bluuebunny/crossref_metadata_2025_split huggingface dataset:
WAVE SCATTERING DECONVOLUTION BY SEISMIC INVERSION*
Authors: SARWAR A.K.M. , SMITH D.L. 
Published: April 2006

ABSTRACTWe propose a wave scattering approach to the problem of deconvolution by the inversion of the reflection seismogram. Rather than using the least‐squares approach, we study the full wave solution of the one‐dimensional wave equation for deconvolution. Randomness of the reflectivity is not a necessary assumption in this method. Both the reflectivity and the section multiple train can be predicted from the boundary data (the reflection seismogram). This is in contrast to the usual statistical approach in which reflectivity is unpredictable and random, and the section multiple train is the only predictable component of the seismogram. The proposed scattering approach also differs from Claerbout's method based on the Kunetz equation.The coupled first‐order hyperbolic wave equations have been obtained from the equation of motion and the law of elasticity. These equations have been transformed in terms of characteristics. A finite‐difference numerical scheme for the downward continuation of the free‐surface reflection seismogram has been developed. The discrete causal solutions for forward and inverse problems have been obtained. The computer algorithm recursively solves for the pressure and particle velocity response and the impedance log. The method accomplishes deconvolution and impedance log reconstruction. We have tested the method by computer model experiments and obtained satisfactory results using noise‐free synthetic data. Further study is recommended for the method's application to real data.
INVERSION OF AN UNMIGRATED STACKED SECTION TO DETERMINE AN INTERVAL VELOCITY MODEL*
Authors: SUTTON G.R. , MOORE B.J. 
Published: April 2006

ABSTRACTA seismic inversion procedure is developed that inverts data available from an unmigrated stacked section to produce an interval velocity model. It attempts to overcome some of the limitations of existing methods by using a generalized linear inversion technique. The inversion process incorporates several features: (i) Lateral interval velocity variations are permitted, (ii) A fast accurate forward model was developed, (iii) Input data is weighted according to the accuracy with which it has been acquired. The procedure is applied to seismic data from the Gippsland Basin, an area offshore South‐East Australia.
SHOULD THE ELECTRIC LINE BE STRAIGHT IN MAGNETOTELLURIC SURVEYS?*
Authors: GOMEZ‐TREVINO E. 
Published: April 2006

ABSTRACTThe usual description of electric field measurements in terms of potential differences is not entirely adequate at high frequencies. In general, the telluric electric field is non‐conservative and voltage measurements depend on the contour described by the cable. A simple error analysis helps to recognize those situations where systematic errors may be significant for present day standards.
EXAMINATION OF SOUNDING CURVE EXTRAPOLATION USED BY THE OFFSET WENNER SYSTEM1
Authors: WHITE P. A. , SCOTT D. M. 
Published: April 2006

ABSTRACTThe Offset Wenner resistivity sounding system provides for the extrapolation of the Wenner resistivity curve. The extrapolation technique was applied to data measured in the Solomon Islands and it is shown to be unreliable. An accurate method of predicting the reliability of extrapolation using measured resistances could not be found.
APPROXIMATE INVERSION OF AIRBORNE EM DATA FROM A MULTILAYERED GROUND1
Authors: SENGPIEL K.‐P. 
Published: April 2006

ABSTRACTA complex transfer function c (or generalized skin depth) can be derived from data for the secondary magnetic field measured by a dipole system with small coil spacing at height h above the ground. This function has a useful property: For a uniform or layered ground, the real part of c yields the‘ centroid depth’z* of the in‐phase current system as a function of frequency.This parameter can be combined with the apparent resistivity ρa derived by conventional methods. The function ρa(z*), if known over a broad frequency range, yields a smoothed approximation of the true distribution ρ(z) without an initial model. The relations between ρa(z*) and ρ(z) are studied for a number of multilayer models. An example of the application of the ρa*) algorithm to data from a groundwater survey is given.
STATISTICAL TECHNIQUES APPLIED TO BOREHOLE GEOPHYSICAL DATA IN GOLD EXPLORATION1
Authors: URBANCIC T. I. , BAILEY R. C. 
Published: April 2006

ABSTRACTGold, as a trace element, is not directly detectable by geophysical borehole logging techniques. Geophysical logging methods have therefore been used to infer the presence of structural features and alteration processes associated with gold. Since these features and processes differ with the style of gold mineralization, a unique relation between geophysical anomalies and gold cannot be established. However, in a particular environment, such an association can be established. We examined geophysical log data (self potential, induced polarization, resistivity, gamma, temperature, and temperature gradient) and drill core from the Barber‐Larder property in NE Ontario. The geophysically detectable alteration processes associated with gold were identified as sericitization and pyritization, by means of factor analysis. A linear discriminant function was constructed which allowed the zones of economic mineralization (nominally 0.05 oz/ton or 1.5 ppm) to be identified with a 75% success rate based on geophysical log data alone. This rate was achieved without sophisticated non‐linear data transformations.
TRANSIENT EM ANALOGUE MODELLING FOR KOREAN TREASURE HUNTING WITH THE SIROTEM SYSTEM1
Authors: LEE SANG KYU , BUSELLI G. 
Published: April 2006

ABSTRACTThe inductive transient electromagnetic method (TEM) shows good potential for the detection of metallic relics of historical interest, such as buried antique bells, Buddhist idols, or precious metal nuggets. The effectiveness of the method was investigated with analogue models, using transmitter‐receiver loop configurations with sizes or receiver spacings slightly different from those usually applied in earth resources exploration with the Sirotem system.The analogue modelling results show that the location and depth of the buried treasure may be obtained from the Sirotem data. A solid metallic object such as an antique bell could be detected to a depth about 12 times greater than its size, and treasure consisting of separate metallic objects, such as gold nuggets, could be detected to a depth 5 times its linear dimensions with a transmitter current of ˜ 20 A.In scaling down the dimensions of a target, its conductivity should be increased in order to preserve the same TEM conditions found in the field. However, since the buried treasure consists of gold or copper objects, it is not possible to properly scale the conductivity. Hence, in the field, the depth detection limit is expected to be greater than that derived from analogue modelling.
A STUDY OF DIFFERENT METHODS OF WAVEFIELD SEPARATION FOR APPLICATION TO VSP DATA1
Authors: KOMMEDAL J. H. , TJØSTHEIM B. A. 
Published: April 2006

AbstractDuring the last couple of years there has been much research in the area of wavefield separation of borehole seismic data, and several articles have been published on various separation techniques. Methods involving the application of two‐dimensional Fourier transformation, the Radon transformation, multi‐level median filters or optimal filters, are all suggested as possible approaches to the wavefield separation problem.This paper compares some of these methods commonly used in the industry.The theories of the chosen methods are described to see how they are related. Using the different methods on synthetic and real data, we show how this theoretical relation is reflected in the relatively similar results obtained. We also show how the different filters treat coherent and random noise.
MAGNETIC WAVETILT MEASUREMENTS FOR GEOLOGICAL FRACTURE MAPPING1
Authors: SINHA AJIT K. 
Published: April 2006

AbstractThe measurement of wavetilt is diagnostic for determining the electrical characteristics of the upper layers of the ground at VLF and LF frequency ranges. Theoretical and field studies have indicated that electric wavetilt using the transverse magnetic (TM) waves detects lateral inhomogeneities virtually instantly as abrupt changes in electrical properties are encountered. Theoretical studies have also indicated that magnetic wavetilt measurements using transverse electric (TE) waves are superior to electric wavetilts for such purposes.An experimental survey was conducted at two locations near Atikokan, Ontario, to verify the theoretical predictions. The survey area, forming a part of a large granitic pluton, was mapped earlier by various geophysical techniques, including the ground VLF‐EM method, to detect weak conductors formed either by the presence of fractures in the bedrock filled with water and/or clay, or overburden filling bedrock depressions.A small, multi‐turn, horizontal loop was used during the survey as the transmitter to generate TE waves at eleven frequencies from 10.7 to 58.5 kHz. The magnetic wavetilt measurements detected all previously known conductors at the two locations. In addition, the survey detected several weak conductors that were missed by the VLF survey. Thus, the survey indicated the usefulness of magnetic wavetilt results for detection of weak conductors at shallow depths, which may have application in engineering geophysical surveys. The multi‐frequency wavetilt data also provided some indications of the depth and depth extent of such conductors.
STATIC CORRECTIONS ON CRYSTALLINE ROCKS1
Authors: DAHL‐JENSEN TRINE 
Published: April 2006

AbstractStatic corrections for reflection seismic data shot on high velocity crystalline rock can be calculated directly from the data using a global, iterative stackpower optimization. The reflections consist of several peaks, creating the possibility of aligning the wrong peak in the signals. Due to misalignments a large number of local maxima in the stackpower exist in a cyclic manner, only slightly smaller than thes stackpower of the‘best’stack. Therefore the search must be of a global nature. A Monte Carlo search requires long run times. A global search method is presented using a varying sequence of parameters within each iteration. The ability for a set of static corrections and the associated stackpower to move from a local maximum to a larger maximum is enhanced. The performance of the iterative process is improved, so a relatively small number (about 20) of iterations is needed to obtain the optimum set of corrections. The risk of misalignment of traces by using the wrong peak in a signal consisting of several peaks is diminished, and it is less important that the initial set of corrections is close to the final set. The method is illustrated by a synthetic example and on a data set shot on the granites of Dalarna, Sweden.
RESOLUTION AND NMO‐STRETCH: IMAGING BY STACKING1
Authors: HALDORSEN JAKOB B. U. , FARMER PAUL A. 
Published: April 2006

AbstractWe analyse the stacking process within the framework of imaging techniques. Our results show how the NMO stretch, traditionally looked upon as giving a negative contribution, can be utilized to improve the vertical resolution of the stacked data from a source deficient in low frequencies. The added bandwidth is provided by the spatial coherency of the energy emitted by a point source.
OPTIMIZED RECEIVER ARRAY SIMULATION BASED UPON RESOLUTION CONSTRAINTS1
Authors: CARLINI A. , MAZZOTTI A. 
Published: April 2006

ABSTRACTThe use of cables with a large number of closely spaced receivers has become increasingly common in marine seismic acquisition; this gives geophysicists the opportunity to improve data quality in processing centres. In particular, arrays need not be fixed in the field, but instead can be flexibly simulated in the computer.The paper defines a strategy to optimize the application of receiver‐array simulation, based on the requirement that signal resolution should not be compromised. The approach consists in the design of k‐domain filters through the use of Chebyshev polynomials that define the spatial response of the desired array, with array length that varies with time according to spatial and vertical resolution constraints set by the interpreter.An example, taken from a marine 240‐channel seismic line, ties the underlying ideas to practical application and shows that appropriate array processes can contribute to improving the quality of seismic data, especially in areas with complex geology and strong coherent noise.
RESIDUAL DEPTH‐MIGRATION: ENHANCING DIPS1
Authors: BENSON ALVIN K. 
Published: April 2006

ABSTRACTResidual migration can correct for migration done with the wrong velocity. The initial migration can be done with a less accurate, fast time‐migration algorithm followed by a residual depth‐migration operator. This combines the speed and accuracy of different migration schemes.The residual depth‐migration equation can be obtained by an appropriate transformation of a typical finite‐difference, depth‐migration equation. Since the cheaper, initial time‐migration has already partially migrated the data, the residual depth‐migration has less work to do than if the depth‐migration were done in a single step. As a result, the overall process has increased the speed and reduced the dip limitations associated with typical depth‐migration operators. Results on a steeply dipping model demonstrate the validity and usefulness of residual depth‐migration.
IMPROVED LOW‐FREQUENCY DECAY ESTIMATION USING THE MULTITAPER SPECTRAL ANALYSIS METHOD1
Authors: WALDEN A. T. 
Published: April 2006

AbstractSpectral analysis is one of the most ubiquitous signal processing tools used in exploration geophysics. Among many applications, it is used simply to look at the frequency content of seismic traces, to find notches, to estimate wavelets under the minimum‐phase assumption, and to match broadband synthetic seismograms to seismic data.Seismic spectra exhibit very large dynamic ranges, particularly at low frequencies. Estimation of low‐frequency decay is very important for accurate modelling. However, when using traditional spectral estimates incorporating smoothing windows, too much sidelobe energy leaks from high power into low power areas, spoiling our ability to estimate low‐frequency spectral decay. The multitaper method of spectral analysis due to D. Thomson does not employ just a single window, but rather a set of orthogonal data tapers. It is possible to have much less sidelobe contamination, while maintaining a stable estimate.The trace is tapered by each of a subset of the orthogonal tapers, and a raw spectral estimate produced in each case. These are combined to produce a final spectral estimate. The technique can be made adaptive by applying different weights to the different raw spectra at different frequencies.A comparison of seismic spectral estimation using this multitaper technique with a traditional approach having the same analysis bandwidth and stability demonstrates the very different estimates of spectral decay in the areas of high dynamic range. The multitaper approach provides estimates with much reduced sidelobe leakage, and hence is a very appealing method for reflection seismology.
SHEAR‐WAVE VELOCITY ESTIMATION FROM FULL SONIC WAVEFORM BY THE EXTENDED‐LATTICE PREDICTOR1
Authors: ANGELERI G. P. , BURRASCANO P. , MARTINELLI G. , ORLANDI G. 
Published: April 2006

AbstractOne of the basic parameters of the rock formation surrounding a fluid‐filled borehole to be estimated is the shear‐wave velocity. In the present contribution a novel method for carrying out this estimate, based on the use of linear prediction techniques, is proposed. It is assumed that the shape and energy content of each wave can be accurately modelled by an ARMA (Auto Regressive Moving Average) impulsive process and by an appropriate delay. The overall seismogram is then considered to be a multiple impulsive ARMA process and the estimation is carried out by using the residual at the output of an extended version of the Burg lattice predictor. The resulting algorithm is very effective as illustrated by several examples performed on synthetic and real seismograms.
RESIDUAL STATICS BY CDP‐LOCALIZED STACK OPTIMIZATION1
Authors: KIRCHHEIMER F. 
Published: April 2006

AbstractMany conventional schemes of automated residual statics estimate time lags between prestack traces (sorted as CDP gathers) and a model section, and transform the collected lags into surface‐consistent residuals. The method discussed in this paper aims at improving the lag estimator. ‘Externally generated’ reference traces are avoided and a principle of localized stack optimization is introduced whereby application of a multichannel filter to the stack and evaluation of the normalized power of every filtered trace gives a measure of the stack quality. One may consider the power as a function of all variable (inconsistent) shifts applied to the prestack traces. To obtain a set of optimal lag estimates the power function must be maximized. This power function is complex and the number of its variables prohibits a straightforward search for the maximum. Thus an iterative method must be employed, and steepest descent schemes have proved the most satisfactory. In the actual calculation, the repeated evaluation of the objective can be replaced by the computation of certain cross‐correlations. At the last iteration (after five to ten coordinate sweeps), the global behaviour of this correlation gives some indications of how well a prestack trace is adapted to the filtered stack. This information is used to compute a weighting factor to be stored with the lag estimate. At this stage simple statistical procedures are run to eliminate the most unlikely estimates. The remaining ones are transformed to surface‐consistent residuals by means of a weighted least‐squares inversion according to a model which takes into account the fact that the lags have no fixed reference datum.The efficiency of the method is demonstrated by a field data example into which synthetic anomalies were introduced, and the effect of the new process is compared with that of a ‘classical’ production program using field data with genuine static problems.
INTERPRETATION OF VLF‐EM IN‐PHASE DATA USING CURRENT DENSITY PSEUDOSECTIONS1
Authors: OGILVY R. D. , LEE A. C. 
Published: April 2006

AbstractThe interpretation of VLF‐EM surveys in terms of buried conductors can be assisted by the application of a linear filter to the observed in‐phase component of the vertical magnetic field. One such scheme is examined critically by using the calculated response from a variety of synthetic models to compute theoretical current density pseudosections.The results confirm that this filter technique provides a useful complementary tool for studying the third (i.e. depth) dimension. For single, steeply dipping plates (> 45°) diagnostic information may be derived concerning the depth, size, lateral location, and direction of dip, since the current density maxima seems always to occur within the conductor or at least within one data interval.However, there are some limitations which do not appear to be widely recognized. For single plates the angle of dip cannot be resolved by the current density transformation. Pattern distortions can occur where targets are in close proximity, where the cross‐sectional form of the conductor is complex or where the dip is shallow (< 45°). In these latter cases the current density maxima may not occur within the conductive structure and therefore cannot be used to infer depth of burial and/or conductor shape with the same degree of confidence.
AN ALTERNATIVE STRATEGY FOR NON‐LINEAR INVERSION OF SEISMIC WAVEFORMS1
Authors: SAMBRIDGE M. S. , TARANTOLA A. , KENNETT B. L. N. 
Published: April 2006

AbstractA common example of a large‐scale non‐linear inverse problem is the inversion of seismic waveforms. Techniques used to solve this type of problem usually involve finding the minimum of some misfit function between observations and theoretical predictions. As the size of the problem increases, techniques requiring the inversion of large matrices become very cumbersome. Considerable storage and computational effort are required to perform the inversion and to avoid stability problems. Consequently methods which do not require any large‐scale matrix inversion have proved to be very popular. Currently, descent type algorithms are in widespread use. Usually at each iteration a descent direction is derived from the gradient of the misfit function and an improvement is made to an existing model based on this, and perhaps previous descent directions.A common feature in nearly all geophysically relevant problems is the existence of separate parameter types in the inversion, i.e. unknowns of different dimension and character. However, this fundamental difference in parameter types is not reflected in the inversion algorithms used. Usually gradient methods either mix parameter types together and take little notice of the individual character or assume some knowledge of their relative importance within the inversion process.We propose a new strategy for the non‐linear inversion of multi‐offset reflection data. The paper is entirely theoretical and its aim is to show how a technique which has been applied in reflection tomography and to the inversion of arrival times for 3D structure, may be used in the waveform case. Specifically we show how to extend the algorithm presented by Tarantola to incorporate the subspace scheme. The proposed strategy involves no large‐scale matrix inversion but pays particular attention to different parameter types in the inversion.We use the formulae of Tarantola to state the problem as one of optimization and derive the same descent vectors. The new technique splits the descent vector so that each part depends on a different parameter type, and proceeds to minimize the misfit function within the sub‐space defined by these individual descent vectors. In this way, optimal use is made of the descent vector components, i.e. one finds the combination which produces the greatest reduction in the misfit function based on a local linearization of the problem within the subspace. This is not the case with other gradient methods. By solving a linearized problem in the chosen subspace, at each iteration one need only invert a small well‐conditioned matrix (the projection of the full Hessian on to the subspace). The method is a hybrid between gradient and matrix inversion methods. The proposed algorithm requires the same gradient vectors to be determined as in the algorithm of Tarantola, although its primary aim is to make better use of those calculations in minimizing the objective function.
TWO‐POINT RAY TRACING IN GENERAL 3D MEDIA1
Authors: PEREYRA V. 
Published: April 2006

AbstractThe modelling of realistic 3D geology and wave‐propagation phenomena is an important aspect of exploration geophysics. Ongoing research on a new model representation in terms of strongly reflecting interfaces described by surface patches, and on two‐point (i.e. source‐receiver) non‐zero‐offset ray tracing in such models, is presented.Examples are used to demonstrate how this model representation facilitates the description of geological unconformities such as pinched‐out layers, overhangs and reverse faults. Neither pseudo‐layers, nor ‘transparent’ or zero‐impedance extensions are needed for surfaces that do not naturally cover the geological window under study. These are therefore no longer layered models and consequently they require new ray‐tracing techniques which are described and exemplified.