bluuebunny/crossref_metadata_2025_split huggingface dataset:
3D PRESTACK DEPTH MIGRATION IN THE GULF OF SUEZ: A CASE HISTORY1
Authors: WESTERN P. G. , BALL G. J. 
Published: April 2006

AbstractSeismic reflection data in the southern Gulf of Suez, offshore Egypt, are commonly severely affected by shallow velocity inhomogeneities in the form of diapiric salt bodies, and depth migration techniques must be used in order to image the presalt structure correctly. Frequently the diapir and the underlying prospective structure are three dimensional rather than two dimensional and thus require 3D techniques to resolve them. In addition, the severity of the problem is sometimes such that the common midpoint (CMP) stack assumptions are invalid and prestack depth migration is therefore required.In 1990, Unocal developed a practical 3D prestack depth‐migration scheme, which was applied to a data set in the Gulf of Suez. The prospect was subsequently drilled and results proved the effectiveness of the technique.This paper describes the use of the technique in the form of a case history. It is expected that the technique will be routinely used to solve similar problems.
SLOPE DATA WATER‐BOTTOM MULTIPLE ATTENUATION1
Authors: HAASE ARNIM B. 
Published: April 2006

AbstractMethods for predicting and attenuating water‐bottom multiples by wavefield extrapolation have been discussed by several investigators. Because these prediction methods operate on shot records, boundary conditions must be specified for every shot record.The approach presented operates in the common‐offset plane; a model of expected water‐bottom multiples is generated from the observed surface wavefield using a finite‐difference wave‐equation migration algorithm with an offset term. An accurate water‐depth profile is required, but there is no restriction on the shape of the water bottom other than a dip limit of approximately 18–20°. In generating a multiple model, the water‐bottom primary and each water‐bottom multiple reflection of the observed surface wavefield are extrapolated to a higher order. Thus, the extrapolated water‐bottom primary of the model is lined up with a water‐bottom multiple in the data and each multiple in the model is lined up with a higher‐order (or later) multiple in the data.Prestack multiple attenuation is achieved, for one offset at a time, by first adapting the model of expected multiples to the observed data and then subtracting the predicted multiple energy. An error‐constrained adaptation algorithm is proposed in order to control instabilities. No assumptions are made about primary reflections and no subwater‐bottom velocities are required.Computational efficiency of modelling and adaptation can be improved by applying this method only to near and intermediate offsets as the stacking process usually provides sufficient multiple attenuation at far offsets. A field data example demonstrates the potential of the proposed method for improving the primary‐to‐multiple ratio in prestack and post‐stack data.
IP RESPONSE FOR A 2D HORIZONTAL CYLINDER1
Authors: BHATTACHARYA B. B. , BISWAS D. 
Published: April 2006

AbstractThe induced polarization response for a 2D horizontal cylinder embedded in a half‐space is calculated for a uniform electric source. Response curves, in the form of apparent charge‐ability taking into account the effect of the air‐earth interface, exhibit a sharp decrease in amplitude with an increase in depth of burial of the target. The resistivity contrast between the cylinder and the host plays a dominant role in determining the IP response, i.e. the amplitude decreases considerably with the increase in resistivity contrast. The decrease is due to the defocusing effect caused by the resistive cylinder. The current lines tend to deviate away from the cylindrical target. In the case of a highly conducting cylinder, apparent defocusing takes place as current lines are confined to the surface of the conducting cylinder. An increase in chargeability contrast is reflected as a steady rise in the response. The peak response at the centre is reduced by about half the magnitude when the air–earth interface is not considered. The variation of response along the profile, though noticeable, is not as high as that obtained at the centre.
SEISMIC HORIZON DETECTION USING IMAGE PROCESSING ALGORITHMS1
Authors: BONDÁR ISTVÁN 
Published: April 2006

AbstractSeismic sections used in interpretation are actually images. We often superimpose colour‐coded pictures of seismic attributes on seismic sections. Thus, it seems straightforward to use image processing algorithms to enhance the quality of the seismic images.From an image processing point of view, seismic horizons can be thought of as edges on the seismic image. We present a novel approach to detecting seismic horizons, which includes the 2D median filtering of the instantaneous phase attribute and applying an edge detection algorithm. The resulting edge magnitude picture provides a skeletonized image of the seismic section, on which the structural and stratigraphic patterns can be better recognized.
MAPPING OF CRUSTAL DISCONTINUITIES BY WAVELENGTH FILTERING OF THE GRAVITY FIELD1
Authors: CHAKRABORTY KUNAL , AGARWAL B. N. P. 
Published: April 2006

AbstractA statistical technique, based on the concept of a 1D energy density spectrum of the observed gravity field, has been used to compute ensemble average depths to various horizons containing causative sources of random geometric shape, size, density, etc. The plot of the logarithm of the energy of the observed Bouguer anomaly versus the angular frequency can be approximated, over a certain frequency band, by a linear segment whose slope is related to an average ensemble depth around which a random distribution of numerous anomalous sources exists. Suitable matched filters, based on the computed values of intercepts and slopes of several linear segments approximating the spectrum, have been used to deconvolve the gravity effects associated with the causative sources, occurring around their respective mean depths. The individual deconvolved gravity effects thus separated out have been modelled using the sinx/xmethod by assuming a fluctuating interface between two formations.The applicability of the present method has been assessed using two observed Bouguer anomaly profiles: one from Ujjain to Mahan, and the other from Jhansi to Mandla where Deep Seismic Sounding (DSS) results are available. The proposed geological crustal models along these two profiles exhibit reasonably good agreement with those obtained from DSS results. A geologically plausible model of the crust in a virgin region has been presented along a Bouguer anomaly profile from Jaipur to Raipur.The following main conclusions have been drawn from the present analysis: (1) The depths to the Moho and Archaean basement interfaces fluctuate between 33.2 and 36.8 km and between 4.6 and 7.0 km respectively. (2) The Narmada‐Son Lineament (NSL) does not coincide exactly with the Moho upwarp beneath it. However, this offset is greater in the eastern part of the NSL rather than in the western part. (3) The development of the Satpura horst structure is due to a rise in the Moho interface in a compressional regime. (4) The intrabasement feature (depth from 5 to 12 km) represents a hybrid massif possibly formed due to an admixture of sialic and simatic crust under a tensional regime in the Ujjain‐Mahan section.
CONTAMINATION INDICATIONS DERIVED FROM ELECTRICAL PROPERTIES IN THE LOW FREQUENCY RANGE1
Authors: BÖRNER F. , GRUHNE M. , SCHÖN J. 
Published: April 2006

AbstractElectrical measurements are an important and integrated component of geophysical investigations connected with environmental problems. As a result of an analysis of the electrical conductivity, basic experiments on sandstones at frequencies below 10 kHz show that the complex behaviour of conductivity is caused exclusively by a complex interface conductivity. Its value is determined mainly by the internal rock interface to porosity ratio, the composition of the pore fluid and connected matrix‐water interactions resulting in a specific microstructure of the interface. Therefore, it can be expected that the interface region of a soil or rock material is very sensitive to changes in composition caused by contamination. Contaminated sandstone and clay samples were investigated using a low‐frequency measurement system. The investigations are directed at the influence of different contaminants and their concentration. Results show that the complex electrical conductivity (real and imaginary parts) is influenced by properties of the pore‐filling contaminant. This influence results in a change of the level of both parts and the shape of their frequency dependence. The imaginary part in particular seems to provide important secondary information; in some cases this part alone allows a differentiation of the various contaminants. The different behaviour of various rock types shows that the effects observed are the result of interactions between pore fluid properties and the internal pore surface structure.
GRADIENT ARRAY PROFILES OVER THIN RESISTIVE VEINS1
Authors: FURNESS PETER 
Published: April 2006

AbstractFurness, P. 1993. Gradient Array Profiles over thin resistive veins. Geophysical Prospecting41, 113–130.Gradient array geoelectric surveys are commonly used to explore for buried sheet‐like targets. It is demonstrated that under certain circumstances the responses of such features can become practically equivalent for a range of models differing considerably in resistivity and thickness. This is the dual of the familiar equivalence phenomenon associated with thin beds in geoelectric sounding operations. Hence the geoelectric response profiles over thin dipping resistive veins are conveniently studied by considering equivalent forms with vanishing thickness and infinite resistivity but with the same resistivity‐thickness product. The effects of varying the dip, depth of burial, depth extent and resistivity–thickness product of such features on the gradient array response profiles are investigated by means of a series of numerical experiments. These demonstrate several effects of significance to survey design and data interpretation.In particular it is found that while the resistivity–thickness product influences the amplitude of the response profiles it has little effect on their form. A similar behaviour is found to be exhibited by veins with extended depth extent when the dip is varied.A complete quantitative interpretation of gradient array profiles over thin dipping resistive veins is thus only possible for veins of limited depth extent. For veins with an extended depth dimension, the data is ambiguous (in a practical sense) in terms of the dip and the resistivity–thickness product. Here, the interpretation can only be expected to deduce the horizontal and vertical locations of the vein apex.
THE MAGNETIC ANOMALY OF 3D SOURCES HAVING ARBITRARY GEOMETRY AND MAGNETIZATION
Authors: RUOTOISTENMÄKI TAPIO 
Published: April 2006

AbstractMagnetic anomalies of complicated 3D sources can be calculated by using a combination of analytical and numerical integration. Two surfaces and the magnetization parameters (the amplitudes of the induced and remanent components and the direction cosines) of the source can be defined by arbitrary functions or by discrete data points in a plane. When combined with a polynomial magnetization function in the direction of the third axis, 3D magnetization distribution can also be modelled.The method gives very general equations for anomaly calculation. It can be used for direct modelling of sources interpreted by seismic or other methods and also for interactive interpretation with fast computers. It is possible to calculate anomalies of, for example, intrusives or folded sedimentary beds whose surfaces are functions of horizontal coordinates and which have polynomial magnetization variations in the vertical direction due to gravitational differentiation and arbitrarily varying magnetization in the horizontal direction due to regional metamorphosis.If the distribution of magnetization parameters in the vertical direction cannot be described satisfactorily by polynomials, models can be used whose surfaces are functions of the vertical coordinate and which can then have any arbitrary magnetization distribution in the vertical direction.
EXAMPLES OF RESOLUTION IMPROVEMENT IN GEOELECTRICAL SOUNDINGS APPLIED TO GROUNDWATER INVESTIGATIONS1
Authors: SANDBERG STEWART K. 
Published: April 2006

AbstractOne simulation and two field examples from New Jersey illustrate resolution improvement in geoelectrical soundings applied to groundwater exploration. Layered‐earth parameter resolution is derived from data obtained with the commonly used methods of resistivity, induced polarization (IP) and transient electromagnetic (TEM) soundings. Resolution improvement is achieved by simultaneous inversion of two or more data sets and by constraining parameters of the inverse problem.A quantitative analysis showing the contribution of IP data to the resolution of geo‐electric sections is presented. Simultaneous inversion of simple IP data with conventional resistivity and resistivity‐TEM data sets resulted in improved parameter resolution. IP data improved resolution in three ways: (1) by decoupling correlated layered‐earth parameters, (2) by adding information to a geological interpretation about a second physical property, namely chargeability, and (3) by increasing the electrical information available.
RADON DEPTH MIGRATION1
Authors: HILDEBRAND S. T. , CARROLL R. J. 
Published: April 2006

AbstractA depth migration method is presented that uses Radon‐transformed common‐source seismograms as input. It is shown that the Radon depth migration method can be extended to spatially varying velocity depth models by using asymptotic ray theory (ART) to construct wavefield continuation operators. These operators downward continue an incident receiver‐array plane wave and an assumed point‐source wavefield into the subsurface. The migration velocity model is constrained to have longer characteristic wavelengths than the dominant source wavelength such that the ART approximations for the continuation operators are valid.This method is used successfully to migrate two synthetic data examples:

a point diffractor, and
a dipping layer and syncline interface model.

It is shown that the Radon migration method has a computational advantage over the standard Kirchhoff migration method in that fewer rays are computed in a main memory implementation.
NUMERICAL SEISMOGRAM COMPUTATIONS FOR INHOMOGENEOUS MEDIA USING A SHORT, VARIABLE LENGTH CONVOLUTIONAL DIFFERENTIATOR1
Authors: ZHOU B. , GREENHALGH S.A. , ZHE J. 
Published: April 2006

AbstractA short convolutional differentiator (CD) for computing second spatial derivatives in the acoustic wave equation is presented. This differentiator is obtained by tapering the inverse Fourier transform of the band‐limited Fourier spectrum of the second‐derivative operator. This new filter has been applied to seismogram computations for inhomogeneous media and results are compared with the conventional high‐order finite‐difference (FD) and Fourier schemes. The operator can be progressively shortened at the model edges to reduce boundary artefacts. The CD method is superior to the conventional FD operator and comparable with the Fourier method in accuracy but faster to run. A strategy to reduce computation time by 20%, which exploits the localized nature of the operator, is given. The method is illustrated using simple 2D models.
SEISMIC STRATIGRAPHY IN HIGH RESOLUTION SHALLOW MARINE SEISMIC DATA OF THE GEMLIK GULF1
Authors: KURTULUS CENGIZ 
Published: April 2006

AbstractSeismic stratigraphy and sedimentological studies of the Gemlik Gulf in the Sea of Marmara, Turkey, have been carried out. For this purpose, 19 lines totalling 189 km of excellent quality, high‐resolution seismic data were recorded.Four major acoustic units were identified in the seismic profiles. Three were sedimentary units: irregular layered, cross‐layered and well‐layered; and the fourth was an acoustic basement which is probably composed of crystalline volcanic rocks.Some local areas in the Neogene formation contain gas accumulations.The formation of faults in E–W and N–S directions can be explained by the existence of shear stresses in the Gulf. The bathymetric map shows good accommodation with the shore line as does the tectonic map.
LINE CURRENT FILTERING OF FIXED‐LOOP TRANSIENT ELECTROMAGNETIC DATA1
Authors: SMITH R. S. , WHITAKER S. A. 
Published: April 2006

AbstractThe VLF filtering technique of Karous and Hjelt has been applied to fixed‐loop step‐response transient electromagnetic data. This allows the data measured in each channel to be converted to an equivalent current‐density pseudosection.For a conductive half‐space, the maximum value of the equivalent current density starts near the transmitter loop and migrates outwards as a function of delay time. The rate of migration tends to increase as a function of delay time, with the increase being faster for a surficial conductive layer than it is for a half‐space.Theoretical and field examples show that the currents tend to be more persistent in the relatively conductive areas, so that a pseudosection which is the average of the current densities at all delay times will highlight the more conductive zones.In resistive ground, it is not so critical to average the pseudosections as a particular delay time may give a better idea of the conductivity structure. For example, the latest possible delay time will reveal the most conductive features.
DEPTH IMAGING OF OFFSET VERTICAL SEISMIC PROFILE DATA1
Authors: AMUNDSEN LASSE , ARNTSEN BØRGE , MITTET RUNE 
Published: April 2006

AbstractDepth migration consists of two different steps: wavefield extrapolation and imaging. The wave propagation is firmly founded on a mathematical frame‐work, and is simulated by solving different types of wave equations, dependent on the physical model under investigation. In contrast, the imaging part of migration is usually based on ad hoc‘principles’, rather than on a physical model with an associated mathematical expression. The imaging is usually performed using the U/D concept of Claerbout (1971), which states that reflectors exist at points in the subsurface where the first arrival of the downgoing wave is time‐coincident with the upgoing wave.Inversion can, as with migration, be divided into the two steps of wavefield extrapolation and imaging. In contrast to the imaging principle in migration, imaging in inversion follows from the mathematical formulation of the problem. The image with respect to the bulk modulus (or velocity) perturbations is proportional to the correlation between the time derivatives of a forward‐propagated field and a backward‐propagated residual field (Lailly 1984; Tarantola 1984).We assume a physical model in which the wave propagation is governed by the 2D acoustic wave equation. The wave equation is solved numerically using an efficient finite‐difference scheme, making simulations in realistically sized models feasible. The two imaging concepts of migration and inversion are tested and compared in depth imaging from a synthetic offset vertical seismic profile section. In order to test the velocity sensitivity of the algorithms, two erroneous input velocity models are tested. We find that the algorithm founded on inverse theory is less sensitive to velocity errors than depth migration using the more ad hoc U/D imaging principle.
The relationships between the velocities, attenuations and petrophysical properties of reservoir sedimentary rocks1
Authors: Best A.I. , McCann C. , Sothcott J. 
Published: April 2006

AbstractWe have measured the velocities and attenuations of compressional and shear waves in 29 water‐saturated samples of sandstones and shales at a confining pressure of 60 MPa and at frequencies of about 0.85 MHz. The measurements were made using a pulse echo method in which the samples (diameter 5 cm, length 1.5 cm to 2.5 cm) were placed between perspex buffer rods inside a high‐pressure cell. The velocity of each seismic wave was determined from the traveltime difference of equivalent phase points (corrected for diffraction effects) of the signals reflected from the top and from the base of each sample. Attenuation was determined in a similar way by comparison of the diffraction corrected amplitudes of the signals. The attenuation data are presented as ‘quality factors’: Qp and Qs for compressional and shear waves respectively. The results show that Qs is strongly correlated with Vs, that Qp is weakly correlated with Vp, and that Qp is strongly correlated with Qs. Qp is strongly dependent on the volume percentage of the assemblage of intra‐pore minerals, whether they are clays or carbonates. It is concluded that the attenuation mechanism is due to the local fluid flow arising from the differential dilation of the solid rock frame and the intra‐pore mineral assemblage, which is a result of their very different elastic moduli.
A study of the geoelectrical properties of peatlands and their influence on ground‐penetrating radar surveying1
Authors: Theimer Brian D. , Nobes David C. , Warner Barry G. 
Published: April 2006

AbstractGeophysical surveys and chemical analyses on cores were carried out in three Ontario peatlands, from which we have gained a better understanding of the peat properties that control the geophysical responses. The electrical conductivity depends linearly on the concentration of total dissolved solids in the peat pore waters and the pore waters in turn bear the ionic signatures of the underlying mineral sediments. The ionic concentration, and thus the electrical conductivity, increase linearly from the surface to basement. The average bulk electrical conductivity of peatlands at Ellice Marsh, near Stratford, and at Wally Creek Area Forest Drainage Project, near Cochrane, are of the order of 25 mS/m. The Mer Bleue peatland, near Ottawa, has extremely high electrical conductivity, reaching levels of up to 380 mS/m near the base of the peat. The Mer Bleue peatland water has correspondingly high values of total dissolved solids, which originate from the underlying Champlain Sea glaciomarine clays. The dielectric permittivity in peats is largely controlled by the bulk water content. Ground penetrating radar can detect changes in water content greater than 3%, occurring within a depth interval less than 15 cm. The principal peatland interfaces detected are the near‐surface aerobic to anaerobic transition and the peat to mineral basement contact. The potential for the successful detection of the basement contact using the radar can be predicted using the radar instrument specifications, estimates of the peatland depth, and either the bulk peat or the peat pore water electrical conductivities. Predicted depths of penetration of up to 10 m for Ellice Marsh and Wally Creek exceed the observed depths of 1 to 2 m. At Mer Bleue, on the other hand, we observe that, as predicted, a 100 MHz signal will penetrate to the base of a 2 m thick peat but a 200 MHz signal will not.