bluuebunny/crossref_metadata_2025_split huggingface dataset:
Backus averaging, scattering and drift1
Authors: Sams M.S. , Williamson P.R. 
Published: April 2006

AbstractDifferences between traveltimes from sonic to seismic frequencies, commonly known as drift, can be attributed to a combination of multiple scattering and absorption. The portion due to scattering can be estimated directly by calculating synthetic seismograms from sonic logs. A simple alternative approach is suggested by the long‐wave equivalent averaging formulae for the effective elastic properties of a stack of thin layers, which gives the same traveltime delays as the low‐frequency limit of the scattering dispersion. We consider the application of these averaging formulae over a frequency‐dependent window with the hope of extending their use to frequencies higher than those allowed by the original validity conditions. However, comparison of the time delay due to window‐averaging with the scattering dispersion predicted by the O'Doherty‐Anstey formula reveals that it is not possible to specify a form of window that will fit the dispersion across the spectrum for arbitrary log statistics. A window with a width proportional to the wavelength squared matches the behaviour at the low‐frequency end of the dispersive range for most logs, and allows an almost exact match of the drift across the entire spectrum for exponential correlation functions.We examine a real log, taken from a hole in nearly plane‐layered geology, which displays strong quasi‐cyclical variations on one scale as well as more random, smaller‐scale fluctuations. The details of its drift behaviour are studied using simple models of the gross features. The form of window which gave a good theoretical fit to the dispersion for an exponential log correlation function can only fit the computed drift at high or low frequencies, confirming that there are at least two significant scale‐lengths of fluctuation. A better overall fit is obtained for a window whose width is proportional to the wavelength. The calculated scattering drift is significantly less than that observed from a vertical seismic profile, but the difference cannot be wholly ascribed to absorption. This is because the source frequency of the sonic tool is not appropriate for its resolution (receiver spacing) so that the scattering drift from sonic to seismic frequencies cannot be fully estimated from the layer model derived from the log.
Using fractal crustal magnetization models in magnetic interpretation1
Authors: Pilkington M. , Gregotski M.E. , Todoeschuck J.P. 
Published: April 2006

AbstractEvidence from borehole susceptibility logs and the spectral analysis of aeromagnetic data suggests that the three‐dimensional distribution of magnetization within the crust can be described as fractal. This property can be exploited in magnetic interpretation methods which explicitly require statistical information on the spatial variation of magnetization. Specifically, we address the problem of magnetic source depth estimation through downward continuation and gridding aeromagnetic survey data using the method of kriging.When magnetic data are continued downwards the depth at which the power spectrum flattens out (the ‘white’ depth) can be taken to be an estimate of the top of the source distribution. This procedure assumes that individual sources are uncorrelated with each other. Taking into account the correlation of the magnetization using a fractal description leads to a reduction in this depth estimate.Gridding of randomly distributed magnetic measurements using kriging requires an estimate of the covariance of the data. Compared with the assumption of a white (uncorrelated) magnetization distribution, using fractal covariances for kriging produces gridded estimates which more closely reflect the statistics of the underlying magnetization process and produce maps with a justifiable degree of smoothness.
Resistivity measurements on the sea bottom to map fracture zones in the bedrock underneath sediments1
Authors: Lile O.B. , Backe K.R. , Elvebakk H. , Buan J.E. 
Published: April 2006

AbstractRefraction seismics with the shotpoints and the hydrophone cable on the sea‐bottom, have become the standard geophysical method for investigating rock quality before constructing offshore tunnels in Norway. In connection with the construction of a sub‐sea tunnel by the Norwegian Public Roads Administration, research work was carried out to compare two low‐velocity zones, indicated by refraction seismics with other methods. A special resistivity cable for pole‐dipole measurements on the sea‐floor, with 10 m between the electrodes, was constructed. A 200 m long profile, crossing the two low‐velocity zones, was measured with all combinations of electrode distances. The two zones were detected as low‐resistivity zones. A special data processing technique to enhance the anomalies is described. Resistivity soundings in a seawater environment to detect fracture zones in the bedrock underneath the bottom sediments, are discussed. It is concluded that severely fractured zones, which may cause difficulties for the tunnel construction, can be detected both with sea‐surface and sea‐floor arrays using long electrode spacings.
Inversion for seismic anisotropy using genetic algorithms1
Authors: Horne Steve , MacBeth Colin 
Published: April 2006

AbstractA general inversion scheme based on a genetic algorithm is developed to invert seismic observations for anisotropic parameters. The technique is applied to the inversion of shear‐wave observations from two azimuthal VSP data sets from the Conoco test site in Oklahoma. Horizontal polarizations and time‐delays are inverted for hexagonal and orthorhombic symmetries. The model solutions are consistent with previous studies using trial and error matching of full waveform synthetics. The shear‐wave splitting observations suggest the presence of a shear‐wave line singularity and are consistent with a dipping fracture system which is known to exist at the test site. Application of the inversion scheme prior to full waveform modelling demonstrates that a considerable saving in time is possible whilst retaining the same degree of accuracy.
Determination of electrical resistivity, its anisotropy and heterogeneity on drill cores: a new method1
Authors: Rauen A. , Soffel H.C. 
Published: April 2006

AbstractThis paper presents a new method for the measurement on core samples of their electrical resistivity, its anisotropy and heterogeneity. The equipment used has been developed in the field laboratory of the German Continental Deep Drilling program KTB in the north‐east of Bavaria on the western rim of the Bohemian Massif. The apparatus measures the resistivity at a fixed frequency as a function of the drill core azimuth and along the core by moving point electrode configurations.From these azimuth and depth dependences, mean values of resistivity and additional information about its anisotropy and heterogeneity are determined. Geometrical averaging is used, because the resistivity data follows a log normal distribution. The quantitative parameters ‘azimuth factor’, corresponding to horizontal anisotropy, and ‘heterogeneity factor’ are introduced.The depth logs of resistivity, azimuth factor and heterogeneity factor, measured on cores obtained from the KTB main drill hole (gneisses and amphibolites) at depths between 4150 m and 8080 m are presented. The geometrically averaged mean values of resistivity of gneisses and amphibolites are in the same range (≅ 103Ωm). The resistivities tend to decrease with depth. The stress release of the drill cores during recovery produces microcracks which may partially account for this effect.Reduced resistivities (down to 150 Ωm) within an amphibolite core correlate with an alteration zone. One sample of this core displays alteration from fresh to completely altered. This sample is also electrically heterogeneous (heterogeneity factor ≅ 2). Other samples with uniform low alteration are more homogeneous heterogeneity factor ≅ 1.4).In general, higher anisotropies are observed in gneisses (mean azimuth factor 2.8), lower anisotropies in amphibolites (mean azimuth factor 1.3). Examples of isotropic and homogeneous samples, as well as anisotropic and heterogeneous samples are also presented.
Median filter behaviour with seismic data1
Authors: Duncan G. , Beresford G. 
Published: April 2006

AbctractMedian filters may be used with seismic data to attenuate coherent wavefields. An example is the attenuation of the downgoing wavefield in VSP data processing. The filter is applied across the traces in the ‘direction’ of the wavefield. The final result is given by subtracting the filtered version of the record from the original record. This method of median filtering may be called ‘median filtering operated in subtraction’. The method may be extended by automatically estimating the slowness of coherent wavefields on a record. The filter is then applied in a time‐ and‐space varying manner across the record on the basis of the slowness values at each point on the record.Median filters are non‐linear and hence their behaviour is more difficult to determine than linear filters. However, there are a number of methods that may be used to analyse median filter behaviour: (1) pseudo‐transfer functions to specific time series; (2) the response of median filters to simple seismic models; and (3) the response of median filters to steps that simulate terminating wavefields, such as faults on stacked data. These simple methods provide an intuitive insight into the behaviour of these filters, as well as providing a semiquantitative measurement of performance. The performance degradation of median filters in the presence of trace‐to‐trace variations in amplitude is shown to be similar to that of linear filters. The performance of median filters (in terms of signal distortion) applied obliquely across a record may be improved by low‐pass filtering (in the t‐dimension). The response of median filters to steps is shown to be affected by background noise levels. The distortion of steps introduced by median filters approaches the distortion of steps introduced by the corresponding linear filter for high levels of noise.
Using the pseudospectral method on curved grids for 2D elastic forward modelling1
Authors: Nielsen Per , If Flemming , Berg Per , Skovgaard Ove 
Published: April 2006

AbstractWhen applying the conventional Fourier pseudospectral method (FSM) on a Cartesian grid that has a sufficient size to propagate a pulse, spurious diffractions from the staircase representation of the curved interfaces appear in the wavefield. It is demonstrated that these non‐physical diffractions can be eliminated by using curved grids that conform to all the interfaces of the subsurface. Methods for solving the 2D acoustic wave equation using such curved grids have been published previously by the authors. Here the extensions to the full 2D elastic wave equations are presented.The curved grids are generated by using the so‐called multiblock strategy which is a well‐known concept in computational fluid dynamics. In principle the sub‐surface is divided into a number of contiguous subdomains. A separate grid is generated for each subdomain patching the grid lines across domain boundaries to obtain a globally continuous grid. Using this approach, even configurations with pinch outs can be handled.The curved grid is taken to constitute a generalized curvilinear coordinate system. Thus, the elastic equations have to be written in a curvilinear frame before applying the numerical scheme. The method implies that twice the number of spatial derivatives have to be evaluated compared to the conventional FSM on a Cartesian grid. However, it is demonstrated that the extra terms are more than compensated for by the fewer grid points needed in the curved approach.
The stochastic inversion of magnetics and resistivity data using the simulated annealing algorithm1
Authors: Dittmer J.K. , Szymanski J.E. 
Published: April 2006

AbstractSimulated annealing is a stochastic combinatorial optimization technique, based on ideas from statistical mechanics, thermodynamics and multivariable probability theory. This paper presents the use of simulated annealing as a means of inversion for both linear magnetics and non‐linear resistivity problems. The subsurface is viewed as being constructed of smaller elemental blocks which possess either uniform internal magnetization or conductivity, enabling larger structures to be modelled. Simulated annealing is employed to calculate the distribution of the particular physical property which causes a measured anomalous field curve.A general description of simulated annealing and its application is given, followed by specific descriptions of its application to the magnetics and resistivity cases.For the magnetics case the subsurface consists of 2D prismatic elements as the basis for the forward model. Synthetic model data is used to test the algorithm and an example of actual field data; a survey across an igneous dike is used to demonstrate the use of the method with real data. In the resistivity case, the finite‐element method is used to generate the forward models. Synthetic vertical profiling data is used to test the application of the simulated annealing method to the resistivity case. Actual data from an archaeological survey is used to show again the use of the method with real data.Simulated annealing is shown to be capable of inverting both the linear and non‐linear methods of magnetic surveying and resistivity surveying respectively.
Long period multiple suppression by predictive deconvolution in the x–t domainl1
Authors: Taner M. Turhan , O'Doherty Ronan F. , Koehler Fulton 
Published: April 2006

AbctractThere are two forms of systematic error in conventional deconvolution as applied to the problem of suppressing multiples with periodicities longer than a hundred milliseconds. One of these is the windowing effect due to the assumption that a true autocorrelation function can be computed from a finite portion of data. The second form of error concerns the assumption of periodicity, which is strictly true only at zero offset for a 1D medium. The seriousness of these errors increased with the lengthening of the multiple period.This paper describes and illustrates a rigorous 2D solution to the predictive deconvolution equations that overcomes both of the systematic errors of conventional 1D approaches. This method is applicable to both the simple or trapped system and to the complex or peg‐leg system of multiples. It does not require that the design window be six to ten times larger compared to the operator dimensions and it is accurate over a wide range of propagation angles. The formulation is kept strictly in the sense of the classical theory of prediction. The solution of normal equations are obtained by a modified conjugate gradient method of solution developed by Koehler. In this algorithm, the normal equations are not modified by the autocorrelation approximation.As with all linear methods, approximate stationary attitude in the multiple generating process is assumed. This method has not been tested in areas where large changes in the characteristic of the multiple‐generating mechanism occur within a seismic spread length.
Reflections from a randomly grooved interface: ultrasonic modelling and finite‐diff erence calculation1
Authors: Schultz Craig A. , Tokcöz M. Nafi 
Published: April 2006

AbstractThis study utilizes ultrasonic water tank modelling to examine the complexities which can result when seismic energy is diffracted by even a simple 3D interface structure. Ultrasonic laboratory modelling is carried out in our in‐house water tank, using an aluminum block to simulate very simple ocean‐bottom topography. The topography consists of parallel depressions which simulate valleys running parallel to a mid‐ocean spreading ridge. This study shows that the phase and amplitude of primary ocean‐bottom reflections can vary strongly with the azimuth of propagation. The trailing energy coda also varies dramatically as a function of propagation direction, showing the importance of developing a seismic model which includes the full 3D azimuthal distribution of interface structure.
Surface NMR applied to an electroconductive medium1
Authors: Trushkin D.V. , Shushakov O.A. , Legchenko A.V. 
Published: April 2006

AbstractAn integrated electromagnetic and nuclear magnetic resonance (NMR) method is proposed for investigating highly conductive soil areas. Using a simple model of a homogeneous half‐space and the same antenna for both methods, a significant improvement in the NMR data interpretation is obtained. A case study has shown fair agreement between the results from computer modelling, field tests, and data from a nearby observation well. The electromagnetic method and the half‐space model were selected for easy integration into an existing instrument used for the NMR method. A more accurate knowledge of the conductivity distribution with depth will further improve the final result.
Monitoring soil contaminations using a contactless conductivity probe1
Authors: Huang Ming , Liu Ce , Shen Liang C. , Shattuck David 
Published: April 2006

AbstractIn order to investigate the possibility of using low‐frequency electromagnetic waves to detect and monitor oil contamination of soils, a series of laboratory measurements were performed. A new measurement system to monitor the resistivities of soil and sand samples while samples are being contaminated by diesel oil is presented. The frequency used in measurements is 100 kHz. Since the measurement system is composed of coil‐type transmitters and receivers, there is no need for electrodes to be in contact with samples. The contamination process is simulated using diesel oil dripping on top of soil and sand samples. The conductivity distributions in samples along the sample length are recorded as a function of time. Water‐wet sand and soil samples were measured during diesel oil contamination. The measured data show that the conductivities of soil and sand samples change during the contamination process. The change in resistivity for measured samples before and after diesel oil contamination is in the range of 20% to 50%, giving a reflection coefficient change in the low‐frequency limit of 4.7% to 7%. This amount of change in the reflection coefficient makes it very challenging to detect and monitor oil contamination based on EM reflection from the contaminants. The results suggest that EM methods based on propagation and induction, such as tomography and borehole induction, could be used for this purpose.
Point‐to‐curve ray tracing in complicated geological models1
Authors: Hanyga Andrzej , Pajchel Jan 
Published: April 2006

AbstractBoundary‐value problems (BVPs) for seismic rays generally have multiple solutions. In practical applications the number of solutions can be large. The algorithm presented below solves a one‐parameter family of BVPs and makes it easy to obtain all the solutions of a BVP.
The stable‐beam seismic modelling method1
Authors: Wang Xiutian , Waltham Dave 
Published: April 2006

AbstractThe stable‐beam method for forward modelling of seismic data is introduced. The method is applicable to geometries which may be approximated by a series of single‐valued (in depth) interfaces separating constant‐velocity layers. For models of this restricted type, the results are of similar accuracy to those from waveequation‐based methods whilst the run times are similar to, or better than, those for simple ray‐tracing approaches. The basis of the method is to approximate interfaces by a series of straight‐line segments. This allows very rapid and stable ray tracing through the model. Pseudodiffractions are then added from all of the interface discontinuities formed between adjacent segments. These pseudodiffractions have the effect of correcting for the phase, amplitude and wavefront continuity errors introduced by the interface approximation. Comparison of the stable‐beam results to analytical, Kirchhoff, finite‐difference and physical model results confirm the accuracy ofthe technique.
Non‐linear inversion of resistivity profiling data for some regular geometrical bodies1
Authors: Chunduru Raghu K. , Sen Mrinal K. , Stoffa Paul L. , Nagendra R. 
Published: April 2006

AbstractThe inversion of resistivity profiling data involves estimation of the spatial distribution of resistivities and thicknesses of rock layers from the apparent resistivity data values measured in the field as a function of electrode separation. The drawbacks of using traditional curve‐matching techniques to solve this inverse problem have been overcome by iterative linear techniques but these require good starting models even if the shape of the causative body is asssumed known. In spite of the recent developments in inversion techniques, no robust method exists for the inversion of resistivity profiling data for the simple model of dikes and spheres which are the classical models of geophysical prospecting.We apply three different non‐linear inversion schemes to invert synthetic resistivity profiling data for the classical models embedded in a uniform matrix of contrasting resistivity. The three non‐linear algorithms used are called the Metropolis simulated annealing (SA), very fast simulated annealing (VFSA) and a genetic algorithm (GA). We compare the performance of the three algorithms using synthetic data for an outcropping vertical dike model. Although all three methods were successful in obtaining optimal solutions for arbitrary starting models, VFSA proved to be computationally the most efficient.
Smoothing seismically derived velocities1
Authors: Grubb H.J. , Walden A.T. 
Published: April 2006

AbstractIt is often very useful to be able to smooth velocity fields estimated from exploration seismic data. For example seismic migration is most successful when accurate but also smooth migration velocity fields are used. Smoothing in one, two and three dimensions is examined using North Sea velocity data.A number of ways for carrying out this smoothing are examined, and the technique of locally weighted regression (LOESS) emerges as most satisfactory. In this method each smoothed value is formed using a local regression on a neighbourhood of points downweighted according to their distance from the point of interest. In addition the method incorporates ‘blending’ which saves computations by using function and derivative information, and ‘weighting and robustness’ which allows the smooth to be biased towards reliable points, or away from unreliable ones.A number of other important factors are also considered: namely, the effect of changing the scales of axes, or of thinning the velocity field, prior to smoothing, as well as the problem of smoothing on to irregular subsurfaces.