bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonbright‐spot AVO: Two examples
Authors: Ross Christopher P. , Kinman Daniel L.
Published: October 2002

The use of amplitude variation with offset (AVO) attribute sections such as the product of the normal incidence trace (A) and the gradient trace (B) have been used extensively in bright spot AVO analysis and interpretation. However, while these sections have often worked well with low acoustic impedance bright spot responses, they are not reliable indicators of nonbright‐spot seismic anomalies. Analyzing nonbright‐spot seismic data with common AVO attribute sections will: (1) not detect the gas‐charged reservoir because of near‐zero acoustic impedance contrast between the sands and encasing shales, or (2) yield an incorrect (negative) AVO product if the normal incidence and gradient values are opposite in sign. We divide nonbright‐spot AVO offset responses into two subcategories: those with phase reversals and those without. An AVO analysis procedure for these anomalies is presented through two examples. The procedure exploits the nature of the prestack response, yielding a more definitive AVO attribute section, and this technique is adaptive to both subcategories of nonbright‐spot AVO responses. This technique identifies the presence of gas‐charged pore fluids within the reservoir when compared to a conventionally processed, relative amplitude seismic section with characteristically low amplitude responses for near‐zero acoustic impedance contrast sands.
Multiscale seismic waveform inversion
Authors: Bunks Carey , Saleck Fatimetou M. , Zaleski S. , Chavent G.
Published: October 2002

Iterative inversion methods have been unsuccessful at inverting seismic data obtained from complicated earth models (e.g. the Marmousi model), the primary difficulty being the presence of numerous local minima in the objective function. The presence of local minima at all scales in the seismic inversion problem prevent iterative methods of inversion from attaining a reasonable degree of convergence to the neighborhood of the global minimum. The multigrid method is a technique that improves the performance of iterative inversion by decomposing the problem by scale. At long scales there are fewer local minima and those that remain are further apart from each other. Thus, at long scales iterative methods can get closer to the neighborhood of the global minimum. We apply the multigrid method to a subsampled, low‐frequency version of the Marmousi data set. Although issues of source estimation, source bandwidth, and noise are not treated, results show that iterative inversion methods perform much better when employed with a decomposition by scale. Furthermore, the method greatly reduces the computational burden of the inversion that will be of importance for 3-D extensions to the method.
Empirical relationships between transverse isotropy parameters and Vp/Vs: Implications for AVO
Authors: Ryan‐Grigor Sarah
Published: October 2002

The transverse isotropy (TI) parameters ε and δ control amplitude variation with offset (AVO) response at most angles of incidence used in exploration, although the value of δ is not usually known and is difficult to measure. Published measurements on TI materials show that there is a useful empirical correlation between [Formula: see text] and δ. The relationship between δ and [Formula: see text] can be simplified by assuming a linear relationship between [Formula: see text] and [Formula: see text]. Anellipticity parameter η also shows a useful empirical correlation with [Formula: see text]. The correlations imply that knowledge of [Formula: see text] is sufficient to make an estimate of the anellipticity of the P‐ and S‐wavefronts in a rock, regardless of the lithology. In this way, the effect of TI on the AVO response of a particular interface may be estimated in the absence of any more accurate data. The empirical relationships indicate that rocks tend to become more anelliptic with increasing [Formula: see text]. Rocks with [Formula: see text] smaller than about 1.8 tend to have zero to small positive values of δ, while rocks with [Formula: see text] larger than around 2 tend to have zero to medium negative δ values. Most previous work has assumed a positive value of δ in shales, but this is not necessarily true. If in fact the δ of a shale is negative ([Formula: see text] is around 2 or higher), and overlies a sandstone, the positive change in δ across the interface could cause a false negative AVO gas indicator. If shale with [Formula: see text] less than 1.8, as measured in organic‐rich and overpressured shales, overlies a water‐filled sandstone, this could cause a false positive AVO gas indicator. However, if the effect of TI can be estimated, then the chances of success for AVO analysis in correctly predicting the presence of hydrocarbons can be increased.
Tomographic velocity estimation with plane‐wave synthesis
Authors: Ji Jun
Published: October 2002

In areas with structurally complex geology, tomographic velocity analysis is often required to estimate velocities. In this paper I describe an algorithm for tomographic velocity estimation that uses plane‐wave synthesis imaging as a prestack migration. The classical iterative two‐step process (measures the traveltime errors with the current velocity model and then update the velocity model) is performed as follows. The events are picked in the image space after prestack migration with surface‐oriented plane‐wave synthesis imaging. the traveltime deviations are measured through residual‐moveout (RMO) velocity analysis in common‐surface‐location (CSL) gathers obtained by reflector‐oriented plane‐wave synthesis imaging, and the velocity update is calculated by inverting the traveltime deviations through a conjugate gradient. The results from synthetic data indicate that the tomographic method successfully estimates interval‐velocity models that lead to depth‐migrated images with no residual moveout.
Nonlinear viscoelastic behavior of sedimentary rocks, Part I: Effect of frequency and strain amplitude
Authors: Tutuncu Azra N. , Podio Augusto L. , Gregory Alvin R. , Sharma Mukul M.
Published: October 2002

Sedimentary rocks display nonlinear elastic behavior. This nonlinearity is a strong function of frequency, strain amplitude, and the properties of the saturating fluid. Experimental observations and potential mechanisms that cause these nonlinearities are presented in this and a companion paper. Young’s moduli and Poisson’s ratios obtained from ultrasonic laboratory measurements (50 kHz, 100 kHz, 180kHz and 1 MHz), low‐frequency measurements (1–2000 Hz) and static measurements (0.001–0.05 Hz) show significant differences under identical stress conditions. A comparison of the laboratory‐measured quantities with log‐derived moduli measured at 20 kHz indicates that [Formula: see text]. This shows clearly that a wide variety of sandstones demonstrate frequency‐dependent elastic behavior (viscoelastic behavior) over a range of frequencies. Differences between static (low‐frequency, high‐strain amplitude) velocities and ultrasonic velocities can be explained partially by differences in frequency as predicted by grain contact models. Such models, however, do not explain the strain amplitude dependence observed in our data. A series of uniaxial stress cycling measurements were carried out to investigate the influence of strain amplitude on elastic moduli. These low‐frequency measurements (0.01 Hz) clearly show that the Young’s modulus decreases with strain amplitude for a wide variety of sandstones. Attenuation increases with strain amplitude. The strain amplitude dependence does not change when the rocks are saturated with brine although the rocks soften measureably.
Model‐driven predictive deconvolution
Authors: Robinson Enders A.
Published: October 2002

A gap‐deconvolution filter with gap α is defined as the prediction error operator with prediction distance α. A spike‐deconvolution filter is defined as the prediction error operator with prediction distance unity. That is, a spike‐deconvolution filter is the special case of a gap‐deconvolution filter with gap equal to one time unit. Generally, the designation “gap deconvolution” is reserved for the case when α is greater than one, and the term “spike deconvolution” is used when α is equal to one. It is often stated that gap deconvolution with gap α shortens an input wavelet of arbitrary length to an output wavelet of length α (or less). Since an arbitrary value of α can be chosen, it would follow that resolution or wavelet contraction may be controlled by use of gap deconvolution. In general, this characterization of gap deconvolution is true for arbitrary α if and only if the wavelet is minimum delay (i.e., minimum phase). The method of model‐driven deconvolution can be used in the case of a nonminimum‐delay wavelet. The wavelet is the convolution of a minimum‐delay reverberation and a short nonminimum‐delay orphan. The model specifies that the given trace is the convolution of the white reflectivity and this nonminimum‐delay wavelet. The given trace yields the spike‐deconvolution filter and its inverse. These two signals are then used to compute the gap‐deconvolution filters and their inverses for various prediction distances. The inverses are examined, and a stable one is picked as the most likely minimum‐delay reverberation. The corresponding gap‐deconvolution filter is the optimum one for the removal of this minimum‐delay reverberation from the given trace. As a byproduct, the minimum‐delay counterpart of the orphan can be obtained. The optimum gap‐deconvolved trace is examined for zones that contain little activity, and the leading edge of the wavelet following such a zone is chosen. Next, the phase of the minimum‐delay counterpart of the orphan is rotated until it fits the extracted leading edge. From the amount of phase rotation, the required phase‐correcting filter can be estimated. Alternatively, downhole information, if available, can be used to estimate the phase‐correcting filter. Application of the phase‐correcting filter to the spike‐deconvolved trace gives the required approximation to the reflectivity. As a final step, wavelet processing can be applied to yield a final interpreter trace made up of zero‐phase wavelets.
Framework for AVO gradient and intercept interpretation
Authors: Castagna John P. , Swan Herbert W. , Foster Douglas J.
Published: October 2002

Amplitude variation with offset (AVO) interpretation may be facilitated by crossplotting the AVO intercept (A) and gradient (B). Under a variety of reasonable petrophysical assumptions, brine‐saturated sandstones and shales follow a well‐defined “background” trend in the A-B plane. Generally, A and B are negatively correlated for “background” rocks, but they may be positively correlated at very high [Formula: see text] ratios, such as may occur in very soft shallow sediments. Thus, even fully brine‐saturated shallow events with large reflection coefficients may exhibit large increases in AVO. Deviations from the background trend may be indicative of hydrocarbons or lithologies with anomalous elastic properties. However, in contrast to the common assumptions that gas‐sand amplitude increases with offset, or that the reflection coefficient becomes more negative with increasing offset, gas sands may exhibit a variety of AVO behaviors. A classification of gas sands based on location in the A-B plane, rather than on normal‐incidence reflection coefficient, is proposed. According to this classification, bright‐spot gas sands fall in quadrant III and have negative AVO intercept and gradient. These sands exhibit the amplitude increase versus offset which has commonly been used as a gas indicator. High‐impedance gas sands fall in quadrant IV and have positive AVO intercept and negative gradient. Consequently, these sands initially exhibit decreasing AVO and may reverse polarity. These behaviors have been previously reported and are addressed adequately by existing classification schemes. However, quadrant II gas sands have negative intercept and positive gradient. Certain “classical” bright spots fall in quadrant II and exhibit decreasing AVO. Examples show that this may occur when the gas‐sand shear‐wave velocity is lower than that of the overlying formation. Common AVO analysis methods such as partial stacks and product (A × B) indicators are complicated by this nonuniform gas‐sand behavior and require prior knowledge of the expected gas‐sand AVO response. However, Smith and Gidlow’s (1987) fluid factor, and related indicators, will theoretically work for gas sands in any quadrant of the A-B plane.
Avoiding pitfalls in shallow seismic reflection surveys
Authors: Steeples Don W. , Miller Richard D.
Published: October 2002

Acquiring shallow reflection data requires the use of high frequencies, preferably accompanied by broad bandwidths. Problems that sometimes arise with this type of seismic information include spatial aliasing of ground roll, erroneous interpretation of processed airwaves and air‐coupled waves as reflected seismic waves, misinterpretation of refractions as reflections on stacked common‐midpoint (CMP) sections, and emergence of processing artifacts. Processing and interpreting near‐surface reflection data correctly often requires more than a simple scaling‐down of the methods used in oil and gas exploration or crustal studies. For example, even under favorable conditions, separating shallow reflections from shallow refractions during processing may prove difficult, if not impossible. Artifacts emanating from inadequate velocity analysis and inaccurate static corrections during processing are at least as troublesome when they emerge on shallow reflection sections as they are on sections typical of petroleum exploration. Consequently, when using shallow seismic reflection, an interpreter must be exceptionally careful not to misinterpret as reflections those many coherent waves that may appear to be reflections but are not. Evaluating the validity of a processed, shallow seismic reflection section therefore requires that the interpreter have access to at least one field record and, ideally, to copies of one or more of the intermediate processing steps to corroborate the interpretation and to monitor for artifacts introduced by digital processing.
Nonlinear refraction traveltime tomography
Authors: Zhang Jie , Toksöz M. Nafi
Published: October 2002

A few important issues for performing nonlinear refraction traveltime tomography have been identified. They include the accuracy of the traveltime and raypath calculations for refraction, the physical information in the refraction traveltime curves, and the characteristics of the refraction traveltime errors. Consequently, we develop a shortest path ray‐tracing method with an optimized node distribution that can calculate refraction traveltimes and raypaths accurately in any velocity model. We find that structure ambiguity caused by short and long rays in the seismic refraction method may influence the inversion solution significantly. Therefore, we pose a nonlinear inverse problem that explicitly minimizes the misfits of the average slownesses (ratios of traveltimes to the corresponding ray lengths) and the apparent slownesses (derivatives of traveltimes with respect to distance). As a result, we enhance the resolution as well as the convergence speed. To regularize our inverse problem, we use the Tikhonov method to avoid solving an ill‐posed inverse problem. Errors in refraction traveltimes are characterized in terms of a common‐shot error, a constant deviation for recordings from the same shot, and a relative traveltime‐gradient error with zero mean with respect to the true gradient of the traveltime curve. Therefore, we measure the uncertainty of our tomography solution using a nonlinear Monte Carlo approach and compute the posterior model covariance associated with two different types of random data vectors and one random model vector. The nonlinear uncertainty analysis indicates that the resolution of a tomography solution may not correspond to the ray coverage. We apply this tomography technique to image the shallow velocity structure at a coastal site near Boston, Massachusetts. The results are consistent with a subsequent drilling survey.
Accurate viscoelastic modeling by frequency‐domain finite differences using rotated operators
Authors: Štekl I. , Pratt R. G.
Published: October 2002

The viscoelastic wave equation is an integro‐differential equation that requires special methods when using time‐domain numerical finite‐difference methods. In the frequency domain, the integral terms are easily represented by complex valued elastic media properties. There are further significant advantages to using the frequency domain if the forward or the inverse problem requires modeling or inverting a large number of prestack source gathers. Numerical modeling is expensive for seismic data because of the large number of wavelenghths typically separating sources from receivers, which results in a need for a large number of grid points. A major obstacle to using frequency‐domain methods is the consequent storage requirements. To reduce these, we maximize the accuracy and simultaneously minimize the spatial extent of the numerical operators. We achieve this by extending earlier published methods introduced for the viscoacoustic case to the viscoelastic case. This requires the formulation of two new numerical operators: a differencing operator in a rotated coordinate frame and a lumped mass term. The new operators are combined with ordinary second‐ order, finite‐difference operators in an optimal manner to minimize numerical errors without increasing the size of the numerical operator. For a fixed number of grid points, the resulting second‐ order differencing scheme is no more expensive than an ordinary second‐order differencing scheme, but a numerical dispersion analysis shows that the number of grid points required per smallest wavelength is reduced from approximately 15 to approximately 4. The new scheme is also capable of handling embedded fluid layers without instability. We demonstrate that no further improvement in performance can be achieved using higher order spatial operators because of the associated computational overheads associated with the larger differencing operators. The new viscoelastic modeling scheme is used to study a crosshole data set in which the exact nature of the seismic coda is unclear. The results of the modeling study indicate this coda is likely related to the generation of mode‐converted shear waves within the complicated, finely layered sediments at the site.
Stress‐dependent seismic anisotropy of shales
Authors: Sayers Colin M.
Published: October 2002

A simple theory for the stress‐dependent seismic anisotropy of shales can be obtained in terms of a second‐rank tensor and a fourth‐rank tensor that depend on the orientation distribution of contacts between clay platelets. The theory allows the normal and shear stiffness of the contact regions between clay platelets to be obtained as a function of stress from measurements of seismic P- and S-wave velocities for shales. The ratio of the normal‐to‐shear compliance, [Formula: see text], of the contact regions between clay particles is found to be sensitive to the saturation state of the shale. Inversion of velocity measurements for fully saturated shales shows a low value of [Formula: see text] when compared with measurements on air‐dry shales, consistent with the expected reduction in normal compliance in a fluid‐saturated, low‐permeability rock. For all shales considered, [Formula: see text] is found to be less than unity. The contacts between clay particles are therefore more compliant in shear than in compression.
3-D traveltime computation using the fast marching method
Authors: Sethian James A. , Popovici A. Mihai
Published: October 2002

We present a fast algorithm for solving the eikonal equation in three dimensions, based on the fast marching method. The algorithm is of the order O(N log N), where N is the total number of grid points in the computational domain. The algorithm can be used in any orthogonal coordinate system and globally constructs the solution to the eikonal equation for each point in the coordinate domain. The method is unconditionally stable and constructs solutions consistent with the exact solution for arbitrarily large gradient jumps in velocity. In addition, the method resolves any overturning propagation wavefronts. We begin with the mathematical foundation for solving the eikonal equation using the fast marching method and follow with the numerical details. We then show examples of traveltime propagation through the SEG/EAGE salt model using point‐source and plane‐wave initial conditions and analyze the error in constant velocity media. The algorithm allows for any shape of the initial wavefront. While a point source is the most commonly used initial condition, initial plane waves can be used for controlled illumination or for downward continuation of the traveltime field from one depth to another or from a topographic depth surface to another. The algorithm presented here is designed for computing first‐arrival traveltimes. Nonetheless, since it exploits the fast marching method for solving the eikonal equation, we believe it is the fastest of all possible consistent schemes to compute first arrivals.
Wavefield extrapolation by nonstationary phase shift
Authors: Margrave Gary F. , Ferguson Robert J.
Published: October 2002

The phase‐shift method of wavefield extrapolation applies a phase shift in the Fourier domain to deduce a scalar wavefield at one depth level given its value at another. The phase‐shift operator varies with frequency and wavenumber, and assumes constant velocity across the extrapolation step. We use nonstationary filter theory to generalize this method to nonstationary phase shift (NSPS), which allows the phase shift to vary laterally depending upon the local propagation velocity. For comparison, we derive an analytic form for the popular phase shift plus interpolation (PSPI) method in the limit of an exhaustive set of reference velocities. NSPS and this limiting form of PSPI can be written as generalized Fourier integrals which reduce to ordinary phase shift in the constant velocity limit. In the (x, ω) domain, these processes are the transpose of each other; however, only NSPS has the physical interpretation of forming the scaled, linear superposition of laterally‐variable impulse responses (i.e.,Huygen’s wavelets). The difference between NSPS and PSPI is clear when they are compared in the case of a piecewise constant velocity variation. Define a set of windows such that the jth window is unity when the propagation velocity is the jth distinct velocity and is zero otherwise. NSPS can be computed by applying the window set to the input data to create a set of windowed wavefields, which are individually phase‐shift extrapolated with the corresponding constant velocity, and the extrapolated set is superimposed. PSPI proceeds by phase‐shift extrapolating the input data for each distinct velocity, applying the jth window to the jth extrapolation, and superimposing. Though neither process is fully correct, PSPI has the unphysical limit that discontinuities in the lateral velocity variation cause discontinuities in the wavefield, whereas NSPS shows the expected wavefront “healing.” We then formulate a finite aperture compensation for NSPS which has the practical result of absorbing lateral boundaries for all incidence angles. Wavefield extrapolation can be regarded as the crosscorrelation of the wavefield with the expected response of a point diffractor at the new depth level. Aperture compensation simply applies a laterally varying window to the infinite, theoretical diffraction response. The crosscorrelation becomes spatially variant, even for constant velocity, and hence is a nonstationary filter. The nonstationary effects of aperture compensation can be simultaneously applied with the NSPS extrapolation through a laterally variable velocity field.
Formation nonlinear constants from sonic measurements at two borehole pressures
Authors: Sinha Bikash K. , Winkler Kenneth W.
Published: October 2002

Granular structure and microcracks in rocks cause large nonlinearities in the constitutive relations that result in the stress dependence of acoustic‐wave velocities. The nonlinear constitutive relations of isotropic materials are described in terms of two linear and three nonlinear elastic constants. For nonhyperelastic materials such as rocks, these constants are defined in terms of strain derivatives of stresses for either the load or unload cycle. Acoustic waveforms at an array of receivers recorded at two different borehole pressures can be used to estimate two of the three formation nonlinear constants. Processing of these time waveforms produced by a monopole or dipole source yields the Stoneley or flexural dispersions, respectively. The differences in the Stoneley and flexural dispersions caused by a known change in the borehole pressure are then utilized in a multifrequency inversion model that yields two of the three independent nonlinear constants of the formation. These two nonlinear constants, c144 and c155, are sufficient to calculate the difference between the maximum and minimum stresses in the azimuthal plane from the dipole anisotropy in the fast and slow shear‐wave velocities. In addition, these two formation nonlinear constants are also sufficient to estimate the stress derivatives, [Formula: see text] and [Formula: see text], for shear‐wave propagation normal to the uniaxial stress direction in a cylindrical rock sample of the same material as that of the in‐situ formation rock. Here ρ0 is the formation mass density in the reference state; [Formula: see text] denotes plane‐wave velocity for propagation along the [Formula: see text]-direction and polarization along the [Formula: see text]-direction; and S is the uniaxial stress magnitude applied normal to the propagation direction. Generally, a positive derivative indicates that the rock sample would stiffen with increasing uniaxial stress and a negative derivative indicates that it would soften.
CSAMT exploration at Sellafield: Characterization of a potential radioactive waste disposal site
Authors: Unsworth Martyn J. , Lu Xinyou , Watts M. Don
Published: October 2002

The long term disposal of radioactive waste in an underground repository requires the detailed geological evaluation of a potential site. Owing to their inherent sensitivity to the presence of fluids in rocks, electromagnetic (EM) methods have an important role in this assessment. Controlled‐source EM techniques are especially useful in strong anthropogenic noise environments such as industrial locations. However the complexity of modeling and inversion can limit the quantitative interpretation of controlled‐source EM data. A potential radioactive waste disposal site at Sellafield in Great Britain has been investigated using a variety of EM exploration techniques. Controlled‐source audio‐frequency magnetotelluric (CSAMT) data have given the best subsurface information in an environment that has a high level of cultural noise. One‐dimensional inversions of the Sellafield CSAMT data were found to be inadequate; 2.5-D forward modeling and inversion were used to interpret the data. The resulting resistivity models show good agreement with well log data collected at the site. These resistivity models show the presence of a large zone of hypersaline groundwater extending 1 km inland towards the potential repository and indicate the effect of faults on the hydrogeology.
Can we quantitatively image complex structures with rays?
Authors: Operto M. Stéphane , Xu Sheng , Lambaré Gilles
Published: October 2002

Ray‐based prestack depth migration fails to image quantitatively complex structures when a single arrival—for example, the first or the strongest one—is taken into account. In this paper, we investigate whether accounting for multiple arrivals in ray‐based preserved amplitude prestack depth migration allows one to improve quantitative imaging of complex media. The asymptotic ray‐Born migration/inversion, originally designed to process one single arrival, is extended to the case of multiple arrivals by accounting for the cross‐contributions of all the source and receiver raypaths. Multiple arrivals in the folded ray fields are computed by a dynamic ray tracing based on a wavefront construction technique. With an application to the complex Marmousi model, we demonstrate that ray‐Born inversion can provide a reliable quantitative estimation of the relative impedance perturbation even in the complex deep part of the model, for which the amplitudes were underestimated drastically when only a single arrival was used, either the first or the strongest one. This 2-D case study shows that complex structures can be imaged quantitatively with rays. Future studies will require the optimizing of the implementation of ray‐Born migration/inversion with multiple arrivals before considering 3-D applications.
3-D simulation of GPR surveys over pipes in dispersive soils
Authors: Wang Tsili , Oristaglio Michael L.
Published: October 2002

The finite‐difference time‐domain method is adapted to simulate radar surveys of objects buried in dispersive soils whose complex permittivity depends on frequency. The method treats dispersion through the constitutive relation between the electric field vector and the electric displacement vector, which is a convolution in the time domain. This convolution is updated recursively, along with Maxwell’s equations, after approximating the dispersion with a Debye (exponential) relaxation model. A novel feature of our work is the inclusion of dispersion in the perfectly‐matched layer formulation of Maxwell’s equations, which gives an absorbing boundary condition for dispersive media. We simulate 200-MHz ground‐penetrating radar surveys over metallic and plastic pipes buried at a depth of 2 m in soils whose electrical properties model are those of clay loams of different moisture contents. Radar reflections modeled for pipes in dispersive soil differ from those for pipes in soils whose electrical properties are constant (at the values of dispersive soil at the central frequency of the radar pulse). Because the permittivity decreases at higher frequencies in the soils modeled, energy in the reflections shifts toward the front of the waveform, and the amplitudes of trailing lobes in the waveform are suppressed. The effects are subtle, but become more pronounced in models of soils with 10% moisture content by weight.
Multisite, multifrequency tensor decomposition of magnetotelluric data
Authors: McNeice Gary W. , Jones Alan G.
Published: October 2002

Accurate interpretation of magnetotelluric data requires an understanding of the directionality and dimensionality inherent in the data, and valid implementation of an appropriate method for removing the effects of shallow, small‐scale galvanic scatterers on the data to yield responses representative of regional‐scale structures. The galvanic distortion analysis approach advocated by Groom and Bailey has become the most adopted method, rightly so given that the approach decomposes the magnetotelluric impedance tensor into determinable and indeterminable parts, and tests statistically the validity of the galvanic distortion assumption. As proposed by Groom and Bailey, one must determine the appropriate frequency‐independent telluric distortion parameters and geoelectric strike by fitting the seven‐parameter model on a frequency‐by‐frequency and site‐by‐site basis independently. Although this approach has the attraction that one gains a more intimate understanding of the data set, it is rather time‐consuming and requires repetitive application. We propose an extension to Groom‐Bailey decomposition in which a global minimum is sought to determine the most appropriate strike direction and telluric distortion parameters for a range of frequencies and a set of sites. Also, we show how an analytically‐derived approximate Hessian of the objective function can reduce the required computing time. We illustrate application of the analysis to two synthetic data sets and to real data. Finally, we show how the analysis can be extended to cover the case of frequency‐dependent distortion caused by the magnetic effects of the galvanic charges.
Understanding seismic attributes
Authors: Brown Alistair R.
Published: October 2002

With the success of 3-D surveys has come the popularity of seismic attributes. Attributes are valuable for gaining insight from the data particularly when displayed spatially over interpreted horizons. However, all the many attributes available are not independent pieces of information but, in fact, simply represent different ways of presenting a limited amount of basic information. The key to success lies in selecting the most applicable attribute for the problem. Furthermore, statistical analysis using attributes must be based on understanding, not simply mathematical correlation.
Time‐lapse seismic reservoir monitoring
Authors: Lumley David E.
Published: October 2002

Time‐lapse seismic reservoir monitoring has advanced rapidly over the past decade. There are currently about 75 active projects worldwide, and more than 100 cumulative projects in the past decade or so. The present total annual expenditures on 4-D seismic projects are on the order of $50–100 million US. This currently represents a much smaller market than 3-D seismic, but the use of 4-D seismic has grown exponentially over the past decade and is expected to continue to do so.
USE OF THE GRAVITY METER IN ESTABLISHMENT OF GRAVITY BENCH MARKS
Authors: McCollum E. V. , Brown Andrew
Published: October 2002

Data are presented showing comparisons between gravity meters and networks of gravity meter bench marks. Methods of adjustment of networks for closure are suggested. Ties to pendulum stations of the U. S. Coast and Geodetic Survey and the Dominion Observatory of Canada afford a basis for the calibration of the gravity meters used and the evaluation of the gravity meter bench marks in terms of the world gravity network. Probable errors and accuracies are discussed.
Reply by author to discussion by Amalendu Roy
Authors: Dampney C. N. G.
Published: October 2002

Fundamentally, in reply to point (3) of Roy’s discussion, the term “potential” has been used for physical functions which obey Laplace’s equation or the potential equation (Courant and Hilbert, 1962, p. 240) [Formula: see text] where [Formula: see text] (1) for [Formula: see text] and [Formula: see text] (with [Formula: see text]). Courant and Hilbert call the solution of equation (1) potential functions or harmonic functions. A large number of authors including Jeffreys (1956), Kellogg (1953), and Grant and West (1965), have considered the properties of “potential fields” for [Formula: see text]. This is the context in which I have used the term “potential.”
Reply by the authors to J. Roy
Authors: Vallee Marc A. , Chouteau Michel , Palacky George J.
Published: October 2002

We agree with J. Roy’s suggestion of using a three‐component system. Ping et al. (1990) also presented preliminary results in this direction. However, we are not aware of any three‐component system presently available on the North American market. Platform attitude measurements are also desirable but have not been integrated in existing geophysical platforms.
Correlation length and fractal dimension interpretation from seismic data using variograms and power spectra
Authors: Mela Ken , Louie John N.
Published: October 2002

Recent modeling techniques for fluid flow in a reservoir or an aquifer require characterizing the statistics of the spatial variability of physical properties such as porosity or reflectivity. The major interest of such modeling is in horizontal variations. We acquired a nontraditional seismic survey and analyzed it for the statistical parameters of correlation length and fractal dimension. Variograms and power spectra can extract these parameters from migrated sections. Statistical parameters are valuable for newly established hydrogeologic modeling techniques, with their definition greatly increasing the understanding of the aquifer under study. Applying similar techniques may also prove valuable for reservoir exploitation in the petroleum industry. We acquired our survey on top of a bench of an open‐pit diatomite quarry and extracted the statistical parameters for an exposed vertical face. The imaged zone of interest is therefore physically accessible to future permeability studies that could validate our interpreted parameters. We can interpret horizontal, but not vertical, correlation lengths and fractal dimensions from either variograms or power spectra of our migrated seismic images. The horizontal correlation length and fractal dimension extracted from the seismic variograms match those extracted from photographs taken of the quarry face.
A comparison of data from airborne, semi‐airborne, and ground electromagnetic systems
Authors: Smith Richard S. , Annan A. Peter , McGowan Patrick D.
Published: October 2002

The region around a small conductive massive sulfide body near Sudbury, Ontario, Canada, was used as a test site to compare airborne and ground electromagnetic (EM) systems with a new experimental EM system that uses a ground‐based transmitter and an airborne receiver. In this test survey, the semi‐airborne data were acquired with the transmitter loop used for the ground survey and the receiver normally used for the airborne system. At the time the data were acquired, there was no synchronization between the semi‐airborne receiver and the ground transmitter. However, subsequent digital processing of the full waveform data allowed the zero‐time position to be defined. The data could then be stacked and windowed. The ratio of the peak signal to the late‐time noise level for the airborne data is about 25:1, the semi‐airborne data shows signal‐to‐noise ratios of 500:1, while the signal‐to‐noise ratio for the ground data has a ratio of 50 000:1. This particular conductor is very close to the ground transmitter and receiver, so the signal‐to‐noise ratio for the ground system is very high. Numerical modeling shows that the marked advantage of the ground system is reduced when the conductor is deeper. However, the semi‐airborne system will generally show signal‐to‐noise intermediate between the airborne and ground systems. From an operational perspective, the semi‐airborne system has features of both the ground and airborne systems. Like the ground system, it is necessary to lay a transmitter loop on the ground; but because an aircraft is used, the semi‐airborne receiver can cover the survey area much more quickly.
Derivation of a Termination‐of‐resuscitation Guideline for Emergency Medical Technicians Using Automated External Defibrillators
Authors: Verbeek P. Richard , Vermeulen Marian J. , Ali Fahim H. , Messenger David W. , Summers Jim , Morrison Laurie J.
Published: October 2002

cardiac arrest and survival to hospital discharge following failed resuscitation by defibrillation‐trained emergency medical technicians (EMT‐Ds), and to propose an out‐of‐hospital termination‐of‐resuscitation (TOR) guideline for EMT‐Ds. Methods: A 22‐month retrospective review of 700 out‐of‐hospital primary cardiac arrest patients in a large emergency medical services (EMS) system who received exclusively EMT‐D care. Results: Seven hundred primary cardiac arrest patients were identified. Follow‐up was obtained in 662 cases (94.6%). Of these, 36 (5.4%) achieved a return of spontaneous circulation (ROSC) prior to transport. Among the 626 patients who failed to achieve ROSC at any time, two (0.3%) survived to discharge. Multivariate analysis showed that ROSC at any time had the strongest association with survival [odds ratio (OR) 45.5; 95% confidence interval (95% CI) = 8.5 to 243.7]. A shock prior to transport (OR 6.9; 95% CI = 1.2 to 40.3) and cardiac arrest witnessed by EMS personnel (OR 4.4; 95% CI = 1.0 to 18.5) were also independently associated with survival. These variables were incorporated into a TOR guideline. The guideline was 100% sensitive (95% CI = 99.1 to 100) in identifying survivors and had 100% negative predictive value (95% CI = 75.3 to 100) for identifying nonsurvivors of out‐of‐hospital cardiac arrest in the study population. Conclusions: In this EMS system, cardiac arrest patients may be considered for out‐of‐hospital TOR following EMT‐D resuscitation attempts when there has been no ROSC, no shock has been given, and the arrest was not witnessed by EMS personnel. These guidelines require prospective validation.
Survival of Patients With Newly Diagnosed Glioblastoma Multiforme Treated With RSR13 and Radiotherapy: Results of a Phase II New Approaches to Brain Tumor Therapy CNS Consortium Safety and Efficacy Study
Authors: Kleinberg L. , Grossman S.A. , Carson K. , Lesser G. , O’Neill A. , Pearlman J. , Phillips P. , Herman T. , Gerber M.
Published: October 2002

PURPOSE: The objectives of this phase II study were to determine survival, safety, pharmacokinetics (PK), and pharmacodynamics (PD) of 2,4-[[(3,5-dimethylanilino)carbonyl]methyl]phenoxy]-2-methylpropionic acid (RSR13, efaproxiral) 100 mg/kg per day administered with standard cranial radiotherapy (RT) for the treatment of glioblastoma multiforme (GBM). RSR13, a synthetic allosteric modifier of hemoglobin, is a radiation-enhancing agent that noncovalently binds to hemoglobin, reduces oxygen-binding affinity, and increases oxygen unloading to hypoxic tissue.

PATIENTS AND METHODS: Fifty patients with newly diagnosed GBM (Karnofsky performance status ≥ 60) were enrolled onto this multicenter phase II study. Patients received daily RSR13 100 mg/kg intravenously infused for 30 minutes immediately before cranial RT (60 Gy in 30 fractions). Supplemental oxygen was given during RSR13 infusion and continued until after the RT treatment was completed. RT was given within 30 minutes of the end of RSR13 infusion. PK and PD determinations were performed.

RESULTS: The median survival for the RSR13-treated patients was 12.3 months with 1-year and 18-month survival rates of 54% and 24%, respectively. Twenty-four percent of patients had greater than grade 2 toxicity, which was generally transient and self-limited. A significant PD effect on hemoglobin-oxygen binding affinity was demonstrated for most patients.

CONCLUSION: RSR13 (100 mg/kg) administered immediately before cranial RT is well tolerated and is pharmacodynamically active. Median survival in excess of 1 year is favorable.
Role of Chest Computed Tomography at Diagnosis in the Management of Wilms’ Tumor: A Study by the United Kingdom Children’s Cancer Study Group
Authors: Owens C. M. , Veys P. A. , Pritchard J. , Levitt G. , Imeson J. , Dicks-Mireaux C.
Published: October 2002

PURPOSE: This study sought to determine whether the identification of minimal pulmonary metastatic disease by chest computed tomography (CT) performed at diagnosis in patients with Wilms’ tumor and normal chest x-rays (CXR) could predict a subgroup of children at increased risk of pulmonary relapse.

PATIENTS AND METHODS: A retrospective analysis was carried out of the records of 449 children entered onto the United Kingdom Childrens’ Cancer Study Group Second Wilms’ Tumor Study between July 1986 and September 1991. The imaging protocol did not stipulate chest CT at diagnosis, but 141 children who had normal frontal and lateral CXRs and a chest CT scan performed at diagnosis were eligible for analysis. After surgery, children with stage I Wilms’ tumor received single-agent chemotherapy (vincristine), whereas children with stages II, III, and bilateral Wilms’ tumor received combination chemotherapy. Most children with stage III tumors were also treated with abdominal radiotherapy (20 Gy).

RESULTS: In 31 patients (22%), pulmonary nodules were visible on chest CT; eight experienced relapse, four (15%) in the lungs. When only stage I patients were analyzed, there was a significant difference between the pulmonary relapse rate of 43% (three of seven) in the CT-positive group and 10% (five of 48) in the CT-negative group (P = .02). Four of eight patients with stage I disease with pulmonary relapse died.

CONCLUSION: CT seemed to identify a subgroup of stage I patients who were at increased risk of pulmonary relapse. These children had received only single-agent chemotherapy. A prospective randomized trial is needed to clarify whether these children would benefit from combination chemotherapy.
Response to Neoadjuvant Chemotherapy Combined With Regional Hyperthermia Predicts Long-Term Survival for Adult Patients With Retroperitoneal and Visceral High-Risk Soft Tissue Sarcomas
Authors: Wendtner Clemens-M. , Abdel-Rahman Sultan , Krych Matthäus , Baumert Jens , Lindner Lars H. , Baur Andrea , Hiddemann Wolfgang , Issels Rolf D.
Published: October 2002

PURPOSE: To determine the efficacy of neoadjuvant chemotherapy combined with regional hyperthermia (RHT) for local tumor control and overall survival (OS) in adult patients with retroperitoneal or visceral (RP/V) high-risk soft tissue sarcomas (HR-STS).

PATIENTS AND METHODS: From 1991 to 1997, 58 patients with HR-STS at RP/V sites were prospectively treated with four cycles of etoposide, ifosfamide, and doxorubicin combined with RHT followed by surgery, adjuvant chemotherapy, and radiation.

RESULTS: Objective response rate assessable in 40 patients was 13% (five partial responses). Including minor responses (n = 8), the radiographic response rate was 33%. The pathologic response rate assessable in 26 patients after surgical resection was 42%. Median OS was 31 months. At a median observation time of 74 months, 5-year probability of local failure-free survival (LFFS), distant metastasis-free survival, event-free survival, and OS were 25%, 51%, 20%, and 32%, respectively. Averaged minimum temperatures (Tmin) and time-averaged temperatures achieved in 50% (T50) and 90% (T90) of all measured tumor sites differed significantly between responders and nonresponders (Tmin, 39.3°C v 38.0°C; P = .002; T50, 40.9°C v 40.3°C; P = .038; T90, 40.1°C v 39.3°C; P = .017). At 5-year follow-up, probability of LFFS (59% v 0%; P < .001) and OS (60% v 10%; P < .001) was significantly in favor of patients responding to neoadjuvant thermochemotherapy.

CONCLUSION: Response to neoadjuvant chemotherapy combined with RHT is predictive for an improved local tumor control resulting in a long-term survival benefit for patients with HR-STS at unfavorable RP/V sites; however, the impact of RHT has to be defined in a randomized phase III trial.