bluuebunny/crossref_metadata_2025_split huggingface dataset:
STOCHASTIC INVERSION BY RAY CONTINUATION: APPLICATION TO SEISMIC TOMOGRAPHY1
Authors: HAAS A. , VIALLIX J. R. 
Published: April 2006

AbstractThe conventional tomographic inversion consists in minimizing residuals between measured and modelled traveltimes. The process tends to be unstable and some additional constraints are required to stabilize it. The stochastic formulation generalizes the technique and sets it on firmer theoretical bases.The Stochastic Inversion by Ray Continuation (Sirc) is a probabilistic approach, which takes a priori geological information into account and uses probability distributions to characterize data correlations and errors. It makes it possible to tie uncertainties to the results.The estimated parameters are interval velocities and B‐spline coefficients used to represent smoothed interfaces. Ray tracing is done by a continuation technique between source and receivers. The ray coordinates are computed from one path to the next by solving a linear system derived from Fermat's principle. The main advantages are fast computations, accurate traveltimes and derivatives.The seismic traces are gathered in CMPs. For a particular CMP, several reflecting elements are characterized by their time gradient measured on the stacked section, and related to a mean emergence direction.The program capabilities are tested on a synthetic example as well as on a field example. The strategy consists in inverting the parameters for one layer, then for the next one down. An inversion step is divided in two parts. First the parameters for the layer concerned are inverted, while the parameters for the upper layers remain fixed. Then all the parameters are reinverted.The velocity‐depth section computed by the program together with the corresponding errors can be used directly for the interpretation, as an initial model for depth migration or for the complete inversion program under development.
GROUND‐PENETRATING RADAR FOR HIGH‐RESOLUTION MAPPING OF SOIL AND ROCK STRATIGRAPHY1
Authors: DAVIS J. L. , ANNAN A. P. 
Published: April 2006

ABSTRACTGround‐penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions. The need to better understanding overburden conditions for activities such as geochemical sampling, geotechnical investigations, and placer exploration, as well as the factors controlling groundwater flow, has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible.The areas of application for ground‐penetrating radar are diverse. The method has been used successfully to map ice thickness, water depth in lakes, bedrock depth, soil stratigraphy, and water table depth. It is also used to delineate rock fabric, detect voids and identify karst features. The effective application of the radar for the high‐resolution definition of soil stratigraphy and fractures in bedrock is highlighted.The basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories. One example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m. Two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine. Another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination. The corroboration of the radar results by borehole investigations demonstrates the power and utility of the high‐resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes. With the advent of new instrumentation and field procedures, the routine application of the radar method is becoming economically viable and the method will see expanded use in the future.
SUGGESTIONS FOR A CONSISTENT TERMINOLOGY FOR SEISMIC ANISOTROPY1
Authors: CRAMPIN STUART 
Published: April 2006

ABSTRACTSeismic anisotropy is an unfamiliar concept to many geophysicists and the use of misleading and ambiguous terminology has made it more difficult to understand. I suggest here a consistent terminology in which simple expressions have specific meanings similar to their colloquial meanings. It is hoped that use of such language will help to make the increasing number of papers reporting seismic anisotropy more readily comprehensible to the non‐specialist. This not a manual of anisotropy, and it is not intended for theoreticians. It is a list of terms which may make anisotropy a little easier to understand for those more familiar with wave propagation in isotropic solids.
ACHIEVEMENTS IN COPPER SULPHIDE EXPLORATION IN ALBANIA WITH IP AND EM METHODS1
Authors: LANGORE L. , ALIKAJ P. , GJOVREKU D. 
Published: April 2006

ABSTRACTThe copper sulphide exploration programme in Albania involves a number of geophysical methods. The most important ones are the Induced Polarization (IP) and the Turam methods. This paper reports some recent achievements in increasing the depth of investigation and in discriminating sulphide ore textures by the IP, spectral IP and Turam methods.
MODELLING OF SOFT SEDIMENTS AND LIQUID‐SOLID INTERFACES: MODIFIED WAVENUMBER SUMMATION METHOD AND APPLICATION1
Authors: KERNER CLAUDIA 
Published: April 2006

AbstractAlekseev and Mikhailenko have developed a wavenumber‐summation method which combines a finite integral transformation with a finite‐difference calculation and involves no approximations other than numerical ones. However, numerical anisotropy causes velocity errors for shear waves which are unacceptable if Poisson's ratios are larger than 0.4 and unless the number of grid points per wavelength is chosen considerably higher than the value generally regarded as sufficient in finite‐difference computations. To overcome this limitation in the applicability of the otherwise very powerful modelling scheme, the method is applied to the elastodynamic equations for the velocity vector. Thus, instead of solving a second‐order hyperbolic system as in the case of the wave equation, solutions to a first‐order hyperbolic system are computed. The finite‐difference iteration is performed in a staggered grid. In addition to mastering the numerical difficulties in cases where the Poisson's ratio is unusually high, this approach results in a code which can be used for the modelling of liquid layers.With the new scheme, water reverberations are investigated in terms of normal modes. It is found that for realistic sea‐bottom velocities the critical and supercritical cases exist only for P‐waves. It means that compressional waves are trapped within the water layer but energy leaks into the substratum through converted shear waves. These leaky compressional normal modes attain properties similar to those of shear normal modes or Pseudo‐Love waves. Due to their origin from conversion of dispersed multi‐modal compressional waves the shear waves generated at the sea‐bottom form a long complex wavetrain. They were found to mask the reflections from the target horizon in an offset‐VSP field section.
INVERSE THEORY APPLIED TO MULTI‐SOURCE CROSS‐HOLE TOMOGRAPHY.
Authors: PRATT R. GERHARD 
Published: April 2006

AbstractIt is advantageous to consider inversion of multi‐source (wide‐aperture) cross‐hole data using methods that (i) are based on the wave equation rather than its high‐frequency ray approximation, and (ii) use the full information content of the recorded wavefield rather than only first‐arrival times. Wave‐theoretical methods require the ability to forward‐model appropriate wave equations for all source positions in arbitrary reference media. This can be achieved using a frequency‐domain elastic wave propagator that facilitates the modelling of multi‐source data at the cost of temporal bandwidth. The trade‐off is deliberate; the propagator is applied to the cross‐hole imaging problem, in which wide spatial bandwidths are more important than temporal bandwidth.By using the frequency‐domain propagator, non‐linear inverse techniques are applied to data from a very large number of source positions. The method can be applied in 2D media of arbitrary a priori complexity. In a synthetic example, compressional and shear‐velocity perturbations are successfully resolved with one iteration using only a single frequency component of wide‐aperture elastic wave cross‐hole data.
WEIGHTED STACKING OF SEISMIC DATA USING AMPLITUDE‐DECAY RATES AND NOISE AMPLITUDES1
Authors: ANDERSON RICHARD G. , McMECHAN GEORGE A. 
Published: April 2006

AbstractThe signal‐to‐noise (S/N) ratio of seismic reflection data can be significantly enhanced by stacking. However, stacking using the arithmetic mean (straight stacking) does not maximize the S/N ratio of the stack if there are trace‐to‐trace variations in the S/N ratio. In this case, the S/N ratio of the stack is maximized by weighting each trace by its signal amplitude divided by its noise power, provided the noise is stationary. We estimate these optimum weights using two criteria: the amplitude‐decay rate and the measured noise amplitude for each trace. The amplitude‐decay rates are measured relative to the median amplitude‐decay rate as a function of midpoint and offset. The noise amplitudes are measured using the data before the first seismic arrivals or at late record times. The optimum stacking weights are estimated from these two quantities using an empirical equation.Tests with synthetic data show that, even after noisy‐trace editing, the S/N ratio of the weighted stack can be more than 10 dB greater than the S/N ratio of the straight stack, but only a few decibels more than the S/N ratio of the trace equalized stack. When the S/N ratio is close to 0 dB, a difference of 4 dB is clearly visible to the eye, but a difference of 1 dB or less is not visible. In many cases the S/N ratio of the trace‐equalized stack is only a few decibels less than that of the optimum stack, so there is little to be gained from weighted stacking. However, when noisy‐trace editing is omitted, the S/N ratio of the weighted stack can be more than 10 dB greater than that of the trace‐equalized stack. Tests using field data show that the results from straight stacking, trace‐equalized stacking, and weighted stacking are often indistinguishable, but weighted stacking can yield slight improvements on isolated portions of the data.
Lp‐NORM DECONVOLUTION1
Authors: DEBEYE H. W. J. , VAN RIEL P. 
Published: April 2006

AbstractThe purpose of deconvolution is to retrieve the reflectivity from seismic data. To do this requires an estimate of the seismic wavelet, which in some techniques is estimated simultaneously with the reflectivity, and in others is assumed known. The most popular deconvolution technique is inverse filtering. It has the property that the deconvolved reflectivity is band‐limited. Band‐limitation implies that reflectors are not sharply resolved, which can lead to serious interpretation problems in detailed delineation.To overcome the adverse effects of band‐limitation, various alternatives for inverse filtering have been proposed. One class of alternatives is Lp‐norm deconvolution, L1norm deconvolution being the best‐known of this class.We show that for an exact convolutional forward model and statistically independent reflectivity and additive noise, the maximum likelihood estimate of the reflectivity can be obtained by Lp‐norm deconvolution for a range of multivariate probability density functions of the reflectivity and the noise. The L∞‐norm corresponds to a uniform distribution, the L2‐norm to a Gaussian distribution, the L1‐norm to an exponential distribution and the L0‐norm to a variable that is sparsely distributed. For instance, if we assume sparse and spiky reflectivity and Gaussian noise with zero mean, the Lp‐norm deconvolution problem is solved best by minimizing the L0‐norm of the reflectivity and the L2‐norm of the noise. However, the L0‐norm is difficult to implement in an algorithm. From a practical point of view, the frequency‐domain mixed‐norm method that minimizes the L1norm of the reflectivity and the L2‐norm of the noise is the best alternative.Lp‐norm deconvolution can be stated in both time and frequency‐domain. We show that both approaches are only equivalent for the case when the noise is minimized with the L2‐norm.Finally, some Lp‐norm deconvolution methods are compared on synthetic and field data. For the practical examples, the wide range of possible Lp‐norm deconvolution methods is narrowed down to three methods with p= 1 and/or 2. Given the assumptions of sparsely distributed reflectivity and Gaussian noise, we conclude that the mixed L1norm (reflectivity) L2‐norm (noise) performs best. However, the problems inherent to single‐trace deconvolution techniques, for example the problem of generating spurious events, remain. For practical application, a greater problem is that only the main, well‐separated events are properly resolved.
IMPROVEMENT OF OBSERVATION ACCURACY OF THE LACOSTE‐ROMBERG (MODEL D) GRAVITY METER BY SUPPLEMENTARY INSTALLATION OF ELECTRONIC FEEDBACK1
Authors: CASTEN U. , HAUSSMANN U. 
Published: April 2006

AbstractRecent years have seen an increasing need for high‐precision gravity meters. A widely used and the most accurate one is the LaCoste‐Romberg, model D (LCR‐D) meter, equipped with electronic readout. According to the manual the reading accuracy is 5 μGal. A way of reducing most of the instrumental factors limiting the accuracy is the use of an electronic feedback system.We have fitted the LCR‐D 34 with a Schnüll‐Röder‐Wenzel, model D (SRW‐D) feedback. After installation the readout voltage was calibrated, the instrumental behaviour tested, and the accuracy of the system determined. By repeat readings in the laboratory without moving the meter, the standard deviation for a single reading is better than 4 μGal in normal mode and better than 1 μGal in feedback mode. The accuracy of gravity differences ‐ this is usually observed in field practice ‐ is the mean value of the repeat errors of several sets of differences observed in a short time to avoid any corrections. This accuracy is better than 9 μGal in normal mode and better than 5 μGal in feedback mode. With this, the accuracy of a single reading becomes more than 6 μGal and more than 3 μGal, respectively.As the described improvement of accuracy was found to be not as good as expected, additional improvements should centre on the use of electronic levels instead of the standard liquid ones.
ALIGNMENT OF NEAR‐SURFACE INCLUSIONS AND APPROPRIATE CRACK GEOMETRIES FOR GEOTHERMAL HOT‐DRY‐ROCK EXPERIMENTS1
Authors: CRAMPIN STUART 
Published: April 2006

AbstractUbiquitous splitting of seismic shear‐waves indicates that most rocks in the upper half of the crust are pervaded by stress‐aligned fluid‐filled inclusions, called EDA‐cracks. These inclusions are expected to be aligned perpendicular to the minimum compressional stress by stress relationships similar to those aligning industrial hydraulic fractures. At depths where the overburden stress is sufficiently large (typically below a few hundred metres), this minimum stress is usually horizontal, so that the EDA‐cracks and hydraulic fractures are typically aligned vertically, striking parallel, or subparallel, to the direction of maximum compression. This is confirmed by the polarizations of the split shear‐waves along raypaths at depth in the crust. At the free surface, however, the vertical stress is zero (or approximately zero) and cracks (and hydraulic fractures) at shallow depths in intact rock tend to be horizontal. Thus, the directions of minimum stress, and the orientations of hydraulic fractures, are likely to swing through 90° near the surface of the Earth. Since the behaviour of cracks and stress is often crucial to drilling operations, the rotation of the crack‐ and stress‐geometry near‐surface has important implications, particularly for optimizing hydrocarbon production and geothermal reservoir management. Consequently, evidence gained from experiments, for example in hot‐dry‐rock geothermal heat extraction, in inappropriate crack geometries at shallow depths, may not be valid when applied to other crack‐ and stress‐geometries at depth in hot rock.
EFFECTS OF WELL CASING ON POTENTIAL FIELD MEASUREMENTS USING DOWNHOLE CURRENT SOURCES1
Authors: SCHENKEL C. J. , MORRISON H. F. 
Published: April 2006

AbstractA mathematical formulation for the electric potential from point current‐sources coaxial with a metal casing has been obtained. The excitation caused by the axial point‐sources will produce currents in the pipe. By assuming that the pipe can be divided into many cylindrical ring segments with constant axially‐directed current, the solution of the fields inside and outside the pipe can be formulated in an integral form. The integral equation applied to the segmented pipe yields a set of simultaneous linear equations which are solved for the currents in the pipe; these are then used to calculate the potentials anywhere outside the pipe in the medium.This solution has been used to study the distribution of the potentials in a half‐space for a single current‐source at and beyond the bottom of a finite length of casing. For a casing 0.1 m in radius and 0.006 m in wall thickness with a conductivity of 106 S/m, in a half‐space of 10‐2 S/m, it was found that only in a region very near the pipe does the pipe exert substantial influence on the fields of a point‐source 100 casing diameters beyond the end of the pipe. It appears that cross‐hole resistivity surveys can be implemented without corrections for the casing if the source is located at least 50–100 casing diameters beyond the end of the casing. Hole‐to‐surface surveys are much more affected by the pipe. For a pipe‐source separation of 100 casing diameters, the surface measurements must not be closer than a half pipe length for a 5% or less field distortion.
CONSTRUCTION OF COMPONENT MAPS FROM AEROMAGNETIC TOTAL FIELD ANOMALY MAPS1
Authors: PEDERSEN L. B. , RASMUSSEN T. M. , DYRELIUS D. 
Published: April 2006

AbstractTotal field anomalies as defined from normal aeromagnetic surveys give good approximations of the anomalous components along the direction of the main geomagnetic field, which is generally much larger than the anomalous field. Using the relations between vertical and horizontal components of the field, the total field anomaly is related to any vertical or horizontal component and the corresponding horizontal and vertical derivatives. An example from the Siljan impact structure indicates that such directional filters may be applied to extract useful information from magnetic maps.
THE RESOLUTION POSSIBLE IN IMAGING WITH DIFFRACTED SEISMIC WAVES1
Authors: PHADKE SUHAS , KANASEWICH ERNEST R. 
Published: April 2006

AbstractThe determination of the vertical and lateral extent of discontinuities is an important aspect of interpreting seismic reflection data. The Common Fault Point (CFP) stacking method appears to be promising in imaging discontinuities in acoustic impedance by making use of diffracted energy from a spatial array of receivers. The problems of vertical and lateral resolution in the method are most important when carrying out an interpretation.Source signature, subsurface velocities and the depth of the discontinuity are the most important parameters affecting the resolution. We use, for a perfectly coherent source, the first derivative of the Gaussian function which is an antisymmetric band‐limited wavelet. Rayleigh's, Ricker's and Widess' criteria are also applicable to this wavelet. The limits of vertical and lateral resolution are illustrated by using a step fault and a dike model respectively. The vertical resolution of the CFP method is found to be of the order of λ/16 which is half the theoretically predicted value for a single receiver. The lateral resolution is still limited by the size of the Fresnel zone which depends upon the velocity, two‐way time and the dominant frequency of the wavelet. The resolution limits of the CFP method are compared with that of the CDP method, prestack migration and post‐stack migration. Obtaining high resolution with real data is limited by the extent to which it is possible to generate a coherent source or to simulate one during computer processing with before stack seismic data. The CFP method is an artificial intelligence approach to imaging diffracting points as it localizes parts of the structure that scatter acoustic waves.
MODELLING 1D WAVE PROPAGATION IN A SYSTEM OF ABSORBING LAYERS1
Authors: McGEE T. M. 
Published: April 2006

AbstractThe purpose of this paper is to derive relationships useful for the analysis of shallow transmission and reflection seismic data. Since the effects of absorption cause such data to be non‐stationary, the derived results do not rely on frequency‐domain representations being independent of the time origin. Rather, results are expressed in the z‐domain and are suitable for implementation in the time domain. In order to give an unambiguous system of notation a classic approach to the problem of 1D wave propagation in a layered system is fully described. Absorption is included in a general but realistic way and the implications of representing it digitally are considered in detail. Computational efficiency is improved by grouping the layers in blocks of similar absorptive effect. The transmissivity and reflectivity of systems composed of such blocks are obtained and a dereverberation filter identified. The filter is described succinctly in terms of a non‐linear recursion relationship. The sequences obtained by convolving the filter with the transmissivity and reflectivity are discussed in detail. The results are illustrated by nominally realistic synthetic examples computed in the time domain.
APPLICATION OF THE PARTIAL KARHUNEN‐LOÈVE TRANSFORM TO SUPPRESS RANDOM NOISE IN SEISMIC SECTIONS1
Authors: AL‐YAHYA KAMAL M. 
Published: April 2006

AbstractThe Karhunen‐Loève (K–L) transform is an effective technique for suppressing spatially uncorrelated noise, but because of its high computational cost, fast transforms, such as the Fourier transform, have been more favoured. Two techniques that combine to make the K–L transform feasible for seismic data processing are discussed. The first technique filters the data for limited dips. For each dip, linear moveout is applied to the seismic sections so that events with this dip are made flat. By interpolation, we can include dips that are fractions of a sample/trace. After linear moveout, zero‐lag K–L filtering is applied followed, by inverse linear moveout; the results from all dips are added to form the final filtered data. The second technique is blocking, in which the seismic section is divided into blocks small enough for each block to be processed using relatively small matrices; the processed blocks are assembled to form the final filtered section. Using a combination of these techniques, seismic sections can be filtered at a reasonable cost using the K‐L transform.
ELASTIC PARAMETER ESTIMATION BY COHERENCY OPTIMIZATION1
Authors: RESHEF MOSHE , LANDA EVGENY , RAVID TAMAR 
Published: April 2006

AbstractWe present a method for estimating P‐ and S‐velocities within defined layers (macromodel), using only kinematic properties (i.e. traveltimes) of the wavefield. The method does not require identification of mixed‐mode events on prestack or post‐stack data. After obtaining a Vp‐depth model by coherency inversion, S‐velocities are determined by coherency optimization along computed traveltime curves for mixed‐mode events on prestack data. Since the method does not involve any dynamic wavefield computations, a simple ray‐tracing algorithm is used to solve the forward problem. The simplicity of the scheme, together with the ability to apply it locally, makes it highly suitable for interactive use. Results of this method may be used to detect Poisson's ratio anomalies within or between layers and may serve as an initial model for more complicated elastic inversion algorithms.