bluuebunny/crossref_metadata_2025 huggingface dataset:
Geophysical approach to the environmental study of a coastal plain
Authors: Balia Roberto , Gavaudò Enrico , Ardau Federica , Ghiglieri Giorgio
Published: September 2003

In the study of coastal plains affected by soil and water salination, a knowledge of several geological aspects, such as structural features, depth to basement, stratigraphy of sedimentary cover, relationships between the phreatic aquifer and underlying aquifers, and the latter's structure, is basic to gaining an adequate understanding of both the causes and possible evolution of salination. In this framework, geophysical techniques can play a very important role.

To improve the available geophysical information about the Muravera coastal plain, Sardinia, Italy, which is affected by severe soil and water salination, previously acquired electrical resistivity, reflection seismic and gravity data have been reprocessed, and a new seismic reflection survey has been conducted. Moreover, in order to give better support to the geological and hydrogeological interpretation of geophysical data, three boreholes were drilled. Reprocessed electrical data indicate the presence of a wide, electrically homogeneous low‐resistivity zone associated with salination phenomena. Reprocessed reflection data provide useful information on the near‐surface stratigraphy. The combined interpretation of resistivity and seismic results, supported by one calibration borehole, elucidates the relationships between the phreatic aquifer and the underlying confined aquifer. A new seismic reflection survey gives information on the depth to, and structure of, the Paleozoic basement, as well as on stratigraphic conditions of Pleistocene‐Holocene sediments. Finally, the combined interpretation of seismic, gravity, and well data results in a geological section containing most of the information considered essential, such as the interface between Holocene alluvium and Pleistocene alluvium, the thickness of the latter, and the structure and composition of the Paleozoic basement.

The work as a whole shows how the combined application of geophysical techniques can in this specific situation provide wide‐ranging and high‐quality information that is essential for the realistic mathematical modeling of aquifer contamination, and can enable the rational planning of exploratory drillings.
Seismic impedance inversion and interpretation of a gas carbonate reservoir in the Alberta Foothills, western Canada
Authors: Madiba Gislain B. , McMechan George A.
Published: September 2003

Acoustic and simultaneous elastic impedance inversions of a 2D land seismic data set are performed to characterize a carbonate reservoir of Mississippian age in the Turner Valley Formation, in the Rocky Mountain foothills of western Canada. The inversions produce P‐wave and S‐wave impedance sections (Ipand Is, respectively), from which Lamé parameter × density (λρ and μρ) sections are derived. The Ipdata provide a separation between the clastics and carbonates. The μρ data provide an estimate of porosity distribution within the dolomitized limestone target. Deviations from baseline curves for water‐saturated carbonates, of λρ versus porosity, λ/μ versus porosity, and Isversus Ip, are interpreted as indicators of gas potential. These indicators all provide similar spatial patterns of areas of high gas potential and are consistent with the gas occurence observed in a well.
Time‐lapse imaging at Bullwinkle field, Green Canyon 65, offshore Gulf of Mexico
Authors: Swanston Alastair M. , Flemings Peter B. , Comisky Joseph T. , Best Kevin D.
Published: September 2003

Two orthogonal preproduction seismic surveys and a regional seismic survey acquired after eight years of production from the Bullwinkle field (Green Canyon 65, Gulf of Mexico) reveal extraordinary seismic differences attributed to production‐induced changes in rock and fluid properties. Amplitude reduction (of up to 71%) occurs where production and log data show that water has replaced hydrocarbons as the oil–water contact moved upward. Separate normalizations of these surveys demonstrate that time‐lapse results are improved by using seismic surveys acquired in similar orientations; also, clearer difference images are obtained from comparing lower‐frequency data sets. Superior stratigraphic illumination in the dip‐oriented survey relative to the strike‐oriented surveys results in nongeological amplitude differences. This documents the danger of using dissimilar baseline and monitor surveys for time‐lapse studies.
Uncertainty estimation for amplitude variation with offset (AVO) inversion
Authors: Riedel Michael , Dosso Stan E. , Beran Laurens
Published: September 2003

This paper uses a Bayesian approach for inverting seismic amplitude versus offset (AVO) data to provide estimates and uncertainties of the viscoelastic physical parameters at an interface. The inversion is based on Gibbs' sampling approach to determine properties of the posterior probability distribution (PPD), such as the posterior mean, maximum a posteriori (MAP) estimate, marginal probability distributions, and covariances. The Bayesian formulation represents a fully nonlinear inversion; the results are compared to those of standard linearized inversion. The nonlinear and linearized approaches are applied to synthetic test cases which consider AVO inversion for shallow marine environments with both unconsolidated and consolidated seabeds. The result of neglecting attenuation in the seabed is investigated, and the effects of data factors such as independent and systematic errors and the range of incident angles are considered. The Bayesian approach is also applied to estimate the physical parameters and uncertainties from AVO data collected at two sites along a seismic line in the Baltic Sea with differing sediment types; it clearly identifies the distinct seabed compositions. Data uncertainties (independent and systematic) required for this analysis are estimated using a maximum‐likelihood approach.
An inclusion‐based model of elastic wave velocities incorporating patch‐scale fluid pressure relaxation
Authors: Taylor S. Richard , Knight Rosemary J.
Published: September 2003

We consider elastic wave velocities in fluid‐saturated porous media with pore fluids distributed in “patches” (i.e., heterogeneity much larger than the typical pore size). We model elastic properties of such materials using inclusion‐based effective medium theory (IBEMT). The standard IBEMT formulation assumes insufficient time during the wave cycle for pore fluids to flow in response to wave‐induced pressure gradients. Our approach accounts for this flow, incorporating wave‐frequency dependent flow effects in the definition of effective elastic moduli for patches. Effective moduli are used in conjunction with IBEMT to estimate elastic moduli of the composite material. In the low‐ and high‐frequency limits, the model reproduces previous theoretical results. At intermediate frequencies, it yields results qualitatively similar to other patch‐scale models. We demonstrate this approach, estimating elastic P‐wave velocities and attenuation in a porous rock that simultaneously contains fluid‐saturated patches of different sizes.
Multicomponent induction response in a borehole environment
Authors: Wang Tsili , Yu Liming , Fanini Otto
Published: September 2003

Field and numerically simulated data show that coplanar induction measurements (σxx and σyy) are affected by both conductive and resistive muds. The effect is, for a resistivity contrast of 10 or less between the formation and mud, approximately proportional to the resistivity contrast. The borehole effect is generally stronger for a decentralized tool. In the perpendicular eccentricity mode, the eccentricity effect on σxx or σyy can be greater than 100% of the true formation responses. We demonstrate that dual‐frequency processing can reduce the eccentricity effect by an order of magnitude. We investigate the effect of invasion for both circular and elliptic invasions. For circular invasion, the effect on σxx, σyy, or the coaxial measurement (σzz) is characterized by a critical invasion thickness. For invasion thinner than the critical thickness, the effect is minimal. The effect increases rapidly as invasion thickness increases beyond the critical thickness. For elliptic invasion, both the σxx and σzz responses are similar to those of the corresponding inner circular invasion. Beyond a long‐to‐short‐axis ratio of three, an elliptic invasion behaves like a long, thin fracture.
Crosshole seismoelectric measurements in borehole models with fractures
Authors: Zhu Zhenya , Toksöz M. Nafi
Published: September 2003

Laboratory experiments are performed in cross‐borehole models with fractures to investigate seismoelectric conversions in the fractures. Using an electrode in a borehole, we measure the electric field induced by an acoustic wave, and we investigate the relationship between the electric signal and the fracture aperture using ultrasonic fracture borehole models. The experimental results confirm that a radiating electromagnetic (EM) wave is induced by a guide wave at a fracture between an acoustic source and electric receiver boreholes. The position of a vertical or inclined fracture between two boreholes can be determined using the arrival times of the EM wave and the formation velocity by placing the acoustic source at a different depth and recording electric signals with real or synthetic arrays of acoustic and electric receivers in the second borehole. Crosshole seismoelectric measurements might be a technique for investigating fractures between boreholes more directly than traditional acoustic measurements.
An efficient finite‐difference scheme for electromagnetic logging in 3D anisotropic inhomogeneous media
Authors: Davydycheva Sofia , Druskin Vladimir , Habashy Tarek
Published: September 2003

We consider a problem of computing the electromagnetic field in 3D anisotropic media for electromagnetic logging. The proposed finite‐difference scheme for Maxwell equations has the following new features based on some recent and not so recent developments in numerical analysis: coercivity (i.e., the complete discrete analogy of all continuous equations in every grid cell, even for nondiagonal conductivity tensors), a special conductivity averaging that does not require the grid to be small compared to layering or fractures, and a spectrally optimal grid refinement minimizing the error at the receiver locations and optimizing the approximation of the boundary conditions at infinity. All of these features significantly reduce the grid size and accelerate the computation of electromagnetic logs in 3D geometries without sacrificing accuracy.
Shipborne electromagnetic measurements of Antarctic sea‐ice thickness
Authors: Reid James E. , Worby Anthony P. , Vrbancich Julian , Munro Angus I. S.
Published: September 2003

We present a study of Antarctic sea‐ice thickness estimates made using a shipborne Geonics EM31 electromagnetic (EM) instrument, based on both 1D and 3D models. Apparent conductivities measured in the vertical coplanar (VCP) geometry are shown to be the measured quantity most sensitive to changes in the height of the instrument above seawater. An analysis of the effect of instrument orientation on the measured VCP apparent conductivity shows that the effects of pitch and roll on the calculated sea‐ice thickness can be neglected except in the case of very thin sea ice.

Because only a single (quadrature) component of the magnetic field is measured at a single frequency, interpretation of shipborne EM31 data must necessarily be based on very simple models. For a typical sea‐ice bulk conductivity of ∼60 mS/m, a uniform half‐space model representing conductive seawater is appropriate for interpretation of VCP EM31 measurements over level sea ice up to ∼2.5 m thick. For thicker, more conductive sea ice, the interpretation model must account for the effect of the finite sea‐ice conductivity. Simultaneous acquisition of EM data at several frequencies and/or transmitter–receiver geometries permits interpretation of the data in terms of multilayered models. A synthetic example shows that 1D inversion of single‐frequency in‐phase and quadrature data from two transmitter–receiver geometries can yield reliable estimates of sea‐ice thickness even when the ice contains thin, highly conductive brine layers.

Our 3D numerical model calculations show that smoothing the measured response over the system footprint means that the sea‐ice thickness recovered over multidimensional sea‐ice structures via half‐space inversion of apparent conductivity data yields a highly smoothed image of the actual keel relief. The dependence of footprint size on the height of the system above seawater results in the interpreted sea‐ice thicknesses being dependent on the deployment height of the instrument.

Sea‐ice thickness data acquired using an EM31 equipped with a hardware processing module can be transformed to apparent conductivity and then inverted assuming a conductive half‐space model. For EM system heights >4.5 m above seawater, corresponding to large altitude and/or thick sea ice, inversion assuming a conductive half‐space model yields an improved estimate of the true sea‐ice thickness compared to that obtained using the processing module. However, the noise level in the estimated depth to seawater is relatively large (±0.1 m) in comparison with typical Antarctic sea‐ice thicknesses, and thickness estimates made using the shipborne system may be significantly in error over thin ice.
Controls on induced polarization in sandy unconsolidated sediments and application to aquifer characterization
Authors: Slater L. D. , Glaser D. R.
Published: September 2003

Resistivity and induced polarization (IP) measurements (0.1–1000 Hz) were made on clay‐free unconsolidated sediments from a sandy, alluvial aquifer in the Kansas River floodplain. The sensitivity of imaginary conductivity σ″, a fundamental IP measurement, to lithological parameters, fluid conductivity, and degree of saturation was assessed. The previously reported power law dependence of IP on surface area and grain size is clearly observed despite the narrow lithologic range encountered in this unconsolidated sedimentary sequence. The grain‐size σ″ relationship is effectively frequency independent between 0.1 and 100 Hz but depends on the representative grain diameter used. For the sediments examined here, d90, the grain diameter of the coarsest sediments in a sample, is well correlated with σ″. The distribution of the internal surface in the well‐sorted, sandy sediments investigated here is such that most of the sample weight is likely required to account for the majority of the internal surface. We find the predictive capability of the Börner model for hydraulic conductivity (K)estimation from IP measurements is limited when applied to this narrow lithologic range.

The relatively weak dependence of σ″ on fluid conductivity (σw) observed for these sediments when saturated with an NaCl solution (0.06–10 S/m) is consistent with competing effects of surface charge density and surface ionic mobility on σ″ as previously inferred for sandstone. Importantly, IP parameters are a function of saturation and exhibit hysteretic behavior over a drainage and imbibition cycle. However, σ″ is less dependent than the real conductivity σ′ on saturation. In the case of evaporative drying, the σ″ saturation exponent is approximately half of the σ′ exponent.

Crosshole IP imaging illustrates the potential for lithologic discrimination of unconsolidated sediments. A fining‐upward sequence correlates with an upward increase in normalized chargeability Mn, a field IP parameter proportional to σ″. The hydraulic conductivity distribution obtained from the Börner model discriminates a hydraulically conductive sand–gravel from overlying medium sand.
Sulfide detection in drill core from the Stillwater Complex using visible/near‐infrared imaging spectroscopy
Authors: Bolin Brock J. , Moon Thomas S.
Published: September 2003

This feasibility study examines the potential of imaging spectroscopy to estimate sulfide percentage in drill core from the Stillwater Complex, Montana. The Stillwater Complex is a layered mafic to ultramafic intrusion hosting ore‐grade platinum group elements within the zone known as the JM Reef. Stillwater Mine geologists indirectly infer the platinum/palladium grade by the presence and abundance of sulfide minerals. In order to discriminate between waste and ore rock, geologists visually inspect the core and working faces for minerals such as chalcopyrite, pentlandite, and pyrrhotite. Iron sulfide minerals have a strong ultraviolet absorption that blends into the blue portion of the visible region and produces their yellow luster. The spectral differences between these pathfinder minerals and the accessory minerals are sufficiently distinct to allow classification of this mineralogy using imaging spectroscopy even in the absence of a particular absorption feature. Five different sections of split core from the JM Reef were chosen for their representative mineralogical character. The surface of each sample was scanned with Montana Tech's prototype Airborne and Laboratory Imaging Spectrometer (ALIS) and the images were analyzed for sulfides. For validation, the amount of sulfides was independently determined visually with counting grids. The imaging spectrometer results correlate well with the point‐count percentage, although all five samples consistently fall below the point‐count average. This underestimation is possibly due to metal ion substitution, linear mixing at mineral boundaries, or anisotropic scattering due to the high spatial resolution of the spectrometer. The success of this experiment suggests possible machine vision applications in future mining operations, such as automation of core logging and downhole instrumentation.
Pore pressure estimation in reservoir rocks from seismic reflection data
Authors: Carcione José M. , Helle Hans B. , Pham Nam H. , Toverud Tommy
Published: September 2003

A method is used to obtain pore pressure in shaly sandstones based upon an acoustic model for seismic velocity versus clay content and effective pressure. Calibration of the model requires log data—porosity, clay content, and sonic velocities—to obtain the dry‐rock moduli and the effective stress coefficients as a function of depth and pore pressure. The seismic P‐wave velocity, derived from reflection tomography, is fitted to the theoretical velocity by using pore pressure as the fitting parameter. This approach, based on a rock‐physics model, is an improvement over existing pore‐pressure prediction methods, which mainly rely on empirical relations between velocity and pressure. The method is applied to the Tune field in the Viking Graben sedimentary basin of the North Sea. We have obtained a high‐resolution velocity map that reveals the sensitivity to pore pressure and fluid saturation in the Tarbert reservoir. The velocity map of the Tarbert reservoir and the inverted pressure distribution agree with the structural features of the Tarbert Formation and its known pressure compartments.
Pressure and fluid saturation prediction in a multicomponent reservoir using combined seismic and electromagnetic imaging
Authors: Hoversten G. Michael , Gritto Roland , Washbourne John , Daley Tom
Published: September 2003

This paper presents a method for combining seismic and electromagnetic (EM) measurements to predict changes in water saturation, pressure, and CO2 gas/oil ratio in a reservoir undergoing CO2 flood. Crosswell seismic and EM data sets taken before and during CO2 flooding of an oil reservoir are inverted to produce crosswell images of the change in compressional velocity, shear velocity, and electrical conductivity during a CO2 injection pilot study. A rock‐properties model is developed using measured log porosity, fluid saturations, pressure, temperature, bulk density, sonic velocity, and electrical conductivity. The parameters of the rock‐properties model are found by an L1‐norm simplex minimization of predicted and observed differences in compressional velocity and density. A separate minimization, using Archie's law, provides parameters for modeling the relations between water saturation, porosity, and electrical conductivity. The rock‐properties model is used to generate relationships between changes in geophysical parameters and changes in reservoir parameters. Electrical conductivity changes are directly mapped to changes in water saturation; estimated changes in water saturation are used along with the observed changes in shear‐wave velocity to predict changes in reservoir pressure. The estimation of the spatial extent and amount of CO2 relies on first removing the effects of the water saturation and pressure changes from the observed compressional velocity changes, producing a residual compressional velocity change. This velocity change is then interpreted in terms of increases in the CO2/oil ratio. Resulting images of the CO2/oil ratio show CO2‐rich zones that are well correlated to the location of injection perforations, with the size of these zones also correlating to the amount of injected CO2. The images produced by this process are better correlated to the location and amount of injected CO2 than are any of the individual images of change in geophysical parameters.
Discrimination between pressure and fluid saturation changes from marine multicomponent time‐lapse seismic data
Authors: Landrø Martin , Veire Helene Hafslund , Duffaut Kenneth , Najjar Nazih
Published: September 2003

Explicit expressions for computation of saturation and pressure‐related changes from marine multicomponent time‐lapse seismic data are presented. Necessary input is PP and PS stacked data for the baseline seismic survey and the repeat survey. Compared to earlier methods based on PP data only, this method is expected to be more robust since two independent measurements are used in the computation. Due to a lack of real marine multicomponent time‐lapse seismic data sets, the methodology is tested on synthetic data sets, illustrating strengths and weaknesses of the proposed technique. Testing ten scenarios for various changes in pore pressure and fluid saturation, we find that it is more robust for most cases to use the proposed 4D PP/PS technique instead of a 4D PP amplitude variation with offset (AVO) technique. The fit between estimated and “real” changes in water saturation and pore pressure were good for most cases. On the average, we find that the deviation in estimated saturation changes is 8% and 0.3 MPa for the estimated pore pressure changes. For PP AVO, we find that the corresponding average errors are 9% and 1.0 MPa. In the present method, only 4D PP and PS amplitude changes are used in the calculations. It is straightforward to include use of 4D traveltime shifts in the algorithm and, if reliable time shifts can be measured, this will most likely further stabilize the presented method.
Quartic moveout coefficient: 3D description and application to tilted TI media
Authors: Pech Andres , Tsvankin Ilya , Grechka Vladimir
Published: September 2003

Nonhyperbolic (long‐spread) moveout provides essential information for a number of seismic inversion/processing applications, particularly for parameter estimation in anisotropic media. Here, we present an analytic expression for the quartic moveout coefficient A4that controls the magnitude of nonhyperbolic moveout of pure (nonconverted) modes. Our result takes into account reflection‐point dispersal on irregular interfaces and is valid for arbitrarily anisotropic, heterogeneous media. All quantities needed to compute A4can be evaluated during the tracing of the zero‐offset ray, so long‐spread moveout can be modeled without time‐consuming multioffset, multiazimuth ray tracing.

The general equation for the quartic coefficient is then used to study azimuthally varying nonhyperbolic moveout of P‐waves in a dipping transversely isotropic (TI) layer with an arbitrary tilt ν of the symmetry axis. Assuming that the symmetry axis is confined to the dip plane, we employed the weak‐anisotropy approximation to analyze the dependence of A4on the anisotropic parameters. The linearized expression for A4is proportional to the anellipticity coefficient η ≈ ε − δ and does not depend on the individual values of the Thomsen parameters. Typically, the magnitude of nonhyperbolic moveout in tilted TI media above a dipping reflector is highest near the reflector strike, whereas deviations from hyperbolic moveout on the dip line are substantial only for mild dips.

The azimuthal variation of the quartic coefficient is governed by the tilt ν and reflector dip φ and has a much more complicated character than the NMO–velocity ellipse. For example, if the symmetry axis is vertical (VTI media, ν = 0) and the dip φ < 30°, A4goes to zero on two lines with different azimuths where it changes sign. If the symmetry axis is orthogonal to the reflector (this model is typical for thrust‐and‐fold belts), the strike‐line quartic coefficient is defined by the well‐known expression for a horizontal VTI layer (i.e., it is independent of dip), while the dip‐line A4is proportional to cos4φ and rapidly decreases with dip. The high sensitivity of the quartic moveout coefficient to the parameter η and the tilt of the symmetry axis can be exploited in the inversion of wide‐azimuth, long‐spread P‐wave data for the parameters of TI media.
Petro‐seismic inversion for sandstone properties
Authors: Koesoemadinata Adam P. , McMechan George A.
Published: September 2003

We use empirical relations derived from laboratory and log data for sandstones to estimate unknown parameters from given parameters. The known and unknown parameters may be any of the following: compressional and shear wave velocities (Vp and Vs), density (ρ), compressional and shear wave quality factors (Qp and Qs), effective pressure (P), frequency (f), water saturation (S), porosity (φ), clay content (C), fluid permeability (k), and Vp/Vs and Qs/Qp ratios. The goal is to obtain estimates of all thirteen of these petro‐seismic variables from subsets of these as input; whether this is achievable depends on the particular combination of input variables.

The inversion typically proceeds through a hierarchy of three levels of parameter estimation, in order of the expected reliability of the estimates. First, we solve for the parameters that can be obtained directly from the fitted empirical equations. Then, a grid search is performed to simultanously fit more than one unknown parameter by finding values that best predict the known parameters. Finally, various approximations are invoked to predict values when the inputs are insufficient to use the direct equations or a grid search. These three procedures can be initiated in any desired order and any number of times.

Even if the complete set of seismic parameters (Vp, and Vs, ρ, Qp, Qs, and f) are given, it is not possible to uniquely obtain any of the remaining petrophysical variables (S, φ, C, P, or k) directly from the fitted equations without constraints on (or assumptions about) some of the other variables. There is a trade‐off between porosity and clay content that can be resolved by simultaneous fitting of multiple seismic parameters (i.e., Vp, Vs, and Vp/Vs). The predictability of the parameters from various combinations of the available data is expressed using correlation coefficients (R2) and standard deviations. With optimal input combinations, parameters with R2 ≥ 0.8 are (in order from best to worst) Vp, 100/Qp, ρ, Vs, and S. Other parameters with 0.63 ≥ R2 ≤, 0.79 are (in order from best to worst) φ, Qs/Qp, Vp/Vs, ln k, 100/Qs, and C. P and f are the least reliably estimated. All of the seismic parameters can be accurately estimated if all of the petrophysical parameters are known.
Controls of traveltime data and problems of the generalized reciprocal method
Authors: Leung Tak Ming
Published: September 2003

Traveltime data required for 2D seismic refraction surveys are 2D first arrivals. To obtain a high degree of consistency between traveltime data and the seismic model, it is important to verify that traveltime data are appropriate for interpretation or an inversion process. Controls or checkpoints presented here inspect compatibility among traveltime data. Similar to the ray‐trace check on the consistency of interpretation, these controls provide an objective means of quality assessment of seismic refraction data.

The theoretical aspects of the generalized reciprocal method (GRM) are studied because concerns have been raised regarding the accuracy of some interpretations using this method. The problem of the GRM is that the optimum XY value, which is the most important parameter in the method, is assumed to be twice the offset distance. Consequently, based on this unproven assumption, the efficacy of the optimum XY value is somewhat exaggerated.
Sparseness‐constrained least‐squares inversion: Application to seismic wave reconstruction
Authors: Wang Yanghua
Published: September 2003

The spectrum of a discrete Fourier transform (DFT) is estimated by linear inversion, and used to produce desirable seismic traces with regular spatial sampling from an irregularly sampled data set. The essence of such a wavefield reconstruction method is to solve the DFT inverse problem with a particular constraint which imposes a sparseness criterion on the least‐squares solution. A working definition for the sparseness constraint is presented to improve the stability and efficiency. Then a sparseness measurement is used to measure the relative sparseness of the two DFT spectra obtained from inversion with or without sparseness constraint. It is a pragmatic indicator about the magnitude of sparseness needed for wavefield reconstruction. For seismic trace regularization, an antialiasing condition must be fulfilled for the regularizing trace interval, whereas optimal trace coordinates in the output can be obtained by minimizing the distances between the newly generated traces and the original traces in the input. Application to real seismic data reveals the effectiveness of the technique and the significance of the sparseness constraint in the least‐squares solution.
Multiscale traveltime tomography
Authors: Zhou Hua‐wei
Published: September 2003

Nonuniqueness in traveltime tomographic velocity analysis is largely the result of insufficiency in model parameterization. Traditional tomography parameterizes the model with nonoverlapping cells. Such single‐scale tomography (SST) from inverting traveltimes only may produce underdeterminacy at places of insufficient ray coverage. To cope with the poor and uneven ray coverage, a multiscale tomography (MST) method is devised, which is a simultaneous application of many overlapping SSTs with different cell sizes. The MST decomposes velocity anomalies into components in a set of submodels of different cell sizes. At each model location the cells containing higher consistency between data contributions gain more from the inversion. The final MST model is a postinversion superposition of all submodel solutions. The MST method is applicable to the determination of interval velocity and interface geometry using turning rays or reflection rays. In comparison with the SST, superior results of the MST are seen in synthetic examples. The MST method is used to construct the 3D crustal velocities of southern California using the first arrivals from local earthquakes. While the SST and MST models may fit the data equally well, the MST model is smoother and geologically more plausible than the SST model.
Velocity continuation and the anatomy of residual prestack time migration
Authors: Fomel Sergey
Published: September 2003

Velocity continuation is an imaginary continuous process of seismic image transformation in the postmigration domain. It generalizes the concepts of residual and cascaded migrations. Understanding the laws of velocity continuation is crucially important for a successful application of time‐migration velocity analysis. These laws predict the changes in the geometry and intensity of reflection events on migrated images with the change of the migration velocity. In this paper, I derive kinematic and dynamic laws for the case of prestack residual migration from simple geometric principles. The main theoretical result is a decomposition of prestack velocity continuation into three different components corresponding to residual normal moveout, residual dip moveout, and residual zero‐offset migration. I analyze the contribution and properties of each of the three components separately. This theory forms the basis for constructing efficient finite‐difference and spectral algorithms for time‐migration velocity analysis.
Time‐migration velocity analysis by velocity continuation
Authors: Fomel Sergey
Published: September 2003

Time‐migration velocity analysis can be performed by velocity continuation, an incremental process that transforms migrated seismic sections according to changes in the migration velocity. Velocity continuation enhances residual normal moveout correction by properly taking into account both vertical and lateral movements of events on seismic images. Finite‐difference and spectral algorithms provide efficient practical implementations for velocity continuation. Synthetic and field data examples demonstrate the performance of the method and confirm theoretical expectations.
Aperture effects in 2.5D Kirchhoff migration: A geometrical explanation
Authors: Hertweck Thomas , Jäger Christoph , Goertz Alexander , Schleicher Jörg
Published: September 2003

Seismic images obtained by Kirchhoff time or depth migration are always accompanied by some artifacts known as migration noise, migration boundary effects, or diffraction smiles, which may severely affect the quality of the migration result. Most of these undesirable effects are caused by a limited aperture if the algorithms make no special disposition to avoid them. Strong amplitude variation along reflection events may cause similar artifacts. All of these effects can be explained mathematically by means of the method of stationary phase. However, such a purely theoretical explication is not always easily understood by applied geophysicists. A geometrical interpretation of the terms of the stationary‐phase approximation in relation to the diffraction and reflection traveltime curves in the time domain can help to develop a more intuitive understanding of the migration artifacts. A simple numerical experiment for poststack (zero‐offset) data indicates the problem and helps to demonstrate the effects and the methods to avoid them.
Autocorrelogram migration: Theory
Authors: Schuster Gerard T. , Followill Fred , Katz Lewis J. , Yu Jianhua , Liu Zhaojun
Published: September 2003

We present the equations for migrating inverse‐vertical‐seismic‐profile‐while‐drilling and common‐midpoint autocorrelograms. These equations partly generalize the 1D autocorrelation imaging methods of Katz and Claerbout to 2D and 3D media, and also provide a formal mathematical procedure for imaging the reflectivity distribution from autocorrelograms. The imaging conditions are designed to migrate specific events in the autocorrelograms, either the direct‐primary correlations or the direct‐ghost correlations. Here, direct stands for direct wave, primary stands for primary reflections, and ghost denotes free‐surface ghost reflections. The main advantage in migrating autocorrelograms is that the source wavelet does not need to be known, which is the case for seismic data generated by a rotating drill bit or for vibroseis data with a corrupted pilot signal. Another advantage is that the source and receiver static problems are mitigated by autocorrelation migration. Two limitations are that autocorrelation of traces amplifies coherent noise such as surface waves, and produces undesirable coherent noise denoted as “virtual multiples.” Similar to “physical multiples,” such noise can, in principle, be partially suppressed by filtering and stacking of migration images obtained from many different shot gathers. Results with both synthetic and field data validate this conjecture, and show that autocorrelogram migration can be a viable alternative to standard migration when the source signal is not adequately known or there are severe static problems.
Reduced‐time migration of transmitted PS waves
Authors: Sheley David , Schuster Gerard T.
Published: September 2003

We develop the novel theory of transmitted PS migration and show that PS transmitted arrivals in a Gulf of Mexico vertical seismic profile (VSP) data set can be migrated to accurately image a salt sheet even though the receiver array is below the transmitting boundary. We also show that migrating transmitted arrivals is effective in illuminating the base of an orebody invisible to PP reflections. In general, interfaces that bisect wavepath propagation (i.e., the source and receiver are on opposite sides of the interface and therefore invisible to PP reflections) can be imaged by migration of PS transmitted waves. These results suggest that migration of PS transmitted waves opens new opportunities in imaging nearly vertical impedance boundaries that are typically invisible to conventional reflection imaging of crosswell and VSP data.

We also present a new interferometric method, denoted as reduced‐time migration, which uses the arrival‐time difference between the direct P‐wave and subsequent events to increase migration accuracy. Reduced‐time migration removes static time shifts in the data, decreases the focusing error due to an incorrect migration velocity model, and relocates reflection or PS transmission events to be closer to their true positions. Although limited to crosswell and VSP geometries, synthetic‐ and field‐data examples show that reduced‐time migration is noticeably more accurate than conventional migration in the presence of static shifts and/or migration velocity errors. The main assumption of reduced‐time migration is that the direct wave samples errors which are representative of errors in the migration aperture. Transmission wavepaths, in general, are subparallel to the direct wave and therefore the two modes encounter similar errors and, hence, reduced‐time migration is effective in improving the focusing of migration energy. For the PP reflection case, the direct wave and the reflected waves often traverse different parts of the earth, therefore, reduced‐time migration will remove static shifts but it is not expected to mitigate velocity errors if the errors are spatially variant. However, if there is a general and consistent bias in the velocity model, reduced‐time migration is expected to deliver improved results over conventional Kirchhoff migration.
Inferring the statistical distribution of velocity heterogeneities by statistical traveltime tomography
Authors: Iooss Bertrand , Geraets David , Mukerji Tapan , Samuelides Yann , Touati Mustafa , Galli Alain
Published: September 2003

Understanding the internal heterogeneities of reservoirs is one of the key issues in better recovery and efficient reservoir management. Seismic data are widely used to map subsurface heterogeneities. These heterogeneities can include variations in wave velocity and rock density, which can be used to interpret variations in reservoir properties such as porosity, lithofacies, and fluids. This paper describes a statistical tomography method to infer the spatial statistics of subsurface velocity heterogeneities from seismic data. We consider an acoustic wave propagating in a medium represented as a single macromodel superimposed on statistically stationary random velocity perturbations. While the macromodel is retrieved by classical seismic methods, the picked traveltimes and their fluctuations are used to estimate the covariance function of the spatially varying velocity perturbations. We present a formulation based on ray‐theoretical results and describe two algorithms: one using the prestack traveltimes and the other using the stacking velocities. The methods are tested with synthetic seismic reflection data in an idealized medium (with a Gaussian spatial covariance) and with synthetic transmission data in a more geologically realistic medium. Then, the two algorithms are applied on real data. The estimates of the spatial statistics obtained from inverting the traveltime statistics match reasonably well with the true parameters of the heterogeneous media.
A 3D cylindrical PML/FDTD method for elastic waves in fluid‐filled pressurized boreholes in triaxially stressed formations
Authors: Liu Qing Huo , Sinha Bikash K.
Published: September 2003

A new 3D cylindrical perfectly matched layer (PML) formulation is developed for elastic wave propagation in a pressurized borehole surrounded by a triaxially stressed solid formation. The linear elastic formation is altered by overburden and tectonic stresses that cause significant changes in the wave propagation characteristics in a borehole. The 3D cylindrical problem with both radial and azimuthal heterogeneities is suitable for numerical solutions of the wave equations by finite‐difference time‐domain (FDTD) and pseudospectral time‐domain (PSTD) methods. Compared to the previous 2.5D formulation with other absorbing boundary conditions, this 3D cylindrical PML formulation allows modeling of a borehole‐conformal, full 3D description of borehole elastic waves in a stress‐induced heterogeneous formation. We have developed an FDTD method using this PML as an absorbing boundary condition. In addition to the ability to solve full 3D problems, this method is found to be advantageous over the previously reported 2.5D finite‐difference formulation because a borehole can now be adequately simulated with fewer grid points. Results from the new FDTD technique confirm the principle of superposition of the influence of various stress components on both the borehole monopole and dipole dispersions. In addition, we confirm that the increase in shear‐wave velocity caused by a uniaxial stress applied in the propagation direction is the same as that applied parallel to the radial polarization direction.
Finite‐difference modeling of elastic wave propagation: A nonsplitting perfectly matched layer approach
Authors: Wang Tsili , Tang Xiaoming
Published: September 2003

In this paper, we present a nonsplitting perfectly matched layer (NPML) method for the finite‐difference simulation of elastic wave propagation. Compared to the conventional split‐field approach, the new formulation solves the same set of equations for the boundary and interior regions. The nonsplitting formulation simplifies the perfectly matched layer (PML) algorithm without sacrificing the accuracy of the PML. In addition, the NPML requires nearly the same amount of computer storage as does the split‐field approach. Using the NPML, we calculate dipole and quadrupole waveforms in a logging‐while‐drilling environment. We show that a dipole source produces a strong pipe flexural wave that distorts the formation arrivals of interest. A quadrupole source, however, produces clean formation arrivals. This result indicates that a quadrupole source is more advantageous over a dipole source for shear velocity measurement while drilling.
Value of Dual-Phase 2-Fluoro-2-Deoxy-d-Glucose Positron Emission Tomography in Cervical Cancer
Authors: Yen Tzu-Chen , Ng Koon-Kwan , Ma Shih-Ya , Chou Hung-Hsueh , Tsai Chien-Sheng , Hsueh Swei , Chang Ting-Chang , Hong Ji-Hong , See Lai-Chu , Lin Wuu-Jyh , Chen Jenn-Tzong , Huang Kuan-Gen , Lui Kar-Wai , Lai Chyong-Huey
Published: September 2003

Purpose: The role of positron emission tomography (PET) with fluorine-18–labeled fluoro-2-deoxy-d-glucose (FDG) in cervical cancer has not yet been well defined. We conducted a prospective study to investigate its efficacy in comparison with magnetic resonance imaging and/or computed tomography (MRI-CT).

Materials and Methods: Patients with untreated locally advanced (35%) or recurrent (65%) cervical cancer were enrolled onto this study. In the first part of this study, 41 patients had a conventional FDG-PET (40 minutes after injection), and in the second part, 94 patients received dual-phase PET (at both 40 minutes and 3 hours after injection). The overall results of PET scans were compared with MRI-CT, and the two protocols of PET were also compared with each other. Lesion status was determined by pathology results or clinical follow-up. The receiver operating characteristic curve method with area under the curve (AUC) calculation was used to evaluate the discriminative power.

Results: Overall (N = 135), FDG-PET was significantly superior to MRI-CT in identifying metastatic lesions (AUC, 0.971 v 0.879; P = .039), although the diagnostic accuracy was similar for local tumors. Dual-phase PET was also significantly better than the 40-minute PET (n = 94). The latter accurately recognized 70% of metastatic lesions and the former detected 90% (AUC, 0.943 v 0.951; P = .007). Dual-phase FDG-PET changed treatment of 29 patients (31%; upstaging 27% and downstaging 4%).

Conclusion: This study shows that dual-phase FDG-PET is superior to conventional FDG-PET or MRI-CT in the evaluation of metastatic lesions in locally advanced or recurrent cervical cancer.
The Phase III Trial in the Era of Targeted Therapy: Unraveling the “Go or No Go” Decision
Authors: Roberts Thomas G. , Lynch Thomas J. , Chabner Bruce A.
Published: September 2003

Purpose: To review characteristics of contemporary phase III oncology trials and create an explicit framework to help clinical researchers prioritize novel therapies for phase III testing.

Methods: We searched the MEDLINE and EMBASE databases for all reviews of phase III trials; cataloged all phase III trials in two national clinical trial databases; and reviewed approval criteria of recently approved oncology drugs from public data provided by the US Food and Drug Administration. Industry data not available elsewhere in the medical literature were obtained from a sourcebook published by a large contract research organization.

Results: Phase III oncology trials are the most expensive and time-consuming aspect of the drug development process. The results of these trials continue to exert the greatest influence on the treatment decision of oncologists and remain pivotal to the granting of drug approval. Making optimal decisions about which agents to advance to phase III testing may decrease the overall cost of cancer drug development and limit the number of patients exposed to ineffective drugs. A conceptual decision model for prioritizing novel therapies for phase III testing is presented.

Conclusion: Cancer drug development has become more complex and expensive, whereas overall clinical progress remains slow. The transition from phase II to phase III requires a strategic decision that is based on new considerations. A greater investment in phase I and II drug trials may be required to provide the information necessary for phase III planning.