratioing. On the basis of the latter technique, a few areas with ore occurrence potential have been indicated.
ON THE INTERPRETATION OF AIRBORNE ELECTROMAGNETIC DATA FOR THE TWO‐LAYER CASE*
Authors: MUNDRY E. 
Published: April 2006

AbstractFor helicopter‐borne electromagnetic systems, the distance between the transmitting and the receiving coils is small compared with the altitude above ground. For this case, a major simplification can be made for the calculation of model curves. Some two‐layer curves for the interpretation of frequency measurements are presented.A very simple procedure is demonstrated for the conversion of the relative secondary field into apparent resistivity and apparent distance for the mapping of airborne electromagnetic data. Furthermore, an approximation is described for the determination of the thickness and the resistivity of a layer lying on a perfectly conducting half‐space.
OFFSET CONTINUATION IN THEORY AND PRACTICE*
Authors: BOLONDI G. , LOINGER E. , ROCCA F. 
Published: April 2006

AbstractOffset continuation is a technique that was recently proposed for the dip moveout correction. This correction can be carried out in the time‐wavenumber domain using a proper partial differential equation that links sections with different offset.It has been shown that a single spike in a common‐offset section—corresponds to a semi‐elliptically shaped reflector with foci located at the source and receiver in the section migrated after dip moveout correction.The sections that result after offset continuation, stack, and migration are thus a superposition not only of semicircles, but also of semi‐ellipses with different lengths of axes. This effect smears the migration alias‐noise which, without offset continuation, would appear as migration circles not close enough together to interfere destructively.Offset continuation can improve the quality of seismic sections in several ways:—the velocity analyses are more readable, less dispersed and dip independent; diffraction tails arrive with the same normal moveout velocity as the apex and thus diffraction‐noise can be “stacked out”;—noise produced by aliasing in the migrated section is reduced.In this paper we give a practical and conceptual interpretation of the offset continuation method, with a generalization to three‐dimensional volumes of data. A critical examination of several synthetic and field data examples shows the actual possibilities and advantages of offset continuation.
STATIC CORRECTIONS FOR SHEAR WAVE SECTIONS*
Authors: WIEST B. , EDELMANN H. A. K. 
Published: April 2006

AbstractThe computation of static corrections requires information about subsurface velocities. This information can be obtained by different methods: surface wave analysis, short refraction lines, downhole times, uphole times and first arrivals from seismograms.For pure shear waves generated by SH sources the analysis of first arrivals from seismograms combined, if necessary, with short refraction lines has proved to be most accurate and economic.A comparison of first‐arrival plots from P‐ and S‐wave surveys of the same line measured in areas of unconsolidated sediments in northern Germany illustrates the characteristic differences between the two velocity models. P‐waves show a marked velocity increase at the water table from about 600 to 1800 m/s. S‐wave velocities of the same strata increase gradually from about 100 to 400 m/s. As a consequence, S‐wave models are vertically and laterally more complex and, in general, show no significant velocity increase at a defined boundary as P‐wave models do. Therefore, other suitable correction levels with specific velocities must be chosen.A comparison of “tgd‐corrections” (correction time between geophone position and datum level) for P‐ and S‐waves in areas of unconsolidated sediments shows that their ratio is different from the P‐/S‐velocity ratio for the respective correction level because of the greater depth of the S‐wave refractor. Therefore, P‐ and S‐waves are influenced by different near‐surface anomalies, and time corrections calculated for both wave types are largely independent.