bluuebunny/crossref_metadata_2025_split huggingface dataset:
CONTINUOUS CALIBRATION OF MARINE SEISMIC SOURCES*
Authors: NEWMAN P. 
Published: April 2006

ABSTRACTThe success of signature deconvolution in optimizing both signal‐to‐noise ratio and time resolution in the seismic section depends critically upon obtaining an accurate estimate of the far‐field source signature. Various deterministic estimation schemes have been proposed in recent years, most of which involve direct monitoring of source output within the water layer.As an alternative to elaborate and error‐prone source monitoring schemes during data acquisition, a simple modification to any source array permits subsequent estimation of far‐field signatures directly from reflected signal. The new method requires the inclusion within any chosen source array of a simple point source, the “reference” source. Initial experiments employed a water gun as the reference source, characterized by a concise implosive signature with peak‐to‐peak amplitude of approximately 2 bar·m within the seismic sprectrum.In operation the reference source is fired shortly before the main array (typically 2 s during initial trials) and the usual record length is extended by a similar amount. Each recorded trace then comprises two results: the subsurface response to the reference source signal followed by the response of the same subsurface to the main array. The disparities in source amplitudes and NMO differentials ensure that interference effects are negligible in the main recording.Time‐ or frequency‐domain methods can be employed to extract the main array signature from the dual dataset or to invert this to some preferred wavelet simultaneously. As an additional benefit the reference source yields excellent high‐resolution profiles of the shallow geology.
MAXIMUM‐LIKELIHOOD ESTIMATION OF SEISMIC IMPULSE RESPONSES*
Authors: URSIN B. , HOLBERG O. 
Published: April 2006

ABSTRACTA seismic trace is assumed to consist of a known signal pulse convolved with a reflection coefficient series plus a moving average noise process (colored noise). Multiple reflections and reverberations are assumed to be removed from the trace by conventional means. The method of maximum likelihood (ML) is used to estimate the reflection coefficients and the unknown noise parameters. If the reflection coefficients are known from well logs, the seismic pulse and the noise parameters can be estimated.The maximum likelihood estimation problem is reduced to a nonlinear least‐squares problem. When the further assumption is made that the noise is white, the method of maximum likelihood is equivalent to the method of least squares (LS). In that case the sampling rate should be chosen approximately equal to the Nyquist rate of the trace. Statistical and numerical properties of the ML‐ and the LS‐estimates are discussed briefly. Synthetic data examples demonstrate that the ML‐method gives better resolution and improved numerical stability compared to the LS‐method.A real data example shows the ML‐ and LS‐method applied to stacked seismic data. The results are compared with reflection coefficients obtained from well log data.
SELF POTENTIAL AND CURRENT CHANNELLING*
Authors: CULL J. P. 
Published: April 2006

ABSTRACTAnomalies in self potential (SP) are usually explained in terms of geochemical activity. However, there are other sources of current which cause complications in the interpretation. In particular, DC telluric currents can be generated outside the survey region. Contrasts in resistivity cause current channelling and secondary voltages can be induced by leakage. The effect can be demonstrated using network theory and internal sources can be emphasized by subtraction. Joint surveys are required to demonstrate correlations with apparent resistivity.
ON DIRECT RECOVERY OF THE IMPULSE RESPONSE*
Authors: SZARANIEC E. 
Published: April 2006

AbstractThe deconvolution equation is solved in the z‐transform domain directly for an impluse response. The principal assumption is the odd‐depth model: two‐way traveltimes to the boundaries are constrained to be odd integers only. It is further assumed that the length of the wavelet sequence is known to be less than half the length of the data sequence.An inverse of the impulse response is constrained by the zero samples of the source function. The resulting underdetermined set of equations is supplemented with the equations provided by the odd‐depth model. The impulse response is found from the inverse by polynomial division.
POSSIBILITIES OF USING VERTICAL GEOPHONE ARRAYS (CVA) IN REFLECTION SEISMOLOGY*
Authors: RÜTER H. , KÖHLER K. , ARNETZL H. 
Published: April 2006

AbstractVertical geophone arrays in boreholes have been used for many years to study seismic velocities by investigating the first arrivals of records. The development of the vertical seismic profiling (VSP) technique shows possibilities of using the reflected events to close the gap between interpretation of conventional seismic data and physical observations made in the well. Reflected events recorded by vertical arrays (as in VSP) generally have higher signal‐to‐noise ratio, larger bandwidth and can easily be separated from multiples. The new Continuous Vertical Array (CVA) technique combines vertical arrays in several boreholes with a line of source points near the surface. The result is a multi‐covered seismic line similar to that of a conventional seismic survey, but it retains the benefits of observations with vertical arrays. The possibilities of the new technique are discussed with the aid of theoretical considerations, model studies, and a first field case using nine boreholes 500 m apart with depths of 400 m.New data acquisition and processing techniques (mainly migration before stack) have been developed. The CVA‐seismic method is still in the development stage but promises new possibilities for detailed surveys in difficult areas.
THE INFLUENCE OF AN ASYMMETRY IN THE SEQUENCE ROCK/COAL/ROCK ON THE PROPAGATION OF RAYLEIGH SEAM WAVES*
Authors: DRESEN L. , KERNER C. , KÜHBACH B. 
Published: April 2006

AbstractSmall offsets in hard coal seams can be detected with the aid of seam (channel) waves. Transmission and reflection of seam waves depend, among other parameters, upon the symmetry properties of the sequence rock/coal/rock. Two typical unsymmetrical sequences are found in European coal deposits: (i) coal seams with roof and floor of differing acoustic impedance and (ii) coal seams interlayered with rock and soil.Two‐dimensional analog models with appropriate impedance contrasts are used to study the effect of the unsymmetrical layers upon the propagation of Rayleigh seam waves. Data analysis is based upon amplitude measurements both parallel and perpendicular to the layers and dispersion curves.The effect of unsymmetrical roof (rock 1) and floor (rock 2) was studied with models containing homogeneous coal seams. Leaky mode wave groups with phase velocities (cR) in the range between the SV‐wave velocities (βr1, 2) of the two rock materials, i.e. βr1≥cR > βr2, form a characteristic part of the Rayleigh seam wave signal. Using Knott's energy coefficient calculations it is shown that in that range energy leakage into the surrounding rock by refracted SV‐waves is restricted to only one of the two interfaces, namely coal/rock 2. At the other interface, coal/rock 1, all waves are totally reflected. Thus, the high amplitudes of these leaky mode wave groups are explained by “quasi‐normal mode” features.The influence of a dirt bed on wave propagation was studied in models where the roof and the floor have the same elastic properties. The maximum thickness of the dirt bed did not exceed 20% of the total seam thickness. The effect of the bed's location within the seam was also investigated.For all recorded normal‐mode wave groups either the total seam or the coal layers could be regarded as wave guides. This was shown by the fact that the phases could be associated with the phase velocity dispersion curves calculated for the symmetrical sequence rock/coal/rock. These curves are relevant under the condition that the thickness of the coal layer assumed under the calculation coincides with the thickness of the effective wave guide of the respective wave groups.Wave groups guided in the total seam are not influenced by either the thickness or the position of the dirt bed. On the other hand, for wave groups guided in the coal layers, the quotient of signal amplitudes in the coal layers is influenced by the position of the dirt bed.
LEAST‐SQUARES FILTERS WITHOUT TRANSIENT ERRORS: AN EXAMINATION OF THE ERRORS IN LEAST‐SQUARES FILTER DESIGN*
Authors: BUNCH A. W. H. , WHITE R. E. 
Published: April 2006

AbstractA new approach has been developed for the design of cross‐equalization filters by the least‐squares method. The filters estimated by this new exact method are subject to only two types of error: bias and random error. Cross‐equalization filters estimated by a more conventional least‐squares method are further subject to “transient error”. This type of error becomes important when designing filters from a data gate of a length comparable with the length of the filter, i.e., less than four times the length of the filter.The effect of altering various design parameters has been investigated for the new method. It has been found that the proportion of bias in the filter decreases as the effective filter length increases, whereas the random error in the filter decreases with increase in either the signal‐to‐noise ratio of the data or the ratio of the data duration to the filter length. The level of whitening applied to the auto‐correlation matrix before inversion was not found to be a critical design parameter. Also, two techniques have been tested for reducing any anomalous d.c. component in the calculated filter.
EM AND IP RESPONSE OF A STEEL WELL CASING FOR A FOUR‐ELECTRODE SURFACE ARRAY. PART II: NUMERICAL RESULTS*
Authors: WILLIAMS J. T. , WAIT J. R. 
Published: April 2006

AbstractSome numerical results for the apparent resistivity of a homogeneous half space containing a vertical steel well casing as measured by a dipole‐dipole array are presented. The basic model includes the electromagnetic and induced polarization effects by allowing for frequency dependent eddy currents and an electrochemical boundary of the well casing. It is shown that the well casing produces an apparent complex resistivity response that could be mistaken for an induced polarization anomaly existing above a hydrocarbon deposit. This response of the well casing is strongly dependent on frequency.
MAXIMUM LIKELIHOOD ESTIMATION OF SEISMIC REFLECTION COEFFICIENTS*
Authors: ÖZDEMİR H. 
Published: April 2006

AbstractA seismic trace is modeled as a moving average (MA) process both in signal and noise: a signal wavelet convolved with a reflection coefficient series plus colored random noise. Seismic reflection coefficients can be estimated from seismic traces using suitable estimation algorithms if the input wavelet is known and vice versa. The maximum likelihood (ML) algorithm is used to estimate the system order and the reflection coefficients. The system order is related to the arrival time of the latest signal in a complex seismic reflection event. The least‐squares (LS) method does not provide such information. The ML algorithm makes assumptions only about the Gaussian nature of the noise. It is better suited for seismic applications since the LS method inherits the white noise assumption. The Gauss‐Newton (G‐N) and Newton‐Raphson (N‐R) optimization algorithms are used to obtain the ML and the LS estimates.Reflection coefficient estimations are affected by the choice of sampling rate of seismic data. Theoretically, the optimum choice in system identification is the Nyquist rate. Experience with synthetic data confirms the theory.In practice, good estimates of reflection coefficients are possible only up to certain pulse separations (or, equivalently, orders). This is mostly due to numerical problems with the optimization algorithms used and partly due to the limited bandwidth of seismic signals. Good estimates from data simulated using three airgun array pulses recorded with 6–128 Hz filter setting are possible up to about 40.0 ms pulse separations. Successful estimations from pinchout and thin layer simulations and well controlled offshore “bright‐spots” are given.
SEPARATION OF P‐ AND S‐WAVES*
Authors: DANKBAAR J. W. M. 
Published: April 2006

AbstractIn combined P‐ and SV‐observations with vertical and in‐line horizontal geophones, recognition and separation of the individual P‐ and S‐arrivals may form a considerable problem. Due to the geophone receiving characteristics, P‐ and S‐waves are observed on both horizontal and vertical geophones. A new method of separating P‐ and S‐waves is presented that inverts the geophone records for the receiving characteristics, thereby separating the two wavetypes.The filter coefficients are determined from the near‐surface P‐ and S‐wave velocity and from the geometry of the geophone groups. The P/S‐separation filter was tested on synthetic data with good results. Even PS‐ and SP‐converted reflections, which are completely coinciding for horizontal reflectors, could be well separated. The filter appears to be stable and to have low sensitivity to noise in the data or to errors in the near‐surface P‐ and S‐wave velocity. The filter is superior to other separation methods, which were applied to the same dataset for comparison. This method can also be applied to VSP data.
TUTORIAL LINEAR INVERSE FILTERS IN ORTHOGONAL COORDINATES*
Authors: DOUZE E. J. 
Published: April 2006

AbstractThe linear filter is used extensively in exploration geophysics, and is usually computed using the least squares normal equations. In the general field of time series, the inverse problem is often solved through eigenvalue expansion solutions to integral equations.The normal equations can be solved in terms of the eigenvalues and eigenvectors of the autocorrelation matrix. It has been suggested that a spectral expansion technique should be used which computes the inverse directly without explicit use of the normal equations. If all possible spiking positions are calculated using the normal equations, the spiking operator matrix is obtained. The matrix operator obtained from the spectral expansion is closely related to the spiking operator matrix. Thus, it is possible to compute the spectral expansion filter using the normal equations. Therefore, it is possible to use the best features of both methods, i.e. obtaining the optimum filter with the normal equations, and discarding the poorly determined parts of the solution based on spectral expansion.
ON TIMING THE VSP FIRST ARRIVAL*
Authors: DILLON P. B. , COLLYER V. A. 
Published: April 2006

AbstractThe advent of signal energy on a VSP or check‐shot trace may be defined as the first break. An accurate pick of this first break would be possible in the absence of noise. However, real data traces are inevitably corrupted by noise and this leads to difficulty in identifying a break because the signal‐to‐noise ratio is low in its neighbourhood.Under such conditions, an obvious alternative is to pick “troughs” where the local signal‐to‐noise ratio is likely to be much higher. Although trough picking is an effective way to minimize the noise problem, it is sensitive to signal properties (such as absorption and multiple reflections) which have no effect upon the accuracy of break picks. Thus, trough picking is signal‐sensitive and break picking is noise‐sensitive.Clearly, an ideal first‐arrival picking scheme would combine the noise‐tolerant features of trough picking with the signal‐tolerant features of break picking. This ideal may be approached by exploiting known properties of the VSP trace using conventional signal processing techniques. The result of such processing is to reduce the problem to that of picking a trough correctly centered about the true break time.
PHASE AND GROUP TIME SECTIONS AND POSSIBILITIES FOR THEIR USE IN SEISMIC INTERPRETATION OF COMPLEX MEDIA*
Authors: SHTIVELMAN V. , LANDA E. , GELCHINSKY B. 
Published: April 2006

ABSTRACTInterpretation problems are discussed for a new class of models for complex seismic media, called heterogeneous models formed by inclusions (HMI). Examples of such models in geology are destructive deformation zones, tectonic raptures, complex folds, magmatogene formations, fronts of metamorphism and of phase transition, etc., which are of importance in interpretation of seismic data. The wavefields in such media have a complicated interferential character and should be considered as complex wave groups characterized by their phase and group properties.To study the phase and group characteristics of such wavefields, a method of construction and comparative analysis of so‐called phase and group sections is introduced. This method is based on a transformation of the wavefield (seismogram) into a normalized seismogram (cos of the phase) and a perigram (a low cut version of the trace envelope). The group sections obtained on the perigrams represent zones of energy concentration and give stable estimates of the average characteristics of model structure. The phase sections are obtained on the normalized seismograms and represent primarily, the inner structure of the model.The method was applied to both synthetic and field data. The results of the combined analysis of the phase and group sections show that in many cases there are significant differences between them. On the basis of this analysis, several types of seismic objects may be distinguished which can serve as a basis for seismic interpretation.
TWO‐AND‐ONE‐HALF DIMENSIONAL IN‐PLANE WAVE PROPAGATION*
Authors: BLEISTEIN N. 
Published: April 2006

ABSTRACTThis paper collects certain results concerning wave propagation in two‐and‐one‐half dimensions, i.e., three‐dimensional (3‐D) wave propagation in a medium that has variations in two dimensions only. The results of interest are for sources and receivers in the plane determined by the two directions of parameter variation. The objective of this work is to reduce the analysis of the in‐plane propagation to 2‐D analysis while retaining–at least asymptotically–the proper 3‐D geometrical spreading. We do this for the free space Green's function and for the Kirchhoff approximate upward scattered field from a single reflector. In both cases the derivation is carried out under the assumption of a background velocity c(x, z) with the special cases c = c0 and c = c(z).
POTENTIAL FIELD OF A STATIONARY ELECTRIC CURRENT USING FREDHOLM'S INTEGRAL EQUATIONS OF THE SECOND KIND*
Authors: ELORANTA E.H. 
Published: April 2006

ABSTRACTAn integral equation method is described for solving the potential problem of a stationary electric current in a medium that is linear, isotropic and piecewise homogeneous in terms of electrical conductivity. The integral equations are Fredholm's equations of the ‘second kind’ developed for the potential of the electric field. In this method the discontinuity‐surfaces of electrical conductivity are divided into ‘sub‐areas’ that are so small that the value of their potential can be regarded as constant.The equations are applied to 3‐D galvanic modeling. In the numerical examples the convergence is examined. The results are also compared with solutions derived with other integral equations. Examples are given of anomalies of apparent resistivity and mise‐a‐la‐masse methods, assuming finite conductivity contrast. We show that the numerical solutions converge more rapidly than compared to solutions published earlier for the electric field. This results from the fact that the potential (as a function of the location coordinate) behaves more regularly than the electric field. The equations are applicable to all cases where conductivity contrast is finite.
GEOPHYSICAL INVESTIGATION OF A FAULT ZONE–CASE HISTORY FROM ILE‐IFE, SOUTHWEST NIGERIA*
Authors: OLORUNFEMI M.O. , OLAREWAJU V.O. , AVCI M. 
Published: April 2006

ABSTRACTA fault zone, inferred as a major linear structure from aerial photographs of the University of Ife Campus, has been investigated.Results of a multi‐method geophysical survey indicate that the zone is characterized by relatively low resistivity and high magnetic effect. They are characteristic of a sheared and perhaps saturated fault zone with magnetic mineral infillings along its plane.The near symmetry of the magnetic anomalies over the fault zone may not be indicative of a step faulting. The geoelectric sections show no indication of any significant displacement. The above supports results of a previous study that the fault is a strike‐slip fault.
THE EXACT SEISMIC RESPONSE OF AN OCEAN AND A N‐LAYER CONFIGURATION*
Authors: DRIJKONINGEN G.G. , FOKKEMA J.T. 
Published: April 2006

ABSTRACTThe space‐time acoustic wave motion generated by an impulsive monopole source is calculated with the aid of the Cagniard‐de Hoop technique. Two configurations with plane interfaces are discussed: an air/fluid/solid configuration with the source and the receiver located in the fluid layer; and a stack of n fluid layers between two acoustic half‐spaces where the source and the receiver are located in the upper half‐space. Synthetic seismograms are generated for the pressure of the reflected wavefield, using the source signature of an airgun.
ATTENUATION OF RANDOM NOISE BY 2‐D AND 3‐D CDP STACKING AND KIRCHHOFF MIGRATION*
Authors: KREY TH.C. 
Published: April 2006

ABSTRACTThree‐dimensional seismic surveys, in general, do not need the same high degree of CDP coverage as 2‐D surveys to achieve a certain signal‐to‐noise ratio after migration.This can be shown theoretically for Kirchhoff migration and laterally uncorrelated noise. More precisely, there exists a formal relationship between the multiplicity of CDP coverage of a 3‐D survey and that of a 2‐D survey with the same signal‐to‐uncorrelated‐noise ratio. Frequency and aperture are parameters in the corresponding expression. Heuristically the relationship can be obtained by applying the concept of the Fresnel zone.Though the mathematics in this paper refer to laterally uncorrelated noise, the underlying concepts can probably also be used for weakly correlated noise, e.g., for multiple reflections and for the low‐frequency remnants of surface waves.
IN SITU MEASUREMENTS OF SHEAR‐WAVE VELOCITY IN SEDIMENTS WITH HIGHER‐MODE RAYLEIGH WAVES*
Authors: GABRIELS P. , SNIEDER R. , NOLET G. 
Published: April 2006

ABSTRACTA seismic survey was carried out on a tidal flat in the SW‐Netherlands in order to determine shear‐wave velocities in sediments by means of higher‐mode Rayleigh waves. The dispersion properties of these Rayleigh waves were measured in the 2‐D amplitude spectrum–or f, k‐spectrum–and resulted in phase velocities for six different modes as a function of frequency (5–30 Hz). These observed phase velocities were inverted for a nine‐layer model for the shear‐wave velocity to a depth of 50 m.