bluuebunny/crossref_metadata_2025 huggingface dataset:
NATURAL SMOOTHNESS CONSTRAINTS IN CROSS‐HOLE SEISMIC TOMOGRAPHY1
Authors: PILKINGTON M. , TODOESCHUCK J. P.
Published: April 2006

Abstract

Tomographic inversion problems are ill‐posed and therefore solutions must be either damped or regularized to produce results that are geologically reasonable. We introduce a priori information in terms of parameter covariances to constrain the solution. We use slowness logs to determine the appropriate parameter covariances for the inversion of traveltime data collected for a cross‐hole geometry. We find that the logs exhibit power spectra proportional to a power α of the frequency. The value of α controls the smoothness of the inversion solution. For α < 0, the solution smoothness is maximized. Thus, knowing the correct value of α, which can be found from well logs, we can specify the appropriate amount of smoothing, rather than using some arbitrary level.

Inversions were carried out on synthetic data for the case of α= 0 and α=−1. The use of α= 0 implies uncorrelated model parameters and is equivalent to standard damped least‐squares methods. We find that solutions for α= 0 show greater complexity than for α=−1 but this level of resolution can be illusory. An example from the Midale field, Saskatchewan, Canada, is inverted using both α= 0 and α=−2, given by the observed parameter covariances. The solution for α= 0 exhibits spurious detail which must be smoothed away. For α=−2, the solution smoothness is maximized and we recover only that structure which is required to fit the data.
NON‐LINEAR AVO INVERSION FOR A STACK OF ANELASTIC LAYERS1
Authors: DAHL TERJE , URSIN BJØRN
Published: April 2006

Abstract

Parameters in a stack of homogeneous anelastic layers are estimated from seismic data, using the amplitude versus offset (AVO) variations and the travel‐times. The unknown parameters in each layer are the layer thickness, the P‐wave velocity, the S‐wave velocity, the density and the quality factor.

Dynamic ray tracing is used to solve the forward problem. Multiple reflections are included, but wave‐mode conversions are not considered. The S‐wave velocities are estimated from the PP reflection and transmission coefficients. The inverse problem is solved using a stabilized least‐squares procedure. The Gauss‐Newton approximation to the Hessian matrix is used, and the derivatives of the dynamic ray‐tracing equation are calculated analytically for each iteration.

A conventional velocity analysis, the common mid‐point (CMP) stack and a set of CMP gathers are used to identify the number of layers and to establish initial estimates for the P‐wave velocities and the layer thicknesses. The inversion is carried out globally for all parameters simultaneously or by a stepwise approach where a smaller number of parameters is considered in each step.

We discuss several practical problems related to inversion of real data. The performance of the algorithm is tested on one synthetic and two real data sets. For the real data inversion, we explained up to 90% of the energy in the data. However, the reliability of the parameter estimates must at this stage be considered as uncertain.
3D PRESTACK DEPTH MIGRATION IN THE GULF OF SUEZ: A CASE HISTORY1
Authors: WESTERN P. G. , BALL G. J.
Published: April 2006

Abstract

Seismic 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

Abstract

Methods 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

Abstract

The 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

Abstract

Seismic 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

Abstract

A 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

Abstract

Electrical 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

Abstract

Furness, 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

Abstract

Magnetic 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

Abstract

One 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

Abstract

A 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

Abstract

A 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

Abstract

Seismic 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

Abstract

The 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

Abstract

Depth 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

We 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

Geophysical 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.
Compound events decomposition and the interaction between AVO and velocity information1
Authors: Spagnolini Umberto
Published: April 2006

Minimization of seismic residuals does not guarantee uniqueness of the model, and this implies ambiguities in the inversion. Amplitude vs. offset (AVO) inversion does not lead to a unique solution of single elastic interface parameters unless converted and S‐wave or critical angle reflections are available. Given the ambiguity of AVO inversion, this paper discusses the interaction between AVO and velocity estimation. The number of independent parameters necessary to describe an isolated reflection with AVO behaviour and residual velocity error is determined. Statistical analysis allows the establishment of an approximate equivalence of the effects of AVO and slight velocity variations; this equivalence cannot be solved without geologicala prioriinformation (kinematic equivalence). The data are then decomposed into compound events (i.e. sequences ofNinterfaces that follow each other at a fixed time lag). The decomposition is obtained by extrapolating the results of the analysis from narrowband to wideband data. Compound events decomposition demonstrates that AVO inversion is ambiguous, not only in the physical parameter space (P‐ and S‐wave velocities, and density) but also kinematically. As an example of compound event decomposition, a medium is derived. This medium is geologically implausible but is kinematically equivalent.
Experimental and finite difference modelling of borehole Mach waves1
Authors: Cheng Ningya , Zhu Zhenya , Cheng C.H. , Toksöz M.N.
Published: April 2006

A series of model experiments was performed in an ultrasonic laboratory to study the radiation of downhole sources in a variety of formations. Three models were used in the experiments. They were a Lucite model, a Lucite model with a free glass pipe in the centre, and a glass‐cased soil model. In addition, a finite‐difference modelling technique was used to simulate the wave propagation in these models and the results of the laboratory and numerical experiments are compared. In the Lucite borehole model the waveforms recorded in the experiment agree very well with the finite‐difference synthetics. The snapshots of the wavefield from the finite‐difference simulation show the radiation pattern of the P‐ and S‐waves in the Lucite formation. These patterns are consistent with the theoretical calculations. In the Lucite model with the free glass pipe, the finite‐difference synthetics are also in good agreement with the experimental observations, especially for the conical P‐wave arrival. The angle between the wavefront of the conical P‐wave and the borehole axis, observed from the snapshot, agrees with the theory. In the cased soil model, the arrival time of the finite‐difference synthetics is in good agreement with the laboratory measurements. The relative amplitudes of the P‐wave and the Mach wave are not correctly modelled because intrinsic attenuation is not included in the finite‐difference calculation. The Mach cone angle from the snapshot agrees with the theoretical prediction. Finally, a finite‐difference method was used to simulate Mach‐wave propagation in a formation with two horizontal layers. In the case of two slow layers, the Mach‐wave generated in the first layer is reflected back from and transmitted through the boundary and another Mach wave is generated at the second layer when the Stoneley wave travels into the second layer. In the case of a formation having one slow and one fast layer, the Mach wave generated in the slow layer is reflected back at the boundary and leaked into the fast layer. There is no Mach wave in the fast layer.
Upward continuation of potential fields from a polyhedral surface1
Authors: Ivan M.
Published: April 2006

An equivalent source procedure is derived for upward continuation of unevenly spaced gravity and magnetic data. The dipole layer is placed on a topographic relief approximated by a polyhedral surface, the stations being the vertices of the triangular faces. The dipoles have linear magnitudes, being directed along the normal vector over each triangle. The unknown values of the dipole magnitudes at each station are obtained by a suitable modification of the usual integral equation considering the discontinuity of the normal vector at each vertex of the dipole surface. Profile data processing is also studied. A numerical test outlines the accuracy and the limitations of the model for the case of a magnetic field significantly perturbed by a rough topographic relief.
Inversion of reflection seismograms by differential semblance analysis: algorithm structure and synthetic examples1
Authors: Symes William W. , Kern Michel
Published: April 2006

Seismograms predicted from acoustic or elastic earth models depend very non‐linearly on the long wavelength components of velocity. This sensitive dependence demands the use of special variational principles in waveform‐based inversion algorithms. The differential semblance variational principle is well‐suited to velocity inversion by gradient methods, since its objective function is smooth and convex over a large range of velocity models. An extension of the adjoint state technique yields an accurate estimate of the differential semblance gradient. Non‐linear conjugate gradient iteration is quite successful in locating the global differential semblance minimum, which is near the ordinary least‐squares global minimum when coherent data noise is small. Several examples, based on the 2D primaries‐only acoustic model, illustrate features of the method and its performance.
Radar experiments in isotropic and anisotropic geological formations (granite and schists)1
Authors: Tillard Sylvie
Published: April 2006

In order to understand various aspects of radar wave propagation, a survey of electromagnetic wave behaviour relative to the geological characteristics of the formations prospected was undertaken. The sites chosen for the tests were a granite quarry and an underground schist working. By investigating an electrically resistive isotropic site and a conductive anisotropic site, it was demonstrated that non‐conventional use of a radar system (antennae raised, various orientations of the transmitter/receiver, etc.) could improve data quality, and could allow information other than reflector depth to be collected (volume scattering intensity, isotropy, etc.). By studying wave propagation velocities, we underlined the difficulties encountered in establishing a velocity versus depth law, despite recourse to seismic data processing, such as NMO corrections. The results of field experiments, complemented by laboratory measurements of dielectric permittivities, clearly showed anisotropy effects: in the case of a path that is perpendicular to the schistosity plane, an electromagnetic wave propagates more slowly and is more attenuated than a wave parallel to the schistosity plane.
Line integrals of potential field data1
Authors: Ivan M.
Published: April 2006

Information on the mass and the spatial location of an arbitrary source body can be obtained by performing suitable integrations of 3D gravity and magnetic data along an infinite straight line. No assumptions on the density/magnetization distribution or the shape and location of the source are required.

For an oblique borehole, a relationship between the lower limit of the source mass and the distance to the body is obtained. The mass contrast and the magnetic moment of the source can also be estimated.

For a vertical borehole, both gravity and vertical magnetic component anomalies have equal areas to the left and right of the depth axis.

The particular case of a horizontal gallery not intersecting the body is also studied. If the source is intersected, a lower limit is estimated for the maximum thickness of the body along the gallery. Information on the vertical coordinate of the centre of mass of the source can also be obtained.

Numerical tests with synthetic gravity data support the theoretical results.
Location of secondary faults from cross‐correlation of the second vertical derivative of gravity anomalies1
Authors: Zeng Hualin , Zhang Qinghe , Liu Jun
Published: April 2006

A method is described to locate secondary faults, which can be difficult to identify on the Bouguer gravity map. The method is based on cross‐correlation between the theoretical anomaly due to a vertical step and the second vertical derivative of the Bouguer anomaly. Faults are located from the closed maxima and minima on the cross‐correlation contour map calculated for two perpendicular directions. One‐dimensional model computations show that the magnitude of the extremum of the cross‐correlation is related to the depth to the top of the hanging wall and the throw of the fault. Application of the method to the Bouguer gravity map of the former mouth of the Yellow River in the Shengli Oilfield area near the Bo Hai Sea shows the effectiveness of the method.
Synthetic seismograms from generalized ray tracing1
Authors: Hanyga Andrzej , Helle Hans B.
Published: April 2006

A new method of numerical computation of elastic wavefields in regions containing caustics is tested. The method is an extension of the asymptotic ray theory (ART). The essential features of the method consist of the application of expressions which are well defined at caustics and expressed in terms of ray tracing combined with complex ray tracing in caustic shadows. The method and an outline of the underlying theory are briefly presented, followed by a comparison with finite differences on a test model involving a caustic cusp. The comparison reveals the unexpectedly high degree of accuracy of the new method.
Bootstrap statistics for velocity tomography: application of a new information criterion1
Authors: Nishizawa Osamu , Nero Harufumi
Published: April 2006

A new information criterion, the extended information criterion (EIC) was applied in order to determine an optimum solution in simultaneous iterative reconstruction technique (SIRT) P‐wave velocity tomography. The EIC is derived from information theory and statistics, and it measures the goodness‐of‐fit between the true (unknown) data distribution and the observed data distribution: the former gives the probability of data realization from the true (unknown) model, whereas the latter gives a probability of data realization calculated from a particular model of which parameters are estimated. The EIC is calculated using bootstrap statistics, a numerical technique for calculating statistical estimators. Bootstrap statistics enables us to obtain the bias between the log likelihood and the expected log likelihood, and then to obtain the expected log likelihood from the log likelihood. Since the EIC is obtained numerically, we can use it for most problems of model parameter estimation without employing the maximum likelihood method. Taking weak anisotropy into account, we reconstructed the P‐wave velocity structure of a rock sample during water infiltration under differential stress loading conditions. The results indicate that we can remove unrealistic solutions sometimes encountered when too many iterations are made. In spite of much computation time, the EIC is a promising technique for the near future, prompted by the rapid progress in current computer technology.
Restoration of missing offsets by parabolic Radon transform1
Authors: Kabir M.M. Nurul , Verschuur D.J.
Published: April 2006

Restoration of missing offsets and trace interpolation is an interesting and important problem in seismic data processing. Based on the parabolic Radon transform, a method is presented for missing offset restoration, resampling and regularization of prestack individual CMP gathers. The method is also valid for resampling spatially aliased seismic data.

The method is based on the parabolic assumption of the seismic events which is generally verified after a partial NMO correction in the CMP organization of the data. The essence of the method consists of a band‐limited forward parabolic Radon transform of the data containing zero traces at the missing offset locations. The curvature range is chosen to map properly the coherent energy while the zero traces map beyond that range. After inverse transform the originally zero traces are partly filled with information. Several iterations of forward and inverse transform, every time replacing the zero traces in the original gather with the partially reconstructed ones, almost fully restore the zero traces.

Efficient and fast algorithms can be built up to process data having a uniform geometry. Examples on synthetic as well as on field data demonstrate clearly the robustness of the method.
Anisotropic velocity analysis1
Authors: Sayers C.M.
Published: April 2006

Results from walkaway VSP and shale laboratory experiments show that shale anisotropy can be significantly anelliptic. Heterogeneity and anellipticity both lead to non‐hyperbolic moveout curves and the resulting ambiguity in velocity analysis is investigated for the case of a factorizable anisotropic medium with a linear dependence of velocity on depth. More information can be obtained if there are several reflectors. The method of Dellinger et al. for anisotropic velocity analysis in layered transversely isotropic media is examined and is shown to be restricted to media having relatively small anellipticity. A new scheme, based on an expansion of the inverse‐squared group velocity in spherical harmonics, is presented. This scheme can be used for larger anellipticity, and is applicable for horizontal layers having monoclinic symmetry with the symmetry plane parallel to the layers. The method is applied to invert the results of anisotropic ray tracing on a model Sand/shale sequence. For transversely isotropic media with small anisotropy, the scheme reduces to the method of Byun et al. and Byun and Corrigan. The expansion in spherical harmonics allows the P‐phase slowness surface of each layer to be determined in analytic form from the layer parameters obtained by inversion without the need to assume that the anisotropy is weak.
A new method of quantitative interpretation of SP anomalies produced by a polarized inclined sheet1
Authors: Skianis George , Papadopoulos Taxiarchis , Vaiopoulos Dimitrios , Nikolaou Sotiris
Published: April 2006

A new method is proposed for the quantitative interpretation of SP field data produced by a polarized ore body and simulated by an inclined sheet. The theoretical concept is based on the study of the amplitude Spectrum. It is shown that the SP amplitude Spectrum is not continuous at zero frequency; this leads to the dip angle determination of the inclined sheet. It is also shown that the SP amplitude spectrum is practically nullified at a characteristic amortization frequency that depends on the depth of the polarized body. The maximum amplitude Spectrum value of the SP gradient is used to estimate the depth to the bottom of the polarized body. Thus, the geometrical parameters h and H, the depths to the top and bottom, respectively, as well as the dip angle of the inclined sheet, can be satisfactorily determined. Some problems may arise in the determination of these parameters, affecting their accuracy, whenever unwanted frequency noise is present.
On the application of geophysics in the exploration for copper and chrome ores in Albania1
Authors: Frasheri Alfred , Lubonja Ligor , Alikaj Perparim
Published: April 2006

Some generalized results of geophysical exploration for copper sulphide and chromite ores in Albania are presented. The most important geophysical methods used are electrical prospecting, gravity, magnetics and electromagnetics. Physical properties of the ores, genesis and geological problems to be solved have determined the proper choice of any of these methods in the complex exploration.
Conventional and modern seismic investigations for rock quality determination at a dam site—a case history1
Authors: Louis J. , Papadopoulos T. , Drakatos G. , Pantzartzis P.
Published: April 2006

Refraction seismics and modern tomographic surveys were employed at the Platanovrissi dam site on the Nestos river in northern Greece in an effort to investigate the dynamic elastic properties and rockmass condition of gneiss‐schist outcropping in the area under investigation. Six seismic refraction traverses were run on the ground surface, inside the exploratory tunnels and between tunnels and the ground surface. The results provided information about the low‐velocity surface layer parameters and the stress relief zones on the walls of all the tunnels in which seismic tomography tests were performed. Ultrasonic tests, conducted both in situ and in the laboratory, enabled the determination of the ratio Vp/Vs and the Poisson's ratio calculation. From the tomography data analysis and the geological interpretation of the results, three main velocity zones, ranging between 3500 and 4000, 3000 and 3500 and less than 3000m/s, were detected in each tunnel. High‐velocity zones were found to be compatible with the hard rockmass of gneiss‐schist material, while moderate velocities are closely related to jointed and fractured zones. Finally, low‐velocity zones were well correlated with weathered gneiss‐schist material.
Finite‐difference elastic wave propagation in 2D heterogeneous transversely isotropic media1
Authors: Juhlin Christopher
Published: April 2006

The velocity‐stress formulation for propagation of elastic seismic waves through 2D heterogeneous transversely isotropic media of arbitrary orientation is presented. The equations are recast into a finite‐difference scheme and solved numerically using fourth‐order spatial operators and a second‐order temporal operator on a staggered grid. Absorbing, free‐surface and symmetry boundary conditions have been implemented. Test cases compare well with other published solutions. Synthetic seismograms are calculated over two idealized models: (i) vertical fractures in granite with a dolerite sill reflector and (ii) a dipping anisotropic shale. Comparisons with the isotropic counterparts show significant differences which may have to be accounted for in seismic processing in the future.
Computation of apparent resistivity profiles from VLF‐EM data using linear filtering1
Authors: Chouteau Michel , Zhang Ping , Chapellier Dominique
Published: April 2006

A simple filter is developed which transforms VLF‐EM real magnetic field transfer functions into apparent resistivities. It is based on the relationship between the horizontal derivative of the surface electric field and the vertical magnetic field at the surface of a two‐dimensional earth model. The performance of this simple autoregressive filter is tested for modelled and real survey data. The technique yields profiles of apparent resistivity very similar, both in magnitude and in wavelength, to those which would have been obtained using VLF‐EM resistivity measurements or d.c. resistivity profiling. This low‐pass filter has the advantage of reducing high‐wavenumber noise in the data; therefore only the major features of the VLF‐EM profile are displayed.
Extreme climate and variation in tree growth: individualistic response in subalpine fir (Abies lasiocarpa)
Authors: ETTL GREGORY J. , PETERSON DAVID L.
Published: April 2006

Dendroecological techniques were used to describe the variation in growth response of subalpine fir (Abies lasiocarpa)to climate across a range of elevations (1350–1850 m) and annual precipitation (125–350 cm y−1) in the Olympic Mountains, Washington. Correlation analysis is used to describe individual growth‐climate relationships. Growth response is quantified in years with unusually warmer, colder, wetter, and drier climates during the period 1895–1990. Combinations of climatic variables that result in unusually fast or slow growth years are also described. Differences in growth‐climate relationships among sites, and among individuals from the same site, emphasize within‐species variability in response to climate. Growth was not significantly faster or slower on the majority of sites for extreme climate years examined. Few climate variables are correlated with growth of the majority of individuals on most sites, suggesting that some individuals are relatively unresponsive to climate. Individual growth‐climate correlations also indicate an increase in the percentage of individuals whose growth is significantly correlated with a climate variable, as the value of the mean site growth correlation increases for that climate variable. Individual differences in growth‐climate relationships probably result from microsite variation (soil depth, soil moisture, wind, insolation) and from individual genetic differences. Descriptions of tree species response to climate change need to incorporate both individual and site variation in growth response to climate in order to accurately represent existing environmental heterogeneity.