bluuebunny/crossref_metadata_2025_split huggingface dataset:
A physical approach to computing magnetic fields1
Authors: Furness Peter 
Published: April 2006

AbstractThe superposition integral expressing the field due to a magnetic source body is relatively simple to evaluate in the case of a homogeneous magnetization. In practice this generally requires that any remnant component is uniform and the susceptibility of the body is sufficiently low to permit the assumption of a uniform induced magnetization. Under these conditions the anomalous magnetic field due to a polyhedral body can be represented in an intuitive and physically appealing manner. It is demonstrated that the components of the magnetic field H can be expressed as a simple combination of the potentials due to two elementary source distributions. These are, firstly, a uniform double layer (normally directed dipole moment density) located on the planar polygonal faces of the body and, secondly, a uniform line source located along its edges. In practice both of these potentials (and thus the required magnetic field components) are easily computed. The technique is applicable to polyhedra with arbitrarily shaped faces and the relevant expressions for the magnetic field components are suitable for numerical evaluation everywhere except along the edges of the body where they display a logarithmic singularity.
True‐amplitude migration of 2D synthetic data1
Authors: Hanitzsch C. , Schleicher J. , Hubral P. 
Published: April 2006

AbstractTrue‐amplitude (TA) migration, which is a Kirchhoff‐type modified weighted diffraction stack, recovers (possibly) complex angle‐dependent reflection coefficients which are important for amplitude‐versus‐offset (AVO) inversion. The method can be implemented using existing prestack or post‐stack Kirchhoff migration and fast Green's function computation programs. Here, it is applied to synthetic single‐shot and constant‐offset seismic data that include post‐critical reflections (complex reflection coefficients) and caustics. Comparisons of the amplitudes of the TA migration image with theoretical reflection coefficients show that the (possibly complex) angle‐dependent reflection coefficients are correctly estimated.
Ground penetrating radar sounding of a temperate glacier; modelling of a multilayered medium1
Authors: Nicollin Florence , Kofman Wlodek 
Published: April 2006

AbstractA numerical simulation of electromagnetic propagation through a multilayered medium is performed in order to explain and interpret the signal received from the radar sounding of a temperate glacier. During the winter of 1990, several radar profiles were obtained on the Mont‐de‐Lans glacier in the French Alps with a ground penetrating radar which uses a phase modulation of the transmitted pulse by coded sequences. The pulse compression is obtained by applying the matched filter to the received signal, which provides a range‐resolution of about 8 m in the ice. The profiles recorded on the temperate glacier do not show a single clear reflection from the ice‐bedrock interface, but they exhibit widely distributed energy decreasing with depth. This may be due to the inhomogeneous inner structure of the temperate glacier and we use a simple model of a layered medium to compute a simulation of the propagation. Thus, partial reflection at each layer and scattering from a rough basal interface may explain the observed signal. A computer‐based technique is used to locate on the data the bottom of the glacier in order to estimate the ice thickness. The results from the different radar profiles are consistent and are a good fit to the thickness which has been determined by other geophysical methods.
Statistical analysis of non‐stationary voltage recordings in geoelectrical prospecting1
Authors: Lapenna V. , Macchiato M. , Patella D. , Satriano C. , Serio C. , Tramutoli V. 
Published: April 2006

AbstractWe study the geoelectrical problem of picking out the useful signal from voltage time series, monitored under conditions of a low signal‐to‐noise ratio and non‐stationary noise. Statistical tests performed at different sites show that geoelectrical noise often belongs to the class of non‐stationary phenomena with non‐Gaussian probability distributions. In such cases, the application of conventional methods of geoelectrical useful signal extraction, based on the stationary white‐noise assumption, gives biased estimates. For the on‐line processing of geoelectrical recordings, we recommend the use of the periodogram technique combined with the Kolmogorov–Smirnov test, a suitable algorithm of which is described in detail. The suggested procedure allows data acquisition to stop as soon as the useful signal power is estimated with a relative error smaller than a predetermined value. Finally, we compare the suggested procedure with the autoregressive approach. The previously used and simpler periodogram method, applied to the solution of problems of this kind, appears to give better performances than the autoregressive analysis.
Generalized acoustic diffraction tomogra phy1
Authors: Gelius Leiv‐J. 
Published: April 2006

AbstractA generalized diffraction tomography algorithm is developed, which in principle can handle irregularly spaced data, curved acquisition lines and non‐uniform background models. By direct comparison with medical diffraction tomography, it is shown that the generalized method involves the same two processing steps: data filtering and back‐propagation. The filter handles the irregular sampling of the model space and the uneven energy coverage, while the back‐propagation operator removes the wave propagation effects. Paraxial ray‐tracing techniques are employed to compute both these quantities.In medical diffraction tomography, the resolution vector (i.e. the Fourier vector of the model space) is defined by the incident and scattered plane‐wave directions. It is shown here that a similar relationship exists for a non‐uniform background, where the resolution vector at a particular image point is defined by the incident and scattered ray directions. Consequently, the impulse response of the generalized algorithm becomes space variant.Finally, a general processing procedure for transmission mode seismic data, based on this generalized algorithm, is proposed. The potential of the method is demonstrated using synthetic cross‐hole data.
A new velocity model for clay‐sand mixtures1
Authors: Xu Shiyu , White Roy E. 
Published: April 2006

AbstractNone of the standard porosity‐velocity models (e.g. the time‐average equation, Raymer's equations) is satisfactory for interpreting well‐logging data over a broad depth range. Clays in the section are the usual source of the difficulty through the bias and scatter that they introduce into the relationship between porosity and P‐wave transit time. Because clays are composed of fine sheet‐like particles, they normally form pores with much smaller aspect ratios than those associated with sand grains. This difference in pore geometry provides the key to obtaining more consistent resistivity and sonic log interpretations.A velocity model for Clay–sand mixtures has been developed in terms of the Kuster and Toksöz, effective medium and Gassmann theories. In this model, the total pore space is assumed to consist of two parts: (1) pores associated with sand grains and (2) pores associated with clays (including bound water). The essential feature of the model is the assumption that the geometry of pores associated with sand grains is significantly different from that associated with clays. Because of this, porosity in shales affects elastic compliance differently from porosity in sand‐Stones. The predictive power of the model is demonstrated by the agreement between its predictions and laboratory measurements and by its ability to predict sonic logs from other logs over large depth intervals where formations vary from unconsolidated to consolidated sandstones and shales.
A joint inversion algorithm to process geoelectric and sutface wave seismic data. Part I: basic ideas1
Authors: Hering A. , Misiek R. , Gyulai A. , Ormos T. , Dobroka M. , Dresen L. 
Published: April 2006

AbstractFor the exploration of near‐surface structures, seismic and geoelectric methods are often applied. Usually, these two types of method give, independently of each other, a sufficiently exact model of the geological structure. However, sometimes the inversion of the seismic or geoelectric data fails.These failures can be avoided by combining various methods in one joint inversion which feads to much better parameter estimations of the model than the independent inversions.A suitable seismic method for exploring near‐surface structures is the use of dispersive surface waves: the dispersive characteristics of Rayleigh and Love surface waves depend strongly on the structural and petrophysical (seismic velocities) features of the near‐surface Underground.Geoelectric exploration of the structure Underground may be carried out with the well‐known methods of DC resistivity sounding, such as the Schlumberger, the radial‐dipole and the two‐electrode arrays.The joint inversion algorithm is tested by means of synthetic data. It is demonstrated that the geoelectric joint inversion of Schlumberger, radial‐dipole and two‐electrode sounding data yields more reliable results than the single inversion of a single set of these data. The same holds for the seismic joint inversion of Love and Rayleigh group slowness data. The best inversion result is achieved by performing a joint inversion of both geoelectric and surface‐wave data.The effect of noise on the accuracy of the solution for both Gaussian and non‐Gaussian (sparsely distributed large) errors is analysed. After a comparison between least‐square (LSQ) and least absolute deviation (LAD) inversion results, the LAD joint inversion is found to be an accurate and robust method.
Direct current electric potential computation in an inhomogeneous and arbitrarily anisotropic layered earthl
Authors: Das Umesh C. 
Published: April 2006

AbstractWe consider the calculation of the electrical field quantities, electric potential and the vertical component of the total volume density of electric current, in a horizontally layered, piecewise homogeneous and arbitrarily anisotropic earth due to a system of direct current point sources. By applying Fourier transformation with respect to the horizontal space coordinates to the static field equations, the field quantities are obtained as the solutions of the system of transform‐domain differential equations in the vertical (depth) coordinates. A recurrence scheme has been given to compute the tranform‐domain field quantities at any depth. The corresponding space‐domain quantities are then obtained by inverse Fast Fourier Transformation (FFT). A complete computer program has been developed for computing the electric potentials at any depth of the layered earth, which is composed of an arbitrary number of anisotropic layers with arbitrary conductivity tensors. By considering the point sources at different depths from the surface, equipotential contours on the surface of arbitrarily anisotropic layered earth models are given.
Imaging beneath an opaque basaltic layer using densely sampled wide‐angle OBS data1
Authors: Samson C. , Barton P.J. , Karwatowski J. 
Published: April 2006

AbstractA combined reflection/refraction (wide‐angle) seismic survey was conducted on the continental shelf north‐west of Britain, using a conventional streamer with an airgun source, and static ocean‐bottom seismometers (OBS) to record wide‐angle energy. The shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection section. The basalts, however, proved to be opaque to the conventional reflection method and prevented the imaging of deeper horizons, where an important velocity inversion was anticipated. This paper reports on the processing, modelling and interpretation of the densely sampled wide‐angle OBS data that were coincident with the reflection profile. Eleven OBS instruments were deployed along a 75 km line and recorded signal from a powerful 149 litre (9100 in.3) airgun array fired every 50 m. Data processing was performed using a standard industrial reflection seismic software package prior to first‐arrival picking. Processing steps included geometry definition, trace summation and display of the data using various scaling algorithms. An initial model was constructed from 1D velocity‐time profiles digitized every 4 km along the stacked section. First arrival traveltime modelling rapidly converged to a detailed model of the structure of the top 5 km of the crust. Modelling revealed the existence of a buried low‐velocity Mesozoic sedimentary basin, of a prominent basement horst and of a normal fault penetrating to the basement.
Cancelling directional EM noise in magnetoteilurics1
Authors: Santarato G. , Spagnolini U. 
Published: April 2006

AbstractThe prospecting of densely urbanized areas by the measurement of magnetic and electric natural fields is severely hampered by electromagnetic (EM) noise. Active man‐made EM noise sources can generally be considered fixed in space, thus affecting the magnetotelluric (MT) signals of a measuring site mainly along their polarization directions.Taking advantage of the impulsive nature of polarized EM noise, a time‐domain directional noise cancelling (DNC) technique is proposed. The comparison of noisy data with data predicted, using a low noise reference signal or with data interpolated whenever no reference is available, allows the detection of the most likely noise sources with prevailing directional patterns using a Bayes's criterion. The DNC approach is general and can be adapted, depending on the reference signal used (single‐site or remote‐reference).In field data, hodograms of the prediction residuals basically confirm the directional noise model assumed in DNC. An example is presented in which the DNC technique has been applied to a single‐site MT survey carried out in northern Italy, where the signal was heavily corrupted by noise with prevailing directional properties due to the densely urbanized area. MT apparent resistivities and phases obtained at the site of the survey before and after DNC are presented and discussed.
Downward continuation of (Vs)L profile in 1P (time‐domain) with linear excitation using point electrodes. A short notet1
Authors: Apparao A. , Sastry R. Sivarama , Sarma V. Subrahmanya 
Published: April 2006

AbstractIt is seen that the apparent chargeability (Ma)L anomaly over a 2D graphite body splits into two distinct (Vs)L anomalies which closely follow the apparent resistivity profile. This suggests that the electric field amplitude is distorted due to a superficial inhomogeneity creating a (Vs)L anomaly, which bears no relation to the polarized body. The target depth obtained by continuation of such a profile is therefore, not acceptable.
3D depth migration by rotated McClellan filters1
Authors: Biondi Biondo , Palacharla Gopal 
Published: April 2006

AbstractThe application of McClellan transformations considerably reduces the computational cost of 3D wavefield depth extrapolation by explicit convolutional methods. The accuracy of migration methods based on McClellan transformation depends on how well the transformation filter (cos !;κ!;) is approximated; errors in this approximation cause anisotropy in the extrapolation operator and frequency dispersion in the migrated results. The anisotropy can be greatly reduced by rotating the approximate filter by 45° and averaging the rotated filter with the original filter. The application of the rotated filter yields a migration method that correctly images very steep dips, with little or no additional computational cost. McClellan migration with the improved circular response enhances the imaging of synthetic and real data.
2.5D full‐wavefield viscoacoustic invercion1
Authors: Liao Oingbo , McMechan George A. 
Published: April 2006

AbstractFull‐wavefield inversion for distributions of acoustic velocity, density and Q on a vertical slice through a25D model is implemented for common‐source gathers in a cross‐hole geometry. The wavefield extrapolation used is 3D, so all geometrical spreading, scattering, reflection, and transmission effects are correctly and automatically compensated for. In order to keep the number of unknowns tractable, application was limited to 2.5D models of known geometry; the latter assurnes a prior step, such as tomography, to fix the layer geometries. With the model geometry fixed, reliable solutions are obtained using synthetic data from only two independent source locations. Solutions from data with noisy and missing traces are comparable to those from noise‐free data, but with higher residuals. When the source locations are spatially widely separated, conunon‐source gathers may be summed and treated as a single wavefield to yield the same model estimates as when the individual source wavefields are treated separately, at substantially reduced cost. Inversions for full 3D parameter distributions can be handled with the same software, requiring only solution for more unknowns.
Interpretation of slingram conductivity mapping in near‐surface geophysics: using a single parameter fitting with 1D model1
Authors: Guérin R. , Méhéni Y. , Rakotondrasoa G. , Tabbagh A. 
Published: April 2006

AbstractElectrical conductivity mapping is a prerequisite tool for hydrogeological or environmental studies. Its interpretation still remains qualitative but advantages can be expected from a quantitative approach. However a full 3D interpretation is too laborious a task in comparison with the limited cost and time which are involved in the majority of such field studies. It is then of value to define the situations where lateral variations are sufficiently smooth for a 1D model to describe correctly the underlying features.For slingram conductivity measurements, criteria allowing an approximate 1D inversion are defined: these mainly consist of a limited rate of variation over three times the intercoil spacing.In geological contexts where the weathering has generated a conductive intermediate layer between the underlying sound rock and the soil, this processing can be applied to determine the thickness of the conductive layer from the apparent resistivity map when the other geoelectrical parameters are known. The examples presented illustrate this application.
Bayesian inference, Gibbs' sampler and uncertainty estimation in geophysical inversion1
Authors: Sen Mrinal K. , Stoffa Paul L. 
Published: April 2006

AbstractThe posterior probability density function (PPD), σ(m|dobs), of earth model m, where dobs are the measured data, describes the solution of a geophysical inverse problem, when a Bayesian inference model is used to describe the problem. In many applications, the PPD is neither analytically tractable nor easily approximated and simple analytic expressions for the mean and variance of the PPD are not available. Since the complete description of the PPD is impossible in the highly multi‐dimensional model space of many geophysical applications, several measures such as the highest posterior density regions, marginal PPD and several orders of moments are often used to describe the solutions. Calculation of such quantities requires evaluation of multidimensional integrals. A faster alternative to enumeration and blind Monte‐Carlo integration is importance sampling which may be useful in several applications. Thus how to draw samples of m from the PPD becomes an important aspect of geophysical inversion such that importance sampling can be used in the evaluation of these multi‐dimensional integrals. Importance sampling can be carried out most efficiently by a Gibbs' sampler (GS). We also introduce a method which we called parallel Gibbs' sampler (PGS) based on genetic algorithms (GA) and show numerically that the results from the two samplers are nearly identical.We first investigate the performance of enumeration and several sampling based techniques such as a GS, PGS and several multiple maximum a posteriori (MAP) algorithms for a simple geophysical problem of inversion of resistivity sounding data. Several non‐linear optimization methods based on simulated annealing (SA), GA and some of their variants can be devised which can be made to reach very close to the maximum of the PPD. Such MAP estimation algorithms also sample different points in the model space. By repeating these MAP inversions several times, it is possible to sample adequately the most significant portion(s) of the PPD and all these models can be used to construct the marginal PPD, mean) covariance, etc. We observe that the GS and PGS results are identical and indistinguishable from the enumeration scheme. Multiple MAP algorithms slightly underestimate the posterior variances although the correlation values obtained by all the methods agree very well. Multiple MAP estimation required 0.3% of the computational effort of enumeration and 40% of the effort of a GS or PGS for this problem. Next, we apply GS to the inversion of a marine seismic data set to quantify uncertainties in the derived model, given the prior distribution determined from several common midpoint gathers.
Reflection from a deep interface in a strongly heterogeneous layered medium1
Authors: Lewicki Pawel , Burridge Robert 
Published: April 2006

AbstractWe consider the problem of acoustic pulse propagation through a layered medium with a reflector at one end. The fluctuations in the medium properties are assumed to be strong, i.e. of finite amplitude, rapid in comparison to the typical wavelength and to have statistical structure. The depth of the reflector is assumed to be large in comparison to the wavelength. In this regime, simple formulae for the reflected pulse and its arrival time at the surface are obtained. The amplitude of the pulse is broadened and attenuated as a result of multiple scattering: the fine‐layered structure of the medium can be characterized by a single constant which appears in the formula for the limiting waveform and which measures the size of the fluctuations in the medium. Within the theory, the commonly observed discrepancy between the integrated sonic traveltime and the seismic traveltime can be studied and understood. The theory is a natural extension of the long‐wavelength effective medium theory of Backus. The analysis is rigorous and based on the invariant embedding technique.
Seismic migration and absorbing boundaries with a one‐way wave system for heterogeneous media1
Authors: Dai Nanxun , Vafidis Antonios , Kanasewich Ernest R. 
Published: April 2006

AbstractA first‐order one‐way wave system has been created based on characteristic analysis of the acoustic wave system and optimization of the dispersion relation. We demonstrate that this system is equivalent to a third‐order scalar partial‐differential equation which, for a homogeneous medium, reduces to a form similar to the 45° paraxial wave equation. This system describes accurately waves propagating in a 2D heterogeneous medium at angles up to 75°.The one‐way wave system representing downgoing waves is used for a modified reverse time migration method. As a wavefield extrapolator in migration, the downgoing wave system propagates the reflection events backwards to their reflectors without scattering at the discontinuities in the velocity model. Hence, images with amplitudes proportional to reflectivity can be obtained from this migration technique. We present examples of the application of the new migration method to synthetic seismic data where P‐P reflections P‐SV converted waves are present.Absorbing boundaries, useful in the generation of synthetic seismograms, have been constructed by using the one‐way wave system. These boundaries absorb effectively waves impinging over a wide range of angles of incidence.