bluuebunny/crossref_metadata_2025_split huggingface dataset:
FIELD RESULTS WITH DOWNWARD CONTINUATION TECHNIQUE IN INDUCED POLARIZATION PROFILING USING POINT ELECTRODES1
Authors: APPARAO A. , RAO T. GANGADHARA , SARMA V. SUBRAHMANYA 
Published: April 2006

AbstractThe method of downward continuation is well known to those working in gravity, magnetic, SP and low‐frequency electromagnetic exploration. It is demonstrated that the method of continuation can also be usefully employed in the interpretation of induced polarization gradient profiling using point electrodes to determine target depth. The apparent resistance Ra and chargeability Ma measurements obtained with point electrode excitation of the ground have been used to compute the values of (Ra)l and (Ma)l that would be obtained with a linear array.Continuation of the apparent polarizability profile thus obtained with the linear array gives a value for the depth of the target which agrees closely with that obtained by the continuation of the SP profile. On the other hand, continuation of the profile of the secondary transient signal (VS)L alone, yields a depth of the target which is in agreement with the borehole information. However, it is seen that the secondary transient voltage profiling response splits into two anomalies which fall on either side of the SP and/or (Ma)l anomaly centre, and does not coincide with that of the latter.
τ‐p SEISMIC DATA FOR VISCOELASTIC MEDIA ‐ PART 2: LINEARIZED INVERSION1
Authors: MARTINEZ RUBEN D. , McMECHAN GEORGE A. 
Published: April 2006

AbstractAn approach to extraction of viscoelastic parameters from seismic data is implemented and succesfully tested. Viscoelastic inversion is performed using adaptive damping factors to control the sensitivity of the viscoelastic parameters in relation to the τ‐p seismic data. A priori information is incorporated through the damping factors as standard deviations of the data and of the viscoelastic model parameters. The stability of the inversion process is controlled by the variation of the damping factors as a function of the residual errors and parameter updates at each iteration.Tests on synthetic and real data show that P‐ and S‐wave quality factors, Qp and Qs, in addition to P‐ and S‐wave velocities and density Cp, Cs and p, can be extracted from τ‐p seismic information. Singular value decomposition analysis demonstrates that estimated Qp and Qs values are more affected by the presence of data inaccuracies and noise than are those of Cp and p. Cs and Qs are not uniquely recovered due to the limited contribution of P‐S converted waves.Knowledge of the viscoelastic parameters is of particular importance in accurately describing petrophysical properties of rocks and pore fluids existing in the subsurface; this is demonstrated with real data from the Gulf of Mexico.
COMPARISON OF SIGNAL PROCESSING TECHNIQUES FOR ESTIMATING THE EFFECTS OF ANISOTROPY1
Authors: MACBETH COLIN , CRAMPIN STUART 
Published: April 2006

AbstractThree‐component recordings of shear‐waves in exploration surveys provide an opportunity to measure crustal anisotropy, which may be important in estimating the geometrical and physical parameters of reservoir rocks. VSPs are particularly important for this purpose as they are less subject to the complex interactions of the shear wavefield with the free surface. The first stage in characterizing the subsurface anisotropy requires that the distinctive phenomenon of shear‐wave splitting must be examined for every arrival at each geo‐phone. This effect may be defined by two parameters: the polarization of the leading shear‐wave and the time‐delay between corresponding split shear‐waves. A variety of techniques have been designed to estimate these parameters of shear‐wave splitting. Here, we classify the published techniques into four main categories and review their properties. Representative procedures from each group are applied to a common synthetic data set contaminated with signal‐generated noise. The results allow some general statements to be made about the utility of these methods for processing shear‐waves in VSP data.
JOINT INVERSION OF SEISMIC AND GEOELECTRIC DATA RECORDED IN AN UNDERGROUND COAL MINE1
Authors: DOBRÓKA M. , GYULAI Á. , ORMOS T. , CSÓKÁS J. , DRESEN L. 
Published: April 2006

AbstractUntil the present time the ‘ rock‐coal‐rock’ layer sequence and offsets in coal‐seams in underground coal mines have been detected with the aid of seismic waves and geoelectric measurements. In order to determine the geometrical and petrophysical parameters of the coal‐seam situation, the data recorded using seismic and geoelectric methods have been inverted independently. In consequence, the inversion of partially inaccurate data resulted in a certain degree of ambiguity. This paper presents the first results of a joint inversion scheme to process underground vertical seismic profiling data, geolectric resistivity and resistance data.The joint inversion algorithm makes use of the damped least‐squares method and its weighted version to solve the linearized set of equations for the seismic and geolectric unknowns. In order to estimate the accuracy and reliability of the derived geometrical and petrophysical layer parameters, both a model covariance matrix and a correlation matrix are calculated. The weighted least‐squares algorithm is based on the method of most frequent values (MFV). The weight factors depend on the difference between measured data and those calculated by an iteration process.The joint inversion algorithm is tested by means of synthetic data. Compared to the damped least‐squares algorithm, the MFV inversion leads to smaller estimation errors as well as lower sensitivities due to the choice of the initial model. It is shown that, compared to an independent inversion, the correlation between the model parameters is definitely reduced, while the accuracy of the parameter estimation is appreciably increased by the joint inversion process. Thus the ambiguity is significantly reduced.Finally, the joint inversion algorithm using the MFV method is applied to underground field data. The model parameters can be derived with a sufficient degree of accuracy, even in the case of noisy data.
A FILTER, DELAY AND SPREAD TECHNIQUE FOR 3D DMO1
Authors: FERBER R.‐G. 
Published: April 2006

AbstractThe calculation of dip moveout involves spreading the amplitudes of each input trace along the source‐receiver axis followed by stacking the results into a 3D zero‐offset data cube. The offset‐traveltime (x–t) domain integral implementation of the DMO operator is very efficient in terms of computation time but suffers from operator aliasing. The log‐stretch approach, using a logarithmic transformation of the time axis to force the DMO operator to be time invariant, can avoid operator aliasing by direct implementation in the frequency‐wavenumber (f–k) domain.An alternative technique for log‐stretch DMO corrections using the anti‐aliasing filters of the f–k approach in the x‐log t domain will be presented. Conventionally, the 2D filter representing the DMO operator is designed and applied in the f–k domain. The new technique uses a 2D convolution filter acting in single input/multiple output trace mode. Each single input trace is passed through several 1D filters to create the overall DMO response of that trace.The resulting traces can be stacked directly in the 3D data cube. The single trace filters are the result of a filter design technique reducing the 2D problem to several ID problems. These filters can be decomposed into a pure time‐delay and a low‐pass filter, representing the kinematic and dynamic behaviour of the DMO operator. The low‐pass filters avoid any incidental operator aliasing. Different types of low‐pass filters can be used to achieve different amplitude‐versus‐offset characteristics of the DMO operator.
DAMPED LEAST‐SQUARES INVERSION OF TIME‐DOMAIN AIRBORNE EM DATA BASED ON SINGULAR VALUE DECOMPOSITION1
Authors: HUANG H. , PALACKY G. J. 
Published: April 2006

AbstractAirborne electromagnetic (AEM) methods are increasingly being used as tools of geological mapping, groundwater exploration and prospecting for coal and lignite. In such applications, quantitative interpretation is commonly based on the layered‐earth model. A new approach, a damped least‐squares inversion with singular value decomposition, is proposed for interpretation of time‐domain, towed‐bird AEM data. Studies using theoretical and field AEM data indicate that inversion techniques are dependable and provide fast converging solutions. An analysis has been made of the accuracy of model parameter determination, which depends on resistivity and thickness distribution. In the common case of conductive overburden, upper‐layer resistivity and thickness are usually well determined, although situations exist where their separation becomes difficult. In the case of a resistive layer overlying a conductive basement, the layer thickness is the best‐determined parameter. In both cases, estimates of basement resistivity are the least reliable. Field data obtained with the Chinese‐made M‐l AEM system in Dongling, Anhui Province, China, were processed using the described inversion algorithm. The survey area comprised fluvial Cenozoic clays and weathered Mesozoic sediments. Inversion of AEM data resulted in accurate depth‐to‐bedrock sections and realistic estimates of the resistivities of overburden and bedrock which agree with the results of drilling and resistivity sounding.
MAKING AVO SECTIONS MORE ROBUST1
Authors: WALDEN A. T. 
Published: April 2006

AbstractWhen large quantities of seismic data are involved it is impossible to examine all gathers by eye for AVO anomalies. The standard approach is to compute, for each amplitude profile (at a specific time) on each gather, the intercept and gradient of a straight‐line fit to seismic amplitudes. These intercepts and gradients are each plotted as a sort of seismic section ‐ an intercept section, and a gradient section.Estimation of the intercept and gradient for a straight‐line fit to each amplitude profile proceeds traditionally via least‐squares. Two undesirable features can be hidden from the user by the fitting procedure, namely (i) the effect of outlying or uncharacteristic amplitudes on the intercept and gradient estimates, and (ii) complete breakdown of the straight‐line model for the amplitudes, thus rendering meaningless the intercept and gradient estimates. It should be remembered that least‐squares can always fit any sequence of numbers to any other sequence of numbers; checks are needed to show that the result is meaningful.It is shown that statistically robust estimation methods can greatly limit the damage done by outlying amplitudes, and that a simple test on the model, the runs‐statistic, is capable of detecting breakdown of the straight‐line assumption. It is demonstrated using two seismic data sets that these two techniques, used in tandem, facilitate much better quality control of AVO intercept and gradient calculations.
THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD1
Authors: PALMER DERECKE 
Published: April 2006

AbstractThe depth to the surface of a refractor and the seismic velocity within the refractor are very often intimately related. In the shallow environment, increased thicknesses of weathering occur in areas of jointing, shearing or lithological variations, and these zones of deeper weathering can have lower subweathering refractor velocities. This association is important in geotechnical investigations and in the measurement of weathering thicknesses and sub‐weathering velocities for statics corrections for reflection seismic surveys.Algorithms, which employ forward and reverse traveltime data and which explicitly accommodate the offset distance through the process known as refraction migration, are necessary if detailed structure on a refractor and rapid lateral variations of the seismic velocity within it are to be resolved. These requirements are satisfied with wavefront construction techniques, Hales’ method and the generalized reciprocal method (GRM).However, these methods employ refraction migration in fundamentally different manners. Most methods compute an offset distance with an often imprecise knowledge of the seismic velocities of the overlying layers. In contrast, the GRM uses a range of offset distances from less than to greater than the optimum value, with the optimum value being selected with a minimum‐variance criterion.The approach of the GRM is essential where there are undetected layers and where there are rapid variations in the depth to a refractor and the seismic velocity within it. In the latter situations the offset distance necessary to define the seismic velocities can differ considerably from the value required to define depths.The efficacy of the GRM in resolving structure and seismic velocity is demonstrated with three model studies and two field examples.
VLF RESISTIVITY MAPPING AND VERTICALIZATION OF THE ELECTRIC FIELD1
Authors: TABBAGH ALAIN , BENDERITTER YVES , ANDRIEUX PIERRE , DECRIAUD JEAN PAUL , GUERIN ROGER 
Published: April 2006

AbstractFor over 20 years, powerful VLF transmitters have been used as electromagnetic sources for subsurface investigations in mining exploration. Measurements initially concerned the vertical component of the magnetic field or the inclination of the field and were later extended to measurement of the horizontal electric field in the direction of the transmitter, to determine the resistivity of the terrain. Measurement of the electric field is usually performed with electric lines, grounded or not, with lengths of at least 5 m.This paper presents the concept of a VLF resistivity meter with a very short electric sensor (1 m) and the results obtained with it. This technique improves the measurement of the electric field, which is in principle a point value. It also permits a higher spatial sampling rate and, by closely linking the electric sensor with the magnetic sensor on a lightweight mount, makes it possible for the instrument to be used by a single operator.In addition, transformation of the electric field data, analogous to reduction to the pole in magnetism, is proposed to correct the horizontal deformation of the anomalies created by polarization of the primary field. Comparison with direct current electrical measurements shows highly satisfactory correlations. This transformation, validated for VLF, can be extended to any electrical or electromagnetic method using a uniform primary field, i.e. gradient array in direct current or low‐frequency magnetotellurics. We call this verticalization of the electric field.Resistivity measurements and mapping using the VLF frequency range can be applied not only to mining but also to a wide range of shallow geophysical studies (hydrology, civil engineering, etc.) and are not limited to problems concerning the location of conductive targets
SEISMIC REFLECTION AMPLITUDES1
Authors: URSIN BJØRN , DAHL TERJE 
Published: April 2006

AbstractSeismic amplitude variations with offset contain information about the elastic parameters. Prestack amplitude analysis seeks to extract this information by using the variations of the reflection coefficients as functions of angle of incidence. Normally, an approximate formula is used for the reflection coefficients, and variations with offset of the geometrical spreading and the anelastic attenuation are often ignored. Using angle of incidence as the dependent variable is also computationally inefficient since the data are recorded as a function of offset.Improved approximations have been derived for the elastic reflection and transmission coefficients, the geometrical spreading and the complex travel‐time (including anelastic attenuation). For a 1 D medium, these approximations are combined to produce seismic reflection amplitudes (P‐wave, S‐wave or converted wave) as a Taylor series in the offset coordinate. The coefficients of the Taylor series are computed directly from the parameters of the medium, without using the ray parameter.For primary reflected P‐waves, dynamic ray tracing has been used to compute the offset variations of the transmission coefficients, the reflection coefficient, the geometrical spreading and the anelastic attenuation. The offset variation of the transmission factor is small, while the variations in the geometrical spreading, absorption and reflection coefficient are all significant.The new approximations have been used for seismic modelling without ray tracing. The amplitude was approximated by a fourth‐order polynomial in offset, the traveltime by the normal square‐root approximation and the absorption factor by a similar expression. This approximate modelling was compared to dynamic ray tracing, and the results are the same for zero offset and very close for offsets less than the reflector depth.
ENHANCED SERVOVALVE TECHNOLOGY FOR SEISMIC VIBRATORS1
Authors: REUST DENNIS K. 
Published: April 2006

AbstractThe Pelton DRTM Servovalve Enhancement causes the natural output of a vibrator to resemble the desired output more closely. This simplifies the control problem and reduces harmonic distortion.The traditional type of servovalve used on seismic vibrators is a flow‐control servovalve. Flow is proportional to a vibrator's baseplate velocity, with respect to its reaction mass.The new servovalve control parameter is pressure rather than flow. The differential pressure applied to a vibrator's actuator piston, multiplied by the area of the piston, equals the force applied to the vibrator's baseplate structure. This may be defined as actuator force. There is a simpler and more linear relationship between actuator force and ground force than between actuator velocity and ground force. Thus, it is better for the servovalve to control pressure into the actuator rather than flow.A flow‐control servovalve can be made to control pressure by sensing the differential pressure across a vibrator's actuator piston and applying it as a negative feedback around the servovalve main stage. This has been carried out and tested. The result is more accurate vibrator control and reduced harmonic distortion.
A STUDY OF MAGNETOTELLURIC STATIC DISTORTION IN THE CONTEXT OF INTRUSIVE VOLCANISM1
Authors: BEAMISH D. , TRAVASSOS J. M. 
Published: April 2006

AbstractMT data from a detailed 84‐site grid array (28 × 12 km) in the Paraná Basin (Brazil) reveal a high degree of frequency‐independent and parallel amplitude distortion. The Palaeozoic sediments across the survey area are covered by 1.2 km of flood basalts. A deep well‐log provides some control regarding the emplacement of thin diabase sills but information regarding vertical feeder dikes is non‐existent. The degree of parallel behaviour is identified using anisotropy ratios which quantify the extent and bandwidth of the distortion characteristics.A 2D modelling study is carried out using the concept of a horizontally layered (1D) basin with superimposed, small‐scale inhomogeneities. Two likely geological distortion structures are considered. The first represents at‐ or near‐surface (0 to 100 m) inhomogeneities. The second represents thin but vertically elongate dikes. The two distortion effects are found to produce different and characteristic behaviour in the H‐polarization (TM) mode model data. The modelled E‐polarization (TE) mode data are undistorted and provide control. The model developed to represent vertical dikes gives rise to a characteristic frequency dependence which is distinct from that of the undistorted response. This model also generates subparallel distortion effects which give rise to a form of variance in the response estimates as a function of location. The behaviour occurs, importantly, in both the amplitude and phase of the distorted data. A comparison of the normalized amplitude data, observed and modelled, indicates a high degree of correspondence down to a low frequency limit (< 0.1 Hz) where additional and deep contributions to the observed response become apparent. The modelling results and data characteristics indicate that the survey area is underlain by a series of closely parallel, thin intrusive dikes (a dike swarm).
MODELLING GUIDED WAVES IN CROSS‐HOLE SURVEYS IN UNCRACKED AND CRACKED ROCK1
Authors: LOU MIN , CRAMPIN STUART 
Published: April 2006

AbstractThe theoretical behaviour of guided waves in cross‐hole seismic surveys is examined with dispersion curves and synthetic seismograms. The modelling suggests, and field observations confirm, that the dominant energy in many cross‐hole seismic surveys propagates as guided waves, as well as body waves, with the energy tied to an interface, or combination of interfaces, whenever there are interfaces with significant velocity contrasts continuous between the wells. The dispersion and waveforms of such modal solutions carry detailed information about the internal structure, particularly the stress‐aligned structure of fluid‐filled inclusions in the immediate neighbourhood of the waveguide. Since the information content can be controlled to some extent by the choice of frequency and source type and the level of source and receiver in the two wells, such guided waves in cross‐hole surveys may be important for monitoring changes in nearly horizontal reservoirs during hydrocarbon production processes. Initially, we investigate the basic properties of propagation in isotropic strata, and then examine the behaviour of guided waves in the stress‐aligned fluid‐filled inclusions.
SIMULATION OF REALISTIC SYNTHETIC REFLECTION SEQUENCES1
Authors: WALDEN A. T. 
Published: April 2006

AbstractIt is useful to be able to calculate synthetic primary reflection sequences from which to generate synthetic seismic sections which can be used for testing new processing algorithms. However, these synthetic reflection sequences should closely match real properties found in recent studies. Using the ARMA(1,1) model resulting from such studies to describe the correlation (or spectral) structure of the sequences, and by matching moments up to fourth order (since the sequences are non‐Gaussian in practice), realistic sequences can be generated. A simple scheme is provided which also eliminates the necessity of throwing away large numbers of simulated values at start‐up. The procedure is illustrated on three real sequences and is seen to reproduce all the important features.
ESTIMATION OF ELASTIC PARAMETERS FROM AVO EFFECTS IN THE TAU‐P DOMAIN1
Authors: HELGESEN J. , LANDRØ M. 
Published: April 2006

AbstractA quantitative AVO algorithm suitable for media with slow lateral parameter variations is developed. The method is based on a target‐oriented inversion scheme for estimation of elastic parameters in a locally horizontally stratified medium. The algorithm uses band‐limited PP reflection coefficients in the τ‐p domain to estimate P‐ and S‐wave velocities, densities and layer thicknesses. To obtain these reflection coefficients, a pre‐processing involving the Radon transform and multiple attenuation is necessary. Furthermore, a macromodel for the velocities above the target zone must be found prior to the inversion.Various inversion tests involving synthetic data with white Gaussian noise and modelling errors that are likely to occur in conjunction with real data have been performed. In general, the inversion algorithm is fairly robust, since it is able to reproduce the main features of the reference model: main interface locations and relative contrasts in the three unknown layer parameters are recovered.From a test combining the effect of source directivity, one thin layer and 20% white Gaussian noise, it was found that neglect of the source directivity in the inversion caused the largest errors in the estimates. This indicates that it is very important either to eliminate the source directivity in a preprocessing step, or to take the directivity into account in the present algorithm. Despite these problems it was concluded that the inversion algorithm was able to reproduce the main features of the reference model.